EXHIBIT 10.12

                           STOCK PURCHASE AGREEMENT


        THIS AGREEMENT is by and among THERMEDICS DETECTION INC. (the
"Company"), a Massachusetts corporation with an office at 220 Mill Road,
Chelmsford, Massachusetts 01824-4178; THERMO ELECTRON CORPORATION ("Thermo
Electron"), a Delaware corporation and the indirect corporate parent of the
Company with an office at 81 Wyman Street, Waltham, Massachusetts 02254; and the
purchaser whose name and address is set forth on the signature page hereof (the
"Purchaser").

        IN CONSIDERATION of the mutual covenants contained in this Agreement,
the parties agree as follows:

        SECTION 1.  AUTHORIZATION OF SALE OF THE SHARES.  The Company has
authorized the sale of up to 383,500 shares of its Common Stock, $.10 par value
(the "Shares"). The number of  Shares may be increased or decreased by
agreement between the Company and the Placement Agents (as defined below).

        SECTION 2. AGREEMENT TO SELL AND PURCHASE THE SHARES. At the Closing (as
defined in Section 3(a)), the Company will sell to the Purchaser, and the
Purchaser will buy from the Company, upon the terms and conditions hereinafter
set forth, up to the maximum number of Shares set forth on the signature page
hereof at a purchase price of $10.75 per Share. The actual number of Shares to
be purchased by the Purchaser, subject to the foregoing maximum, will be
determined by the Company and the Placement Agents, and will be set forth on the
Company's signature page to this Agreement. The Placement Agents will notify the
Purchaser prior to the Closing of the number of Shares to be so purchased by the
Purchaser. Any excess funds advanced by the Purchaser will be promptly refunded
by the Placement Agents after the Closing.

        The Company represents and warrants that, at the Closing or subsequent
closings, the Company is proposing to enter into substantially this same form
of purchase agreement with certain other investors (the "Other Purchasers") and
expects to complete sales of Shares to them.  The Purchaser and the Other
Purchasers are hereinafter sometimes collectively referred to as the
"Purchasers," and this Agreement and the agreements executed by the Other
Purchasers are hereinafter sometimes collectively referred to as the
"Agreements."  The term "Placement Agents" shall mean NatWest Securities
Limited and National Westminster Bank PLC and, unless the context requires
otherwise, their affiliates.

        SECTION 3.  DELIVERY OF THE SHARES AT THE CLOSING. The completion of the
purchase and sale of the Shares (the "Closing") shall occur at a place and time
(the "Closing Date") specified by the Company and the Placement Agents and of
which the Purchasers will be notified in advance by the Placement Agents. At the
Closing, the Company shall deliver to the Purchaser one or more stock
certificates registered in the name of the Purchaser, or, if so indicated on the
signature page hereof, in the name of a nominee designated by the Purchaser,
representing the number of Shares to be purchased by it. The Company's
obligation to deliver

 
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such certificate to the Purchaser at the Closing shall be subject to the
following conditions, any one or more of which may be waived by the Company: (a)
receipt by the Company of a certified or official bank check or checks or wire
transfer of funds in the full amount of the purchase price for the Shares being
purchased hereunder; (b) completion of the purchases and sales under the
Agreements with Other Purchasers; and (c) the accuracy of the representations
and warranties made by the Purchasers and the fulfillment of those undertakings
of the Purchasers to be fulfilled prior to the Closing. The Purchaser's
obligation to accept delivery of such certificate and to pay for the Shares
evidenced thereby shall be subject to the accuracy of the representations and
warranties made by the Company and Thermo Electron herein as of the Closing and
the fulfillment of those undertakings of the Company and Thermo Electron to be
fulfilled prior to Closing as set forth herein or in the placement agreement
between the Company and the Placement Agents.

        SECTION 4.  REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE COMPANY
THERMO ELECTRON.  The Company and Thermo Electron hereby jointly and severally
represent and warrant to, and covenant with, the Purchaser as follows.
following representations and warranties shall be deemed to apply to each
subsidiary of the Company, unless the context clearly requires otherwise. 

