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, 2 0 0 4

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.
1. 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 ra 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 ra d ia tio n . So t h a t , m aking efficiency
c a lib ra 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 ra d io a c tiv ity source
I - ra 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 pho 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 ra 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 = 1 (1 ,^ '
2. 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 1 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 ra 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 1 a n d plot of e x p e rim e n ta l efficiency c a lib ra 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 2 cm from detector

Order
Number

Count

Energy
(KeV)

It
(%)

1

121.7793

30.6788

14020

9

344.31

97 9

3

411.13

4

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%

5

964.05

14.3344

1633

5.44500

493.9075

0.0110*4%

6

1085.81

10.0966

1072

3.57367

347.8895

0.0103±3%

7

1112.08

13.4042

1244

4.14667

461.8565

0.0090±4%

8

1408.08

20.7264

1562

5.20667

714.1509

0.0073±4%

9

(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 ) 1

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 3 -2008730 E 1

rj2(B) =-3.09414(lnE) 1+73.7661(lnE)'2-687.098(lnE )'3+ 2 9 6 7 .1 l(ln E ) 4-4814.21(lnE)'5
(3)
Absolute
efficiency

0.04

0.0 3

0.02

0.01

200

400

600

800

1000

1200

1400 Energy

Fig. 1 The dependence of efficiency of detector on energies a t 2 cm from detector

47

Surveying the HPGc g a m m a detector absolute efficiency
T a b le 2 . The efficiency of detector depend on energies a t 5 cm from detector
Order
Number

Energy
(KeV)

Iy
(%)

Count

Count per
second

Activity
of source

Detection
efficiency

1

121.7793

30.6788

37400

10.3889

1057.0718

0.0098±1.72%

9

244.6927

7.7193

295.96

0.4324

1.5944
0.0747

265.9770

3

0.0060±2.71%
0.0050±1.16%

4
5

344.32
444.03

97 9

5740
269
15500

2.8832

1320

6
7

688.62
778.91

0.8514
12.7187

220

8
9

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 1 - 30.281 E '2 - 3103.08 E 3
e2(E) = 0 .3 6 0 4 5 8 (ln E )1-7.62022 (InE)-2 +43.9916(lnE )'3 -84.6 5 7 1 (ln E )4 (4)

F ig .3. 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 2 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.

3.

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 ra 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 ra 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
1.

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.

2.

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 .

3.

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.

4.

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.