Exhibit 4.1 [FRONT OF STOCK CERTIFICATE] THERMEDICS DETECTION INC. TDX COMMON STOCK PAR Incorporated under the laws of the COMMON STOCK PAR VALUE $.10 Commonwealth of Massachusetts CUSIP 88355E 10 5 SEE REVERSE SIDE FOR CERTAIN DEFINITIONS This certifies that is the owner of FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK, $.10 PAR VALUE, OF THERMEDICS DETECTION INC. transferable on the books of the Corporation by the holder hereof in person or by duly authorized attorney upon surrender of this Certificate properly endorsed. The Certificate and the shares represented hereby are issued under and shall be subject to the laws of the Commonwealth of Massachusetts and all the provisions of the Articles of Organization and the By-Laws of the Corporation, and all the amendments from time to time made thereto. This Certificate is not valid unless countersigned and registered by the Transfer Agent and Registrar. Witness the facsimile seal of the Corporation and the facsimile signatures of its duly authorized officers. Dated: [ THERMEDICS DETECTION INC. ] [ Corporate Seal ] Treasurer President and Chief Executive Officer [BACK OF STOCK CERTIFICATE] THERMEDICS DETECTION INC. This Corporation will furnish without charge to each stockholder who so requests, a copy of the designations, powers, preferences and relative, participating, optional or other special rights of each class of stock or series thereof, and the qualifications, limitation or restrictions of such preferences and/or rights. Any such requests may be addressed to the Clerk of the Corporation. The following abbreviations when used in the inscription on the face of this certificate, shall be construed as though they were written out in full according to applicable laws or regulations: TEN COM - as tenants in common TEN ENT - as tenants by the entirety JT TEN - as joint tenants with right of survivorship and not as tenants in common UNIF GIFT MIN ACT - __________ Custodian ___________ (Cust) (Minor) under Uniform Gifts to Minors Act __________________________ (State) Additional abbreviations may also be used though not in the above list. For value received, ______________________ hereby sell, assign and transfer unto PLEASE INSERT SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE _____________________________________________________________________________ _____________________________________________________________________________ (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS INCLUDING POSTAL ZIP CODE OF ASSIGNEE) _____________________________________________________________________________ _________________________________________________________ Shares of the Stock represented by the within Certificate, and do hereby irrevocably constitute and appoint _________________________________________________________________ _____________________________________________________________________________ Attorney to transfer the said Stock on the books of the within-named Corporation with full power of substitution in the premises. Dated _______________________________ _________________________________ Signature ___________________________________ THE SIGNATURES TO THIS ASSIGNMENT MUST CORRESPOND WITH THE CERTIFICATE IN EVERY PARTICULAR WITHOUT ALTERATION OR ENLARGEMENT OR ANY CHANGE WHATSOEVER.