The new BNL high-energy gamma-ray spectrometers

Nuclear Instruments and Methods in Physics Research 222 (1984) 479-495 North-Holland, Amsterdam

479

THE NEW BNL HIGH-ENERGY GAMMA-RAY S P E C T R O M E T E R S A.M. S A N D O R F I and M.T. C O L L I N S * Physics Department, Brookha~,enNational Laboratory, Upton, N Y 11973, USA Received 17 October 1983

Two new plastic-shielded NaI gamma-ray spectrometers (BNL-MK I1 and MK Iii) have been developed for the detection of photons between 5 and 50 MeV. Both spectrometers are capable of operating stably and reliably at extremely high counting rates (over 1 MHz above 1 MeV). The superior resolution and line shape of the MK III detector represents a remarkable improvement in response, achieving 2.18_+0.05% fwhm and 6.44+0.12% fwtm at 22 MeV. This is due to a large number of factors, the two most important being a new technique for grading the reflective surfaces of the NaI(TI) crystal, and segmentation of the plastic anticoincidence shield. The design, construction, and response of these detectors are discussed in detail.

1. Introduction The radiative capture of light particles (_< alphas) has long been a popular mechanism utilized in a wide v~riety of studies in nuclear physics such as the spectroscopy of shell-model states, the decay of giant resonances, and the properties of isobaric analogue resonances. In recent years radiative capture studies have even been extended to heavy ions to investigate the unusual states formed in these complex reactions. The first detector designed for high energies (E~, > 10 MeV) was developed at Stanford University more than 15 years ago [1]. Since then, many laboratories have developed similar detectors. Of all of these, the two with the best reported resolution (3.4% at 22 MeV) have been the spectrometer developed by the University of British Columbia group for use at the University of Washington, referred to here as the Seattle detector [2], and the spectrometer developed at Osaka University [3]. Descriptions of earlier poorer-resolution detectors are given in refs. [2] and [3] and the articles cited therein. Most existing high energy gamma-ray spectrometers are of similar overall design, consisting of a large NaI(T1) crystal surrounded by plastic scintillator that is used in anticoincidence with the central crystal to reject cosmic rays and events in which some of the shower products, resulting from the gamma rays, escape from the NaI. This assembly is then surrounded by lead which serves as a shield against background radiation as well as a muon-converter for efficient cosmic-ray rejection. The performance of such detectors depends very strongly on the detailed designs of the NaI(T1) crystal, the plastic * Present address: General Electric Corp., Schenectady, NY 12301, USA. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

shield, the pulse generation and processing electronics, and the gain stabilization. Some of the general considerations may be found in the 1974 review by Paul [4]. In this paper we describe three generations of highenergy gamma-ray spectrometers developed at the double MP-tandem Van de Graaff facility of Brookhaven National Laboratory. The first spectrometer, BNL-MK I, was built as a copy of the detector described by Diener et al. [5]. Although of relatively poor resolution (7% at 22 MeV), it was utilized in a fruitful research program from 1971 until 1977 (see refs. [6-12], for example). The BNL-MK II spectrometer was constructed in 1979. It differs from the MK I detector mainly in that the annular plastic anticoincidence shield was segmented into six optically isolated sections, and the 24 cm x 25 cm NaI(T1) crystal, manufactured by Harshaw Chemical Co. [13], is now viewed by an array of seven 7.6 cm diameter phototubes (RCA 4900), rather than a single 23 cm diameter tube. The response of this detector ( A E / E = 3.3% at 22 MeV) is comparable to that of the Seattle and Osaka detectors. The BNL-MK Ill spectrometer was completed in 1981. This consists of a 24 cm × 36 cm NaI(TI) crystal, manufactured by Bicron Corp. [14] using new techniques described below, and a second segmented plastic-anticoincidence shield. This spectrometer obtains a resolution of 2.2% fullwidth-half-maximum (fwhm) at 22 MeV, and a very high suppression of the low-energy tail resulting in a full-width-tenth-maximum (fwtm) of 6.4%. The performance improves slightly at higher energies giving 2.0% fwhm at 46t MeV. Both MK It and MK lII detectors function reliably at extremely high counting rates (in excess of 1 MHz above 1 MeV) without appreciable gain shifts and without severe degradation in resolution. These results represent a very significant improvement

480

A.M. Sandorfi. M.T. Co~/ins/ BNL high .energy y-ray spectrometers

Fig. 1. The BNL-MK II detector is located in the upper right of this photograph, while the MK II1 spectrometer is in the lower left. For scale, the beam line height is 142 cm above the target room floor. Variable potentiometers are mounted in the relay racks that are fixed to the top of the lead shields of both spectrometers. These are used to adjust the gains of the various phot0multipiier tiabes.

in high-energy detector technology. The MK II and MK III detectors are presently mounted on separate arms of a common goniometer, shown in fig. 1. The detailed design considerations and performance of the MK III detector are presented in sections 2 and 3. The MK II detector, which uses an identical anticoincidence shield, is discussed only for comparison purposes. Sample applications are presented in section 4. Designs and some results relevant for future generation spectrometers are discussed in section 5,

eration by the plastic shield, and hence upon the neutron spectrum, but in most reactions the 6LiH reduces the 6.83 MeV peak from t271(n,~,) typically by factors of 8-10. 2.1. The segmented plastic shield The anticoincidence shields are made from polyvinyltolvene scintillator having 65% of the light output

SHIELDED BEAM DUMP .... ~ , : . \\ ', 2. Spectrometer design An exploded view of one of the spectrometer systems is shown schematically in fig. 2. Not shown in this figure is a 1.6 cm thick cylinder and front disk of 6LiH (encased in a thin layer of epoxy) which fits around the NaI, between it and the plastic shield, and is used as a thermal neutron absorber. Many of the fast neutrons generated in the course of an experiment, either from beam slits, the target, or the beam dump, are moderated by the plastic shield and absorbed by the 6Li. Those that penetrate into the NaI are either radiatively capture in flight or thermalized and subsequently captured, mostly by 1271, the latter process being dominant. The effectiveness of the. 6LiH shield depends upon the mod-