                4.1.   ORGANIZATION; COMPLIANCE WITH LAW. Each of the Company
and Thermo Electron is duly organized and validly existing in good standing
under the laws of the jurisdiction of its organization. The Company has full
corporate power and authority to own, operate and occupy its properties and to
conduct its business as presently conducted and as described in the private
placement memorandum, dated November 11, 1996, distributed in connection with
the sale of the Shares (the "Placement Memorandum") and is registered or
qualified to do business and in good standing in each jurisdiction in which it
owns or leases property or transacts business and where the failure to be so
qualified would have a material adverse effect upon the business, financial
condition, properties or operations of the Company. The Company is in compliance
with all laws, ordinances, regulations and decrees applicable to its properties
(whether owned or leased) and its business as described in the Placement
Memorandum, and all licenses, franchises, governmental approvals, permits and
other authorizations currently applicable to it or its business or properties
are in full force and effect and the Company is in full compliance therewith,
except where noncompliance with such laws, ordinances, regulations, decrees,
licenses, franchises, governmental approvals, permits and authorizations would
not, separately or in the aggregate, have a material adverse effect on the value
or use of such properties or the results of operations of such business.

                4.2.   DUE AUTHORIZATION. Each of the Company and Thermo
Electron has all requisite corporate power and authority to execute, deliver and
perform its obligations under the Agreements, and the Agreements have been duly
authorized and validly executed and delivered by the Company and Thermo Electron
and constitute legal, valid and binding agreements of the Company and Thermo
Electron, enforceable against the Company and Thermo Electron in accordance with
their terms, except as rights to indemnity and contribution may be limited by
state or federal securities laws or the public policy underlying such laws,
except as enforceability 

 
                                       3



may be limited by applicable bankruptcy, insolvency, reorganization, moratorium
or similar laws affecting creditors' and contracting parties' rights generally,
except as enforceability may be subject to general principles of equity
(regardless of whether such enforceability is considered in a proceeding in
equity or at law) and except as otherwise described in the Placement Memorandum.

                4.3.   NON-CONTRAVENTION. The execution and delivery of the
Agreements, the issuance and sale of the Shares pursuant thereto, the
fulfillment of the terms of the Agreements and the consummation of the
transactions contemplated thereby will not conflict with or constitute a
violation of, or default (with the passage of time or otherwise) under, any
material agreement or instrument to which the Company or Thermo Electron is a
party or by which it is bound or the charter, by-laws or other organizational
documents of the Company or Thermo Electron, nor result in the creation or
imposition of any lien, encumbrance, claim, security interest or restriction
whatsoever upon any of the material property or assets of the Company or Thermo
Electron, or an acceleration of indebtedness pursuant to any obligation,
agreement or condition contained in any material bond, debenture, note or any
other evidence of indebtedness or any material indenture, mortgage, deed of
trust or any other material agreement or instrument to which the Company or
Thermo Electron is a party or by which any of them is bound or to which any of
the property or assets of the Company or Thermo Electron is subject, nor
conflict with, or result in a violation of, any law, administrative regulation,
ordinance or order of any court or governmental agency, arbitration panel or
authority applicable to the Company or Thermo Electron. Except for permits and
similar authorizations required under the securities or "Blue Sky" laws of
certain jurisdictions and for such permits and authorizations as have been
obtained, no consent, approval, authorization or other order of any regulatory
body, administrative agency, or other governmental body in the United States is
required for the valid issuance and sale of the Shares to be sold pursuant to
the Agreements.

                4.4.   CAPITALIZATION. The capitalization of the Company as of
September 28, 1996 is as set forth in the Placement Memorandum. The Company has
not issued any capital stock since that date except as contemplated by the
Placement Memorandum. The Shares to be sold pursuant to the Agreements have been
duly authorized, and when issued and paid for in accordance with the terms of
the Agreements will be validly issued, fully paid and nonassessable. The Shares
will conform to the description thereof in the Placement Memorandum. The
outstanding shares of capital stock of the Company have been duly and validly
issued to Thermedics Inc. (a subsidiary of Thermo Electron), and such shares are
fully paid and nonassessable and conform to the description thereof in the
Placement Memorandum. Except as set forth in or contemplated by the Placement
Memorandum, there are no outstanding rights (including, without limitation,
preemptive rights), warrants or options to acquire, or instruments convertible
into or exchangeable for, any shares of capital stock or other equity interest
in the Company or any contract, commitment, agreement, understanding or
arrangement of any kind relating to the issuance of any capital stock of the
Company, any such convertible or exchangeable securities or any such rights,
warrants or options.

