TUDOR INVESTMENT CORP ET AL
SC 13D, 1998-02-19
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<PAGE>
 
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                             ______________________



                                  SCHEDULE 13D
                   Under the Securities Exchange Act of 1934



                               (Amendment No.  )



                        North American Scientific, Inc.
                     -------------------------------------
                               (Name of Issuer)



                         Common Stock, $0.01 par value
                        ------------------------------
                        (Title of Class of Securities)



                                   65715D100
                                ---------------
                                (CUSIP Number)



                              Andrew S. Paul, Esq.
                        c/o Tudor Investment Corporation
                         One Liberty Plaza (51st Floor)
                           New York, New York  10006
                                (212) 602-6700
                          ---------------------------
                 (Name, Address and Telephone Number of Person
               Authorized to Receive Notices and Communications)



                               November 13, 1997
                               -----------------
            (Date of Event which Requires Filing of this Statement)



If the filing person has previously filed a statement on Schedule 13G to report
the acquisition which is the subject of this Schedule 13D, and is filing this
schedule because of Rule 13d-1(b)(3) or (4), check the following box [ ].
<PAGE>
 
- --------------------
CUSIP NO. 65715D100
- --------------------
- -----------------------------------------------------------------

1    NAME OF REPORTING PERSON

     S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
     Tudor Investment Corporation
     TIN: 22-2514825
- -----------------------------------------------------------------

2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

                                                  (a)  [  ]
                                                  (b)  [X ]
- -----------------------------------------------------------------
3.  SEC USE ONLY

- -----------------------------------------------------------------

4.   SOURCE OF FUNDS

     OO
- -----------------------------------------------------------------

5.   CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
     REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)
 
                                                       [X ]
- -----------------------------------------------------------------

6.   CITIZENSHIP OR PLACE OF ORGANIZATION
     Delaware
- -----------------------------------------------------------------

                7     SOLE VOTING POWER
  NUMBER OF           0
  SHARES        -------------------------------------------------
BENEFICIALLY    8     SHARED VOTING POWER
  OWNED BY            284,700
  EACH          -------------------------------------------------
 REPORTING      9     SOLE DISPOSITIVE POWER
  PERSON              0
  WITH          -------------------------------------------------
                10    SHARED DISPOSITIVE POWER
                      284,700
- -----------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
     PERSON
                      284,700
- -----------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
     CERTAIN SHARES
                                                  [  ]
- -----------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
                      6.7%
- -----------------------------------------------------------------
14   TYPE OF REPORTING PERSON
     CO
- -----------------------------------------------------------------
<PAGE>
 
- --------------------
CUSIP NO. 65715D100
- --------------------
- -----------------------------------------------------------------

1   NAME OF REPORTING PERSON

    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
    Paul Tudor Jones, II
    TIN:
- -----------------------------------------------------------------

2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                          (a)  [  ]
                                          (b)  [X ]
- -----------------------------------------------------------------
3.  SEC USE ONLY

- -----------------------------------------------------------------

4.  SOURCE OF FUNDS

    OO
- -----------------------------------------------------------------

5.  CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
    REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)
 
                                                [  ]
- ----------------------------------------------------------------
 6. CITIZENSHIP OR PLACE OF ORGANIZATION
    United States of America
- ----------------------------------------------------------------
                        7   SOLE VOTING POWER
  NUMBER OF                 0
    SHARES              ----------------------------------------
BENEFICIALLY            8   SHARED VOTING POWER
  OWNED BY                  300,000
    EACH                ----------------------------------------
 REPORTING              9   SOLE DISPOSITIVE POWER
   PERSON                   0
    WITH                ----------------------------------------
                        10  SHARED DISPOSITIVE POWER
                            300,000
- ----------------------------------------------------------------
11  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
    PERSON
                            300,000
- -----------------------------------------------------------------
12  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES
                                                  [  ]
- -----------------------------------------------------------------
13  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
                            7.1%
- -----------------------------------------------------------------
14  TYPE OF REPORTING PERSON
    IN
- -----------------------------------------------------------------
<PAGE>
 
- --------------------
CUSIP NO. 65715D100
- --------------------
- -----------------------------------------------------------------

1   NAME OF REPORTING PERSON

    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
    The Raptor Global Fund Ltd.
    TIN:  n/a
- -----------------------------------------------------------------

2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

                                                  (a)  [  ]
                                                  (b)  [X ]
- -----------------------------------------------------------------
3.  SEC USE ONLY

- -----------------------------------------------------------------

4.   SOURCE OF FUNDS
     OO
- -----------------------------------------------------------------

5.   CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
     REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)
                                                         [  ]
- -----------------------------------------------------------------
6.   CITIZENSHIP OR PLACE OF ORGANIZATION
     Cayman Islands
- -----------------------------------------------------------------
                  7   SOLE VOTING POWER
  NUMBER OF           0
    SHARES        -----------------------------------------------
BENEFICIALLY      8   SHARED VOTING POWER
  OWNED BY            125,100
    EACH          -----------------------------------------------
 REPORTING        9   SOLE DISPOSITIVE POWER
   PERSON             0
    WITH          -----------------------------------------------
                  10  SHARED DISPOSITIVE POWER
                      125,100
- -----------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
     PERSON
                      125,100
- -----------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
     CERTAIN SHARES
                                                          [  ]
- -----------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
                      2.9%
- -----------------------------------------------------------------
14   TYPE OF REPORTING PERSON
     CO
- -----------------------------------------------------------------
<PAGE>
 
- --------------------
CUSIP NO. 65715D100
- --------------------
- -----------------------------------------------------------------

1   NAME OF REPORTING PERSON

    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
    The Raptor Global Fund L.P.
    TIN: 13-3735415
- -----------------------------------------------------------------

2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                          (a)  [  ]
                                          (b)  [X ]
- -----------------------------------------------------------------
3.  SEC USE ONLY

- -----------------------------------------------------------------
4.   SOURCE OF FUNDS
     OO
- -----------------------------------------------------------------
5.   CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
     REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)
 
                                                [  ]
- -----------------------------------------------------------------
6.   CITIZENSHIP OR PLACE OF ORGANIZATION
     Delaware
- -----------------------------------------------------------------
                7   SOLE VOTING POWER
  NUMBER OF          0
  SHARES        -------------------------------------------------
BENEFICIALLY    8   SHARED VOTING POWER
  OWNED BY          45,300
  EACH          -------------------------------------------------
 REPORTING      9   SOLE DISPOSITIVE POWER
  PERSON            0
  WITH          -------------------------------------------------
                10  SHARED DISPOSITIVE POWER
                    45,300
- -----------------------------------------------------------------
11  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
    PERSON
                    45,300
- -----------------------------------------------------------------
12  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
    CERTAIN SHARES
                                                  [  ]
- -----------------------------------------------------------------
13  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
                     1.1%
- -----------------------------------------------------------------
14  TYPE OF REPORTING PERSON
    PN
- -----------------------------------------------------------------
<PAGE>
 
- --------------------
CUSIP NO. 65715D100
- --------------------
- -----------------------------------------------------------------

1   NAME OF REPORTING PERSON

    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
    Tudor Arbitrage Partners L.P.
    TIN:  13-3496979
- -----------------------------------------------------------------
2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                               (a)  [  ]
                                               (b)  [X ]
- -----------------------------------------------------------------
3.  SEC USE ONLY

- -----------------------------------------------------------------
4.  SOURCE OF FUNDS
    OO
- -----------------------------------------------------------------
5.  CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
    REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)
                                                [  ]
- -----------------------------------------------------------------
6.  CITIZENSHIP OR PLACE OF ORGANIZATION
    Delaware
- -----------------------------------------------------------------
                7   SOLE VOTING POWER
  NUMBER OF         0
  SHARES        -------------------------------------------------
BENEFICIALLY    8   SHARED VOTING POWER
  OWNED BY          15,300  
   EACH         ------------------------------------------------- 
  REPORTING     9   SOLE DISPOSITIVE POWER                        
  PERSON            0                                             
  WITH          ------------------------------------------------- 
                10  SHARED DISPOSITIVE POWER                      
                    15,300                                         
- -----------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
     PERSON
                    15,300
- -----------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
     CERTAIN SHARES
                                                  [  ]
- -----------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
                     0.4%
- -----------------------------------------------------------------
14   TYPE OF REPORTING PERSON
     PN
- -----------------------------------------------------------------
<PAGE>
 
- --------------------
CUSIP NO. 65715D100
- --------------------
- -----------------------------------------------------------------

1   NAME OF REPORTING PERSON

    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
    Tudor Global Trading LLC
    TIN:  13-3862744
- -----------------------------------------------------------------
2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                          (a)  [  ]
                                          (b)  [X ]
- -----------------------------------------------------------------
3.  SEC USE ONLY

