On the design of NaI(Tl) total absorption detectors for strongly interacting particles at GeV energies

]NUCLEAR I N S T R U M E N T S A N D M E T H O D S 75 (1969)

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O N THE DESIGN OF NaI(Ti) TOTAL ABSORPTION DETECTORS FOR STRONGLY INTERACTING PARTICLES AT GeV ENERGIES* E. B. H U G H E S , R. H O F S T A D T E R , W. L. L A K I N and I. SICK

High Energy Physics Laboratory, Stanford University, Stanford, California, U.S.A. Received 16 June 1969 The properties o f a large N a I ( T I ) detector as a total absorption detector for pions at GeV energies are reported. These observations are supplemented by measurements on the three-dimensional distribution of deposited energy in a large absorber of

metallic tin by 8 GeV pions. The latter data are compared with existing Monte Carlo calculations of the nucleon-meson cascade and their application to the design of total absgrption detectors with large containment factors is discussed.

1. Introduction

large fraction of the nucleon-meson cascade produced by GeV pions (the detector is 3.8 nuclear absorption units t in length but at most 0.44 absorption units in radius), these preliminary measurements can be usefully compared with theoretical estimates of the performance of a detector of this limited size. Even more important the measurements reveal the fluctuations which can occur in the degree of containment. Following these tests, and in order to obtain the necessary information on which to base a design for a successful total absorption detector, we have made measurements on the three-dimensional development of the nucleon-meson cascade produced by 8 GeV pions in a large absorber of metallic tin. These data are of importance not only to the design of total absorption detectors but also to the understanding of the development of the nucleon-meson cascade itself. We report in this article both the results obtained with the large NaI(T1) detector and the subsequent investigations on the development of the nucleon-meson cascade.

In a recent article 1) we reported on the properties of a large NaI(TI) total absorption detector for electrons and y-rays in the GeV energy range. The attractive features of this detector, in particular the excellent energy resolution, raise the question of how large a detector of this type would be required in order to achieve comparably attractive features for the detection of strongly interacting particles. A suggestion for a detector (TANC counter) of this type was made recently by Hofstadter2). This is an especially appealing idea at super-high energies when one considers that the fraction of the energy of a strongly interacting particle which eventually appears in the form of neutral pions, and therefore y-rays, is expected to become increasingly dominant as the incident energy increases. For example, Murzin 3) has estimated this fraction to be 44% at 40GeV, 75% at 300GeV, and 85% at 1000 GeV. Even at energies less favorable to rc° production it is possible, in principle, to absorb a very large fraction of the energy of the incoming particle in a manner entirely analogous to the total absorption of the electromagnetic cascade. For a detector which is sufficiently large the resolution is limited only by fluctuations in the energy needed to overcome the forces of nuclear binding and in the number of positively charged pions, kaons, etc., brought to rest in the absorber. The latter subsequently give rise to neutrinos and the relatively long-lived muons during their decay. Our study of the response of a large NaI(TI) detector to electrons in the GeV range provided an opportunity to make a preliminary investigation of the response of such detectors to pions of the same energy. While it will be immeditely apparent that the detector presently available to use is of insufficient size to contain a very * Work supported in part by t h e U . S. Office of]Naval Research, Contract [Nonr 225(67)].

2. The NaI(TI) detector The experimental apparatus, electronics and crystal equalization procedure used in operating the NaI(T1) detector are identical to those described earlier1). For this study of pion interactions the detector was increased in size to consist of 11 separate NaI(T1) crystals ranging in size from 9-] in to 13½ in in diameter and from 3¼ in to 7 i n in thickness. These crystals were mounted coaxially to simulate one large crystal 56 in thick and placed in a momentum analyzed, negatively charged, 3 ° secondary beam at the Stanford Linear Accelerator Center. The chosen configuration t Throughout this article the nuclear absorption length is calculated from the proton-nucleus absorption cross sections reported by Belletini et al. 4) at a proton energy of 19 GeV. For NaI(T1) this length is 15.0 in.

