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                                                                     EXHIBIT 4.4

                         NORTH AMERICAN SCIENTIFIC, INC.
                        2000 EMPLOYEE STOCK PURCHASE PLAN

                                    ARTICLE I

                                  INTRODUCTION

1.01   PURPOSE OF THE PLAN. The North American Scientific, Inc. Employee Stock
       Purchase Plan (the "Plan") is intended to provide a method whereby
       employees of North American Scientific, Inc. (the "Company") and its
       Eligible Subsidiary Corporations (as defined below) will have an
       opportunity to acquire a proprietary interest in the Company through the
       purchase of shares of the Common Stock of the Company. It is the
       intention of the Company to have the Plan qualify as an "employee stock
       purchase plan" under Section 423 of the Internal Revenue Code of 1986, as
       amended (the "Code"). The provisions of the Plan shall be construed in a
       manner consistent with the requirements of that section of the Code.

                                   ARTICLE II

                                   DEFINITIONS

2.01   "COMPENSATION" shall mean the gross cash compensation (including wage,
       salary and overtime earnings) paid by the Company or any Eligible
       Subsidiary Corporation to a participant in accordance with the terms of
       employment, but excluding all bonus payments, expense allowances,
       nonqualified deferred compensation and compensation paid in a form other
       than cash.

2.02   "COMMITTEE" shall mean the individuals described in Article XI.

2.03   "ELIGIBLE SUBSIDIARY CORPORATION" shall mean any present or future
       corporation which (i) is or becomes a "subsidiary corporation" of North
       American Scientific Inc., as that term is defined in Section 424 of the
       Code, and (ii) is designated as a participating employer in the Plan by
       the Committee on Schedule 2.03 hereto.

2.04   "EMPLOYEE" shall mean any person, including an officer, who is
       customarily employed by the Company or any Eligible Subsidiary
       Corporation for at least twenty (20) hours per week and more than five
       (5) months in a calendar year.

2.05   "OPTION" shall mean the right to purchase shares of Common Stock from the
       Company at a price per share equal to the option price, as determined
       under Section 6.02 below.

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2.06   "PLAN REPRESENTATIVE" shall mean any person designated from time to time
       by the Committee to receive certain notices and take certain other
       administrative actions relating to participation in the Plan.

                                   ARTICLE III

                          ELIGIBILITY AND PARTICIPATION

3.01   INITIAL ELIGIBILITY. Each Employee who shall have completed six
       consecutive months of employment with the Company or any corporation or
       entity acquired by the Company or any Eligible Subsidiary Corporation and
       shall be employed by the Company or any Eligible Subsidiary Corporation
       on the date his or her participation in the Plan is to become effective
       shall be eligible to participate in Offerings (as defined below) under
       the Plan which commence after such six-month period has concluded.
       Persons who are not Employees shall not be eligible to participate in the
       Plan.

3.02   RESTRICTIONS ON PARTICIPATION. Notwithstanding any provision of the Plan
       to the contrary, no Employee shall be granted an option to purchase
       shares of Common Stock under the Plan:

       (a)    if, immediately after the grant, such Employee would own stock
              and/or hold outstanding options to purchase stock possessing 5% or
              more of the total combined voting power or value of all classes of
              stock of the Company (for purposes of this paragraph, the rules of
              Section 424(d) of the Code shall apply in determining stock
              ownership of any Employee); or

       (b)    which permits such Employee's rights to purchase stock under all
              Employee stock purchase plans of the Company to accrue at a rate
              which exceeds $25,000 of fair market value of the stock
              (determined at the time such option is granted) for each calendar
              year in which such Option is outstanding at any time.

3.03   COMMENCEMENT OF PARTICIPATION. An eligible Employee may become a
       participant by completing an authorization for payroll deductions on the
       form provided by the Company and filing the completed form with the Plan
       Representative on or before the filing date set therefor by the
       Committee, which date shall be prior to the Offering Commencement Date
       (as defined below) for the next following Offering. Payroll deductions
       for a participant shall commence on the next following Offering
       Commencement Date after the Employee's authorization for payroll
       deductions becomes effective and shall continue until termination of the
       Plan or the participant's earlier termination of participation in the
       Plan. Each participant in the Plan shall be deemed to continue
       participation until termination of the Plan or such participant's earlier
       termination of participation in the Plan pursuant to Article VIII below.