- -----------------------------------------------------------------
4.   SOURCE OF FUNDS
     OO
- -----------------------------------------------------------------
5.   CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
     REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)
                                                [  ]
- -----------------------------------------------------------------
6.   CITIZENSHIP OR PLACE OF ORGANIZATION
     Delaware
- -----------------------------------------------------------------
                7   SOLE VOTING POWER
  NUMBER OF         0
  SHARES        -------------------------------------------------
BENEFICIALLY    8   SHARED VOTING POWER
  OWNED BY          15,300
  EACH          -------------------------------------------------
 REPORTING      9   SOLE DISPOSITIVE POWER 
  PERSON            0
  WITH          -------------------------------------------------
                10  SHARED DISPOSITIVE POWER
                    15,300
- -----------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
     PERSON
                    15,300
- -----------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
     CERTAIN SHARES
                                                       [  ]
- -----------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
                     0.4%
- -----------------------------------------------------------------
14  TYPE OF REPORTING PERSON
    OO
- -----------------------------------------------------------------
<PAGE>
 
- --------------------
CUSIP NO. 65715D100
- --------------------
- -----------------------------------------------------------------
1   NAME OF REPORTING PERSON

    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
    Tudor BVI Futures, Ltd.
    TIN: n/a
- -----------------------------------------------------------------
2   CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
                                          (a)  [  ]
                                          (b)  [X ]
- -----------------------------------------------------------------
3.  SEC USE ONLY

- -----------------------------------------------------------------
4.  SOURCE OF FUNDS
    OO
- -----------------------------------------------------------------
5.  CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS
    REQUIRED PURSUANT TO ITEMS 2(d) OR 2(e)
                                                [  ]
- -----------------------------------------------------------------
6.  CITIZENSHIP OR PLACE OF ORGANIZATION
    British Virgin Islands
- -----------------------------------------------------------------
                7   SOLE VOTING POWER
  NUMBER OF         0
    SHARES      -------------------------------------------------
BENEFICIALLY    8   SHARED VOTING POWER
  OWNED BY          114,300
    EACH        -------------------------------------------------
 REPORTING      9   SOLE DISPOSITIVE POWER
   PERSON           0
    WITH        -------------------------------------------------
                10  SHARED DISPOSITIVE POWER
                    114,300
- -----------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING
     PERSON
                    114,300
- -----------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES
     CERTAIN SHARES
                                                       [  ]
- -----------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
                    2.7%
- -----------------------------------------------------------------
14   TYPE OF REPORTING PERSON
     CO
- -----------------------------------------------------------------
<PAGE>
 
Item 1.  Security and Issuer
         -------------------

  This Schedule 13D, dated February 19, 1998, relates to the Common Stock, $0.01
par value per share ("Common Stock"), of North American Scientific, Inc., a
Delaware corporation (the "Company"), and is filed by Tudor Investment
Corporation, a Delaware corporation ("TIC"), Paul Tudor Jones, II, a natural
person and a citizen of the United States ("Mr. Jones"), The Raptor Global Fund
Ltd., a company organized under the laws of the Cayman Islands ("Raptor Ltd."),
The Raptor Global Fund L.P., a Delaware limited partnership ("Raptor L.P."),
Tudor Arbitrage Partners L.P., a Delaware limited partnership ("TAP"), Tudor
Global Trading LLC, a Delaware limited liability company ("TGT"), and Tudor BVI
Futures, Ltd., a corporation organized under the laws of the British Virgin
Islands ("Tudor BVI" and collectively with TIC, Mr. Jones, Raptor Ltd., Raptor
L.P., TAP and TGT, the "Reporting Persons").*  The summaries of information on
schedules attached hereto are qualified in their entirety by reference to such
schedules, which are incorporated by reference herein.

  The Company's principal executive offices are located at 7435 Greenbush
Avenue, North Hollywood, CA 91605.


Item 2.  Identity and Background.
         ----------------------- 

  The Reporting Persons are TIC, Mr. Jones, Raptor Ltd., Raptor L.P., TAP, TGT
and Tudor BVI.  The business address of each of TIC, Mr. Jones, Raptor L.P., TAP
and TGT is c/o Tudor Investment Corporation, 600 Steamboat Road, Greenwich,
Connecticut 06830.  The business address of each of Raptor Ltd. and Tudor BVI is
c/o Curacao International Trust Company N.V., Kaya Flamboyan 9, Curacao,
Netherlands Antilles.

  TIC is an international money management firm that provides investment advice
to Raptor Ltd., Raptor L.P. and Tudor BVI, among others. TIC is also the sole
general partner of Raptor L.P.

  Mr. Jones' principal employment is as Chairman and Chief Executive Officer of
TIC of which he owns a majority of the capital stock and voting securities.

  Each of Raptor Ltd., Raptor L.P., TAP and Tudor BVI is an investment fund
which principally invests in debt, equity, derivative securities and other
financial instruments for the benefit of the holders of its partnership, stock
and other capital securities.

- --------
*  For purposes of this Statement on Schedule 13D the Reporting Persons have
filed as a "group".  Nevertheless, the Reporting Persons hereby disclaim that
they are members of a "group" for purposes of Section 13(d) of the Securities
Exchange Act of 1934 or for any other purpose.
<PAGE>
 
  TGT is the sole general partner of TAP.

  The name, residence or business address, present principal occupation or
employment, the name, principal business and address of any corporation or other
organization in which such employment is conducted and the citizenship of each
natural person that is a director or executive officer of TIC, TGT, Raptor Ltd.
or Tudor BVI is set forth on Schedule I hereto and is incorporated by reference
herein.

  During the last five years, none of the Reporting Persons, or to the best
knowledge and belief of the Reporting Persons, any of the individuals listed on
Schedule I hereto, has been convicted in a criminal proceeding (excluding
traffic violations or similar misdemeanors) or has been a party to a civil
proceeding of a judicial or administrative body of competent jurisdiction as a
result of which any Reporting Person or any person listed on Schedule I was or
is subject to a judgment, decree or final order enjoining future violations of,
or prohibiting activities subject to, federal or state securities laws or
finding any violation of such law, except as follows:

  On September 12, 1996, TIC settled a proceeding with the Securities and
Exchange Commission (the "SEC") relating to alleged violations of the "uptick
rule" in connection with certain sales of stock over a two day period in March
1994.  Without admitting or denying the SEC's findings, TIC paid a civil penalty
of $800,000, and agreed not to violate the uptick rule in the future.

Item 3.  Source and Amount of Funds or Other Consideration.
         ------------------------------------------------- 

  As more fully set forth on Schedule II hereto, on November 13, 1997, Raptor
L.P., Raptor Ltd., TAP and Tudor BVI purchased 125,100, 45,300, 15,300 and
114,300 shares of Common Stock, respectively.  Such shares were acquired by each
such Reporting Person in a private transaction with the Company for cash funded
by (i) capital contributions to such Reporting Person and (ii) the proceeds of
transactions with respect to prior investments held by such Reporting Person.


Item 4.  Purpose of Transaction.
         ---------------------- 

  Each of Raptor Ltd., Raptor L.P., TAP and Tudor BVI initially acquired the
Common Stock beneficially owned by such Reporting Person for investment
purposes.  Each of such Reporting Persons intends to continue to evaluate their
respective investments in Common Stock based on the Company's financial
condition, results of operations and prospects as well as other then existing or
anticipated facts and circumstances including general economic, market and other
financial conditions.  
<PAGE>
 
Accordingly, each of the Reporting Persons reserves the right to change its
plans and intentions with respect to its investment in securities of the Company
at any time, as it deems appropriate. In particular, each of the Reporting
Persons may at any time and from time to time (i) acquire additional shares of
Common Stock, (ii) dispose of Common Stock, or (iii) enter into privately
negotiated derivative transactions with institutional counterparties to hedge
the market risk of some or all of its positions in the Common Stock. There can
be no assurance that any Reporting Person will increase or decrease its
investment in the Company or as to the number of shares of Common Stock or
derivative securities that may be bought or sold in any such transactions.

  Except as set forth in this Schedule 13D, none of the Reporting Persons
currently has any plans or proposals which relate to or would result in any of
the transactions, actions or events enumerated in paragraphs (a) through (j) of
the instructions to Item 4 of Schedule 13D.

Item 5.  Interest in Securities of the Issuer.
         ------------------------------------ 

  Attached hereto as Schedule II is a list of the transactions by each of the
Reporting Persons in Common Stock since November 13, 1997.

  Because TIC is the sole general partner of Raptor L.P. and provides investment
advisory services to Raptor Ltd., Raptor L.P. and Tudor BVI, TIC may be deemed
to beneficially own the shares of Common Stock owned by each of such Reporting
Persons.  TGT, as the sole general partner of TAP, may be deemed to beneficially
own the shares of Common Stock owned by TAP.  In addition, because Mr. Jones
owns a majority of the capital stock and voting securities of TIC and indirectly
owns a majority of the equity interests in TGT, Mr. Jones may be deemed to
beneficially own the shares of Common Stock deemed beneficially owned by TIC and
TGT.