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A,M. Sandorfi, M.T. Collins / BNL high-energy y-ray spectrometers

of anthracene and a decay constant of 2.4 ns (NE102, ND120). The 10 cm thick annulus surrounding the NaI is segmented into six optically isolated sections (only four are shown in fig. 2), each viewed at the back surface by two phototubes (Amperex 2202). Each segment is polished to a high surface finish and is wrapped with aluminum foil layed flat against all surfaces. We have tested various methods of surface preparation: a rough surface painted with TiO 2 reflector, a polished surface coated with TiO 2, a polished surface coated with a white paper reflector, and a polished surface wrapped in aluminum foil. The latter method gives by far the largest pulse heights and best uniformity. The surface of the front plastic, a single 10 cm thick slab viewed at the top surface by three phototubes (Amperex 2202), is treated in the same manner. The only surfaces of the annular segments and of the front slab that are not covered with reflector are those faces on which the phototubes are attached. The portions of these faces which are not coupled to a photocathode are covered with black tape. This improves the apparent fall time of the scintillator pulses (20 ns) by damping out the reflection of light across the long length of the scintillators, while having almost no effect on the peak pulse-height or rise time (3 ns). The six-segment annular shield fits tightly into a thin-walled (0.5 mm) stainless-steel housing. A thicker (1.3 cm) back plate is used to support the phototubes. All tubes are equipped with electrostatic and mu-metal shields and are operated at negative high voltage. The front plastic is wrapped in black tape and held in a lead cradle. Each segment of the scintillator has a resolution for 137Cs of about 20%, and the response of the entire plastic shield is uniform to 10% as a collimated 137Cs is moved about all surfaces. This is an important attribute that is difficult to achieve with a single non-segmented annulus, because the geometry of a torus hinders the collection of light at its backplane. The signal to noise ratio is thus much worse and, since all phototubes viewing the annulus must be summed together to collect as much of the light as possible, the final effective threshold for pulse processing is determined by the most non-uniform region of the plastic. The signals from the seven segments of the anticoincidence shield of fig. 2 are processes separately, each having its own amplifers and discriminators with thresholds set at about 30 keV. The seven logical signals are finally ORed before comparison with the Nal. During a typical measurement, when the counting rate in the NaI is 75 kHz above 1 MeV, the corresponding rate in the anticoincidence shield is about 1 MHz above 30 keV. Of this rate, about 700 kHz is due to the front plastic which is exposed to the low-energy gamma rays and neutrons coming from the target, and the rest is split equally among the six segments of the annulus. The plastic anticoincidence shield has been operated at counting rates as high as 10 MHz above 30 keV without

481

loss in effectiveness and without appreciable degradation in the ratio of accepted pulses (those having only a NaI trigger) to totals. 2.2. Nal crystal quality and canning

When a high energy gamma ray enters the NaI it almost immediately pair-produces and creates a cascade shower along the incident trajectory. As such, the effect of such a gamma ray is to produce a line source of light in the NaI. The variations in the light production efficiency for a fixed energy deposition, and in the efficiency of collection of this light from this and similar lines in the crystal, is one of the most important factors effecting the final energy resolution. Most of the earlier large Nal detectors, including the BNL-MK I and Seattle detectors were made from two ingots cemented together. This causes a jump in the amount of light collected when the emitting source crosses the boundary between the two crystals. Traditionally, the NaI manufacturers have used surface compensations to reduce this effect. That is, a map of the sides of the crystal was made with a collimated X37Cs source and the crystal was polished wherever the peak height was higher than desired. However, the collimated 137Cs source produces point sources of light at depths of only about 3.5 cm into the NaI. The optical properties of the central region of the crystal where the gamma-ray showers deposit their energy is never probed with this technique. Both the BNL-MK II and MK III detectors have been fabricated from single ingots. The crystal for the MK III detector was selected from seven ingots on the basis of clarity and maximum light transmission. This is an essential starting point for the new method of surface preparation described in the next section. All subsequent tests were conducted with the MK III detector in its aluminum can, packed with MgO reflector. The walls of this aluminum housing were reduced to a minimum, 0.16 cm on the sides and 0.13 cm across the front face. 2.3. Nal surface compensation at 6.13 MeV

There are two factors affecting the spatial uniformity of response in a NaI(T1) crystal. One is the reflective properties of the boundaries, and the other is the non-uniformity of the thallium doping. Ideally, one would like to treat these effects separately. However, once the ingot is grown there is no way of changing the local concentrations of dopant. In the fabrication of the MK III detector, we have chosen to try to couple these two factors in the hope that one can be used to cancel adverse effects from the other. The detector is viewed by an array of seven 7.6 cm diameter photomultiplier tubes (RCA 4900). After the gains of the tubes were matched to produce the

482

A.M. Sandorfi, M.T. Collins / BNL high-energy y-ray 6pectromete/',s

same pulse height at 0.661 MeV, the seven signals were added passively. The detector was then mapped with 0.661 MeV gamma rays from a 137Cs source, collimated to expose only 1.6 cm 2 of the crystal, at 72 points along its side and at 17 points across its front face. The surface was then compensated in the manner described in the previous section. Several iterations of this procedure were necessary before achieving the optimum uniformity ( + 0.6%). At this point the crystal was mapped with 6.13 MeV gamma rays from a collimated 244Cm-~3C source [15]. supplied by BNL. The 6.13 MeV photons penetrate an average of 8 cm into the crystal, and pair-produce half of the time so that the resulting shower extends even further in. The observed uniformity of + 1.2% at 6.13 MeV illustrates the inadequacy of refining the crystal behavior based only on tests with a low energy source. With the phototubes gain matched at 6.13 MeV, the MK III crystal was recompensated until the non-uniformity at this energy was reduced to + 0.4%. Through this process the resolution at 6.13 MeV improved from 4.2% to 3.2%. These results are summarized in table 1. The performance of the detector at some intermediate energy like 6.13 MeV is much more indicative of what can be expected at high energies. The resolution of the MK III crystal at 0,661 MeV is 6.9%, and the ratio of the 1.17 MeV peak to the valley between the lines from a 6°Co source is 15/1. In comparison, the M K II crystal, which was surface compensated only at 0.661 MeV, has a resolution of 7.1% for ]37Cs, and a 6°Co peak-to-valley ratio of 12/1. This is not significantly different from the MK III behavior. However, the MK II resolution at 6.13 MeV is 4.7%, which 'anticipates the poorer resolution at higher energies. The phototube gain matching conditions provide yet another indication that low energy sources cannot be used to predict behavior at high energies. We have noticed with both M K II and MK III detectors that once the tube gains are matched at 0.661 MeV, they are not simultaneously matched at 6.13 MeV. To match gains at 6.13 MeV requires readjustments of up to 8%. We have rechecked the gain match at 20 MeV, using photons from the H B(p,'y) reaction, and found much smaller differences. The final resolution of the MK II at 20 MeV does not strongly depend upon this gain match-

ing. However, when the intrinsic resolution becomes as good as that of the MK 1II crystal, almost everything affects the high energy behavior, tn particular, gain matching at 6.13 MeV, instead of 0.661 MeV, improves the MK III resolution at 20 MeV by about 0.3%. Rematching at 20 MeV does not provide any further improvement. A typical MK I11 spectrum for the 244Cm--13(_: source is shown in fig. 3. Accepted events, those not accompanied by signals in the plastic anticoincidence shield, are shown in (a). The corresponding events rejected by the shield are shown in (b). The resolution of the 6.13(I MeV peak is 3.2% fwhm. The plastic shield is still not thick enough to be 100% efficient, and the residual 1-escape peaks which leak into the accepted spectrum are indicated by arrows connecting them with the corre-