                4.5.   LEGAL PROCEEDINGS. There is no material legal or
governmental proceeding pending or, to the knowledge of the Company, threatened
or contemplated to which the Company is or may be a party or of which the
business or Placement Memorandum.

 
                                       4


                4.6.   NO VIOLATION OF AGREEMENTS. The Company is not in
violation of its charter, bylaws, or other organizational document, in violation
of any law, administrative regulation, ordinance or order of any court or
governmental agency, arbitration panel or authority applicable to the Company,
which violation, individually or in the aggregate, would have a material adverse
effect on the business or financial condition of the Company and its
subsidiaries taken as a whole and is not in default in any material respect in
the performance of any obligation, agreement or condition contained in any bond,
debenture, note or any other evidence of indebtedness, in any indenture,
mortgage, deed of trust or any other agreement or instrument to which the
Company is a party or by which the Company is bound or by which the properties
of the Company are bound, and there exists no condition which, with the passage
of time or otherwise, would constitute a material default under any such
document or instrument or result in the imposition of any penalty or the
acceleration of any material indebtedness.

                4.7.   GOVERNMENTAL PERMITS, ETC.  The Company has all necessary
franchises, licenses, certificates and other authorizations from any foreign,
federal, state or local government or governmental agency, department, or body
that are currently necessary for the operation of the business of the Company
as currently conducted and as described in the Placement Memorandum, the
absence of which would have a material adverse effect on the Company.

                4.8.   FINANCIAL STATEMENTS. The consolidated financial
statements of the Company included in the Placement Memorandum present fairly,
in all material respects, the consolidated financial condition of the Company as
of the dates and for the periods indicated. Such financial statements are fairly
presented in accordance with generally accepted accounting principles (except
for the absence of footnote disclosure and the absence of consolidated
statements of cash flows).

                4.9.   NO MATERIAL ADVERSE CHANGE. Subsequent to the respective
dates as of which information is given in the Placement Memorandum, and except
as contemplated in the Placement Memorandum, the Company has not incurred any
material liabilities or obligations, direct or contingent, other than in the
ordinary course of business, and there has not been any material adverse change
in the Company's condition (in each case, financial or other), results of
operations, business, prospects, key personnel or capitalization on a
consolidated basis.

                4.10.   PLACEMENT MEMORANDUM. The information contained or
incorporated by reference in the Placement Memorandum is true and correct in all
material respects as of the date thereof; and the Placement Memorandum does not
contain an untrue statement of a material fact or omit to state a material fact
required to be stated therein or necessary to make the statements therein not
misleading.

 
                                       5


        SECTION 5.  REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE PURCHASER.

                    (a)  The Purchaser represents and warrants to, and covenants
with, the Company that: (i) the Purchaser is an "accredited investor" as defined
in the regulations under the United States Securities Act of 1933, as amended
(the "Securities Act"); (ii) the Purchaser is acquiring the Shares being
purchased by it for investment and with no present intention of distributing
such Shares; (iii) the Purchaser will not, directly or indirectly, voluntarily
offer, sell, pledge, transfer or otherwise dispose of (or solicit any offers to
buy, purchase or otherwise acquire or take a pledge of) any of the Shares except
in compliance with the Securities Act and the rules and regulations promulgated
thereunder; (iv) the Purchaser has had an opportunity to ask questions and
receive answers from the management of the Company regarding the Company, its
business and the offering of the Shares; (v) the Purchaser has, in connection
with its decision to purchase Shares relied solely upon the Placement Memorandum
and the representations and warranties of the Company and Thermo Electron
contained herein; and (vi) the purchase price of the Shares being purchased by
the Purchaser does not represent more than 5% of the net worth of the Purchaser
(exclusive of homes and furnishings).