  Based on information contained in the Company's Annual Report on Form 10-KSB
that there were 4,251,201 shares of Common Stock issued and outstanding as of
January 21, 1998, each Reporting Person beneficially owns (or, with respect to
TIC, Mr. Jones and TGT, may be deemed to beneficially own) the number and
percentage of outstanding shares of Common Stock listed in the responses to
Items 11 and 13, respectively, of the cover page filed herewith for such
Reporting Person.  In addition, the number of shares of Common Stock
beneficially owned (or, with respect to TIC, Mr. Jones and TGT, which may be
deemed beneficially owned) by each Reporting Person with respect to which such
Reporting Person (i) has sole voting power, (ii) shares voting power, (iii) has
sole dispositive power and (iv) shares dispositive power are listed in the
responses to Items 7, 8, 9 and 10, respectively, of the cover page filed
herewith for such Reporting Person.  The 
<PAGE>
 
voting and dispositive power is reported as shared because each of Raptor Ltd.,
Raptor L.P. and Tudor BVI has the power to remove TIC as its investment advisor
or general partner and because TAP has the power to remove TGT as its general
partner. Each of Raptor Ltd., Raptor L.P., TAP and Tudor BVI expressly disclaim
beneficial ownership of the shares of Common Stock beneficially owned by any
other such Reporting Person and each of TIC, Mr. Jones and TGT disclaim
beneficial ownership of the Common Stock beneficially owned by Raptor Ltd.,
Raptor L.P., TAP and Tudor BVI and, in the case of Mr. Jones, TIC and TGT.

Item 6.  Contracts, Arrangements, Understandings
         or Relationships with Respect to Securities
         of the Issuer
         -------------------------------------------

  Each of Raptor Ltd., Raptor L.P., TAP and Tudor BVI purchased their shares of
Common Stock pursuant to a Common Stock Purchase Agreement, made as of November
10, 1997 (the "Purchase Agreement"), between the Company and the investors (the
"Investors") named therein.

  Pursuant to the Purchase Agreement, the Company agreed to file with the SEC
within 15 days following the closing (the "Closing Date") of the sale of Common
Stock under the Purchase Agreement a registration statement on Form S-3 (the
"Registration Statement") covering resales of the shares of Common Stock
purchased by the Investors.  The Purchase Agreement provides that the Company
will use its best efforts to keep such registration statement effective until
the earlier of (a) the second anniversary of the Closing Date, (b) such date as
all of the shares of Common Stock so registered have been resold of (c) such
times as all of the shares of Common Stock so registered can be sold within a
given three-month period pursuant to Rule 144.

  The Company filed the Registration Statement with the SEC on November 26,
1997.  The SEC declared the Registration Effective on December 5, 1997.

  A form of the Purchase Agreement is attached as Exhibit 1 hereto.

Item 7.  Material to Be Filed as Exhibits
         --------------------------------

         (1)  Form of Common Stock Purchase Agreement
<PAGE>
 
  SIGNATURE


  After reasonable inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this Schedule 13D is true, complete and
correct.

Dated:  February 19, 1998


                            TUDOR INVESTMENT CORPORATION



                            By:   \s\ Andrew S. Paul
                                --------------------
                            Name:  Andrew S. Paul
                            Title: Vice President &
                                General Counsel



                               \s\ Paul Tudor Jones, II
                            ---------------------------
                            Paul Tudor Jones, II



                            THE RAPTOR GLOBAL FUND LTD.

                            By: TUDOR INVESTMENT CORPORATION,
                                  Trading Advisor


                                By:   \s\ Andrew S. Paul
                                    --------------------
                                Name:  Andrew S. Paul
                                Title: Vice President &
                                       General Counsel



                            THE RAPTOR GLOBAL FUND L.P.

                            By: TUDOR INVESTMENT CORPORATION,
                                  General Partner



                                By:   \s\ Andrew S. Paul
                                    --------------------
                                Name:  Andrew S. Paul
                                Title: Vice President &
                                       General Counsel
<PAGE>
 
                            TUDOR ARBITRAGE PARTNERS L.P.

                            By: TUDOR GLOBAL TRADING LLC,
                                  General Partner



                                By:   /s/ Andrew S. Paul
                                    --------------------
                                Name:  Andrew S. Paul
                                Title: Vice President &
                                       General Counsel



                            TUDOR GLOBAL TRADING LLC



                            By:   /s/ Andrew S. Paul
                                --------------------
                            Name:  Andrew S. Paul
                            Title: Vice President &
                                   General Counsel



                            TUDOR BVI FUTURES, LTD.

                            By: TUDOR INVESTMENT CORPORATION,
                                  Trading Advisor



                                By:   /s/ Andrew S. Paul
                                    --------------------
                                Name:  Andrew S. Paul
                                Title: Vice President &
                                       General Counsel


<PAGE>
 
                                                            Schedule I



                              EXECUTIVE OFFICERS AND DIRECTORS
                              OF THE REPORTING PERSONS


I.  TIC


  The name, residence or business address, present principal occupation or
employment, the name, principal business and address of any corporation or other
organization in which such employment is conducted and the citizenship of each
executive officer or director of TIC is set forth below. Unless otherwise
indicated (i) the principal occupation or employment of such person is as an
executive officer or director of TIC, (ii) the business address of such person
is c/o TIC at the address of TIC set forth under Item 2 of this Schedule 13D,
(iii) such person is a citizen of the United States and (iv) to the knowledge of
the Reporting Persons, such person does not beneficially own and has not had any
transactions in the Common Stock.


                         Principal Occupation/
Name                     Business Address
- ----                     -----------------------------

Paul Tudor Jones, II*    Chairman of the Board of Directors, Chief Executive
                         Officer of TIC.

Mark F. Dalton           Director, President and Chief Operating Officer of TIC.

James J. Pallotta        Director and Vice President of TIC.

David E. Allanson        Director of TIC.  Mr. Allanson trades in
Citizen of United        securities and other financial instruments
Kingdom                  for Tudor Proprietary Trading, L.L.C. and certain
                         investment funds advised by Tudor Capital (U.K.), L.P.,
                         both of which are located at The Upper Mill, Kingston
                         Road, Ewell, Surrey  KT17 2AF, England.

Andrew S. Paul           Director, Vice President, General Counsel and Corporate
                         Secretary of TIC.

Richard L. Fisher        Director of TIC.  Mr. Fisher is Vice President of
                         Investments and Acquisitions and a Managing Director of
                         Dunavant Enterprises, Inc., 3797 Getwell Road, Memphis,
                         TN  38118.

Mark Pickard             Director, Vice President and Chief Financial Officer of
                         TIC.

Robert P. Forlenza       Director and Vice President of TIC.

- ----------
* See Item 5 of this Schedule 13D for a discussion of Mr. Jones' potential
beneficial ownership of Common Stock.
<PAGE>
 
II.  RAPTOR LTD.

  The name, residence or business address, present principal occupation or
employment, the name, principal business and address of any corporation or other
organization in which such employment is conducted and the citizenship of each
executive officer or director of Raptor Ltd. is set forth below.  To the
knowledge of the Reporting Persons, such person does not beneficially own and
has not had any transactions in the Common Stock.


                         Principal Occupation/
Name                     Business Address
- ----                     -------------------------------


Inter Caribbean          Director of Raptor Ltd.  The principal
Services Ltd.            place of business of Inter Caribbean
Incorporated in          Services Ltd. is c/o Curacao
 British Virgin          International Trust Company N.V.
 Islands                 ("CITCO"), Kaya Flamboyan 9, Curacao, Netherlands
                         Antilles.

Bernard A. Loze          Director of Raptor Ltd.  Chairman and
Citizen of France        Chief Executive Officer of Loze et Associe, 43, Avenue
                         Marceau, Paris 75116, France, which is a consulting
                         firm that provides financial and investment advice to
                         international clients.

Arpad A. Busson          Director of Raptor Ltd.  Chairman of
Citizen of France        European Investment Managers, c/o Harney, Westwood &
                         Riegels, P.O. Box 71, Road Town, Tortola, British
                         Virgin Islands, which is a consulting and money
                         management firm.

Jean-Pierre Jacquemoud   Director of Raptor Ltd.  Attorney,
Citizen of Switzerland   Jacquemoud & Stanislas, 2, rue Bellow, Geneva 1206,
                         Switzerland.
<PAGE>
 
III.  TUDOR GLOBAL TRADING LLC


  The name and present principal occupation or employment of each executive
officer of TGT is set forth below.  Each of such persons holds identical titles
for TIC and TGT.  The business address of such person is c/o TIC at the address
of TIC set forth under Item 2 of this Schedule 13D, each such person is a
citizen of the United States and to the knowledge of the Reporting Persons, such
person does not beneficially own and has not had any transactions in the Common
Stock.

Name                   Principal Occupation
- ----                   ---------------------------------------

Paul Tudor Jones, II*  Chairman of the Board of Directors, Chief Executive
                       Officer of TIC.

Mark F. Dalton         Director, President and Chief Operating Officer of TIC.

James J. Pallotta      Director and Vice President of TIC.

Andrew S. Paul         Director, Vice President, General Counsel and Corporate
                       Secretary of TIC.

Mark Pickard           Director, Vice President and Chief Financial Officer of
                       TIC.

Robert P. Forlenza     Director and Vice President of TIC.

- --------
* See Item 5 of this Schedule 13D for a discussion of Mr. Jones' potential
beneficial ownership of Common Stock.
<PAGE>
 
IV.  TUDOR BVI

  The name, residence or business address, present principal occupation or
employment, the name, principal business and address of any corporation or other
organization in which such employment is conducted and the citizenship of each
executive officer or director of Tudor BVI is set forth below.  To the knowledge
of the Reporting Persons, the below listed persons neither currently
beneficially own nor have had any transactions in the Common Stock.