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ON THE DESIGN OF NaI(TI) TOTAL ABSORPTION DETECTORS of the separate crystals in the complete detector is indicated in fig. 1, together with a schematic representation of the beam-defining system and the electronics. An essentially pure beam of pions was obtained by inserting approximately one radiation length of lead in the beam at an appropriate point to degrade and remove the electron flux. Fig. 2 shows the pulse height distribution observed when 8 GeV pions are incident on the crystals. The sharp peak at 8 GeV is a calibration peak produced by the total absorption of 8 GeV electrons and the Landau peak at 824 MeV is produced by pions ( ~ 2% in number) and muons (from pion decay in the beam transport system) which pass completely through the 56 in of crystal losing energy only by Bethe-Bloch collision loss. Electrons were obtained for calibration purposes by temporarily removing the lead radiator from the beam. It is apparent from fig. 2 that on average approximately 49 % of the incident energy is deposited in the detector and that the observed fluctuations correspond to an energy resolution of about 65% (fwhm). It is important to note that some of the pulses observed in even the present small detector represent nearly the full energy of the incident pions. Such pulses confirm that it is possible in principle to contain the whole energy of the incident particle. It is evident from fig. 2 that a considerable fraction of the incident pion energy is escaping from the crystal volume. Pulse height distributions were also measured at 4, 12 and 16 GeV but no significant change in either the average containment or the energy resolution was found over this entire energy range. A measurement was also made at 8 GeV with the order of the crystals in the detector reversed. This configuration provided a detector with a smaller ( ~ 8%) r,!li:T~ i

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Fig. 2. The pulse height distribution observed when 8 GeV pions are incident on the crystals. Signals from the four photo-multipliers on each crystal were passively mixed and subsequently mixed with the signals from each of the other crystals to provide a single output pulse. Plastic scintillators S1 and $2 were used to define the incident beam and scintillators V1 and V2 to reject unwanted off-axis particles. The momentum width in the beam, defined by the 4 ft steel collimator, was ~0.8 %. effective crystal diameter and revealed an average containment of ~ 44% ; a reduction of ~ 10% relative to the normal crystal configuration. These observations are consistent with a substantial portion of the leakage occurring in the radial direction. In order to verify this and to establish, if possible, the nature of the escaping radiation, an attempt was made to observe this radiation directly. Particle detectors were set up adjacent to the crystals but not themselves directly in the beam. These detectors, plastic scintillator telescopes and other NaI(T1) crystals, were able to measure the angular distributions of both charged and neutral secondaries emerging from the crystals and to provide estimates of the energy flux carried by this radiation. The measurements were able to show that neutral secondaries exceeded charged secondaries by a factor of approximately five to one in number and that the total energy carried by these particles is sufficient to account for the average leakage implied by fig. 2. In particular, a measurement of the attenuation length of the neutral secondaries strongly suggested that this component o f the energy leakage is transported predominantly by neutrons.

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E.B. HUGHES et al. were made in an attempt both to understand the observed containment and to determine the size of the detector required for an arbitrarily large degree of containment. The experimental technique is similar to that used in studies of the development of the electromagnetic cascadeS'6). The apparatus is shown schematically in fig. 3 and consists of an assembly of tin plates 12 in x 12in x 30in, eachplate being 12 in x 12 in x 1 in. One of these plates, the so-called probe plate,

3. The probe measurements While studies of the radiation escaping from the NaI(TI) crystals are informative they cannot be used to predict with confidence the absorber dimensions necessary to contain an arbitrarily large fraction of the pion energy. In the absence of suitably large crystal absorbers this question is most directly answered by measurements on the nucleon-meson cascade of the type which we will now describe. These measurements

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was pierced by a cylindrical cavity into which a small NaI(T1) detector 1 in in diameter and 1 in thick could be lowered. With this probe the radial rate of energy deposition in the nucleon-meson cascade could be measured at any depth in the absorber. By changing the position of the probe plate the complete threedimensional deposition of the energy in the cascade could be explored. A tin absorber was chosen because of its proximity to iodine in the periodic table [The iodine nuclei dominate the development of both the nucleon-meson and the secondary electromagnetic cascades in NaI(T1)]. To the extent that the NaI(T1) probe correctly simulates the surrounding absorber in its response to the products of the nucleon-meson cascade, the probe measurements should provide a good description of the development of the nucleonmeson cascade in both tin and NaI(T1). Several points of technique should be mentioned. The incident beam, ~ 6 pions/pulse, was monitored with a counter telescope and appropriate corrections made for electronic dead-time effects. The output of the NaI(T1) probe detector was integrated to measure the total energy deposited at each position in the

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absorber. The radial profiles, at more than 2 in from the beam axis, were measured by withdrawing the probe from the tin such that the light pipe did not intersect the beam. In order to probe to distances between 5 and 8 in from the beam axis the absorber was raised such that the beam was incident 3 in lower than normal. A smooth overlap of these two portions of the radial profile was always observed, which proved the approximate independence of the profiles to edge effects and to the use of a finite absorber. At one particular axial depth in the absorber (N 5 in) the radial profile was also measured with a cylindrical plug of tin, 3 in in length and 1 in in diameter, permanently attached to the under-side of the probe detector. It was found that the profile was independent of the presence of this plug to within the accuracy of the measurements, which showed the unimportance of channeling effects. The basic data, which consist of radial profiles of the nucleon-meson cascade at different depths in the absorber, are shown in~'part in fig. 4. A profile was also measured in the absence of the absorber in order

