DSpace at VNU: Surveying the HPGe Gamma detector absolute efficiency

V N U J O U R N A L O F S C IE N C E , M a th e m a tic s - P h y s ic s , T.xx, N 02, 0 S U R V E Y IN G T H E H P G e G A M M A D E T E C T O R A B SO L U T E E F F IC IE N C Y T r a n Tri V ie n , D o a n Q u a n g T u y e n , T r a n V ie t N h a n H ao , D o a n T h a n h S o n , N g u y e n T r u n g T in h College o f Science, V N U A b stra c t In many nuclear experiments, the energy efficiency of detector is a parameter without negligibility In this paper, the absolute efficiency of HPGe detector is surveyed and mearsured at different distances from detector and different gamma energies I n t r o d u c t i o n In m any n u c le a r e x p e rim e n ta l m e a s u re m e n ts , t h e d e te rm in e d resu lts d epend on e x p e rim e n ta l p a m e te r s , one of th e s e p a r a m e t e r s is absolute detector efficiency B u t u n f o r tu n a te ly th e efficiency of n u c le a r d e te cto r is n o t constant, it dep en d on th e energy of m e a s u re m e n ta l d ia tio n So t h a t , m aking efficiency c a lib tio n of d etector is n e ce ssa ry In th is p a p er, th e a b so lu te efficiency calibration of H P G e g a m m a d etector is surveyed A b s o l u t e e f f i c i e n c y o f d e t e c t o r TỊabs a t a e n e r g y v a l u e i s d e t e r m i n e d t h r o u g h , ph oto peak a re a, s , by e q u a tio n below: s nlabs = — — A.I.t ị V (1) w here: A -activity of d io a c tiv ity source I - d io a c tiv ity em issio n probability t - m e a s u re m e n ta l tim e As e q u a tio n (1), t h e e rro r of efficiency d ep end on t h e p h o to p ea k area (S) In order to d ecrease stro n g ly th e e rro r of efficiency T h e m e a s u re m e n ta l are a of ph otopeak should d e te rm in e w ith h ig h precision To resolve th is problem, some d e te rm in a tio n p h o to p ea k a re a stu died : - T he to ta l p e ak a re a a p p ro x im a tio n - T he Covell m e th o d - T he W asso n m eth o d Beside, th e su p e rp o sitio n , d e ad tim e, th e effect a r e consided In th is p a p e r, th e g a m m a H P G e detector efficiency is d e te rm in e d through th e to p eak a re a of s t a n d a r d sources, th e d e te rm in a tio n of detector efficiency is able to be p erfo rm ed by c alculation B u t w ith th is m eth o d , th e d etector geometry 44 Surveying the HPGe g a m m a d e te c to r abso lute efficiency 45 has to be known So, in e x p e rim e n t, th e efficiency c a lib tio n m eth o d is combined w ith sem i-em pirial re la tio n be come m ost reliable In order to reject th e influence of th e d isto rtio n of p h o to p e a k s h a p e due to th e high activity a n d by th e co u n tin g loss due to th e pile-up effects, th e sam p le should be p u t at place w ith different d istan c es to detector For fitting th e e x p e rim e n ta l d etector efficiency d a ta w ith th e o ric a l fuction In th is paper, two th eo rica l fu nctions describing th e d ep en d en ce of efficiency on energy are used, such as: r| = y H.lnte) 1 = (1 ,^ ' E x p e r im e n t a l a b s o l u t e e f f i c i e n c y c a l i b r a t i o n For g e ttin g e x p e rim e n ta l efficiency value T he sources w ith different g am m a ray energies a re u s e d in our e x p erim en t: E u 152, C s137 