                         Principal Occupation/
Name                     Business Address
- ----                     -------------------------------

Inter Caribbean          Director of Tudor BVI.  See information
Services Ltd.            under Raptor Ltd.

David P. d'Abrumenil     Director of Tudor BVI.  Chairman,
Citizen of United        Lionspring Enterprises Limited,
Kingdom                  36 John Street, London WC1N 2AT, England, which is a
                         financial consulting firm.

Jean-Pierre Jacquemoud   Director of Tudor BVI.  See information under Raptor
                         Ltd.

Bernard A. Loze          Director of Tudor BVI.  See information under Raptor
                         Ltd.

Arpad A. Busson          Director of Tudor BVI.  See information under Raptor
                         Ltd.

Karl-Erbo G. Kageneck    Director of Tudor BVI. Attorney,
Citizen of Germany       TMW/AG, Wittelsbacher Platz 1, Munich Germany 80333
<PAGE>
 
                                                                     Schedule II

                                TRANSACTIONS

  All shares of Common Stock were acquired in connection with the Exchange.

Tudor BVI
- ---------

Date    Transaction  # Shares  $/Share
- ------  -----------  --------  --------

11/13/97  Purchase    114,300  $18.0000

Total Common Stock beneficially owned as of
the date of this Schedule 13D: 114,300


Raptor L.P.
- -----------

Date    Transaction  # Shares  $/Share
- ------  -----------  --------  --------

11/13/97  Purchase    45,300  $18.0000

Total Common Stock beneficially owned as of
the date of this Schedule 13D: 45,300


TAP
- ---

Date    Transaction  # Shares  $/Share
- ------  -----------  --------  --------

11/13/97  Purchase    15,300  $18.0000

Total Common Stock beneficially owned as of
the date of this Schedule 13D: 15,300


Raptor Ltd.
- -----------

Date    Transaction  # Shares  $/Share
- ------  -----------  --------  --------

11/13/97  Purchase    125,100  $18.0000

Total Common Stock beneficially owned as of
the date of this Schedule 13D: 125,100
<PAGE>
 
EXHIBIT INDEX
- -------------



Exhibit No.            Description
- -----------            ----------------------

    (1)                Form of Common Stock Purchase Agreement

<PAGE>
 
                                                                       EXHIBIT 1


                        NORTH AMERICAN SCIENTIFIC, INC.

                        COMMON STOCK PURCHASE AGREEMENT

                               NOVEMBER  , 1997
                                       __
<PAGE>
 
                               TABLE OF CONTENTS

                                                                     Page

SECTION 1.      AUTHORIZATION AND SALE OF SHARES......................  1

        1.1     Authorization ........................................  1
        1.2     Sale of Shares........................................  1

SECTION 2.      CLOSING DATE; DELIVERY................................  1

        2.1     Closing Date..........................................  1
        2.2     Delivery .............................................  1

SECTION 3.      REPRESENTATIONS AND WARRANTIES OF THE COMPANY.........  1

        3.1     Organization and Standing.............................  1
        3.2     Corporate Power; Authorization........................  2
        3.3     Issuance and Delivery of the Shares...................  2
        3.4     Memorandum; SEC Documents; Financial Statements.......  3
        3.5     Intellectual Property.................................  3
        3.6     Properties ...........................................  3
        3.7     Capitalization........................................  4
        3.8     Litigation............................................  4
        3.9     No Defaults...........................................  4
        3.10    Governmental Consents.................................  4
        3.11    Taxes ................................................  4
        3.12    Insurance.............................................  5
        3.13    No Material Adverse Change............................  5
        3.14    Disclosure ...........................................  5
        3.15    Nasdaq NMS Qualification .............................  5
        3.16    FDA Approval..........................................  5
        3.17    Investment Company....................................  5
        3.18    RadioMed..............................................  5

Section 4.      Representations, Warranties and Covenants
                of Purchasers.........................................  5

        4.1     Authorization ........................................  5
        4.2     Investment Experience ................................  6
        4.3     Investment Intent.....................................  6
        4.4     Registration or Exemption Requirements................  6
        4.5     Restriction on Short Sales ...........................  7
        4.6     No Legal, Tax or Investment Advice ...................  7

SECTION 5.      CONDITIONS TO CLOSING OF PURCHASERS...................  7

        5.1     Representations and Warranties........................  7
        5.2     Legal Opinion.........................................  7
        5.3     Officer's Certificate.................................  7
        5.4     Registration Statement ...............................  7
        5.5     SAS 71 Review.........................................  7
        5.6     Covenants ............................................  8
        5.7     Additional Documents .................................  8

SECTION 6.      CONDITIONS TO CLOSING OF COMPANY......................  8

        6.1     Receipt of Payment....................................  8
        6.2     Representations and Warranties........................  8
        6.3     Covenants ............................................  8

SECTION 7.      AFFIRMATIVE COVENANTS OF THE COMPANY..................  8

                                       i.
<PAGE>
 
                               TABLE OF CONTENTS
                                  (continued)

                                                                     Page

        7.1     Financial Information.................................  8
        7.2     Registration Requirements.............................  9
        7.3     Indemnification and Contribution ....................  11

  SECTION 8.    RESTRICTIONS ON TRANSFERABILITY OF SHARES: COMPLIANCE
                WITH SECURITIES ACT................................... 13

        8.1     Restrictions on Transferability ...................... 13
        8.2     Restrictive Legend.................................... 13
        8.3     Transfer of Shares after Registration ................ 13
        8.4     Purchaser Information................................. 14

  SECTION 9.    MISCELLANEOUS......................................... 14

        9.1     Waivers and Amendments................................ 14
        9.2     Broker's Fee.......................................... 14
        9.3     Governing Law......................................... 14
        9.4     Survival.............................................. 14
        9.5     Successors and Assigns................................ 14
        9.6     Entire Agreement ..................................... 14
        9.7     Notices, etc ......................................... 14
        9.8     Severability of this Agreement ....................... 15
        9.9     Counterparts.......................................... 15
        9.10    Further Assurances ................................... 15
        9.11    Expenses ............................................. 15


  Exhibit A     Schedule of Investors
  Exhibit B     Instruction Sheet for Purchaser

        Appendix I      Stock Certificate Questionnaire
        Appendix II     Registration Statement Questionnaire

  Exhibit C     Form of Legal Opinion
  Exhibit D     Purchaser's Certificate of Subsequent Sale

                                      ii.
<PAGE>
 
                        COMMON STOCK PURCHASE AGREEMENT

        This COMMON STOCK PURCHASE AGREEMENT (the "Agreement") is made as of
November ___, 1997 by and among NORTH AMERICAN SCIENTIFIC, INC., a Delaware
corporation (the "Company"), with its principal office at 7435 Greenbush Avenue,
North Hollywood, California 92618, and the persons listed on the Schedule of
Investors attached hereto as Exhibit A (the "Purchasers").

SECTION 1. AUTHORIZATION AND SALE OF SHARES

        1.1 Authorization. The Company has authorized the sale and issuance of
up to ___ shares of its common stock, par value $0.01 per share ("Common
Stock"), pursuant to this Agreement.

        1.2 Sale of Shares. Subject to the terms and conditions of this
Agreement, the Company agrees to issue and sell to each Purchaser and each
Purchaser severally agrees to purchase from the Company on the Closing Date
hereinafter defined, the number of shares of Common Stock set forth opposite
each Purchaser's name on Exhibit A (the "Shares") for a purchase price (the
"Purchase Price") $18.00 per share.

SECTION 2. CLOSING DATE; DELIVERY

        2.1 Closing Date. The closing of the purchase and sale of the Shares
(the "Closing") shall be held at the offices of D'Ancona & Pflaum, 30 N. LaSalle
Street, Suite 2900, Chicago, Illinois at 8:00 a.m. (Pacific Daylight Time), on
November ___, 1997 or at such other time and place upon which the Company and
Purchasers purchasing the majority of the Shares shall agree. The date of the
Closing is hereinafter referred to as the "Closing Date".

        2.2 Delivery. At the Closing, the Company shall deliver to each
Purchaser a certificate, registered in the Purchaser's name as shown on Exhibit
A, representing the number of Shares to be purchased by the Purchaser. Such
delivery shall be against payment of the Purchase Price therefor by wire
transfer of immediately available funds in the amount set forth opposite such
Purchaser's name on Exhibit A to a bank account designated in writing by the
Company to each Purchaser at least two (2) business days prior to Closing Date.
Each Purchaser shall only be obligated to pay the Purchase Price of the Shares
purchased by it.

