SECURITIES AND EXCHANGE COMMISSION

                             Washington, D.C. 20549

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


                                    FORM 8-K

                                 CURRENT REPORT



                       Pursuant to Section 13 or 15(d) of
                       the Securities Exchange Act of 1934



                                 Date of Report
                             (Date of earliest event
                                   reported):

                                   May 6, 1998

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


                            THERMEDICS DETECTION INC.
             (Exact name of Registrant as specified in its charter)


Massachusetts                   1-12745                           04-3106698
(State or other               (Commission                    (I.R.S. Employer
jurisdiction of               File Number)               Identification Number) 
incorporation or
organization)


220 Mill Road
Chelmsford, Massachusetts                                            01824-4178
(Address of principal executive offices)                             (Zip Code)


                                 (781) 622-1000
                         (Registrant's telephone number
                              including area code)













Item 2.  Acquisition or Disposition of Assets

         On May 6, 1998,  Thermedics  Detection  Inc. (the  "Company")  acquired
Orion Research Inc. ("Orion Research"),  a wholly owned subsidiary of Thermedics
Inc.  ("Thermedics"),  in exchange for the right to receive  5,961,225 shares of
the  Company's  common  stock.  Orion  Research   manufactures   electrode-based
chemical-measurement  products  that  determine the quality of a wide variety of
substances  by measuring  components,  such as pH, ion,  dissolved  oxygen,  and
conductivity  levels.  These products are used in the  agricultural,  biomedical
research,  food-processing,   and  pharmaceutical  industries.  In  1997,  Orion
Research's   revenues   and  net  income  were   $53,054,000   and   $6,437,000,
respectively.

         The  acquisition  was  made  pursuant  to  an  Agreement  and  Plan  of
Reorganization  dated as of May 6, 1998 (the  "Agreement"),  among the  Company,
Orion   Acquisition   Inc.,   a  wholly   owned   subsidiary   of  the   Company
("Acquisition"),   Thermedics  and  Orion  Research.  Under  the  terms  of  the
Agreement,  (i)  Acquisition  will  merge  with and into  Orion  Research,  (ii)
outstanding  shares of Orion  Research's  common  stock  will be  cancelled  and
converted  into the right to receive  5,961,225  shares of the Company's  common
stock,  (iii)  each  outstanding  share of  Acquisition's  common  stock will be
cancelled  and converted  into one share of the common stock of Orion  Research,
and (iv) Orion Research will become a wholly owned subsidiary of the Company.

         The shares of the  Company's  common  stock to be issued in  connection
with the  acquisition  will be so issued as soon as such  shares  are listed for
trading upon the American Stock  Exchange,  Inc. The exchange  requires that the
listing be approved by the  holders of a majority of the  Company's  outstanding
shares present and voting at a shareholders' meeting. The meeting is expected to
be held before the end of fiscal 1998.  Thermedics has agreed to vote all of the
shares of the  Company's  common  stock held by it as of the record  date of the
meeting in favor of the listing of the Company's  shares and all matters related
thereto.  Before  giving  effect  to the  issuance  of the  shares  to be issued
pursuant to the Agreement, Thermedics owned approximately 77% of the outstanding
common  stock of the  Company.  Giving  effect to the  issuance of such  shares,
Thermedics owns approximately 84% of such outstanding common stock.

         The  consideration  to be paid  for  Orion  Research  was  based on the
Company's  determination of the fair market value of Orion Research's  business.
Based on the  average of the closing  prices of the  Company's  common  stock as
reported on the  American  Stock  Exchange  for the five  trading days ending on
April 13, 1998 (the date preceding the Company's  announcement  of its intention
to acquire Orion Research), the shares to be issued to Thermedics had a value of
$65,800,000 prior such announcement.

         Because  the  Company and Orion  Research  were  deemed for  accounting
purposes to be under control of their common  majority  owner,  Thermedics,  the
transaction  has been accounted for at historical  cost in a manner similar to a
pooling of interests. Accordingly, the Company's financial statements (including
the financial  statements  included as part of the Company's Quarterly Report on
Form 10-Q for the  quarter  ended  April 4, 1998)  include  the results of Orion
Research  from  December  1,  1995,  the date Orion  Research  was  acquired  by
Thermedics, and the shares issuable subject to listing on the Exchange have been
deemed outstanding from that date.

         The Company has no present intention to use Orion Research's assets for
purposes materially  different from the purposes for which such assets were used
prior to the  acquisition.  However,  the Company will review  Orion  Research's
business   and  assets,   corporate   structure,   capitalization,   operations,
properties,  policies,  management  and personnel  and, upon  completion of this
review, may develop alternative plans or proposals, including mergers, transfers
of a material amount of assets or other transactions or changes relating to such
business.

     Item 7.  Financial  Statements,  Pro  Forma  Combined  Condensed  Financial
Information and Exhibits
             (a)         Financial Statements of Business Acquired:  Information
                         meeting  the  requirements  of this  Item  7(a) will be
                         filed by amendment  within the time period permitted by
                         Item 7(a)(4) of Form 8-K.

             (b)         Pro Forma  Combined  Condensed  Financial  Information:
                         Information  meeting the requirements of this Item 7(b)
                         will be  filed by  amendment  within  the  time  period
                         permitted by Item 7(a)(4) of Form 8-K.

             (c)         Exhibits

                         2.   Agreement  and Plan of  Reorganization  dated as  
                              of May 6,  1998 by and among the Company,  Orion 
                              Acquisition Inc.,  Thermedics Inc., and Orion 
                              Research Inc.  (filed as Exhibit 2.3 to the 
                              Company's  Quarterly  Report on Form 10-Q and
                              incorporated herein by reference).



                                   SIGNATURES

     Pursuant to the  requirements  of the Securities  Exchange Act of 1934, the
Registrant  has duly  caused  this  report  to be  signed  on its  behalf by the
undersigned thereunto duly authorized, on this 20th day of May, 1998.


                                                THERMEDICS DETECTION INC.


                                                 By: /s/ Melissa F. Riordan
                                                 Melissa F. Riordan
                                                 Treasurer