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                                   ARTICLE IV

                     STOCK SUBJECT TO THE PLAN AND OFFERINGS

4.01   STOCK SUBJECT TO THE PLAN. Subject to the provision of Section 12.04 of
       the Plan, the Company's Board of Directors shall reserve initially for
       issuance under the Plan an aggregate of three hundred thousand shares
       (300,000) shares of the Company's common stock (the "Common Stock"),
       which shares shall be authorized but unissued shares of Common Stock. The
       Company's Board of Directors may from time to time reserve additional
       shares of authorized and unissued Common Stock for issuance pursuant to
       the Plan; provided, however, that at no time shall the number of shares
       of Common Stock reserved be greater than permitted by applicable law.
       Shares of Common Stock subject to the Plan may be newly issued shares or
       shares reacquired in private transactions or open market purchases.

4.02   OFFERINGS. Except as described below with respect to the first year the
       Plan is in effect, the Plan will be implemented by two annual offerings
       of the Company's Common Stock each calendar year (the "Offerings"). In
       each year that the Plan is in effect, the first Offering will begin on
       January 1 and end on June 30, the second Offering will begin on July 1
       and end on December 31. The first day of each Offering shall be deemed
       the "Offering Commencement Date" and the last day the "Offering
       Termination Date" for such Offering. The Committee reserves the right to
       change the duration and frequency of offering periods without prior
       shareholder approval.

                                    ARTICLE V

                               PAYROLL DEDUCTIONS

5.01   AMOUNT OF DEDUCTION. The form described in Section 2.03 will permit a
       participant to elect payroll deductions (after tax withholding) of zero
       percent (0%) or any whole percentage from one percent (1%) through
       fifteen percent (15%) of such participant's Compensation for each pay
       period during an Offering.

5.02   PARTICIPANT'S ACCOUNT. All payroll deductions made for a participant
       shall be credited to an account established for such participant under
       the Plan. A participant may not make any separate cash payment into such
       account.

5.03   CHANGES IN PAYROLL DEDUCTIONS. A participant may reduce or increase
       future payroll deductions (within the limits described in Section 5.01)
       by filing with the Plan Representative a form provided by the Company for
       such purpose. Such a reduction or increase in future payroll deductions
       may only be performed once per participant during an Offering. The
       effective date of any increase or reduction in future payroll deductions
       will be the first day of the next pay period succeeding processing of the
       change form.

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5.04   WITHHOLDING. At the time the option is exercised, in whole or in part, or
       at the time some or all of the Company's Common Stock issued under the
       Plan is disposed of, the participant must make adequate provision for the
       Company's federal, state, or other tax withholding obligations, if any,
       which arise upon the exercise of the option or the disposition of the
       Common Stock. At any time, the Company may, but shall not be obligated
       to, withhold from the participant's compensation the amount necessary for
       the Company to meet applicable withholding obligations, including any
       withholding required to make available to the Company any tax deduction
       or benefits attributable to sale or early disposition of Common Stock by
       the Employee.

                                   ARTICLE VI

                               GRANTING OF OPTION

6.01   NUMBER OF OPTION SHARES. On the Offering Commencement Date, each
       participating Employee shall be deemed to have been granted an option to
       purchase a maximum number of shares of Common Stock on the Offering
       Termination Date equal to (i) that percentage of the Employee's
       Compensation which the Employee has elected to have withheld (but not in
       any case in excess of 15%) multiplied by (ii) the Employee's Compensation
       during the Offering then divided by (iii) the applicable Option Price
       determined as provided in Section 6.02 below.