SECTION 3. REPRESENTATIONS AND WARRANTIES OF THE COMPANY

        The Company hereby represents and warrants as of the date hereof to, and
covenants with, the Purchasers as follows:

        3.1 Organization and Standing. The Company has been duly incorporated
and is validly existing as a corporation in good standing under the laws of the
jurisdiction of its incorporation, has full corporate power and authority to own
or lease its properties and conduct

                                       1.
<PAGE>
 
its business presently and as proposed to be conducted and as described in the
Confidential Offering Memorandum, dated November 3, 1997, as amended by that
Addendum dated November 7, 1997 (together with the attachments thereto, the
"Memorandum") and is duly qualified as a foreign corporation and in good
standing in all jurisdictions in which the character of the property owned or
leased or the nature of the business transacted by it makes qualification
necessary (except where the failure to be so qualified would not have a Material
Adverse Effect (as defined below) on the business, properties, financial
condition or results or operations of the Company). Except for the Company's
ownership of all the outstanding shares of North American Scientific, Inc., a
California corporation, and certain shares of RadioMed Corporation ("RadioMed")
held on behalf of the Company by L. Michael Cutrer and Irwin J. Gruverman, the
Company has no subsidiaries or equity interest in any other entity.

        3.2 Corporate Power; Authorization. The Company has all requisite
corporate power, and has taken all requisite corporate action, to execute and
deliver this Agreement, to sell and issue the Shares and to carry out and
perform all of its obligations under this Agreement. This Agreement constitutes
the legal, valid and binding obligation of the Company, enforceable in
accordance with its terms, except (i) as limited by applicable bankruptcy,
insolvency, reorganization, moratorium or similar laws relating to or affecting
the enforcement of creditors' rights generally, (ii) as limited by equitable
principles generally and (iii) as to those provisions of Section 7.3 hereof
relating to indemnity or contribution. The execution and delivery of this
Agreement does not, and the performance of this Agreement and the compliance
with the provisions hereof and the issuance, sale and delivery of the Shares by
the Company will not conflict with, or result in a breach or violation of the
terms, conditions or provisions of, or constitute a default (or an event that
with notice or lapse of time or both would constitute a default) under, or
result in the creation or imposition of any lien pursuant to the terms of, the
Company's certificate of incorporation (the "Certificate of Incorporation") or
its bylaws (the "Bylaws") or, to the Company's knowledge, any statute, law, rule
applicable to the Company or regulation or any state or Federal order, judgment
or decree applicable to the Company or any indenture, mortgage, lease or other
agreement or instrument to which the Company or any of its properties is
subject, except as would not individually or in the aggregate have a material
adverse effect on, or result in a material adverse change in, the business,
properties, operation, condition (financial or other) or results of operations
of the Company, taken as a whole, or render this Agreement, or any portion
hereof, invalid or unenforceable or impair in any material respect the ability
of the Company to perform fully its obligations hereunder (any of the foregoing
shall be referred to herein as a "Material Adverse Effect").

        3.3 Issuance and Delivery of the Shares. The Shares have been duly
authorized and, when issued and paid for in compliance with the provisions of
this Agreement, will be validly issued, fully paid and nonassessable and will
conform to the description thereof contained in the Registration Statement (as
defined below). The issuance and delivery of the Shares is not subject to
preemptive, co-sale, right of first refusal or any other similar rights of the
stockholders of the Company or any liens or encumbrances. Except for certain
"piggyback" registration rights granted to M.H. Meyerson & Co., Inc.
("Meyerson") pursuant to that Letter Agreement dated as of December 11, 1996
between the Company and Meyerson, the Company has not granted any

                                       2.
<PAGE>
 
presently effective registration rights with respect to its securities other
than the registration rights set forth herein. No further approval or authority
of the stockholders or Board of Directors of the Company will be required for
the issuance and sale of the Shares.

        3.4 Memorandum; SEC Documents; Financial Statements. Each complete or
partial statement, report, or proxy statement included in the Memorandum is a
true and complete copy of or excerpt from such document as filed by the Company
with the Securities and Exchange Commission (the "SEC"). The Company has filed
in a timely manner all documents that the Company was required to file with the
SEC under Sections 13, 14(a) and 15(d) of the Securities Exchange Act of 1934,
as amended (the "Exchange Act"), during the twelve (12) months preceding the
date of this Agreement. As of their respective filing dates, all documents filed
by the Company with the SEC (the "SEC Documents") complied in all material
respects with the requirements of the Exchange Act. Neither the Memorandum nor
any of the SEC Documents as of their respective dates contained any untrue
statement of material fact or omitted to state a material fact required to be
stated therein or necessary to make the statements made therein, in light of the
circumstances under which they were made, not misleading. The financial
statements of the Company included in the SEC Documents and the Memorandum (the
"Financial Statements") comply as to form in all material respects with
applicable accounting requirements and with the published rules and regulations
of the SEC with respect thereto. The Financial Statements have been prepared in
accordance with generally accepted accounting principles consistently applied
and fairly present the financial position of the Company at the dates thereof
and the results of its operations and cash flows for the periods then ended
(subject, in the case of unaudited statements, to normal, recurring
adjustments). There is no material commitment of the Company which is not
reflected in the Financial Statements except commitments made in the ordinary
course of business. There have not been any changes in the assets, liabilities,
financial condition or operations of the Company from that reflected in the
Financial Statements, except changes in the ordinary course of business that
have not had a Material Adverse Effect.

        3.5 Intellectual Property. The Company owns or possesses adequate rights
to use all patents, patent rights, inventions, trade secrets and know-how
described or referred to in the Memorandum as owned or used by it or that are
necessary for the conduct of its business as presently conducted and proposed to
be conducted as described in the Memorandum. Except as disclosed under 
"BUSINESS - Legal Proceedings" in the Memorandum, the Company has not received
any notice of, nor has any knowledge of, any infringement of or conflict with
asserted rights of others with respect to any patent, patent right, invention,
trade secret or know-how that, individually or in the aggregate, if the subject
of an unfavorable decision, ruling or finding, would have a Material Adverse
Effect.

        3.6 Properties. The Company has good and valid title to all of the
properties and assets reflected as owned in the Financial Statements, free and
clear of all liens, mortgages, (statutory or otherwise), security interests,
pledges, claims or encumbrances except those, if any, disclosed in the Financial
Statements. The Company holds its leased properties under valid and binding
leases, with such exceptions as are not materially significant in relation to
the business of the

                                       3.
<PAGE>
 
Company. The Company owns or leases all of such properties which, to its
knowledge, are necessary to its operations as now conducted.

        3.7 Capitalization. All of the Company's outstanding shares of capital
stock have been duly authorized and validly issued and are fully paid and
nonassessable, have been issued in compliance with all Federal and state
securities laws, and were not issued in violation of or subject to any
preemptive right or other rights to subscribe for or purchase securities. The
actual authorized and outstanding capital stock of the Company as of the date
hereof and as of the Closing Date is set forth in the Memorandum under the
caption "Capitalization," assuming 500,000 shares are issued in the Offering.
Except as disclosed under "EXECUTIVE SUMMARY - The Offering" in the Memorandum,
there are no outstanding options to purchase, or any preemptive rights or other
rights to subscribe for or to purchase, any securities or obligations
convertible into, or any contracts or commitments to issue or sell shares of the
Company's capital stock or any such options, rights, convertible securities or
obligations.

        3.8 Litigation. Except as disclosed under "BUSINESS - Legal Proceedings"
in the Memorandum, there is no pending or, to the Company's knowledge,
threatened action, suit or other proceeding before any court, governmental body
or authority, or arbitrator to which the Company is a party or to which its
property or assets are subject.

        3.9 No Defaults. The Company is not in violation or default of any
provision of the Certificate of Incorporation or Bylaws, or any organizational
documents, or, to the Company's knowledge, is in breach with respect to any
provision of any agreement (including without limitation, the Stock Purchase
Agreement dated June 16, 1997 between the Company and Mentor Corporation),
judgement, decree, order, mortgage, deed of trust, lease, franchise, license,
indenture, permit or other instrument to which it is a party or by which it or
any of its properties are bound which violation, default or breach would have a
Material Adverse Effect; and the Company is not aware of any fact which
constitutes an event of default on the part of the Company as defined in such
documents or which, with notice or lapse of time or both, would constitute such
an event of default.

        3.10 Governmental Consents. No consent, approval, order or authorization
of, or registration, qualification, designation, declaration or filing with, any
Federal, state, or local governmental authority on the part of the Company is
required in connection with the consummation of the transactions contemplated by
this Agreement except for (a) compliance with the securities and blue sky laws
in the states and other jurisdictions in which Shares are offered and/or sold,
which compliance will be effected in accordance with such laws, and (b) the
filing of the Registration Statement and all amendments thereto with the SEC as
contemplated by Section 7.2 hereof. The Company has not been advised, and has no
reason to believe, that it is not conducting business in compliance in all
material respects with all applicable laws, rules and regulations of the
jurisdictions in which it is conducting business including but not limited to,
all applicable local, state and Federal environmental laws and regulations.

                                       4.
<PAGE>
 
        3.11 Taxes. The Company has accurately prepared and timely filed all
Federal, state and other tax returns which are required to be filed by it and
has timely paid all taxes covered by such returns which have become due and
payable except for such taxes as are being contested in good faith. The Company
has no knowledge of any tax deficiency which has been or might be asserted or
threatened against the Company which would have a Material Adverse Effect.

        3.12 Insurance. The Company maintains insurance of the types and in the
amounts it deems adequate for its business covering all risks customarily
insured against, all of which insurance is in full force and effect.