                    (b)   The Purchaser acknowledges, represents and agrees
that:

               (i)  no action has been or will be taken in any jurisdiction
outside the United States by the Company or the Placement Agents that would
permit an offering of the Shares, or possession or distribution of offering
material in connection with the issue of the Shares, in any country or
jurisdiction outside the United States where action for that purpose is
required. Each Purchaser outside the United States will comply with all
applicable laws and regulations in each foreign jurisdiction in which it
purchases, offers, sells or delivers Shares or has in its possession or
distributes any offering material, in all cases at its own expense. The
Placement Agents are not authorized to make any representation or use any
information in connection with the issue, placement, purchase and sale of the
Shares other than as contained in the Placement Memorandum;
        
               (ii)  certificates evidencing the Shares will be delivered to it
upon the purchase thereof with a legend substantially to the following effect:
        
THE SECURITIES EVIDENCED BY THIS CERTIFICATE WERE ACQUIRED FOR INVESTMENT
AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED. 
SUCH SECURITIES MAY NOT BE OFFERED, SOLD, TRANSFERRED, PLEDGED, HYPOTHECATED OR
OTHERWISE DISPOSED OF, IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT
UNDER SAID ACT OR AN OPINION (WHICH SHALL BE IN FORM AND SUBSTANCE SATISFACTORY
TO THE COMPANY) OF COUNSEL SATISFACTORY TO THE COMPANY, THAT SUCH REGISTRATION
IS NOT REQUIRED.
        
The Purchaser agrees that any sale, transfer, pledge, hypothecation or other
disposition made by it shall be made in compliance with such legend; 

 
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               (iii)  it understands that it must bear the economic risk of its
investment for an indefinite period of time because the Shares have not been
registered under the Securities Act and, therefore, cannot be sold unless
subsequently registered under the Securities Act or an exemption from such
registration is available; and        

               (iv)   it understands that there is no public market for the
Shares.
        
                    (c)   The Purchaser further represents and warrants to, and
covenants with, the Company that (i) the Purchaser has full right, power,
authority and capacity to enter into this Agreement and to consummate the
transactions contemplated hereby and has taken all necessary action to authorize
the execution, delivery and performance of this Agreement, and (ii) upon the
execution and delivery of this Agreement, this Agreement shall constitute a
valid and binding obligation of the Purchaser enforceable in accordance with its
terms, except as enforceability may be limited by applicable bankruptcy,
insolvency, reorganization, moratorium or similar laws affecting creditors' and
contracting parties' rights generally and except as enforceability may be
subject to general principles of equity (regardless of whether such
enforceability is considered in a proceeding in equity or at law) and except as
the indemnification agreements of the Purchaser herein may be legally
unenforceable.

        SECTION 6.  SURVIVAL OF REPRESENTATIONS, WARRANTIES AND AGREEMENTS.
Notwithstanding any investigation made by any party to this Agreement or by the
Placement Agents, all covenants, agreements, representations and warranties
made by the Company and the Purchaser herein and in the certificates for the
securities delivered pursuant hereto shall survive the execution of this
Agreement, the delivery to the Purchaser of the securities being purchased and
the payment therefor.

        SECTION 7.  REGISTRATION OF THE SHARES; COMPLIANCE WITH THE SECURITIES
ACT.

                7.1.   REGISTRATION REQUIREMENTS.

                       (a)     On the first occasion that the Company registers
any of its securities under the Securities Act for sale to the public, whether
for its own account or for the account of other security holders or both (except
with respect to registration statements on Forms S-4, S-8 or another form not
available for registering the Shares for sale to the public), it will give
written notice to the Purchasers of its intention to do so. Upon the written
request of any Purchaser received by the Company within 10 days after the giving
of any such notice by the Company, to register any of such Purchaser's Shares
(which request shall state the intended method of disposition thereof), the
Company will cause the Shares as to which registration shall have been so
requested to be included in the securities to be covered by the registration
statement (the "IPO Registration Statement") proposed to be filed by the
Company, all to the extent requisite to permit the sale or other disposition by
such Purchaser (in accordance with its written request) of the Shares so
registered. In the event that the IPO Registration Statement shall be for an
underwritten public offering of Common Stock, the number of Shares to be
included in such an 

 
                                       7

underwriting may be reduced (pro rata among the Purchasers and other persons or
entities (other than the Company) requesting registration based upon the number
of Shares requested to be included) if and to the extent that the managing
underwriter shall be of the opinion that such inclusion would adversely affect
the marketing of the securities to be sold by the Company therein. In any such
underwritten public offering, the selling Purchasers shall become parties to the
underwriting agreement in the form agreed to by the Company and the
underwriters. If the managing underwriter shall be of the opinion that inclusion
of any securities in such registration for sale by selling shareholders would
adversely affect the marketing of the securities to be sold by the Company
therein, and no such secondary securities are to be included therein, the notice
provisions of the first sentence of this paragraph (a) shall be inapplicable.