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A.M. Sandorfi, M. 72 Collins / BNL high- energy y- ray spectrometers

sponding full-energy peaks. The 4.95 MeV line arises from neutron capture in the 12C of the polyvinyltoluene shield. Since most of these gamma rays enter the NaI through its side, there is a high probability that a 0.511 MeV photon from pair-production will escape back into the plastic. As a result, most of these events are rejected. The same would be true of the 2.22 MeV line from neutron capture in the hydrogen of the shield, except that at this energy the pair-production cross section is almost an order of magnitude smaller. The resolution of the 6.826 MeV ~27I(n,y) line always appears somewhat worse than that of the 6.130 MeV peak, since the former arises from thermal neutron capture throughout the full volume of the crystal. As such this is actually a good measure of crystal non-uniformity, although it is probably too severe a test since showers from gamma rays of a fixed energy never sample as much of the detector as do thermal neutrons. 2.4 N a l gain stabilization

The amplitude of the signals from the photomultiplier tubes viewing both MK II and MK III detectors is related to the number of photoelectrons striking the first dynode. The 10-stage bialkali RCA4900 tubes were chosen both for their high quantum-efficiency as well as their inherent gain stability, and all are operated at negative high voltage. There are three main effects that can alter the gain of these tubes at high counting rates. First, spray from the large number of electrons flowing in the tubes can charge up the insulating structures which support the dynode chain. This can change the focusing potential and the gain. The dynode support structure in the RCA 4900 tube is coated with a layer of CrO 2 which rapidly bleeds off static charge. We have tested a number of these tubes in which this CrO 2 coating has been left out and have observed large gain shifts at high counting rates. Even when the rates are subsequently reduced, the gain can take as much as an hour to recover to its original value. The CrO 2 coating dramatically reduces this problem. The second potential source of gain shifts is a change in the work-function of the dynode surfaces, particularly the last dynode, caused by the bombardment of a large number of electrons. The MK III detector is equipped with a variant of the 4900, now marketed as the RCA $83021E. In this tube, designed for BNL by RCA, the coating on the last dynode has been changed (details are proprietary) to reduce such shifts at high rates, and the grid structure has also been altered to slightly improve the photoelectron collection efficiency. The third source of gain shifts results directly from the increased current flowing down the dynode chain at high rates. This current changes the effective potentials at the various stages of the tube. To combat this problem, each tube of the MK II and MK III detectors is

483

equipped with an active transistor stabilized base. The design of this base follows a philosophy similar to that used with the SLAC crystal ball tubes. The circuit is shown in fig. 4. A voltage stabilized source (labeled HIGH VOLTAGE) is used to define the levels along the resistive divider chain, and the first six dynodes are connected directly to these points. However, the last four dynodes are connected to the emitters of p - n - p transistors (Q3 through Q6 in fig. 4) whose bases are now tied to the primary voltage divider chain. The collectors of these transistors are connected to a separate power supply (labeled BOOSTER) which is used as a current source. Whenever a dynode voltage, and hence an emitter level, drops below that of the base, the transistor provides current from the BOOST supply to the dynode until its voltage returns to its quiescent level. The two n - p - n transistors (Q1 and Q2 in fig. 4) are used to temperature compensate the last two stages. All of the active elements of the base (those within the dashed box of fig. 4) are mounted on a printed circuit board to facilitate service whenever necessary. A test point (labeled MON, and located after Q3 between R7A and R7B) is used to monitor the active elements. The voltage here (typically 8 V when the cathode is at - 1 5 0 0 V) can be used to monitor the gain correction that is applied by the active stages. The transistor-stabilized bases are capable of responding in less than 0.5 /zs so that even rapid fluctations in counting rate can be effectively controlled. We have compared the response of the MK III detector to 22 MeV gamma rays from the UB(p,7) reaction, using these active bases and using high-current passive bases. Even at a low counting rate, 70 kHz above 1 MeV, and with what would appear to be very stable beam currents, the active bases improved the detector resolution by 0.3%. We attribute this to the ability of these bases to respond to microstructure in the beam. With no further stabilization other than that provided by the active bases, the gain of both MK II and MK II! detectors, equipped with RCA 4900 tubes, changes by less than 1.7% when the counting rate above 1 MeV changes from 10 kHz to 150 kHz. The variation in gain over the same range in counting rate is about 1.0% with the MK III crystal when equipped with RCA $83021E tubes. When the counting rate, averaged over a few minutes, is kept constant for a total of 24 h, no perceptible gain changes are observed. The shifts in gain that follow changes in counting rate do not occur immediately, but rather several minutes after the rate has been changed, and this gives a clue to their source. When the inter-dynode current is increased, the compensating current flowing through the p - n - p transistors of fig. 4 increases. This heats up the transistors and changes their base-to-emitter leakage currents. The bases of these transistors serve as the reference points to which the corresponding dynode

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A.M. Sandorfi. M.T. Collins / BNL high-energv y-ra~' spectrometers

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voltages are stabilized. Thus, the increase in temperature which follows an increase in rate causes a change in these reference points, and hence a shift in tube gain. The matched n - p - n elements (Q1 and Q2 in fig. 4) reduce this effect but never completely remove it. To remove the temperature associated shifts and to extend the stable operating range of the detectors to even higher counting rates, we have implemented a stabilized light pulsing system. We have examined