        3.13 No Material Adverse Change. Since July 31, 1997, there have not
been any changes in the assets, liabilities, financial condition or operations
of the Company from that reflected in the Financial Statements except changes in
the ordinary course of business that have not had a Material Adverse Effect.

        3.14 Disclosure. This Agreement does not contain any untrue statement of
fact or omit to state a fact necessary in order to make the statements contained
herein and therein not materially misleading.

        3.15 Nasdaq NMS Qualification. The Company has submitted a listing
application and listing agreement with the Nasdaq National Market System
("Nasdaq NMS") to list the Common Stock, including the Shares, for quotation on
the Nasdaq NMS. If such listing is approved, for so long as the Company is
obligated to keep in effect the Registration Statement provided under Section
7.2 hereof, the Company shall use its reasonable best efforts to maintain such
listing on the Nasdaq NMS, the Nasdaq SmallCap Market or a national securities
exchange, as defined in the Exchange Act.

        3.16 FDA Approval. To the Company's knowledge, the Company's 1-125
Brachytherapy Source has received all required approvals from the Federal Drug
Administration ("FDA") for commercial distribution in the United States and the
Company has delivered to the Placement Agent copies of all correspondence with
the FDA relating to such approvals.

        3.17 Investment Company. The Company is not a registered investment
company within the meaning of the Investment Company Act of 1940, as amended.

        3.18 RadioMed. The Company is under no contractual obligation to
purchase any additional equity securities from, or to make any additional
capital contribution to, RadioMed.

Section 4.   Representations, Warranties and Covenants of Purchasers

        Each Purchaser hereby severally and not jointly, represents and warrants
to the Company, effective as of the Closing Date, as follows:

                                       5.
<PAGE>
 
        4.1 Authorization. Purchaser represents and warrants to the Company
that: (i) Purchaser has all requisite legal and corporate or other power and
capacity and has taken all requisite corporate or other action to execute and
deliver this Agreement, to purchase the Shares to be purchased by it and to
carry out and perform all of its obligations under this Agreement; and (ii) this
Agreement constitutes the legal, valid and binding obligation of Purchaser,
enforceable in accordance with its terms, except (a) as limited by applicable
bankruptcy, insolvency, reorganization, or similar laws relating to or affecting
the enforcement of creditors' rights generally and (b) as limited by equitable
principles generally.

        4.2 Investment Experience. Purchaser is an "accredited investor" as
defined in Rule 501(a) under the Securities Act of 1933, as amended (the
"Securities Act"). Purchaser is aware of the Company's business affairs and
financial condition and has had access to and has acquired sufficient
information about the Company to reach an informed and knowledgeable decision to
acquire the Shares. Purchaser has such business and financial experience as is
required to give it the capacity to protect its own interests in connection with
the purchase of the Shares. Purchaser is able to bear the economic risk of
holding the Shares for an indefinite period, including the loss of Purchaser's
entire investment. The Shares were not offered or sold to Purchaser by any form
of general solicitation or advertising.

        4.3 Investment Intent. Purchaser is purchasing the Shares for its own
account as principal, for investment purposes only, and not with a view to, or
for, resale, distribution or fractionalization thereof, in whole or in part,
within the meaning of the Securities Act. Purchaser understands that its
acquisition of the Shares has not been registered under the Securities Act or
registered or qualified under any state securities law in reliance on specific
exemptions therefrom which exemptions may depend upon, among other things, the
bona fide nature of Purchaser's investment intent as expressed herein. Purchaser
has completed or caused to be completed the Stock Certificate Questionnaire and
the Registration Questionnaire attached as Appendix I and Appendix II,
respectively, of Exhibit B hereto and the responses provided therein shall be
true and correct as of the Closing Date and will be true and correct as of the
effective date of the Registration Statement. Purchaser has, in connection with
its decision to purchase the number of shares set forth in Exhibit A hereto,
relied solely upon the Memorandum and the documents attached as appendices
thereto and the representations and warranties of the Company contained herein.

        4.4   Registration or Exemption Requirements.

              (a) Purchaser further acknowledges and understands that the Shares
may be required to be held indefinitely, and they may not be resold or otherwise
transferred except in a transaction registered under the Securities Act or where
an exemption from such registration is available. Purchaser understands that the
certificate(s) evidencing the Shares will be imprinted with a legend that
prohibits the transfer of the Shares unless (i) they are registered or such
registration is not required, and (ii) if the transfer is pursuant to an
exemption from registration other than Rule 144 promulgated under the Securities
Act ("Rule 144 ") and, if the Company shall

                                       6.
<PAGE>
 
so request in writing, an opinion of counsel satisfactory to the Company is
obtained to the effect that the transaction is so exempt and in compliance with
applicable state law.

              (b) Purchaser further acknowledges that because the Common Stock
is not listed on the Nasdaq NMS or a national securities exchange, resale of the
Shares may be limited by applicable state law, even where a registration
statement covering resale of the Shares has been declared effective under the
Securities Act. For example, certain states may limit resale of the Shares to
qualified institutions or in unsolicited qualified broker transactions in the
absence of qualification or another exemption in such state.

        4.5 Restriction on Short Sales. Purchaser represents and warrants to and
covenants with the Company that neither Purchaser nor its affiliates has engaged
or will engage in any short sales of the Common Stock prior to the effectiveness
of the Registration Statement, except to the extent that any such short sale is
fully covered by shares of Common Stock other than the Shares.

        4.6 No Legal, Tax or Investment Advice. Purchaser understands that
nothing in the Memorandum, this Agreement or any other materials presented to
Purchaser in connection with the purchase and sale of the Shares constitutes
legal, tax or investment advice. Purchaser has consulted such legal, tax and
investment advisors as it, in its sole discretion, has deemed necessary or
appropriate in connection with its purchase of the Shares.

SECTION 5. CONDITIONS TO CLOSING OF PURCHASERS

        Each Purchaser's obligation to purchase the Shares at the Closing is, at
the option of such Purchaser, subject to the fulfillment or waiver as of the
Closing Date of the following conditions:

        5.1 Representations and Warranties. The representations and warranties
made by the Company in Section 3 hereof shall be true and correct in all
material respects when made, and shall be true and correct in all material
respects on the Closing Date with the same force and effect as if they had been
made on and as of said date.

        5.2 Legal Opinion. The Company shall have delivered a legal opinion from
D'Ancona & Pflaum, counsel to the Company, addressed to the Purchasers and
Oppenheimer & Co., Inc. (the "Placement Agent") in the form attached hereto as
Exhibit C-1 with respect to the sale of the Shares hereunder.

        5.3 Officer's Certificate. The Company shall deliver to the Purchasers a
certificate, dated as of the Closing Date, signed by the President of the
Company, stating that the signer of said certificate has carefully examined the
Memorandum and the SEC Documents and that the representations and warranties set
forth in Section 3 hereof are true as of and all of the closing conditions set
forth in Section 5 hereof have been satisfied on the Closing Date.

                                       7.
<PAGE>
 
        5.4 Registration Statement. The Company shall have delivered a draft of
the Registration Statement that is acceptable to the Placement Agent in a form
that may be filed with the SEC as required herein.

        5.5 SAS 71 Review. The Company shall have received a SAS 71 review from
its independent auditors addressed to the Company, in a form acceptable to the
Placement Agent, relating to the Company's financial statements for the three
and nine month periods ended July 31, 1997.

        5.6 Covenants. All covenants, agreements and conditions contained in
this Agreement to be performed by the Company on or prior to the Closing Date
shall have been performed or complied with in all material respects.

        5.7 Additional Documents. The Placement Agent shall have received such
other documents as the Placement Agent and its counsel may reasonably request
for the purpose of facilitating the consummation or performance of the sale of
the Shares and the Placement Agent and its counsel shall be satisfied with all
matters and proceedings described in this Agreement or the Memorandum.

SECTION 6. CONDITIONS TO CLOSING OF COMPANY

        The Company's obligation to sell and issue the Shares at the Closing is,
at the option of the Company, subject to the fulfillment or waiver of the
following conditions:

        6.1 Receipt of Payment. The Company shall have received payment, by
check or wire transfer of immediately available funds, in the full amount of the
Purchase Price for the Shares sold.

        6.2 Representations and Warranties. The representations made by the
Purchasers in Section 4 hereof shall be true and correct in all material
respects when made, and shall be true and correct in all material respects on
the Closing Date with the same force and effect as if they had been made on and
as of such date.

        6.3 Covenants. All covenants, agreements and conditions contained in
this Agreement to be performed by the Purchasers on or prior to the Closing Date
shall have been performed or complied with in all material respects.

SECTION 7. AFFIRMATIVE COVENANTS OF THE COMPANY

        The Company hereby covenants and agrees as follows:

        7.1 Financial Information. The Company will mail the following reports
to each Purchaser until such Purchaser transfers, assigns or sells more than
fifty percent (50%) of the Shares purchased by such Purchaser pursuant to this
Agreement:

                                       8.
<PAGE>
 
                (a) Within one hundred (100) days after the end of each fiscal
year, a copy of its Annual Report on Form 10-KSB.

