total-ratios for NaI(Tl)-crystals

NUCLEAR

INSTRUMENTS

AND METHODS

40

0966) 257-260;

© NORTH-HOLLAND

PUBLISHING

CO.

PEAK/TOTAL-RATIOS FOR NaI(TI)-CRYSTALS H. LEUTZ and G. SCHULZ II Physikahsches lnstitut der Universttat Heidelberg and L. VAN GELDEREN Harshaw-van der Hoorn, Utrecht

Received 12 October 1965 The fraction of )'-quanta totally absorbed in NaI(Tl)-crystals was determined for eight sclntdlation assembhes having dimensions from 1" dia. x 1" hgt through 8" dla x 8" hgt. The )'-energies varied from 122 keV to 2.62 MeV and the ),-sources

were placed at &stances of 10 cm and 50 cm from crystal face. For the crystals up to 5" daa x 5" hgt we also used collimated 7-beams. The measured ratios are about 10 % to 30 % lower than the ratms calculated by Miller et al.

1. Introduction M a n y scientists w o r k i n g with d e t e c t o r a r r a n g e m e n t s in nuclear physics a n d related fields are d e a l i n g with intensity d e t e r m i n a t i o n s o f y - q u a n t a e m i t t e d either f r o m r a d i o a c t i v e isotopes or f r o m r e a c t i o n s i n d u c e d by accelerator beams. I f there is only one y-energy present, the p r o b l e m is r e d u c e d to the e v a l u a t i o n o f the geometric c o u n t i n g efficiency a n d the a b s o r p t i o n p r o b a bility o f the detector. The intensity d e t e r m i n a t i o n , however, b e c o m e s difficult in all cases where the e m i t t e d s p e c t r a are r a t h e r c o m p l e x a n d where c o n s i d e r a b l e b a c k g r o u n d intensities are present. Since in the energy region, where C o m p t o n i n t e r a c t i o n d o m i n a t e s , all y-lines are c o n n e c t e d with a c o n t i n u o u s C o m p t o n d i s t r i b u t i o n , the o v e r l a p o f these C o m p t o n spectra m a k e s detailed intensity d e t e r m i n a t i o n s impossible. Only the t o t a l energy a b s o r p t i o n p e a k s can m o r e or less accurate be s e p a r a t e d f r o m these c o m p l e x spectra. W i t h the use o f an a n t i - C o m p t o n - s p e c t r o m e t e r , one m a y decrease the C o m p t o n b a c k g r o u n d with respect to the t o t a l a b s o r p t i o n p e a k s c o n s i d e r a b l y . Therefore it is

i m p o r t a n t to k n o w the r e l a t i o n o f such p e a k intensities to the t o t a l y-intensities shared between these t o t a l a b s o r p t i o n p e a k s a n d the c o r r e s p o n d i n g C o m p t o n distributions. F u r t h e r on we call this relation the p e a k / t o t a l - r a t i o . This r a t i o was calculated for several dimensions o f NaI(T1)-crystals by Miller et al.1). E x p e r i m e n t a l d e t e r m i n a t i o n s o f the p e a k / t o t a l - r a t i o were r e p o r t e d by K r e g e r a n d B r o w n 2) for two special s h a p e d NaI(T1)-crystals a n d for a 4" dla. x 4" hgt cylindrical NaI(T1)-crystal. H e a t h 3) a n d W e l t k a m p 4) measured the p e a k / t o t a l - r a t i o for a 3" dia. x 3" hgt a n d for a 4" dia. x 6" hgt. NaI(Tl)-crystal, respectively. Since, in several cases, the calculated p e a k / t o t a l - r a t i o s d o n o t agree with the e x p e r i m e n t a l l y o b t a i n e d ones, we m e a s u r e d the p e a k / t o t a l - r a t i o s o f eight NaI(T1)crystals with different sizes.

