Experimental determination of the photofraction of a cylindrical 4″ × 6″ NaI(Tl) scintillation detector

NUCLEAR

INSTRUMENTS

AND' METHODS

23

(1963)

10712;

NORTH-HOLLAND

PUBLISHING

CO.

EXPERIMENTAL DETERMINATION OF THE PHOTOFRACTION OF A CYLINDRICAL 4" × 6" NaI(TI) SCINTILLATION DETECTOR C. W E I T K A M P

Institut ]i2r Neutronenphysik und Reaktortechnik, tfern/orschungszentrum Ifarlsruhe R e c e i v e d 30 N o v e m b e r 1962

The p h o t o f r a c t i o n of a N a I ( T I ) c r y s t a l of 4" d i a m e t e r X 6" l e n g t h is d e t e r m i n e d for g a m m a - r a y s b e t w e e n 0.32 a n d 2.76 MeV. T h e m e t h o d used eliminates all p e r t u r b a t i v e effects e x c e p t C o m p t o n s c a t t e r i n g in t h e source which is corrected for analytically.

1. Introduction Measurement of the photofraction of NaI(T1) scintillation detectors 1-5) in most cases yield results considerably lower than the theoretical values. As the experiments can never exactly represent the simple conditions assumed in the calculations, and as few of the measurements try to remove or correct for these effects, it appeared interesting to set up a simple arrangement allowing the determination of the photofraction under elimination of background, backscatter from the crystal housing and from the wall of the laboratory, counting rate dependence of the amplification factor and deadtime effects in the pulse height analyzer. Compton scattering in the source was allowed for by analytical corrections. The measurements were performed with uncollimated radiation and with a e~ollimator of the aperture 0 = ½0..... 0=.x being the total angle subtended by the crystal. Compared to the method of Ricci 6) the present arrangement has the advantage of using activities smaller by a factor of 103 , thus requiring no shielding and hence reducing scattering a priori to a minimum.

lines of these nuclides (32 and 69 keV) are well suited for calibration purposes. The activities of the sources had to be small enough to avoid the relatively strong counting rate dependence of the amplification factor of the photomultiplier 7) but high enough to maintain good statistics when subtracting background radiation. The optimum activities were between 2 and 7 ~C, depending upon the different ~-energies of the nuclides. The source material was enclosed in small plexiglas cylinders. Background and radiation scattered from the walls of the laboratory were subtracted from the original spectra. The direct radiation was kept aloof by a lead cone (see fig. la). This lead cone, however, prevented not only the direct radiation of the source from reaching the crystal but also a certain amount of background and backscatter. To compensate this effect, the "subtracting t i m e " was increased by an appropriate factor. When subtracting background and backscatter radiation from the spectrum, the influence of gammas scattered in the crystal container is not 1) S. H. Vegors, L. L. Marsden a n d R. L. H e a t h , IDO-16370 (1958). ~) XV. E. Kreger, Phys. Rev. 96 (1954) 1554. z) E. R. R a t h b u r n a n d C. E. Croutharnel, Applied G a m m a R a y S p e c t r o m e t r y (C. E. C r o u t h a m c l ed.); ( P e r g a m o n Press, Oxford 1960). 4) W'. E. K r e g e r a n d R. M. Brown, Nucl. I n s t r . a n d Metb. 11 (196I) 290. 5) H . M. Childers, R e v . Sci. Instr. 30 (1959) 810. 6) R. A. Rieci, P h y s i c a 24 (1958) 289. 7) C. W e i t k a m p , D i p l o m a r b e i t Karlsruhe 1962 (unpublished).