a n d A m 241 T he sources w ith shap of disk w ith Cm r a d iu s a re su pplied by IAEA w ith p a r a m e t e r s as following: S o u r c e : E u 152 Half-life: 12.7 Y ear D a te of produce: A u g u s t l 8t 2002 A ctivity initial: 3672.62 Bq S o u r c e : A m 241 Half-life: 433 Y ear D a te of produce: J u ly 15th 2002 A ctivity in itial: 3759.2 Bq S o u r c e : C s 137 lf-life: 30.1 Y ear D a te of produce: D ecem ber l 8t 1994 A ctivity in itial: 36445 Bq For efficiency c a lib tio n of detector, in our e x p e rim e n t we h a v e to know th e activity of source a t th e e x p e rim e n ta l tim e T h is w ork is n o t difficult by using equation: A=A0e Xt The p a r a m e t e r s of e x p e rim e n t w ith H P G e g a m m a v isio n sp e ctro m etry are given in table a n d plot of e x p e rim e n ta l efficiency c a lib tio n in fig l 46 T r a n Tri Vien, D o a n Q u a n g Tuyen, T r a n Viet N h a n Hao T a b le l The efficiency of detector depend on energies a t cm from detector Order Number Count Energy (KeV) It (%) 121.7793 30.6788 14020 344.31 97 411.13 Count pel' second Activity of source Detection efficiency 46.7350 1057.0718 0.0442±2% 6977 23.2591 937.2056 0.0248±2.3% 2.2848 544 1.8189 78.7253 0.0230±9% 778.91 12.7187 1517 5.05667 438.2368 0.0115±4% 964.05 14.3344 1633 5.44500 493.9075 0.0110*4% 1085.81 10.0966 1072 3.57367 347.8895 0.0103±3% 1112.08 13.4042 1244 4.14667 461.8565 0.0090±4% 1408.08 20.7264 1562 5.20667 714.1509 0.0073±4% (Am) 59.739 35.75 5983 9.97200 1341.13114 0.0074±1.7% 10 (Cs)661.38 85.05 37674 418.5944 25278.6461 0.0166±6% After fitting the exp eim ental d a ta with the theorical function: ’1 = r| = Ỵ a M E ) and the absolute efficiency function of HPGe gam m avision sp e c tro m e try is as following: 11, (E) = 11.5436 E '1- 1110.34 E'2 + 60344.9 E -2008730 E rj2(B) =-3.09414(lnE) 1+73.7661(lnE)'2-687.098(lnE )'3+ l(ln E ) 4-4814.21(lnE)'5 (3) Absolute efficiency 0.04 0.0 0.02 0.01 200 400 600 800 1000 1200 1400 Energy Fig The dependence of efficiency of detector on energies a t cm from detector 47 Surveying the HPGc g a m m a detector absolute efficiency T a b le The efficiency of detector depend on energies a t cm from detector Order Number Energy (KeV) Iy (%) Count Count per second Activity of source Detection efficiency 121.7793 30.6788 37400 10.3889 1057.0718 0.0098±1.72% 244.6927 7.7193 295.96 0.4324 1.5944 0.0747 265.9770 0.0060±2.71% 0.0050±1.16% 344.32 444.03 97 5740 269 15500 2.8832 1320 688.62 778.91 0.8514 12.7187 220 4.0963 10 867.39 964.05 1005.06 14.3344 0.6364 852 2990 134 11 1085.81 10.0966 1960 0.0372 0.5444 12 13 1089.73 1112.08 1.8115 14 15 1212.94 230 1299.2 13.4042 1.496 1.74624 356 2560 16 1408.08 20.7264 14.8988 937.2059 4.3056 0.3667 0.0046±2.01% 99.3438 0.0037±4.82% 0.0611 0.9194 29.3359 438.2368 0.0021±13.6% 0.0021±3.16% 0.2367 141.1425 493.9075 21.9306 0.0017±6.36% 0.0017±3.17% 347.8895 0.0016±3.6% 0.0989 0.0711 62.4172 461.8565 0.0016±1.16% 0.0015±3.02% 246 0.0639 0.0683 51.5463 60.1686 0.0012±11.24% 0.0011±1.16% 3140 0.8722 714.1509 0.0012±2.81% 3310 0.8306 0.0017±16.81% A b so lu te efficiency 200 400 600 800 1000 1200 1400 Fig.2 The dependence of detector efficiency on th e energies a t 5cm from detector After fiting th e e x p eim e n tal d a ta with th e theorical function: and TỊ = ^ a-ln(i?)