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several methods of injecting light into the tubes viewing the NaI. Of these, the optimum is shown in figs. 5 and 6. Light from a green LED is guided into a bundle of 8 optical fibers (0.16 cm diameter Dupont Corfon). Seven of the fibers provide light to the NaI tubes. These fibers are layed in channels machined in the aluminum base plate and in the quartz windows used to view the Nal. The ends of the fibers extend under the face of the tubes and are polished at 45" to the normal, so as to reflect the light directly into the photocathodes, and are coated with TiO 2. As shown schematically in fig. 6, the eighth fiber from the bundle is viewed by a separate RCA 4900 tube. This tube also views a small Nal crystal, which is exposed to a 22Na source. Events, within an SCA window on the 1.274 MeV line from this source, trigger the LED pulse driver and the ratio of the LED signal to the 1.274 MeV pulse, which is independent of the gain of the eighth tube, is fed back to stabilize the absolute light output from the LED. Under normal operation the LED amplitude is adjusted so that the light injected into the seven tubes of the big Nal produces a pulse (equivalent to 50-100 MeV) substantially higher than the region of interest. The height of this pulse is then monitored by an active gain-control amplifer (Harshaw NA-22), which adjusts the gain of the linear signals entering the ADC so as to keep the position of the LED peak fixed. The combined active base and LED stabilization systems have kept the gain of the MK II and MK Ill spectrometers constant to 0.2% for periods as long as a week during which the counting rates, ranging from 50 kHz to 300 kHz, were radically changed at frequent intervals. Significantly higher rates are accompanied by somewhat larger gain shifts, 0.6% at 1.1 MHz above 1 MeV. This is probably due to a change in the work-function of the dynode surfaces under these extreme conditions, although these very high rate tests

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A. M, Sandorfi, M. T Collins / BNL high- enerKv Y ray spectrometers

486

have been carried out only with the RCA 4900 tubes. We expect the performance to be somewhat better with the RCA S83021E tubes. For periods of operations significantly longer than a week, sudden jumps of as much as 20 % can occur in the LED light output. These result from changes in the diode junction which is being driven quite hard to produce the equivalent of 50-100 MeV signals. We have tested Zenon flash lamps but have found similar instabilities over much shorter time periods. A superior light source that should be considered in the design of future NaI(TI) spectrometers is a Q-switched HeCd laser. Not only are much higher intensities readily available, but the 4420 A light from this laser almost matches that from thallium photoemission.

2.5 Signal processing The pulse processing electronics used with the MK II and MK III detectors is similar to those described in refs. [2-4]. A block diagram is shown in fig. 7. The seven anode signals from the tubes viewing the NaI are passively summed and split into LINEAR and LOGIC components. The linear signals are delay-line clipped [16], usually to 400 ns, amplified (Lecroy 612), delayed, and passed to a linear gate (Tennelec 301) which is opened by the LOGIC circuit for about 500 ns. Clipping to 225 ns, to improve pileup rejection, costs about 0.2% in resolution. Further dipping to times shorter than the decay constant due to the thallium doping has disastrous effects on the resolution. Clipping times longer than 400 ns produce essentially insignificant improvements to the high-energy performance. The NaI input to the logic circuit is RC-clipped to 30 ns. Constant-fraction discriminators (Ortec 934) are

used to remove time walks. One of these, labeled HI in fig. 7, is used to reduce low-low pileup [17]. We have found it extremely useful to also use a second higher level discriminator, labeled HI 2 in fig~ 7. Not only does this double the chance of optimizing the position of the high level during a long experiment, but we have also found that the difference between the accepted spectra above these two levels, labeled ACC(HI) and ACC(HI 2) in fig. 7, is a good monitor of the low ~high pileup that leaks through the pileup rejector into the ACC(HI 2) spectrum. In addition to the > 30 keV triggers from the common-OR of the plastic shield segments, labeled SHIELD in fig. 7, a second level set at about 3 MeV is derived from a linear fan-in of all of the shield anode signals and is used to identify pulses from cosmic rays. This level, labeled NO COSMIC in fig 7 is used to generate a total-spectrum (accepted + rejected) in which the cosmic ray component is drastically reduced.

3. Performance at high energies Since the plastic anticoincidence shields are not 100% efficient, the final spectrometer resolution depends upon the fraction of gamma rays that enter off of the crystal axis and produce showers along lines that traverse thinner regions of Nal. All of the data presented in this section were taken with a conical lead aperture which projected the solid-angle-cone subtended at the target onto the full back face of the M K III crystal (For comparison, projecting onto the full crystal diameter at a depth of only 15 cm into the NaI deteriorates the resolution at 22 MeV by about 0.3%,)

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A.M. Sandorfi, M.T Collins / BNL high-energy 3'-ray spectrometers 3.1. Resolution and line shape at 22 Me V

Accepted and rejected spectra from the llB(p,T)12C reaction at Ep = 7.0 MeV are shown in figs. 8a and b. The FWHM of the 22.4 MeV peak in the accepted spectrum is 2.2%, and its fwtm is &4%. Both of these widths represent a substantial improvement in the technology of high energy gamma-ray spectrometers. A double-subtracted spectrum [1], formed by removing another 28% of the rejected spectrum from the accepted events, is plotted in fig. 8c. The resulting improvement in

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resolution is relatively minor. The fraction of the rejected spectrum that was resubtracted was chosen to make the spectrum to go to zero in the region between peaks. For the MK III results of fig. 8c, this fraction is much smaller than that found necessary with earlier detectors [1-5], and reflects the increased sensitivity of our plastic anticoincidence shield. We have found that the analysis of data, using this double-subtraction technique of enhancing the line shape, is free of compounded errors only when studying isolated peaks that are relatively intense compared with the background, or in other works, only when it is not necessary. The data of fig. 8c is presented here only for purposes of comparison with other detectors. The ratio of accepted-to-total (accepted + rejected) events for the 22.4 MeV gamma rays of fig. 8 is 44%. This is slightly smaller than that observed with earlier detectors. However, the increased rejection is mainly in the region of the low-energy tail and not in the full-energy peak. This is best illustrated in fig. 9 where the MK III results of fig. 8a are compared with those of the Osaka [3] and Seattle [2] detectors. The MK III response displays not only a large improvement in resolution, but also a significant reduction in the low-energy tail. A detailed comparison of the fwhm and fwtm from these three detectors, for both 'accepted' and 'doublesubtracted' spectra, is given in table 2. The resolution of the Osaka detector, 3.4% at 22.6 MeV, was determined by using the separation of the Y0 and Yl peaks of fig. 9b (taken from fig. 5 of ref. [3]) for an energy calibration. A resolution of 2.8% is quoted in ref. [3]. This either refers to the double-subtracted spectrum (fig. 4 of ref. [3] the results of which are summarized here in table 2), or to some other unpublished data. We attribute the residual low-energy tail in fig. 9a mainly to the lack of perfect rejection by the plastic shield. The Osaka NaI is segmented into a central 15.2 cm diameter crystal surrounded by four optically isolated sections that fit together to form a 28 cm diameter annulus. The spectrum of fig. 9b was taken by collimating the high energy gamma rays to the back face of the

Table 2 Comparison of the response of the BNL MK III, Osaka (fig. 4 of ref. 3) and Seattle (table 2 of ref. [2]) detectors to 22.4 MeV gamma rays from 11B(p,7 )12C. The results for earlier detectors are tabulated in ref. [2]. Single subtraction Double subtraction of rejected events of rejected events fwhm(%) fwtm(%) fwhm(%) fwtm(%)

28

Fig. 8. Accepted, rejected, and double subtracted BNL-MK III spectra of two high energy gamma-ray lines from the 11B(p,y ) reaction.