                    (b)  Within 120 days after the closing under the IPO
Registration Statement (or, if earlier, within 10 days after the effective date
of the registration of the Common Stock under Section 12(g) of the Securities
Exchange Act of 1934 if such registration is undertaken otherwise than in
connection with the Company's initial underwritten public offering of its Common
Stock), the Company will file a registration statement (the "Resale Registration
Statement", the IPO Registration Statement and/or the Resale Registration
Statement being referred to herein as the "Registration Statement") under the
Securities Act with respect to the resale of all Shares held by the Purchasers
and not sold pursuant to the IPO Registration Statement, and the Company will
use its best efforts to cause the Resale Registration Statement to become
effective as soon as practicable. Each Purchaser undertakes in connection
therewith to provide in a timely manner all such information and materials and
take all such action as may be required in order to permit the Company to comply
with all applicable legal requirements and to obtain the acceleration of the
effective date of the Resale Registration Statement. Each Purchaser also agrees
that such Purchaser will not publicly re-sell any Shares held by such Purchaser
and not sold pursuant to the IPO Registration Statement for a period of 120 days
after the closing under the IPO Registration Statement provided that all of the
Company's executive officers and directors similarly agree (or are otherwise
effectively precluded during such period from making public resales of shares of
Common Stock owned by them).

                The Company will prepare and file with the Securities and
Exchange Commission (the "Commission") such amendments and supplements to the
Resale Registration Statement and the prospectus used in connection therewith as
may be necessary to keep the Resale Registration Statement effective until the
earliest to occur of (i) all of the securities registered thereunder have been
sold pursuant thereto, (ii) the third anniversary of the date of the Closing, or
(iii) until, by reason of Rule 144(k) under the Securities Act or any other rule
of similar effect, the Shares are no longer required to be registered for the
sale thereof by the Purchasers without restriction. The Purchaser acknowledges
that there may occasionally be times when the Company must suspend the use of
the prospectus forming a part of the Resale Registration Statement until such
time as an amendment to the Resale Registration Statement has been filed by the
Company and declared effective by the Commission, or until such time as the
Company has filed an appropriate report with the Commission pursuant to the
Securities Exchange Act of 1934, as amended. The Purchaser hereby covenants that
it will not sell any Shares pursuant to said prospectus during the period
commencing at the time at which the Company gives the Purchaser notice of the

 
                                       8

suspension of the use of said prospectus and ending at the time the Company
gives the Purchaser notice that the Purchaser may thereafter effect sales
pursuant to said prospectus.

                7.2.  REGISTRATION PROCEDURES

                      (a)   The Company will furnish to the Purchaser with
respect to the securities registered under the Registration Statement (and to
each underwriter, if any, of such securities) such number of copies of
prospectuses and preliminary prospectuses in conformity with the requirements of
the Securities Act and such other documents as the Purchaser may reasonably
request, in order to facilitate the public sale or other disposition of all or
any of such securities by the Purchaser; provided, however, that the obligation
of the Company to deliver copies of prospectuses or preliminary prospectuses to
the Purchaser shall be subject to the receipt by the Company of reasonable
assurances from the Purchaser that the Purchaser will comply with the applicable
provisions of the Securities Act and of such other securities or blue sky laws
as may be applicable in connection with any use of such prospectuses or
preliminary prospectuses;

                       (b)  The Company will file documents required of the
Company for blue sky clearance in states specified in writing by the Purchaser;
provided, however, that the Company shall not be required to qualify to do
business or consent to service of process in any jurisdiction in which it is not
now so qualified or has not so consented; and

                        (c)  The Company will bear all expenses in connection
with each Registration Statement and with the procedures in paragraphs (a) and
(b) of this Section 7.2 and the registration of the Shares pursuant to the
Registration Statement, other than fees and expenses, if any, of underwriters
and of counsel or other advisers to the Purchaser or the Other Purchasers.