_l 48 T r a n Tri Vien, D o a n Q u a n g Tuyen, T r a n Viet N h a n Hao th e absolu te efficiency fun ction of H PG e g a m m av isio n sp e c tro m e try is as following: 6l(E) = 1.65095 E - 30.281 E '2 - 3103.08 E e2(E) = (ln E )1-7.62022 (InE)-2 +43.9916(lnE )'3 -84.6 (ln E )4 (4) F ig T he d ep en d en ce of a b so lu te efficiency of d etector on e n erg ies of HPGe gam m a G a m m a v isio n spectrom etry Absolute efficienc Fig.4 The depen dence of a b so lu te efficiency of detector on e n erg ies of HPGe gam m a G e n n ie 2000 sp e ctro m etry Su rveying the HPGc g a m m a dete ctor absolute efficiency 49 In order to d e te rm in e th e dependence of ab so lu te d etector efficiency on gam m a energies at d ifferen t d istan c es from d etector to source G a m m a sources are placed at different d ista n c e s from detector surface In o u r e x p erim e n t, g am m a sources are placed a t position from cm to 16 cm to su rfa ce of detector The absolute efficiency of H P G e g a m m a G a m m a v isio n sp e ctro m etry are show n in fig.3 The absolute efficiency of H PG e g am m a G en nie 2000 s p e c tro m e try a re show n in fig.4 R e s u lt s a n d d i s c u s s i o n F ittin g th e e x p e rim e n t d a ta for d e te rm in in g a b so lu te efficiency of detector w ith theorical functions is c a rr ie d out In order to select th e m ost su itab le theorical fu n ctio n for fittin g with ex p erim en tal data T he fittin g p a m e te r s of e x p e rim e n ta l d a ta wich theorical functions: T! = £ and a,:l n ( E ) ' n=x > £ ‘ (5) (6) a re compared In general, th e fittin g d ia g m s of th ese two fu n ctio n s a re closing to ex p erim en tal points However, th e function (2) is m ore s u ita b le to h igh energentic rad iatio n s because B coefficients h a v e sm a lle r failu res, th e re fo re , e rro r a re sm all A c k n o w l e d g e m e n t s : T he V ie tn a m N a tio n a l U n iv ersity , H anoi su p p o rts this work throug h th e subject QG-04-02 R eferen ces Boston M., E r d u r a n M N ,Sirin M a n d S u b a st M., Isom eric cross-section ratio for the (n,2n) re a c tio n on Sc from 13.6 to 14.9 MeV , Phys Rev, New York, V 56, No 2(1997), pp 918-921 w M a a h a r t a n d H Vonach, T he g am m a-ray a b so rp tio n coefficients for Nal(Tl), Nucl.Instr Meth E lsevier, V.134, No 4(1976), pp 347-351 Kolev D., S tu dies of som e Isom eric Yield R atios P ro d u ced w ith B re m s s tra h lu n g , Appl R ad ia ti h o t , G r e a t B rita in , V.49, No 8(1998), p p 989-995 Seuyng-Gy Ro, , A bsolute dection efficienies of cy lin d ric a l N al(T l) c ry s ta l for point source g a m m a-ra y s, J.Kor.Asso.Radiat Prot, Korea, V 8, No 3(2002), pp 235-241  ... (3) Absolute efficiency 0.04 0.0 0.02 0.01 200 400 600 800 1000 1200 1400 Energy Fig The dependence of efficiency of detector on energies a t cm from detector 47 Surveying the HPGc g a m m a detector. .. 418.5944 25278.6461 0.0166±6% After fitting the exp eim ental d a ta with the theorical function: ’1 = r| = Ỵ a M E ) and the absolute efficiency function of HPGe gam m avision sp e c tro m e try is... detector absolute efficiency T a b le The efficiency of detector depend on energies a t cm from detector Order Number Energy (KeV) Iy (%) Count Count per second Activity of source Detection efficiency