2. Experimental Procedure Tile scintillation detector was a 4"diam. x 6" Harshaw NaI(T1) crystal mounted on an RCA 7046 photomultiplier tube. The pulses of the tenth dynode were fed to a 256 channel pulse-height analyzer. The zero level was determined by means of a weak source of Cs 137 or Au 19s. The conversion 10

DETERMINATION

OF T H E P H O T O F R A C T I O N

taken into account. To eliminate this effect, the following simple extrapolation method was used: A tube of thickness equivalent to the thickness of the crystal container, reflector material etc. was placed over the front part of the detector in order to double the number of gammas scattered into the crystal, and a second tube to treble it. Each measurement was then performed without any tube, with one tube, and with two tubes over the crystal housing; the three photofraction results were linearly extrapolated to container thickness zero. In the case of collimated radiation the collimator kept direct radiation nearly completely off the crystal housing so that no such correction was necessary. The timer unit used allowed direct deadtime correction

OF A S C I N T I L L A T I O N

DETECTOR

II

3) The sources are infinitely long cylinders of radius R. Then I is given by (see fig. 2)

i=

~-o,=o,=o,=o

- 2n

n

R

-=

{R.

2'~

fsinOdfdf,d, 0

0

0

0

I

~

J

Fig. 2. E x p l i c a t i o n of parameters i n v o l v e d in the calculation of the mean p a t h i of p h o t o n s in the source.

fo)

As the scattered gammas have a smaller energy than the unscattered, the mean efficiency T 2 of scattered photons is larger than the efficiency T 1 of the unscattered photons. Calling P * = P + A P the " t r u e " photofraction, one has

(b)

Fig. 1. G e o m e t r y of the arrangement. S source K NaI(T1) crystal L lead cone H crystal housing C collimator T 1, T 2 a l u m i n u m tubes.

P =

T 1 e -~J + T2(1 - e-Zd)

and, hence, the correction AP = P'T2/T I"B.

3. T r e a t m e n t of E x p e r i m e n t a l D a t a

The influence of photons scattered in the source material was not negligible in all cases, especially for low g a m m a energies. To calculate the correction of the measured photofractions, the following simplifying assumptions were made : 1) Photons subject to a Compton effect within the source can no longer cause a pulse contributing to the full energy peak of the spectrum. 2) A mean path, f, of the gammas in the source m a y be defined so that the ratio of the number of scattered photons to the number of photons not scattered is B = (1 - e-~°~e-Z3-= e ~ 7 - 1 (Z c = macroscopic Compton cross section).

TIP* e - Z :

For collimated radiation another correction should be made taking into account the influence of gammas scattered by the collimator. A quantitative calculation of this effect is difficult because of the strong dependence on the dimensions of the source and on its adjustment. But it can be seen qualitatively that the correction will be small for the lower energies, where primary photons striking the collimator as well as secondary gammas are very likely to be absorbed within the lead, whilst for higher energies the effect m a y be rather significant. 4. Results

Fig. 3 gives the corrected results, P * , for uncollimated and for collimated radiation as a

12

C. W E I T K A M P

function of the gamma-ray energy E~. Agreement with the calculated photofractions s) is very good except for collimated radiation of energy > 1 MeV ~0

0.9

i

I

I

I

I I ] I

~

I

i

I

I

I

I I I

(a)

08

by reducing the mass of the source material or by using several sources of various thicknesses and extrapolating to source thickness zero. A vertical 10

i

b

t

JiJl~

i

~

09 Q8

uncollimoted

I

i

iiJi

{b) col Limated, e = 05emo x

O'7 06

P~

06

"X~

05 04 0302

P~

/

Q5 Q4

Er ° I 10151 itttl

t

2

i

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i

£r

I t l

5HeY 10

1

2

5MeV 10

Fig. 3. Comparison of e x p e r i m e n t a l ( . . . ) a n d theoretical (--) photofractions of a 4"4 × 6 ~ NaI(TI) crystal for uncollimated (a) and collimated radiation (b) as a function of t h e g a m m a energy E 7.

where the collimator influence, as discussed above, should not be neglected. The accuracy of the method might be improved

position of the detector would allow to diminish considerably the mass of the crystal container. 8) C. W e i t k a m p , Nucl. Instr. and Meth. 23 (1963) 13.