BNL-MK III Osaka Seattle

2.2 3.4 3.4

6.4 10.0 15.0

2.1 2.8 3.0

4.4 6.7 5.0

A.M. Sandorfi, M.T. Collins

488

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800 600

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BNL high- energy 7 - r a y spectrometers

We have found a useful analytical formula, capable of accurately describing the response of all of the BNL spectrometers [18]. The line shape, as a function of

"

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I

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18 20 22 24 GAMMA-RAY ENERGY (MeV)

26

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f

Fig. 9. The response of the BNL-MK III spectrometer in (a) is compared to that of the Osaka detector [3] in (b) and the Seattle detector [2] in (c), under nearly identical conditions. central 15.2 cm diameter crystal and by using the other four NaI segments in anticoincidence. In principle, the combination of these four NaI sections and their plastic annulus should constitute an extremely efficient shield. The poor fwtm is thus rather puzzling. It may in part be due to the incomplete front plastic used with this detector which covers only the outer circumference of the NaI front face. Thus backscattered shower products cannot be detected. We have noticed that the resolution of the BNL detectors deteriorates by about 0.6% if the front plastic slab is removed.

12

14

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20 I

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F i g . 10. The intrinsic :line shape of the M K I I I detector, fitted to the 11B(p,T) spectrum Using the paxameterization of eq. (1),

is shown here as the dotted line whieh merges with the fitto the full spectrum (solid curve) in the r ~ above the T1 transition in (a). T h e c o r r e s p o n ~ line shape Observed w i t h a 50 cm paraffin neutron absorber in front of the detoetor is shown in (b).

A.M. Sandorfi, M. 72 Collins / BNL high-energvy-ray spectrometers channel number X, for a peak with centroid I, is parameterized as the sum of two components, a Gaussian P(X) and a tail T(X ), defined as

P(X)-

489

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Here, A is the area of the Gaussian part of the peak and 2 o is its associated width. C and D are parameters that define the exponential tail, and G defines how this tail smoothly merges with the Gaussian. During a line-shape fit A, I, o, C, D, and G are all allowed to vary, and B is unity. When stripping the area out of a peak with the line-shape parameters determined from a reference peak at I o, B is chosen to reflect the change in detector resolution with energy; e.g., typically B = (1/lo) 1/3. A two peak plus background fit to an accepted ]lB(p,~,) spectrum is shown in fig. 10a by the full line. The intrinsic line shape of the 70 peak is shown as the dotted curve. In this case the fitted values of the constants of eq. (1) are A = 2801, 10 = 375.6, o = 3.1328, C = 0.0184, D = 0.0370, and G = 1.1947. To reduce the counting rate due to high-energy neutrons produced in reactions it is often helpful to use a moderator such as paraffin between the target and the spectrometer. There have been several conflicting opinions on how the presence of such material effects the intrinsic line-shape [19-21]. The line shape observed with 50 cm of paraffin in front of the M K III detector is shown in fig. 10b. The change in the resolution is relatively minor, and the increase in the low-energy tail, although significant, is much less than has been observed with other detectors under similar conditions. We presume this is due to a lower effective energy threshold in the front plastic.

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60

Fig. 11. The response of the MK Ill spectrometer to 45.6 MeV gamma rays is shown in (a). For comparison, the 22.4 MeV data of fig. 8a is plotted in (b).

the NaI(T1) detector was perfectly uniform, the amount of light reaching the photocathodes of the tubes would be directly proportional to the energy deposited by a gamma ray. In this case, the final resolution would be determined only by the statistical fluctuations on the number of light quanta emitted by the thallium. The

I

r

I

r

3.2. Response at higher energies 3.0

As the gamma-ray energy increases above 22 MeV the spectrometer resolution becomes almost constant. Only a slight improvement is observed for 45.6 MeV gamma rays, as shown in fig. 11. The M K III resolution (fwhm of the accepted spectrum), measured with various reactions yielding gamma rays in the range from 6.1 MeV to 45.6 MeV, is plotted in fig. 12. The region from 6 to 33 MeV is fairly well reproduced by A E / E v ( i n %) = 5.92(E~, in MeV) 0.31. In fact, this is a fairly good representation of the response from 661 keV all the way up to 45.6 MeV, although it falls slightly below the data at the highest energies. If

I ~1

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I

I

I

1.0

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Fig. 12. The MK llI resolution (fwhm of the accepted spectrum) is shown here as a function of gamma-ray energy.

490

A.M. Sandorfi, M, T. Collins / BNL high-energyy-ray spectrometers

detector resolution would thus be expected to decrease, with increasing gamma-ray energy, as E r t/2. In reality. the improvement in the resolution of most high-energy spectrometers is a much slower function of increasing energy, typically decreasing as E v- 1/4 [22]. This is usually attributed to the effects of crystal non-uniformity that are discussed in sections 2.2 and 2.3. The fact that the resolution of the MK III detector improves as E -°31 reflects the improvement in crystal uniformity. In analyzing the energy variation in the fwhm of the accepted spectrum, the effects of the residual one-escape peak must be properly treated. At low energies, such as those of fig. 3a, the one-escape peak is so well separated from the full-energy peak that it does not alter the resolution of the latter. At sufficiently high energies, such as those of fig. l l a , the width due to photon statistics alone is sufficiently larger than 0.511 MeV that the effects of the one-escape peak are irrelevant. However, at intermediate energies the merging of the residual one-escape peak with the full-energy peak can significantly affect the resolution. In fig. 3a the first-escape to the 6.13 MeV peak accounts for 16% of the total area of both 1-escape and full-energy peaks. At 11.57 MeV, which is just about the highest energy at which the first-escape is clearly resolved, the latter is again 16% of the total area of both one-escape and full energy peaks. If the data of fig. 12 is reanalyzed by fitting every observed peak as a sum of two peaks separated by 0.511 MeV, and with the area of the lower-energy peak fixed at 16% of the total, then the width of the higher full-energy peak decreases with E~-°55 as E~ increases from 6 to 33 MeV. This is very nearly E~-1/2 and suggests that the statistics of thallium photoemission dominates the response of the MK III detector. This is an attractive picture, but a bit too simple. On the average 12% of the incident gamma-ray energy is transferred to the thallium and reemitted in the form of 3.0 eV photons [22]. Typically, the quantum efficiency of the photomultiplier tubes is about 25% so that, if the crystal surfaces were perfectly reflecting, an average 6610 electrons would strike a first tube-dynode following the absorption of a 661 keV gamma ray. However, the surfaces and coatings of the Nal are not perfectly reflective. In particular, the MgO reflects the 3.0 eV thallium light with an efficiency of only 97%. The 50 ns rise times observed from crystals as large as the M K III indicates that the light emitted by the NaI(T1) spends a long time bouncing back and forth before finally reaching a photocathode. (The rise time of signals from small NaI's can be a factor of 10 shorter.) This is most likely due to the large index of refraction (1.8 in Nal) which causes many of the thallium photons to be totally internally reflected at the quartz windows. In the 24 cm × 36 cm M K III crystal, the average separation between collisions with the MgO coating must be about 30 cm, or equivalently about 1 ns. The observed rise

