Nuclear Instruments and Methods m Physics Research 227 (1984) 257 258 257 North-Holland, Amsterdam

A HIGH ENERGY GAMMA RAY C A L I B R A T I O N S O U R C E

K i r k J. K I N G a n d T o m m y L. J O H N S O N

Health Physt¢s Staff (Code 6070), Naval Research Laboratory, Washington, DC 20375, USA

Recoved 26 April 1984

A high energy gamma ray cahbration source is obtained by the fast neutron lrradmtlon of polytetrafluonde and lead

High energy gamma ray cahbra t lon s tandards are essential for cahbra t lng gamma ray spectrum analyzers used to identify unknown fissionable materials. The 4 439 MeV gamma ray from the 9Be(or, T/)12C* react ion in all a lpha -be ry l l ium neu t ron sources is often used as a high energy s tandard. A more desirable source would be one having higher energy gamma rays which do not exhibi t the characteris t ic Dopple r -broadening of the 4.439 MeV peak c o m m o n to all a, T/ sources. To meet the need for a bet ter high energy cal ibrat ion s tandard, we have constructed a source holder for a small A m - B e neu t ron source which uses fast neu t ron reactions in the holder materials to produce gamma ray peaks having energies from 0 5 to 7.1 MeV at roughly 0 5 MeV intervals. The details of the source holder are shown in fig 1

Two high energy gamma rays, one at 7.117 MeV (5%), the other at 6.128 MeV (69%) are emit ted from 16N which is produced by the fast neu t ron reaction 19F(T/, a)16N in fluorine a toms of the Teflon (polytetra- fluoride). A lower energy gamma ray of 2 614 MeV is produced by the 2°8Pb(~/, p)2°STL reaction in the lead insert This is a bonus since the pr imary purpose of the lead is to reduce the intensi ty of the 4 439 MeV gamma rays from the neut ron source relative to the higher energy gamma rays from the 16N. Many addi t ional gamma rays with energies less than 2 MeV are produced by neut ron reactions in the Teflon and lead of the source holder, and also by a lpha and neut ron reactions in the stainless steel encapsula t ion of the neu t ron source. In order to reduce the intensi ty of these lower-energy gamma rays, a 6.4 m m thick tungsten absorber is added

lO c m ~ , ~

S O U R C E 1

L E A D - -

0 64 ¢m

" 4 TUNGSTEN --

Fig 1 Diagram of source holder.

0168 -9002 /84 /$03 00 © Elsevier Science Publishers B.V. (Nor th -Hol l and Physics Publ ishing Division)

258 K J Kmg, T L Johnson /Htgh energy 7-ray cahbrat~on source

10 7

z Z

-r" (..)

I-- Z

o (D

10 6

10 5

10 4

lO a

,,,.~ ~ ~ * , ,

z =z =u_

Lt3 t.~ r,D

~. Z

z

r,~

10t 0 I I I I I I I 1.0 2 .0 3 .0 4 .0 5 .0 6 0 7 0 8

E N E R G Y (MeV)

Fig. 2 G a m m a ray spectrum, obtained using a 0 1 C1 A m - B e neutron source m the source holder shown m fig. 1, placing it on top of a 110 cm 3 h t luum-dnf ted germanium detector and counting for 1000 mln

to t he b o t t o m of the sou rce h o l d e r as s h o w n m fig. 1.

T h e s p e c t r u m s h o w n m fig. 2 was o b t a i n e d wi th a 0.1

C~ A m - B e sou rce ( = 2.5 × 105 , / / s ) m the sou rce h o l d e r

s h o w n m fig 1. T h e sou rce ho l de r was p l aced d i rec t ly

o n top o f a 110 c m 3 l l t t u u m - d r l f t e d g e r m a n i u m d e t e c t o r

a n d c o u n t e d for 1000 m m . T h e m o s t p r o / m n e n t p e a k s

a n d thei r ene rg ies [1] a n d or ig ins a re n o t e d m fig. 2.

T h e a u t h o r s wish to t h a n k M e s s r s R. Lue r sen , W

P o w e r s a n d E. R a n k for the i r t echn ica l a s s i s t ance .

Reference [1] G Erdtmann and W. Soyka, The gamma rays of the

radlonuchdes tables for apphed gamma ray spectroscopy (Wemhelm, Verlag Chemle, New York, 1979)