6.02   OPTION PRICE. The option price of stock purchased with payroll deductions
       made during any Offering (the "Offering Price") for a participant therein
       shall be the lesser of:

       (a)    85% of the closing price of the stock on the Offering Commencement
              Date for such Offering or the nearest prior business day on which
              trading occurred on the NASDAQ National Market System; or

       (b)    85% of the closing price on the Offering Termination Date for such
              Offering or the nearest prior business day on which trading
              occurred on the NASDAQ National Market System. If the Common Stock
              of the Company is not admitted to trading on any of the aforesaid
              dates for which closing prices of the stock are to be determined,
              then reference shall be made to the fair market value of the stock
              on each such date, as determined on such basis as shall be
              established or specified for the purpose by the Committee.

                                   ARTICLE VII

                               EXERCISE OF OPTION

7.01   AUTOMATIC EXERCISE. Each Plan participant's option for the purchase of
       stock with payroll deductions made during any Offering will be deemed to
       have been exercised automatically on the applicable Offering Termination
       Date for the purchase of the number of full shares of Common Stock which
       the accumulated payroll deductions in the participant's account at the
       time will purchase at the applicable Option Price (but not in excess of
       the number of

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       shares for which outstanding options have been granted to the participant
       pursuant to Section 6.01).

7.02   FRACTIONAL SHARES. Fractional shares of Common Stock will not be issued
       under the Plan. Any accumulated payroll deductions which would have been
       used to purchase fractional shares, unless refunded pursuant to Section
       8.01, will be held for the purchase of Common Stock in the next following
       Offering, without interest.

7.03   EXERCISE OF OPTIONS. During a participant's lifetime, options held by
       such participant shall be exercisable only by such participant and is not
       transferable by such participant.

7.04   DELIVERY OF STOCK. As promptly as practicable after the Offering
       Termination Date of each Offering, the Company will deliver to each
       participant in such Offering, as appropriate, the shares of Common Stock
       purchased therein upon exercise of such participant's option. The Company
       may deliver such shares in certificated or book entry form, at the
       Company's sole election.

7.05   STOCK TRANSFER RESTRICTIONS. The Plan is intended to satisfy the
       requirements of Section 423 of the Code. A participant will not obtain
       the tax benefits of Section 423 of the Code if such participant disposes
       of shares of Common Stock acquired pursuant to the Plan within two (2)
       years from the Offering Commencement Date or within one (1) year from the
       date such Common Stock is purchased by the participant, whichever is
       later (the "Notice Period."). Each participant shall notify the Committee
       if the participant disposes of any of the shares purchased during an
       Offering pursuant to this Plan if such disposition occurs within the
       Notice Period. Unless such participant is disposing of any of such shares
       during the Notice Period, such participant shall keep the certificates
       representing such shares in his or her name (and not in the name of a
       nominee) during the Notice Period. The Company may, at any time during
       the Notice Period, place a legend or legends or any certificate
       representing shares acquired pursuant to this Plan requesting the
       Company's transfer agent to notify the Company of any transfer of the
       shares. The obligation of the participant to provide such notice shall
       continue notwithstanding the placement of any such legend on the
       certificates.

                                  ARTICLE VIII

                     WITHDRAWAL/TERMINATION OF PARTICIPATION

8.01   IN GENERAL. A participant may stop participating in the Plan at any time
       by giving written notice to the Plan Representative. Upon processing of
       any such written notice, no further payroll deductions will be made from
       the participant's Compensation during such Offering or thereafter, unless
       and until such participant elects to resume participation in the Plan by
       providing written notice to the Plan Representative pursuant to Section
       3.03, above. A participant may withdraw all but not less than all of the
       payroll deductions credited to his or her account and not yet used to
       exercise his or her option under the Plan at any time by giving written
       notice to the Company in the form of Schedule 8.01 to this Plan. If the
       participant elects not to withdraw all (but not less than all) of his or
       her payroll deductions

<PAGE>

       credited to his or her account, the Employee's funds in his or her
       account will be considered suspended and will be used to purchase the
       Company's Common Stock on the next Offering Termination Date and no
       further payroll deductions will be made under the Plan. If the
       participant elects to withdraw all (but not less than all) of his or her
       payroll deductions credited to his or her account, all of the
       participant's payroll deductions credited to his or her account shall be
       paid to such participant promptly after receipt of notice of withdrawal
       and such participant's option for the current Offering shall be
       automatically terminated, and no further payroll deductions for the
       purchase of shares shall be made for such Offering. If a participant
       withdraws from an Offering, payroll deductions shall not resume at the
       beginning of the succeeding Offering unless the participant delivers to
       the Company a new subscription agreement.