                (b) Within fifty-five (55) days after the end of the first,
second and third quarterly accounting periods of each fiscal year of the
Company, a copy of its Quarterly Report on Form 10-QSB.

        7.2     Registration Requirements

                (a) No later than fifteen (15) business days following the
Closing Date, the Company shall file with the SEC one or more registration
statements on Form S-3 (together with the prospectus included therein, a
"Registration Statement") pursuant to Rule 415 of the Securities Act in order to
register with the SEC the continuous resale by the Purchasers, from time to
time, of the Shares through the Nasdaq NMS or the facilities of any national
securities exchange on which Common Stock is then traded, or in privately-
negotiated transactions. The Company shall use its best efforts to cause such
Registration Statement to be declared effective as soon thereafter as possible
and in any event within sixty (60) days thereafter. Each Purchaser agrees to
furnish promptly to the Company in writing all information required from time to
time to be disclosed in order to make the information previously furnished to
the Company by such holder not misleading.

                (b) The Company shall pay all Registration Expenses (as defined
below) in connection with any registration, qualification or compliance
hereunder, and each Purchaser shall pay all Selling Expenses (as defined below)
and other expenses that are not Registration Expenses relating to the Shares
resold by such Purchaser. "Registration Expenses" shall mean all expenses,
except for Selling Expenses, incurred by the Company in complying with the
registration provisions herein described, including, without limitation, all
registration, qualification and filing fees, printing expenses, escrow fees,
fees and disbursements of counsel for the Company, blue sky fees and expenses
and the expense of any special audits incident to or required by any such
registration. "Selling Expenses" shall mean selling commissions, underwriting
fees and stock transfer taxes applicable to the Shares and all fees and
disbursements of counsel for any Purchaser.

                (c) In the case of the registration effected by the Company
pursuant to these registration provisions, the Company will use its best efforts
to:

                        (1) keep such registration effective until the earlier
of (A) the second anniversary of the Closing Date, (B) such date as all of the
Shares have been resold or (C) such time as all of the Shares held by the
Purchasers can be sold within a given three-month period without compliance with
the registration requirements of the Securities Act pursuant to Rule 144;

                        (2) prepare and file with the SEC such amendments and
supplements to the Registration Statement and the prospectus used in connection
therewith as may be necessary to

                                       9.
<PAGE>
 
comply with the provisions of the Securities Act with respect to the disposition
of all securities covered by the Registration Statement;

                        (3) furnish such number of prospectuses and other
documents incident thereto, including any amendment of or supplement to the
prospectus, as a Purchaser from time to time may reasonably request;

                        (4) cause all Shares registered as described herein to
be listed on each securities exchange and quoted on each quotation service on
which the Common Stock are then listed or quoted;

                        (5) provide a transfer agent and registrar for all
Shares registered pursuant to the Registration Statement and a CUSIP number for
all such Shares;

                        (6) otherwise use its best efforts promptly to comply
with all applicable rules and regulations of the SEC;

                        (7) file the documents required of the Company and 
otherwise use its best efforts promptly to obtain, if applicable, and maintain
requisite blue sky clearance in (A) all jurisdictions in which any of the Shares
are originally sold and (B) all other states specified in writing by a
Purchaser, provided as to clause (B) however, that the Company shall not be
required to qualify to do business or consent to service of process in any state
in which it is not now so qualified or has not so consented; and

                        (8) with respect to the initial filing of the
Registration Statement, as of the date of declaration of effectiveness, obtain
an opinion of counsel to the Company in the form of opinion attached hereto as
Exhibit C-2, addressed to each Purchaser selling registrable securities pursuant
to the Registration Statement. The Company shall use its best efforts to
qualify for use of Form S-3 under the Securities Act to register the resale of
the Shares and to maintain such qualification during the periods described in
subsection (c)(1) hereof.

                (d) The Company shall furnish to each Purchaser upon request a
reasonable number of copies of a supplement to or an amendment of such
prospectus as may be necessary in order to facilitate the public sale or other
disposition of all or any of the Shares held by the Purchaser.

                (e) With a view to making available to the Purchasers the
benefits of Rule 144 and any other rule or regulation of the SEC that may at any
time permit a Purchaser to sell Shares to the public without registration or
pursuant to registration, the Company covenants and agrees to: (i) make and keep
public information available, as those terms are understood and defined in Rule
144, until the earlier of (A) the second anniversary of the Closing Date or (B)
such date as all of the Shares shall have been resold; (ii) file with the SEC in
a timely manner all reports and other documents required of the Company under
the Exchange Act; and (iii) furnish to any Purchaser upon request, as long as
the Purchaser owns any Shares, (A) a written statement by the

                                      10.
<PAGE>
 
Company that it has complied with the reporting requirements of the Exchange
Act, (B) a copy of the most recent annual or quarterly report of the Company,
and (C) such other information as may be reasonably requested in order to avail
any Purchaser of any rule or regulation of the SEC that permits the selling of
any such Shares without registration.

                (f) At any time the Company may refuse to permit a Purchaser to
resell any Shares pursuant to the Registration Statement; provided, however,
that in order to exercise this right at any time the Company does not qualify
for Form S-3, the Company must deliver a certificate in writing to the
Purchasers to the effect that suspension of the sale of shares under the
Registration Statement, until such time as the Company can make an appropriate
filing with the SEC, is necessary because a sale pursuant to the Registration
Statement, in its then-current form, could constitute a violation of the
Federal securities laws. In such an event, the Company shall use its best
efforts to amend the Registration Statement if necessary and take all other
actions necessary to allow such sale under the Federal securities laws, and
shall notify the Purchasers and the Placement Agent promptly after it has
determined that such sale has become permissible under the Federal securities
laws. Notwithstanding the foregoing, the Company shall not under any
circumstances be entitled to exercise its right to suspend sales under the
registration statement more than two (2) times in any twelve (12) month period,
and the period during which such Registration Statement may be withdrawn shall
not exceed thirty (30) days.

        7.3     Indemnification and Contribution.

                (a) The Company agrees to indemnify and hold harmless each
Purchaser and its officers, directors, controlling persons and affiliates from
and against any losses, claims, damages or liabilities (or actions or
proceedings in respect thereof) to which such Purchaser may become subject
(under the Securities Act, state law, common law or otherwise) insofar as such
losses, claims, damages or liabilities (or actions or proceedings in respect
thereof) arise out of, or are based upon, any untrue statement of a material
fact contained in, or omission of a material fact from, the Registration
Statement, or arise out of any failure by the Company to fulfill any undertaking
included in the Registration Statement or this Agreement, and the Company will,
as incurred, reimburse such Purchaser for any legal or other expenses reasonably
incurred in investigating, defending or preparing to defend any such action,
proceeding or claim; provided, however, that the Company shall not be liable in
any such case to the extent that such loss, claim, damage or liability arises
out of, or is based upon an untrue statement made in such Registration Statement
in reliance upon and in conformity with information furnished to the Company by
or on behalf of such Purchaser specifically for use in preparation of the
Registration Statement. The Company will reimburse the Purchasers for any legal
or other expenses reasonably incurred in investigating, defending or preparing
to defend any such action, proceeding or claim notwithstanding the absence of a
judicial determination as to the propriety and enforceability of the obligations
under this section and the possibility that such payments might later be held to
be improper, provided, that to the extent any such payment is ultimately held to
be improper, the persons receiving such payments shall promptly refund them.

                                      11.
<PAGE>
 
                (b) Each Purchaser, severally and not jointly, agrees to
indemnify and hold harmless the Company from and against any losses, claims,
damages or liabilities (or actions or proceedings in respect thereof) to which
the Company may become subject (under the Securities Act, state law, common law
or otherwise) insofar as such losses, claims, damages or liabilities (or actions
or proceedings in respect thereof) arise out of, or are based upon an untrue
statement made in such Registration Statement in reliance upon and in conformity
with information furnished to the Company by or on behalf of such Purchaser
specifically for use in preparation of the Registration Statement; provided,
however, that no Purchaser shall be liable in any such case for any untrue
statement included in any Prospectus which statement has been corrected, in
writing, by such Purchaser and delivered to the Company before the sale from
which such loss occurred and in no event shall any Purchaser be liable for any
amount in excess of the net proceeds received for the sale of its Shares
pursuant to such Registration Statement.

                (c) Promptly after receipt by any indemnified person of a notice
of a claim or the beginning of any action in respect of which indemnity is to
be sought against an indemnifying person pursuant to this Section 7.3, such
indemnified person shall notify the indemnifying person in writing of such claim
or of the commencement of such action, and, subject to the provisions
hereinafter stated, in case any such action shall be brought against an
indemnified person and the indemnifying person shall have been notified thereof,
the indemnifying person shall be entitled to participate therein, and, to the
extent that it shall wish, to assume the defense thereof, with counsel
reasonably satisfactory to the indemnified person. After notice from the
indemnifying person to such indemnified person of the indemnifying person's
election to assume the defense thereof, the indemnifying person shall not be
liable to such indemnified person for any legal expenses subsequently incurred
by such indemnified person in connection with the defense thereof; provided,
however, that if there exists or shall exist a conflict of interest that would
make it inappropriate in the reasonable judgment of the indemnified person for
the same counsel to represent both the indemnified person and such indemnifying
person or any affiliate or associate thereof, the indemnified person shall be
entitled to retain its own counsel at the expense of such indemnifying person.