2. Technique of measurement and apparatus The interesting d a t a o f the crystals used are c o m p i l e d in table 1; all scintillation assemblies were H a r s h a w units. The a n o d e pulses o f the p h o t o m u l t i p h e r s were

TABLE 1 Interesting data of the crystals used for the determination of peak/total-ratios. All scmtdlatlon assembles are Harshaw units. Crystal dimensions dla. x height 1" × 1" 2" × 2" 3" × 3" 4"x4" 5"x5" 8" x 4" 9" x 6.5" 8" x 8"

Mounting procedure

Photomultiplier

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6199 6342A 8054 8055 8055 8054 8054 8054

Resolution for 137Cs(662 keV) (%) 7.8 7.6 7.5 7.1 8.3 8.5 8.7 10.0

258

H. LEUTZ

directly fed to a preamplifier and then coupled to the internal amphfier of a Vlctoreen 400 channel pulse height analyzer. Llnearity between the energies of incident y-quanta and the corresponding channel numbers of the registered pulse height distributions was found to be in order. Distortions due to electronic noise and overload effects were reduced to a sufficiently low level as to not to interfere with our measurements The most serious problem in these measurements arose from the undesired intensity contributions of yquanta emitted from the source and Compton-scattered in the surroundings of the source-detector arrangement To avoid wrong shapes of the Compton distributions, induced by such scattering effects, attention had to be paid to the geometric arrangement of source and detector and to the surroundings of the entire arrangement. To cover a rather wide range of current experimental arrangements we measured the peak/total-ratios in three different geometries shown m fig. 1 In a first series of measurements we located the sources at a distance of 50 cm from crystal face (fig. la), thus providing an almost parallel beam of y-quanta at least for the smaller crystals In this geometry the ratio of y-quanta Compton-scattered in the surroundings to 7-quanta directly striking the crystals is very disadvantageous. Therefore we shielded the sources by a lead box as shown m the drawing. To provide a better detection efficiency we located the y-sources in the b)

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TABLE2 Isotopes and 7-energies used for the determmaUon of peak/total-ratios. Isotope 57Co

139Ce 2O3Hg 1138/1 22Na 137Cs

54Mn 6oCo

88y 2O~Tl(ThC")

T-energy (keV) 122 166 279 392 511, 1274 662 835 1173, 1333 900, 1841 2620

second series of measurements at a distance of 10 cm from crystal face (fig. lb). In this arrangement the best conditions resulted from totally unshielded sources. Arrangements with collimated y-beams are often applied for smaller crystals. Therefore we used this geometry in a third series of measurements (fig. lc). Since the contribution of 7-quanta Compton-scattered in the surroundings of the arrangement varies approximately with the squared inverse of the dxstance from the detector, we located all crystals at distances of at least 150 cm from floor, ceding and walls of the laboratory. All y-sources used for these invesUgations consisted in thin layers of radioactive materials canned in about 0.5 mm lucite to prevent scattering effects m the very near source region. 3. Measurements and results

Besides the 8" dla. × 8" hgt, 8" dia. x 4" hgt and 9" dla. x 6.5" hgt Na(T1)-crystals the other five scintillators, respectively 1" dla. x 1" hgt through 5" dla. x 5" hgt, were measured at twelve different y-energies using the ~sotopes hsted in table 2 From all spectra collected the background radiation was subtracted. As an example for our peak/total determination we depicted in fig 2a the spectrum obtained by the detection of 662-keV y-quanta from 137Cs with a 4" dia. x 4" hgt crystal. This spectrum consists of the total absorption peak, the Compton continuum and the peak due to back-scattered radiation. The ranges of these three lntensmes are indicated by dashed lines The backscatterpeak is mainly caused by the phototube connected with the crystal, prowded that any matertal behind the source is avoided. Therefore this peak does not contribute to the y-intensity directly absorbed in the crystal. The absolute intensity of backscattered radiaUon is mainly propomonal to the mass of glass of the photomultlpher envelope and to the y-intensity transmitted