8.02   EFFECT ON SUBSEQUENT PARTICIPATION. A participant's withdrawal from any
       Offering will not have any effect upon such participant's eligibility to
       participate in any succeeding Offering or in any similar plan which may
       hereafter be adopted by the Company and for which such participant is
       otherwise eligible. An approved leave of absence of less than ninety days
       shall not be treated as a termination of employment.

8.03   TERMINATION OF EMPLOYMENT. Upon a participant's ceasing to be an Employee
       for any reason, he or she shall be deemed to have elected to withdraw
       from the Plan and the payroll deductions credited to such participant's
       account during the Offering but not yet used to exercise the option shall
       be returned to such participant, or, in the case of his or her death, to
       the person or persons entitled thereto under Section 12.01, and his or
       her participation in the Plan shall be deemed to be terminated.

                                   ARTICLE IX

                                    INTEREST

9.01   PAYMENT OF INTEREST. No interest will be paid or allowed on any money
       paid into the Plan or credited to the account of or distributed to any
       participant Employee.

                                    ARTICLE X

                                      STOCK

10.01  PARTICIPANT'S INTEREST IN OPTION STOCK. No participant will have any
       interest in shares of Common Stock covered by any option held by such
       participant until such option has been exercised as provided in Section
       7.01 above.

10.02  REGISTRATION OF STOCK. Shares of Common Stock purchased by a participant
       under the Plan will be registered in the name of the participant, or, if
       the participant so directs by written notice to the Plan Representative
       prior to the Offering Termination Date applicable thereto, in the names
       of the participant and one such other person as may be designated by the
       participant, as joint tenants with rights of survivorship or as tenants
       by the entireties, to the extent permitted by applicable law.

<PAGE>

10.03  RESTRICTIONS ON EXERCISE. The Board of Directors may, in its discretion,
       require as condition to the exercise of any option that the shares of
       Common Stock reserved for issuance upon the exercise of such option shall
       have been duly listed, upon official notice of issuance, upon a stock
       exchange or market, and that either:

       (a)    a registration statement under the Securities Act of 1933, as
              amended, with respect to said shares shall be effective, or

       (b)    the participant shall have represented at the time of purchase, in
              form and substance satisfactory to the Company, that it is his or
              her intention to purchase the shares for investment and not for
              resale or distribution.

                                   ARTICLE XI

                                 ADMINISTRATION

11.01  APPOINTMENT OF COMMITTEE. The Plan shall be administered by the Board of
       Directors of the Company (the "Board") and/or by a duly appointed
       committee of the Board having such powers as shall be specified by the
       Board. Any subsequent references to the Board shall also mean the
       committee if a committee has been appointed.

11.02  AUTHORITY OF BOARD. Subject to the express provisions of the Plan, the
       Board shall have plenary authority in its discretion to interpret and
       construe any and all provisions of the Plan, to adopt rules and
       regulations for administering the Plan, and to make all other
       determinations deemed necessary or advisable for administering the Plan.
       The Board's determination of the foregoing matters shall be conclusive.
       The Board may request advice or assistance or employ such other persons
       as it in its absolute discretion deems necessary or appropriate for the
       proper administration of the Plan, including but not limited to employing
       a brokerage firm, bank, or other financial institution to assist in the
       purchase of shares, delivery of reports, or other administrative aspects
       of the Plan. All expenses incurred in connection with the administration
       of the Plan shall be paid by the Company.