             7.3.   INDEMNIFICATION.  For the purpose of this Section 7.3:

                    (a)     the term "Selling Shareholder" shall mean any
Purchaser selling securities pursuant to a Registration Statement, and any
affiliate of such Purchaser;

                    (b)     the term "Registration Statement" shall include any
preliminary prospectus, final prospectus, exhibit, supplement or amendment
included in or relating to either Registration Statement referred to in Section
7.1; and

                    (c)     the term "untrue statement" shall mean any untrue
statement or alleged untrue statement of a material fact in the Registration
Statement, or any omission or alleged omission to state in the Registration
Statement a material fact required to be stated therein or necessary to make the
statements therein, in the light of the circumstances under which they were
made, not misleading.

                The Company and Thermo Electron agree to indemnify and hold
harmless each Selling Shareholder from and against any losses, claims, damages
or liabilities to which such Selling Shareholder may become subject (under the
Securities Act or otherwise) insofar as such

 
                                       9


losses, claims, damages or liabilities (or actions or proceedings in respect
thereof) arise out of, or are based upon, any untrue statement contained in the
Registration Statement, and the Company and Thermo Electron will reimburse such
Selling Shareholder 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 and Thermo Electron 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 written information furnished
to the Company by or on behalf of such Selling Shareholder specifically for use
in preparation of the Registration Statement, or the failure of such Selling
Shareholder to comply with the covenants and agreements contained herein
respecting sale of its securities.

                The Purchaser agrees to indemnify and Electron (and each other
person, if any, who controls the Company within the meaning of Section 15 of the
Securities Act, each officer of the Company who signs the Registration Statement
and each director of the Company) from and against any losses, claims, damages
or liabilities to which the Company or Thermo Electron (or any such officer,
director or controlling person) may become subject (under the Securities Act 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 failure
of the Purchaser to comply with the covenants and agreements contained herein,
or any untrue statement contained in the Registration Statement if such untrue
statement was made in reliance upon and in conformity with written information
furnished by or on behalf of the Purchaser specifically for use in preparation
of the Registration Statement, and the Purchaser will reimburse the Company and
Thermo Electron (or such officer, director or controlling person), as the case
may be, for any legal or other expenses reasonably incurred in investigating,
defending or preparing to defend any such action, proceeding or claim.

                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 such indemnifying person shall have been notified
thereof, such indemnifying person shall be entitled to participate therein, and,
to the extent it shall wish, to assume the defense thereof, with counsel
reasonably satisfactory to such indemnified person. After notice from the
indemnifying person to such indemnified person of its election to assume the
defense thereof, such 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 opinion of counsel to 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; provided,
however, that no indemnifying person shall be responsible for the fees and
expenses of more than one separate counsel for all indemnified parties.

 
                                      10

        The obligations of the Company and Thermo Electron under this Section
7.3 shall be joint and several.

                7.4.   TERMINATION OF CONDITIONS AND OBLIGATIONS. The conditions
precedent imposed by this Agreement upon the transferability of the Shares as
relates to securities laws matters shall cease and terminate as to any
particular number of the Shares when such securities shall have been effectively
registered under the Securities Act and sold or otherwise disposed of in
accordance with the intended method of disposition set forth in the Registration
Statement covering such securities or at such time as an opinion of counsel
satisfactory to the Company shall have been rendered to the effect that such
conditions are not necessary in order to comply with the Securities Act.
 
                7.5.   INFORMATION AVAILABLE. Until the first Registration
Statement is effective, the Company will furnish to the Purchaser:

                       (a)     as soon as practicable after available one copy
of its year-end and quarterly consolidated financial statements prepared in
accordance with generally accepted accounting principles, such-year end
statements to be audited; and

                       (b)     upon the reasonable request of the Purchaser, any
other information concerning the Company.

        SECTION 8.  BROKER'S FEE.  The Purchaser acknowledges that the Company
intends to pay to the Placement Agents a fee in respect of this transaction, as
well as reimbursement of their expenses.  The parties hereto hereby represent
that there are no other brokers or finders entitled to compensation in
connection with the transactions contemplated hereby except for arrangements by
the Placement Agents to share a portion of their fee with certain other brokers
as selling group members.