                (d) If the indemnification provided for in this Section 7.3 is
unavailable to or insufficient to hold harmless an indemnified party under
subsection (a) or (b) hereof in respect of any losses, claims, damages or
liabilities (or actions or proceedings in respect thereof) referred to therein,
then each indemnifying party shall contribute to the amount paid or payable by
such indemnified party as result of such losses, claims, damages or liabilities
(or actions in respect thereof) in such proportion as is appropriate to reflect
the relative fault of the Company on the one hand and the Purchasers on the
other in connection with the statements or omissions which resulted in such
losses, claims, damages or liabilities (or actions in respect thereof), as well
as any other relevant equitable considerations. The relative fault shall be
determined by reference to, among other things, whether the untrue or alleged
untrue statement of a material fact or the omission or alleged omission to state
a material fact relates to information supplied by the Company on the one hand
or a Purchaser on the other and the parties' relative intent, knowledge, access
to information and opportunity to correct or prevent such statement or omission.
The Company and the Purchasers agree that it would not be just and equitable if
contribution pursuant



                                      12.
<PAGE>


to this subsection (d) were determined by pro rata allocation (even if the
Purchasers were treated as one entity for such purpose) or by any other method
of allocation which does not take account of the equitable considerations
referred to above in this subsection (d). The amount paid or payable by an
indemnified party as a result of the losses, claims, damages, or liabilities (or
actions in respect thereof) referred to above in this subsection (d) shall be
deemed to include any legal or other expenses reasonably incurred by such
indemnified party in connection with investigating or defending any such action
or claim. Notwithstanding the provisions of this subsection (d), no Purchaser
shall be required to contribute any amount in excess of the amount by which the
amount received by the Purchaser from the sale of the Shares to which such loss
relates exceeds the amount of any damages which such Purchaser has otherwise
been required to pay by reason of such untrue or alleged untrue statement or
omission or alleged omission. No person guilty of fraudulent misrepresentation
(within the meaning of Section 11 (f) of the Securities Act) shall be entitled
to contribution from any person who was not guilty of such fraudulent
misrepresentation. The Purchasers' obligations in this subsection (d) to
contribute are several in proportion to their respective sales of Shares to
which such loss relates and not joint.

                (e) The obligations of the Company and the Purchasers under this
Section 7.3 shall be in addition to any liability which the Company and the
respective Purchasers may otherwise have.

SECTION 8       RESTRICTIONS ON TRANSFERABILITY OF SHARES: COMPLIANCE WITH
                SECURITIES ACT

        8.1 Restrictions on Transferability. The Shares shall not be
transferable in the absence of a registration under the Securities Act or an
exemption therefrom or in the absence of compliance with any term of this
Agreement. The Company shall be entitled to give stop transfer instructions to
its transfer agent with respect to the Shares in order to enforce the foregoing
restrictions.

        8.2 Restrictive Legend. Each certificate representing Shares shall bear
substantially the following legends (in addition to any legends required under
applicable securities laws):

        THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
        INVESTMENT PURPOSES ONLY AND HAVE NOT BEEN REGISTERED UNDER THE
        SECURITIES ACT OF 1933, AS AMENDED, OR ANY STATE SECURITIES LAWS. THE
        SHARES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SUCH
        REGISTRATION OR AN EXEMPTION THEREFROM.

        ADDITIONALLY, THE TRANSFER OF THE SHARES REPRESENTED BY THIS CERTIFICATE
        IS SUBJECT TO CERTAIN RESTRICTIONS SPECIFIED IN THE COMMON STOCK
        PURCHASE AGREEMENT DATED NOVEMBER , 1997 (THE "AGREEMENT") BETWEEN THE
        COMPANY AND THE ORIGINAL PURCHASER, AND NO TRANSFER OF SHARES SHALL BE
        VALID OR EFFECTIVE ABSENT COMPLIANCE WITH SUCH RESTRICTIONS. ALL

                                      13.
<PAGE>
 
        SUBSEQUENT HOLDERS OF THIS CERTIFICATE WILL HAVE AGREED TO BE BOUND BY
        CERTAIN OF THE TERMS OF THE AGREEMENT, INCLUDING SECTIONS 7.2 AND 8.3 OF
        THE AGREEMENT. COPIES OF THE AGREEMENT MAY BE OBTAINED AT NO COST BY
        WRITTEN REQUEST MADE BY THE REGISTERED HOLDER OF THIS CERTIFICATE TO THE
        SECRETARY OF THE COMPANY.

        8.3 Transfer of Shares after Registration. Each Purchaser hereby
covenants with the Company not to make any sale of the Shares except either (i)
in accordance with the Registration Statement, in which case Purchaser covenants
to comply with the requirement of delivering a current prospectus, (ii) in
accordance with Rule 144, in which case Purchaser covenants to comply with Rule
144, or (iii) pursuant to another exemption under the Securities Act. Purchaser
further acknowledges and agrees that such Shares are not transferable on the
books of the Company unless the certificate submitted to the Company's transfer
agent evidencing such Shares is accompanied by a separate certificate executed
by an officer of, or other person duly authorized by, the Purchaser in the form
attached hereto as Exhibit D.

        8.4 Purchaser Information. Each Purchaser covenants that it will
promptly notify the Company in writing of any changes in the information set
forth in the Registration Statement regarding such Purchaser or such Purchaser's
"Plan of Distribution."

SECTION 9. MISCELLANEOUS

        9.1 Waivers and Amendments. With the exception of Section 7 hereof, the
terms of this Agreement may be waived or amended with the written consent of the
Company and each Purchaser. With respect to Section 7 hereof, with the written
consent of the Company and the record holders of more than fifty percent (50%)
of the Shares then outstanding and held by Purchasers, the terms of this
Agreement may be waived or amended and any such amendment or waiver shall be
binding upon the Company and all holders of Shares.

        9.2 Broker's Fee. Each Purchaser acknowledges that the Company intends
to pay a fee to Oppenheimer & Co., Inc. in respect of the sale of the Shares to
the Purchaser. Each of the parties hereto hereby represents that, on the basis
of any actions and agreements by it, there are no other brokers or finders
entitled to compensation in connection with the sale of the Shares to the
Purchasers.

        9.3 Governing Law. This Agreement shall be governed in all respects by
and construed in accordance with the laws of the State of California without any
regard to conflicts of laws principles.

        9.4 Survival. The representations, warranties, covenants and agreements
made in this Agreement shall survive any investigation made by the Company or
the Purchasers and the Closing.

                                      14.
<PAGE>
 
        9.5 Successors and Assigns. The provisions hereof shall inure to the
benefit of, and be binding upon, the successors, assigns, heirs, executors and
administrators of the parties to this Agreement.

        9.6 Entire Agreement. This Agreement constitutes the full and entire
understanding and agreement between the parties with regard to the subjects
thereof.

        9.7 Notices, etc. All notices and other communications required or
permitted under this Agreement shall be effective upon receipt and shall be in
writing and may be delivered in person, by telecopy, overnight delivery service
or registered or certified United States mail, addressed to the Company or the
Purchasers, as the case may be, at their respective addresses set forth at the
beginning of this Agreement or on Exhibit A, or at such other address as the
Company or the Purchasers shall have furnished to the other party in writing
with a copy to Michael Fekete, Oppenheimer & Co., Inc., 10880 Wilshire
Boulevard, Los Angeles, California 90024. All notices and other communications
shall be effective upon the earlier of actual receipt thereof by the person to
whom notice is directed or (i) in the case of notices and communications sent by
personal delivery or telecopy, one business day after such notice or
communication arrives at the applicable address or was successfully sent to the
applicable telecopy number, (B) in the case of notices and communications sent
by overnight delivery service, at noon (local time) on the second business day
following the day such notice or communication was sent, and (c) in the case of
notices and communications sent by United States mail, seven (7) days after such
notice or communication shall have been deposited in the United States mail.

        9.8 Severability of this Agreement. If any provision of this Agreement
shall be judicially determined to be invalid, illegal or unenforceable, the
validity, legality and enforceability of the remaining provisions shall not in
any way be affected or impaired thereby.

        9.9 Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

        9.10 Further Assurances. Each party to this Agreement shall do and
perform or cause to be done and performed all such further acts and things and
shall execute and deliver all such other agreements, certificates, instruments
and documents as the other party hereto may reasonably request in order to carry
out the intent and accomplish the purposes of this Agreement and the
consummation of the transactions contemplated hereby.

        9.11 Expenses. The Company and each such Purchaser shall bear its own
expenses incurred on its behalf with respect to this Agreement and the
transactions contemplated hereby, including fees of legal counsel.

                                      15.
<PAGE>
 
        IN WITNESS WHEREOF, this Agreement is hereby executed as of the date
first above written.



                                        the "Company"

                                        NORTH AMERICAN SCIENTIFIC, INC.


                                        By:
                                             ---------------------------------

                                                
                                             ---------------------------------
                                                 [Print Name and Title]
                                        




                                      16.


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