                                   ARTICLE XII

                                  MISCELLANEOUS

12.01  DESIGNATION OF BENEFICIARY. A participant may file with the Plan
       Representative a written designation of a beneficiary who is to receive
       any shares of Common Stock and/or cash under the Plan upon the
       participant's death. Such designation of beneficiary may be changed by
       the participant at any time by written notice to the Plan Representative.
       Upon the death of a participant and receipt by the Company of proof of
       identity and existence at the participant's death of a beneficiary
       validly designated by the participant under the Plan, and subject to
       Article VIII, above, concerning withdrawal from the Plan, the Company
       shall deliver such shares of Common Stock and/or cash to such
       beneficiary. In the event

<PAGE>

       of the death of a participant lacking a beneficiary validly designated
       under the Plan who is living at the time of such participant's death, the
       Company shall deliver such shares of Common Stock and/or cash to the
       executor or administrator of the estate of the participant, or if no such
       executor or administrator has been appointed (to the knowledge of the
       Company), the Company, in its discretion, may deliver such shares of
       Common Stock and/or cash to the spouse or to any one or more dependents
       of the participant, in each case without any further liability of the
       Company whatsoever under or relating to the Plan. No beneficiary shall,
       prior to the death of the participant by whom he or she has been
       designated, acquire any interest in the shares of Common Stock and/or
       cash credited to the participant under the Plan.

12.02  TRANSFERABILITY. Neither payroll deductions credited to any participant's
       account nor any option or rights with regard to the exercise of an option
       or to receive Common Stock under the Plan may be assigned, transferred,
       pledged, or otherwise disposed of in any way by the participant other
       than by will or the laws of descent and distribution. Any such attempted
       assignment, transfer, pledge or other disposition shall be without
       effect, except that the Company may, in its discretion, treat such act as
       an election to withdraw from participation in the Plan in accordance with
       Section 8.01.

12.03  USE OF FUNDS. All payroll deductions received or held by the Company
       under the Plan may be used by the Company for any corporate purpose. The
       Company shall not be obligated to segregate such payroll deductions.

12.04  ADJUSTMENT UPON CHANGES IN CAPITALIZATION.

       (a)    If, while any options are outstanding under the Plan, the
              outstanding shares of Common Stock of the Company have increased,
              decreased, changed into, or been exchanged for a different number
              or kind of shares or securities of the Company through any
              reorganization, merger, recapitalization, reclassification, stock
              split, reverse stock split or similar transaction, appropriate and
              proportionate adjustments may be made by the Committee in the
              number and/or kind of shares which are subject to purchase under
              outstanding options and in the Option Price or Prices applicable
              to such outstanding options. In addition, in any such event, the
              number and/or kind of shares which may be offered in the Offerings
              described in Article IV hereof shall also be proportionately
              adjusted. No such adjustments shall be made for or in respect of
              stock dividends. For purposes of this paragraph, any distribution
              of shares of Common Stock to shareholders in an amount aggregating
              20% or more of the outstanding shares of Common Stock shall be
              deemed a stock split, and any distribution of shares aggregating
              less than 20% of the outstanding shares of Common Stock shall be
              deemed a stock dividend.

       (b)    Upon the dissolution or liquidation of the Company, or upon a
              reorganization, merger or consolidation of the Company with one or
              more corporation as a result of which the Company is not the
              surviving corporation, or upon a sale of substantially all of the
              property or capital stock of the Company to another corporation,
              the holder of each option then outstanding under the Plan will
              thereafter be entitled to receive at the next Offering Termination
              Date, upon the exercise of such option, for each share as to which
              such option

<PAGE>

       shall be exercised, as nearly as reasonably may be determined, the cash,
       securities and/or property which a holder of one share of the Common
       Stock was entitled to receive upon and at the time of such transaction.
       The Board of Directors shall take such steps in connection with such
       transactions as the Board shall deem necessary to assure that the
       provisions of this Section 12.04 shall thereafter be applicable, as
       nearly as reasonably may be determined, in relation to the said cash,
       securities and/or property as to which each such holder of any such
       option might hereafter be entitled to receive.