time thus implies that the 3.0 eV light photons make about 50 collisions with the MgO before successfully reaching a photocathode. During this process, the fraction of light not absorbed by the MgO is about (0.97) 5o = 0.22. This reduces the number of photoelectrons at a first-dynode to 6610 × 0.22 = 1441, following the absorption of a 661 keV gamma ray. The statistical fluctuations in this number (lo = 38) imply a resolution of 6.2%, which is very close to the observed value of 6.9%. It is likely that the large number of collisions with MgO walls that has been deduced here for the MK II1 detector also takes place in much smaller Nal's although this does not deteriorate their rise time because the path lengths are much shorter. The best reported resolution at 661 keV, with a 7.6 cm diameter × 7.6 cm crystal, is in fact 6.2% [13,14]. Nonetheless, similar analyses would predict resolutions of 2.0% at 6.13 MeV. and 1.1% at 22 MeV. It would appear that between 6 and 33 MeV the MK III resolution, although nearly following E~- 1/2, is offset by a constant value of about 1% from the photon-statistics limit. There are two clues that suggest that this has something to do with crystal non-uniformity. First, the behavior is different below 6 MeV where there is very little pair-production so that the light emitting region of the Nal is rather well confined. Second, the response curve of fig. 12 is flattening off to a nearly constant value at the highest energies as the lengths of the gamma-ray showers increase. It would appear that the crystal preparation techniques described in sections 2.2 and 2.3 have still not removed all of the effects of non-uniformity. Nonetheless, the E~ °'~5 behaviour at high energies strongly suggests that they have reduced the NaI uniformity problem to the point that a significant improvement in the quantum efficiency of light collection should be the next goal towards improving the resolution of NaI(T1) based high-energy spectrometers, This may in fact be possible with low-noise silicon-photodiodes although, at present, the time constants of these devices would severely limit the counting rate capabilities. 3.3. Counting rate stability

The performance of the M K 111 spectrometer at counting rates ranging as high as 1.1 MHz above 1 MeV is illustrated by fig. 13. Using the full stabilization system described in section 2.4, no significant deterioration in resolution and no shifts in gain larger than 0.2% are observed* for counting rates as high as 300 kHz above 1 MeV. At 1.1 M H z the aceepted-fwhm is broadened to 3.2% at 23.3 MeV, although a large fraction of this increased width is very likely due to pileup. If the detector were used at this rate in a measurement in which some other reaction product was detected in coincidence, the pileup would be dramatically reduced

A.M. Sandorfi, M.T. Collins / BNL high-energy y-ray spectrometers [

I

liB(p, 180

I

J

3.4. Timing resolution

I

Ero =25.30 MeV

y)

Ep = 8 . 0

The time resolution, measured with 0.511 MeV annihilation photons, between the plastic anticoincidence shield and the NaI crystals of either the MK II or MK III detectors is about 3 ns. The resolution between MK II and MK III detectors, with the MK II used to trigger on gamma rays over a very large dynamic range, is 5.5 ns [23].

MeV

(c) 150

R=I.I

491

MHz

120

3.5. Cosmic ray rejection 90

3.2% FWHM CENTROID SHIFT= 0.6%

I'

60

[

30

Ld Z Z < I

i

I

I

I

The average cosmic ray rate observed in the MK Ili crystal at Brookhaven is 3 counts, min -1. MeV t. These are rejected by the anticoincidence shield with an efficiency of 99.5 + 0.2%. The deviation chiefly reflects the care with which the electronic timing is set-up.

4. Sample applications

nr" Ld

a_ co I.-z D 0

(b) 56

R = 200

KHz

2.2% FWHM CENTROID SHAFT= 0.2%

24

12

(o) 360

R=7OKHz 2.3%

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I

I

I

I

200

240

280

320

CHANNEL

360

I

I

400

440

NUMBER

Fig. 13. The behavior of the MK Ill spectrometer at various counting rates (R) above 1 MeV in the NaI crystal.

and the intrinsic line shape should appear much narrower. As mentioned in section 2.4, the spectra of fig. 13 were taken with RCA 4900 tubes. We anticipate that the 0.6% gain shift observed at 1.1 MHz should be significantly reduced when RCA $83021E tubes are used.

The MK II detector has been used in a number of light- and heavy-ion capture experiments [18,24-26], with resolutions in the accepted spectrum ranging from 3.2% to 3.4% at 20 MeV depending upon collimation. The stability and superior resolution of the MK III detector has been exploited in a number of experiments in which transitions to particular final states had to be isolated from a large number of other possible gamma decays. We discuss here two examples. A spectrum from the first, an investigation of the 27Al(p,3,)28Si reaction, is shown in fig. 14 [27]. The peaks observed in this spectrum shift with bombarding energy according to E r = (27/28)Ep + Q for fixed values of Q, indicating that they arise from radiative capture to final states in 28Si. The final excitation energy scale in 28Si (E f) is shown, along with the gamma ray energy, at the bottom of the figure. The strongest peaks correspond primarily to the population of the E f ( J = , T ) = 14.36 MeV (6-,1), 13.25 MeV (5-,1), 12.65 MeV (4-,1), and 11.58 MeV (6-,0) levels, all of which are known from transfer reactions. In fact, all structures in this and similar spectra can be correlated with states with significant proton transfer strength, the locations of which are shown at the top of fig. 14. Accordingly, we have fitted this and similar spectra with a sum of line shapes of variable amplitude, but with positions fixed (relative to 3o) at the locations of states with significant proton transfer strength, as shown in fig. 14. This procedure has yielded excellent fits to the data from Ep = 14 MeV all the way up to Ep = 39 MeV. The resolution of the MK 1II detector during these measurements (2.3% at 20 MeV) was crucial in identifying the final 28Si levels that were being populated, and in minimizing the errors in the extracted cross sections that can accumulate during such fitting procedures. The excitation functions of the