134

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HUGHES et al.

to determine the background intensity surrounding the beam. This profile, which is also shown in fig. 4, showed the flux in the halo to be approximately 0.25% of that in the incident beam. From geometrical considerations alone the propagation of this distributed beam through the absorber should be considerably different from the cascade induced by the beam itself. Distortion of the radial profiles is therefore possible in those regions of the absorber where the intensity is of the order of, or less than, 0.25% of the incident flux. This intensity is reached at radial distances of approximately 8 in at all depths in the absorber. In the following analysis the radial curves have been extrapolated to radial distances larger than the maximum observed at each depth using a radial attenuation length equal to the nuclear absorption length. This exponential attenuation function, when multiplied by the appropriate geometrical factor, provides a good representation of the limiting shapes of the observed radial profiles at radial distances of 7-8 in. An extrapolation procedure is also necessary to determine the energy flux propagating to absorber depths larger than 30 in. The basic data of fig. 4 are replotted in fig. 5 to show the development of the cascade along the axis and in directions parallel to the axis. After an initial transition region an exponential decrease is observed at all distances from the axis with an attenuation length which increases with increasing displacement from the axis. The extrapolation to absorber depths larger than 30 in is based on a continuation of these exponentials. By numerical integration of the curves shown in figs. 4 and 5 the total energy deposited within an absorber of arbitrary volume can be determined. For example, figs. 6a and 6b illustrate the energy containment to be expected for cylindrical absorbers as a function of radius and axial depth. In particular, it can be seen that an absorber equivalent in volume to the NaI(TI) detector should contain on average ,,~ 44% of the incident energy. This is in good agreement with the pulse height distribution shown in fig. 2. In addition fig. 6a shows that a reduction of the absorber radius from 0.38 to 0.35 absorption lengths, which corresponds to the transition from the normal to the reversed crystal configurations, should cause a decrease of 12% in the average containment. This compares very well with what was observed. The containment curves in figs. 6a and 6b can also be compared with the results of Monte Carlo calculations of the three-dimensional development of the nucleon-meson cascade. The most extensive calculations have been made by Geibel et al. 7) and RanftS),

and predict the distribution of stars and tracks produced in an iron absorber by 10 GeV incident protons. The results of these calculations are also shown in figs. 6a and 6b. It can be seen that there are systematic differences between the measured and calculated containment curves in both the axial and radial directions. In the axial direction the energy containment appears to be distinctly more rapid than the containment expected for either tracks or stars. This comparison is displayed in more detail in fig. 7 which shows the measured energy density and the calculated track and star densities both along the cascade axis and integrated over directions transverse to the cascade axis. On the axis all three distributions at depths larger than one nuclear absorption length decrease exponentially with an attenuation length essentially equal to the nuclear absorption length. The energy density, unlike either calculated,'i distribution, exhibits a transition region in which the density is larger than or

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equal to the incident density. This effect may, at least partly, arise f r o m the finite size o f the p r o b e detector. F o r example, calculations o f the star density have also been m a d e for a c o m p a r a b l y b r o a d incident beam, which also result in a density increase o f ~ 50% in the transition region7). T h e discrepancy between the observed a n d calculated results a p p e a r s only when the d i s t r i b u t i o n s are integrated over directions transverse to the cascade axis. This is p r o b a b l y n o t an u n r e a s o n a b l e result when one considers t h a t all tracks with m o m e n t a larger t h a n 80 M e V / c a n d all stars initiated by particles with m o m e n t a larger t h a n 80 M e V / c are considered collectively for the p u r p o s e s o f the M o n t e C a r l o calculation. In a d d i t i o n the calculated densities d o n o t reflect the energy transfer due to n ° decay which is p r o b a b l y o f greatest i m p o r tance in the early stages o f the cascade. Fig. 7 also includes the m e a s u r e d t r a c k densities r e p o r t e d by Childers et al. 9) for 10 G e V p r o t o n s incident on steel. These d a t a are in g o o d overall a g r e e m e n t with the M o n t e C a r l o results. In the radial direction, as shown in fig. 6a, both the track and star densities a p p e a r to be significantly m o r e c o n c e n t r a t e d at smaller radii t h a n is the energy density.