P E A K / T O T A L - R A T I O S FOR N a I ( T 1 ) - C R Y S T A L S

through the crystal. The ratio of backscattered to directly absorbed radiation is therefore more important for thin crystals because it is favoured by the high ratio of transmitted to incident radmtlon for these crystals. As in fig. 2a also in all other measured pulse height distributions the backscatter-peaks could clearly be recognized and deducted from the C o m p t o n continua. In fig. 2b the absorption spectrum of the 900-keV and 1841-keV 7-lines from 88y is displayed as an example for the treatment of spectra containing two v-lines. To get the peak/total-ratios for both energies, we separated the two Compton continua by extrapolating the Compton distribution of the 1841-keV quanta to an energy which equals zero. The backscatterpeaks from 900-keV and 1841-keV quanta have approximately the same energy and can not be separated from each other but can be clearly deducted from the total Compton continuum. In this case also a pair escape peak at 1330-keV and a peak at 511-keV due to annihilation quanta can be distinguished, since 88y also emits positrons with an intensity of about 0 . 5 ~ . The intensity of the pair escape peak was added to the total intensity while the 511-keV peak was deducted from the spectrum. In those instances where applicable, we corrected in this way for contributions of such side

259

effects calculating the peak/total-ratios. This procedure is very important for the evaluation of the 2°8T1 (ThC")spectrum. Finally, the peak/total-ratios for the three geometries mentioned above, are given as a function of the energy for all crystals in fig. 3. Although it is difficult to calculate, we estimate that the errors in our measurements are not greater than about _+ 5 ~ due to the deviations of the individual experimental points from the fitted curves. 4. Discussion

As can be seen from figs. 3a, 3b and 3c the peak/totalratios vary for the various source-detector arrangements. The highest ratios resulted from the arrangement with a collimated 7-beam (fig. lc and fig. 3c). This is obvious, because in this geometry the path for onefold Compton-scattered quanta is longer and hence the probability for further collisions m the crystal leading finally to total absorptions of the quantum-energies is greater than in the other geometries mentioned above. Miller et al. 1) have calculated peak/total-ratios applying the Monte Carlo method to evaluate the interaction probabilities of the incident 7-quanta. Assuming a parallel 7-beam they obtained for 2" dia. x 2" hgt,

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Fig. 3. Peak/total-ratios in the absorptlonspectra of eight NaI(T1)-crystals vs y-energy a, b, c Measured with the arrangement shown in fig la, b, c respecnvely. 4" dla. x 4" hgt, 8" dla. x 4" hgt and 8" dla. x 8" hgt crystals p e a k / t o t a l - r a t i o s which are from 10~o to 30% higher t h a n our e x p e r i m e n t a l d a t a The same holds for the a r r a n g e m e n t with a collimated 7-beam at y-energies a b o v e 1 M e V using a 4" dia. x 4" hgt crystal This difference can be partly induced by the scattering events in the A1203 reflector and in the containers o f the crystals, because the calculations did not regard such effects F o r practical applications, however, scattering events in reflector- and canning materials have to be t a k e n in account. H e a t h 3) measured the p e a k / t o t a l - r a t i o s with a 3" dIa. x 3" hgt crystal at a source distance o f 10 cm. His results are up to an energy o f 2.5 M e V in g o o d agreement with our d a t a obtained with the corres p o n d i n g a r r a n g e m e n t The p e a k / t o t a l - r a t i o s of Kreger

and Brown 2) measured with a 4" dla. × 4" hgt crystal using collimated and parallel 7-beams agree rather g o o d with our results up to 7-energies o f a b o u t 1 MeV. F o r higher 7-energms there are considerable differences a m o u n t i n g to a b o u t 40% at 1 8 MeV for the c o l h m a t e d beam arrangement. W e wish to acknowledge the assistance o f Mr. J. N. Slump in p e r f o r m i n g several measurements.

References 1) w F. Miller, J Reynolds and W. J Snow, Rev. Scl. Instr. 28 (1957) 717. 2) W. E. Kreger and R M Brown, Nucl Instr. and Meth 11 (1961) 290. 3) R L. Heath, IDO-16408 Phlhps Petroleum Co. 4) C Weltkamp, Nucl Instr. and Meth 23 (1963) 10.