492

A.M. Sandorfi, M.T. Collins / BNL high energy -

y

-

ray l~'pectrorneters 12C( 160,y 28Si )

Z;'AI (3He, d) 2eSi* Spectroscopic Strength -- - o

oo

o

o

E,so= 20MeV 8)., = 120°

o

OSi : " h 2 °

Nol

/=0,2

o

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"["

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,

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o

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i

o

o

i I

i i

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. . . . . .

.

TARGET

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/',E-E .

FUSION RECOILS

I

I

I

oi

E0. 22 Me~ i t

9o ta

!ii

HI

60

3o~

" E~ (MeV) 210

/

cO

2

]1-

BEAM STOP

150!

m

~- ,doJ -

.

I

i i g

,I

-b

21-S Ey (MeV)

i

6°°° 1 JO 3000

Fig. 14. An accepted 27Al(p,7)spectrum taken with the MK III detector is shown at the bottom, together with a line-shape decomposition [27]. Single-proton spectroscopic factors are shown at the top. The vertical lines indicate the correspondence between proton capture and stripping strength for some of the prominent gamma transitions,

extracted transitions revealed giant dipole resonances (GDRs) built upon one-particle one-hole excited states• This experiment provided the first measurement of the dependence of the G D R width and strength as a function of the energy of the final states on top of which the GDRs are built. In another experiment, the MK III detector was used in coincidence with the crossed E and B fields of a velocity (Wien) selector to study the 12C(160,y28Si) reaction [28]. The geometry is shown schematically at the top of fig. 15. The cross sections for heavy-ion radiative capture are generally quite small compared with those of light-ion capture, and measurements of transition rates to excited final states, where the ?-ray energy is significantly less than the maximum (E~o), are extremely difficult because of large pileup backgrounds. With the MK III detector at 120 °, the recoiling 28Si nuclei were focused through the Wien filter, positioned at - 1.2 ° to compensate for the kick due to the gamma ray momentum, and entered an AE - E ionization chamber. To maximize the coincidence counting rate, the collimator used with the MK III detector projected the solid angle cone onto the full crystal diameter at a depth of only 16 cm into the NaI. As shown at the bottom of

F F

~

I

J~ ji~ °

~

i

tO

15

ZO

=

i

~

>~i\ i

riB( p , y ) 1 2 C

8), = 9 0 °

I

Z5

EX iMev) I

tI

/

Ep = 7 . 2 5 MeV

I~

-*ti--2.4%

Fig. 15. An arrangement of the MK Ill detector and a velocity filter are shown schematically at the top of the figure. An accepted gamma ray+ fusion coincidence spectrum from the 12C(160,y28Si) reaction is shown in the center of the figure [28]. The MK I11 response in the geometry used for this experiment is shown at the bottom.

fig. 15, the resolution in this geometry was 2.4% at 22.6 MeV. The large pileup backgrounds present in the singles spectra were reduced by up to four orders of magnitude by the coincidence requirement. A coincidence spectrum is shown in the middle of fig. 15, and the gamma-ray yields go almost to zero between many of the peaks. The velocity acceptance of the Wien filter used in these measurements limited the effective target thickness to only 20 /xg/cm 2. As a result, rather long data collection times were required (12-18 h), and the gain stability of the MK III spectrometer was essential in maintaining the quality of the gamma-ray spectra. The decay to the 0~- state at 6:69 MeV in 2Ssi, indicated by the heavy arrow in fig. 15, is clearly the most prominent capture transition. It appears in this coincidence spectrum as a well-isolated peak, although the width of this peak, together with the known line shape of the MK III detector, requires a smalt contribution from gamma rays slightly lower in energy, to states at Esi = 7.4 MeV. This latter component appears as a well defined peak at higher energies where the yield to the 0~- is reduced. Although the ground a n d low-lying

A.M. Sandorfi, M.T. Collins / BNL high-energy "f-ray spectrometers states of 28Si are oblate, this 0~- has been identified as the bandhead of a K = 0 prolate shape-isomeric state. We associate the transitions to states at Esi -- 7.4 MeV with decays to the 2 + member of this prolate band. Transitions to the 4 + prolate state at Esi = 9.16 MeV are also evident in fig. 15, although these too are much stronger at higher energies. The exictation functions of these transitions are consistent with giant dipole resonances built on the prolate intrinsic shape of 28Si. The observed radiative decays appear enhanced in heavy-ion capture because the entrance channel is inherently prolate. The width of the gamma-ray strength built on the excited prolate states is narrower than the G D R of the oblate ground state, while all models coupling deformation degrees of freedom to the G D R would predict a significantly broader prolate resonance. This is the first time that such a comparison has been possible within a single nucleus.

5. Design considerations for future spectrometers While developing the MK III detector we have investigated several other types of spectrometers, all aimed at the gamma-ray energy range from 20 to 30 MeV. Some preliminary results are described here.

493

same as that of the MK III crystal. This volume was segmented into seven optically isolated sections: an 8 cm diameter core surrounded by six pie-shaped sections. To maintain a large efficiency-solid angle product, the solid-angle cone would project onto the full back face of the assembly, and the high energy showers would, on the average, cross three crystal boundaries. Because of this, it was necessary to require a high level of uniformity along each detector as well as between adjacent detector surfaces. Several such assemblies were manufactured by Bicron [14], although none achieved the desired uniformity and the best gave a 7% resolution at 20 MeV. At high energies, gain matching in such a detector is extremely critical, and rather difficult. Ideally, one would like to match tube gains at 20 MeV since, as discussed in section 2.3, the matching conditions can be somewhat energy dependent. However, since each tube can now only sample part of the shower, a well defined peak cannot be observed and gain matching at high energies is impossible. The gains can be equalized with a light pulser, but this is only relevant if the response is extremely uniform throughout all the sections. It would appear to us that the uniformity of light emission and collection was the key problem, and this is not yet under sufficient control.