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136

E.B. HUGHES et al.

reported by Citron et al. 1°) also reveal a systematic underestimation of the measured densities in the radial direction. Part of this latter disagreement can perhaps be attributed to background radiations present in the experiments. However, until this assertion is verified, we cannot conclude that the Monte Carlo calculations successfully account for the radial development of the track and star distributions. 4. Discussion In summary, the probe measurements confirm the fact that the diameter of our NaI(T1) detector is inadequate for good energy resolution, and are able to account for the observed average containment of ~ 49%. They also indicate, in principle, the size and shape of the absorber necessary for an arbitrary degree of containment. Fig. 8 shows the results expressed in a form convenient for that purpose. These data show, for example, that an infinitely long NaI(T1) detector needs to be --, 16.5 in in radius to contain on the average 75% of the energy of an 8 GeV pion. It should, of course, be possible to economize on crystal length, at the expense of detector efficiency, by requiring the incoming particle to intiate the cascade in the first few absorption lengths of the crystal. The probe measurements do not indicate the fluctuations to be expected in the amount of energy contained for a given detector size. We emphasize, however, that it might not be necessary to contain all the energy to achieve a useful resolution. It is perhaps more important to have sufficient material to degrade the high energy components of the nucleon cascade so that the energy escaping is carried away mostly by low energy particles. Even though many such particles may escape, carrying away significant fractions of the total energy, the fluctuations in the amount of energy so lost need not be large. Moreover, it may be possible to improve the resolution by a method analogous to that already employed in small NaI(T1) total absorption detectors for ~-rays 11). In this technique the detectoris surrounded by an anti-coincidence shield which samples those pulses accompanied by significant energy loss. This information is then used, at the expense of detector efficiency, to isolate those pulses for which the energy loss is negligible. The comparison between the probe measurements and the Monte Carlo calculations of the nucleonmeson cascade demonstrates that only an approximate description of the rate of energy deposition during the cascade is provided by the calculated densities of stars and tracks and that such calculations probably cannot be directly applied to the design of total absorption

detectors. It would appear desirable to compute directly the energy density deposited in the absorber and, most importantly, the fluctuations in the total energy contained. It is apparent that an anomaly exists between the radial development of the cascade predicted by the Monte Carlo calculations and that observed in the present experiment. Although the calculated results refer to cascades initiated by protons at 10 GeV and the probe measurements to pions at 8 GeV we see no compelling reason to attribute this radial discrepancy to either of these differences nor do we think it unreasonable to make such comparisons in terms of the nuclear absorption length. To clarify this discrepancy it would seem desirable in future experimental studies to avoid an extrapolation of the measurements to radii sufficiently large to achieve complete containment of the incident pion energy, particularly if such data are to be confidently applied to the design of total absorption detectors possessing very large containment factors. We gratefully acknowledge the cooperation of Dr. J.J. Murray of the Stanford Linear Accelerator Center for making available to us the secondary beam at SLAC and, with Mr. R. A. Gearhart, for his help in the use of this beam. We also are indebted to our research assistants, Mr. D . J . Norman, Mr. M. Damashek and Mr. R . P . Hermann for their help at various stages of this work, to Mr. R. Parks for his design of the probe apparatus, to the Harshaw Chemical Company for the loan of some of the crystals used in this research, and to Dr. R. H. Thomas of Stanford University for a critical reading of the manuscript. References 1) R. Hofstadter, E. B. Hughes, W. L. Lakin and I. Sick, Nature 221 (1969) 228. 9) R. Hofstadter, Nucl. Instr. and Meth. 63 0968) 136. a) V. S. Murzin, Prog. Elem. Particle Cosmic Ray Phys. 9 (1967) 247. 4) G. Bellettini, G. Cocconi, A. N. Diddens, E. Lillethun, G. Matthiae, J. P. Scanlon and A. M. Wetherell, Nucl. Phys. 79 0966) 609. 5) A. Kantz and R. Hofstadter, Phys. Rev. 89 (1953) 607; Nucleonics 12 0954) 36. 6) C. Crannell, Phys. Rev. 161 (1967) 310. 7) j. A. Geibel and J. Ranft, Nucl. Instr. and Meth. 32 (1965)65. s) j. Ranft, Nucl. Instr. and Meth. 48 (1967) 133; 48 (1967) 261. 9) R. L. Childers, C. D. Zerby, C. M. Fischer and R. H. Thomas, Nucl. Instr. and Meth. 32 (1965) 53. 1% A. Citron, L. Hoffman, C. Passow, W. R. Nelson and M. Whitehead, Nucl. Instr. and Meth. 32 (1965) 48. 11) M. Suffert, W. Feldman, J. Mahieux and S. Hanna, Nucl. Instr. and Meth. 63 (1968) 1.