        SECTION 9. INDEMNIFICATION. The Company and Thermo Electron jointly and
severally agree to indemnify and hold harmless the Purchaser from and against
any losses, claims, damages or liabilities, together with all reasonable costs
and expenses related thereto (including, without limitation, reasonable legal
fees and expenses), which would not have been incurred if (a) all of the
representations and warranties of the Company and Thermo Electron herein had
been true and correct when made or (b) all of the covenants and agreements of
the Company and Thermo Electron herein had been duly complied with and
performed.

 
                                      11

        SECTION 10.  NOTICES.  All notices, requests, consents and other
communications hereunder shall be in writing, shall be sent by first-class
registered or certified mail, or by an established courier service, and shall
be deemed given when so sent:

             (a)  if to the Company, to:  
                  President               
                  Thermedics Detection Inc.
                  220 Mill Road            
                  Chelmsford, Massachusetts  01824-4178

                  with a copy to:              
                  General Counsel              
                  Thermo Electron Corporation  
                  81 Wyman Street              
                  Waltham, Massachusetts  02254 

or to such other person at such other place as the Company shall designate to 
the Purchaser in writing; and

            (b)   if to Thermo Electron, to:
                  General Counsel
                  Thermo Electron Corporation
                  81 Wyman Street
                  Waltham, Massachusetts 02254

or to such other person at such other place as Thermo Electron shall
designate to the Purchaser in writing; and

        (c)     if to the Purchaser, at its address as set forth at the end of
this Agreement, or at such other address or addresses as may have been furnished
to the Company in writing.

        SECTION 11.  CHANGES.  Any term of the Agreements  may be amended or
compliance therewith waived with the written consent of the Company, Thermo
Electron and the holders (other than Thermo Electron and its subsidiaries) of a
majority of the Shares purchased pursuant to the Agreements (whether at the
Closing or subsequent closings).

        SECTION 12.  HEADINGS.  The headings of the various sections of this
Agreement have been inserted for convenience of reference only and shall not be
deemed to be part of this Agreement.

        SECTION 13.  SEVERABILITY.  In case any provision contained in this
Agreement should be invalid, illegal or unenforceable in any respect, the
validity, legality and enforceability of the remaining provisions contained
herein shall not in any way be affected or impaired thereby.

        SECTION 14.  GOVERNING LAW.  This Agreement shall be governed by and
construed in accordance with the internal laws of the Commonwealth of
Massachusetts and United States federal law.

        SECTION 15.  COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall constitute an original, but all of which, when
taken together, shall constitute but one instrument, and shall become effective
when one or more counterparts 

 
                                      12

have been signed by each party hereto and an original or conformed copy
delivered to the other parties.

[Remainder of page intentionally left blank]

 
                                      13
 
        IN WITNESS WHEREOF, the parties hereto have caused this Stock Purchase
Agreement to be executed by their duly authorized representatives as of the
following date. 

Dated: November 19, 1996                        THERMEDICS DETECTION INC.


No. of Shares Allocated to                      By: /s/ Jonathan W. Painter
the Purchaser: 10,000                                   Title:  Treasurer


                                                THERMO ELECTRON CORPORATION


                                                By: /s/ Jonathan W. Painter
                                                        Title:  Treasurer

 
                                      14

PURCHASER SIGNATURE PAGE

        The undersigned Purchaser hereby executes the Stock Purchase Agreement
with Thermedics Detection Inc. and Thermo Electron Corporation and hereby
authorizes this signature page to be attached to a counterpart of such document
executed by a duly authorized officer of Thermedics Detection Inc. and Thermo
Electron Corporation.

Maximum Number of Shares                    Jeffrey J. Langan
to be Purchased: 10,000                     Name of Purchaser -- PLEASE
                                            PRINT OR TYPE
        
U. S. Taxpayer ID No., if any:              [SIGN HERE]:
        
________________________                    By: /s/ Jeffrey J. Langan
        
                                            Title: N/A
        
                                            Address:

                                            ---------------------------
        
                                            ---------------------------
         
                                            ---------------------------
        
        Please set out below your registration requirements:

        Name in which Shares
        are to be registered:   Jeffrey J. Langan

        Address of registered holder (if different
        from above):  

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

        Contact name and telephone
                number regarding settlement
                and registration:
                                 ---------------------------
                                            Name

                                 ---------------------------
                                      Telephone Number