5.2. A large BGO crystal 5.1. A segmented Nal(Tl) The NaI crystal used by the OSAKA group is segmented into five optically isolated sections, the core being a 15 cm diameter by 28 cm long cylinder [3]. Their purpose in doing this was to improve the final resolution although, as seen in fig. 9, they achieved essentially no improvement over the response of the Seattle detector. As pointed out in their article (ref. [3]). collimating to the back of the central 15 cm diameter crystal does not affect the detection efficiency. However, the important quantity in any physics measurement is the product of efficiency and solid angle, and this is reduced by more than a factor of 3 in this approach. We have also designed and tested a segmented NaI detector, although the goals of our effort were rather different. In high energy gamma-ray measurements, the use of a high-level discriminator to reduce low-low pileup always limits the range of transitions that can be studied, and it is almost always advantageous to run with as low a high-threshold as possible (see fig. 14, for example). The spatial energy distribution of two 10 MeV gammas is quite different from that of a single 20 MeV photon. Our hope was to operate with a very low high-level threshold by using the spatial information from a segmented NaI for low-low pileup rejection, while maintaining a resolution comparable to that of the MK II detector. The overall NaI dimensions were the

Recent measurements have suggested that bismuth germanate (BGO) might be an attractive alternative to NaI(T1) as a high energy gamma-ray detector. Resolutions around 3.5% at 22 MeV have been reported for 7.6 cm × 7.6 cm crystals [29], although these widths have been computed using only the high energy edge of the full energy peak. Because of the higher density and average Z, a radiation length in BGO (1.12 cm) is 2.3 times smaller than in NaI, and the Moli~re radius (2.24 cm) is 2.0 times shorter. The crystal used in the measurements of ref. [29] was thus equivalent in shower-stopping-power to a 14.9 cm diameter × 17.5 cm long Nal. This is too short to propertly contain 20 MeV showers, and a longer crystal is necessary if resolutions approaching that of the MK III are desired. We have tested a 7.6 cm diameter × 15.3 cm long BGO crystal, grown as a single ingot by Harshaw [13], and viewed by a single tube. This detector had a resolution of 17.5% at 661 keV, and was uniform to _+1% when measured with a collimated 137Cs source. However, veils of small bubbles could be clearly observed in the center region of the crystal. The BGO was surrounded by a NaI annulus. A 10 cm thick slab of plastic scintillator in frontlof the crystal completed the anticoincidence shield, and a conical lead aperture collimated gamma rays to the back face of the BGO. This anticoincidence shield, although rate limited by the NaI, was extremely efficient; the one-escape peak from a 4.44 MeV source

494

A.M. Sandorfi, M.T. (_'ollins / BNL high-energy y- r~(~, ~pectrometers

was not observable in the accepted spectrum. Nonetheless, the fwhm of the accepted spectrum at 22 MeV was only 7%. Again we believe that crystal non-uniformity is the chief problem. With the present manufacturing techniques it does not yet seem possible to grow a sufficiently large ingot that is even free of inclusions. If resolution is an important factor, a significant refinement in technology is necessary before BGO can compete with NaI as a detector of high energy photons. 5.3. A large intrinsic Ge + N a l detector

We have designed a MK IV spectrometer with a philosophy similar to that of the OSAKA detector. However, the central core of the M K IV would be a 7.6 cm diameter × 20.3 cm long crystal of intrinsic Ge, housed in a thin-walled cryostat. Surrounding and behind this would be an annulus of NaI(TI) with a wall thickness of 15.3 cm. Gamma rays entering this detector would be collimated by a conical lead aperture which projected the solid angle cone onto the full back face of the germanium crystal. This thickness of Ge is insufficient to contain the full shower produced by a 20 MeV photon, and 5 to 10 percent of the energy (1-2 MeV) would escape. This residual shower energy would be detected in the NaI with about 5% resolution. Estimates from potential manufacturers indicate that the intrinsic resolution of the large Ge crystal would be between 4 and 8 keV. When the energy detected in the Ge and in the NaI(T1) crystals are added together, the net resolution of the full energy peak should be between 54 and 108 keV, or between 0.3 and 0.5% at 20 MeV. At worst, the NaI annulus would be 72% efficient for absorbing 1-2 MeV of low energy shower products. However, most of this residual energy would be in the form of 511 keV annihilation photons, traveling at acute angles through the NaI, so that their detection efficiency would actually be greater than 99.5%. Thus it is likely that an annular plastic anticoincidence shield would not be necessary. Cosmic rays could easily be distinguished by their large energy deposition in the NaI. Only a front plastic slab would be needed to reject backscattered radiation. The efficiency of this spectrometer should be nearly one, although the solid angle would be limited by the diameter of the Ge crystal. Due to the time constants of the Ge detector, the total counting rate would probably be limited to about 100 kHz. The Ge crystal is of course sensitive to neutrons. However, since the final resolution would be completely dominated by the N a I , a considerable deterioration in the resolution of the intrinsic Ge could be tolerated before annealing was required, and 6LiH shielding would prolong this even further. The dramatic improvement in the expected resolution of the M K IV spectrometer would undoubtedly

open new areas of research. Unfortunately. construction of this spectrometer has been suspended due to a lack of funds.

6. Summary The fwhm and fwtm that have been achieved with the BNL-MK III spectrometer, along with its stability at extremely high counting rates, represent a remarkable improvement over previous detectors of 10-50 M e v photons. We attribute this to a large number of detailed design considerations, the most important of which is the NaI(TI) crystal uniformity. Here, the greatest single improvement has come from the new technique of surface compensation at an intermediate energy (6.13 MeV). The same technique, using the same source supplied by BNL, has since been used to surface compensate large Nal detectors for the University of Illinois and for the University of British Columbia. In both cases, significant improvements in resolution were made. The improved MK III fwtm is largely the result of the segmentation of the plastic anticoincidence annulus. The response of the MK III detector appears to be nearly limited by the photoelectron statistics at the first dynodes of the photomultipler tubes. This being the case, future improvements in Nal,based high'energy spectrometers are most likely to come if new light-collecting devices, with substantially higher quantum efficiencies, can be developed. We would like to acknowledge several discussions with J. Roelf of the High Energy Physics Laboratory at Stanford University regarding the design of the active photomultiplier bases, and with T.J. Bowles of Los Alamos Scientific Laboratory regarding NaI surface compensation at intermediate energies. We are indebted to the staff of Bicron Corp., particularly R. Dayton, for their painstaking cooperation during the fabrication of the MK III NaI(TI) crystal. This work was supported by the US Department of Energy under Contract No. DE-AC02-76CH00016.

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