12.05  AMENDMENT AND TERMINATION. The Board of Directors shall have complete
       power and authority to terminate or amend the Plan; provided, however,
       that the Board of Directors shall not, without the approval of the
       shareholders of the Company, alter (i) the aggregate number of shares of
       Common Stock which may be issued under the Plan (except pursuant to
       Section 12.04 above) or (ii) the class of employees eligible to receive
       options under the Plan, other than to designate additional Eligible
       Subsidiary Corporations; and provided further, however, that no
       termination, modification, or amendment of the Plan may, without the
       consent of an Employee then having an option under the Plan to purchase
       shares of Common Stock, adversely affect the rights of such Employee
       under such option.

12.06  TERM OF PLAN. The Plan shall become effective upon the earlier to occur
       of its adoption by the Board of Directors or its approval by the
       stockholders of the Company, provided that stockholder approval occurs
       within one year of the adoption of this Plan. It shall continue in effect
       for a term of ten (10) years unless sooner terminated under Section 12.05
       herein.

12.07  NO EMPLOYMENT RIGHTS. The Plan does not, directly or indirectly, create
       in any person any right with respect to continuation of employment by the
       Company or any Subsidiary Corporation, and it shall not be deemed to
       interfere in any way with the Company's or any Subsidiary Corporation's
       right to terminate, or otherwise modify, any employee's employment at any
       time.

12.08  REPORTS. Individual accounts shall be maintained for each participant in
       the Plan. Statements of account shall be given to participating Employees
       at least annually, which statements shall set forth the amounts of
       payroll deductions, the purchase price, the number of shares purchased
       and the remaining cash balance, if any.

12.09  COMPANY POLICY PROHIBITING INSIDER TRADING. Nothing in this Plan shall be
       construed to replace, supersede, nullify, amend, or eliminate the
       Company's "Statement of Policy Requiring Confidentiality and Prohibiting
       `Insider Trading.'" Accordingly, all participants in the Plan shall be
       prohibited from trading in Company Common Stock during the period
       commencing on the twenty-fifth day of the last month of each calendar
       quarter and through the second business day following the public release
       of such prior quarter's earnings results.

12.10  EFFECT OF PLAN. The provisions of the Plan shall, in accordance with its
       terms, be binding upon, and inure to the benefit of, all successors of
       each Employee participating in the Plan, including, without limitation,
       such Employee's estate and the executors, administrators or

<PAGE>

       trustees thereof, heirs and legatees, and any receiver, trustee in
       bankruptcy or representative of creditors of such Employee.

12.11  GOVERNING LAW. The law of the State of California will govern all matters
       relating to this Plan except to the extent superseded by the federal laws
       of the United States.<PAGE>

INDEPENDENT AUDITORS' CONSENT

We consent to the use in this Post-Effective Amendment No. 3 to the
Registration Statement on Form N-4 (Reg. No. 333-37907) of Sun Life of Canada
(U.S.) Variable Account F of our report dated February 10, 2000 accompanying
the financial statements of the Futurity, Futurity II, Futurity Focus and
Futurity Accolade Sub-Accounts included in Sun Life of Canada (U.S.) Variable
Account F appearing in the Statement of Additional Information, which is part
of such Registration Statement, to the use of our report dated February 10,
2000 accompanying the statutory financial statements of Sun Life Assurance
Company of Canada (U.S.), which includes explanatory paragraphs relating to
the use of statutory accounting practices which differ from generally
accepted accounting principles, appearing in the Prospectus, which is part of
such Registration Statement, and to the incorporation by reference of our
report dated February 10, 2000 appearing on the Annual Report on Form 10-K of
Sun Life Assurance Company of Canada (U.S.) for the year ended December 31,
1999, which includes explanatory paragraphs relating to the use of statutory
accounting practices which differ from generally accepted accounting
principles.

We also consent to the reference to us under the headings "Accountants" and
"Appendix B -- Condensed Financial Information -- Accumulation Unit Values"
appearing in such Prospectus and under the heading "Financial Statements" in
such Statement of Additional Information.

DELOITTE & TOUCHE LLP
Boston, Massachusetts

April 6, 2000

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