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A new high energy gamma radiation measuring system (HERMES)
Nuclear Instruments and Methods 198 (1982) 269-276 North-Holland Publishing Company
269
A NEW HIGH ENERGY GAMMA RADIATION MEASURING SYSTEM (HERMES) Tadafumi KISHIMOTO Hiroyasu EJIRI
*, T o k u s h i S H I B A T A , M a m i k o S A S A O **, M a s a h a r u N O U M A C H I
and
Department of Physics, Osaka University, Toyonaka, Osaka 560, Japan Received 6 July 1981 and in revised form 27 November 1981
A high energy gamma radiation measuring system (HERMES) for 10-50 MeV high energy gamma rays was designed and was constructed successfully. It is very useful for high resolution study of radiative hadron capture reactions. It consists of an 1I" ~ × 11" NaI(T1) detector surrounded by a plastic scintillator for anticoincidence requirement. The large NaI(TI) detector is composed of a 6"~ X 1I" central NaI(TI) crystal and an 11"~ × 11" annular NaI(TI) crystal. These NaI(T1) crystals are optically isolated from each other. Total energy of a gamma ray is obtained by summing up signals from the central 6"~ x 11" crystal and the annular 1I " ~ X I 1" one. The central crystal has the best energy resolution (7.1% for 662 keV gamma rays) among conventional large NaI detectors. It leads to the good energy resolution of the HERMES (2.8% for 22.5 MeV gamma rays) because high energy gamma rays collimated to the central crystal deposit most of their energy in the central crystal. Also the signal-to-noise ratio has been improved very much compared to conventional large NaI detectors mainly due to the optical separation of the large NaI detector into the central and annular parts.
1. Introduction The study of nuclear gamma rays is one of the powerful probes to investigate nuclear properties because the electro-magnetic interaction responsible for a g a m m a transition is well known. The long wavelength approximation for the gamma emission greatly simplifies the multipolarity of the transition operator and the selection rule. Recent investigations of the giant multipole resonances gave a momentous impact to the study of high energy gamma rays [1-3]. It demands a good detector for high energy gamma rays in the 10-50 MeV range. This paper deals with a new type of large N a I spectrometer for high energy gamma rays. The energy resolution and the signal-to-noise ( S / N ) ratio of the new spectrometer are much better than those previously reported. Several large NaI spectrometers have been developed previously [4-8]. Characteristics for some of them are listed in ref. 8. The basic idea of these detectors is essentially the same. They consist of a large NaI(T1) crystal of typically 1 0 " ~ x 10". High energy g a m m a rays entering the NaI crystal give rise to a shower of photons and electrons in the crystal. The total energy of the gamma ray is deposited in the detector if all of the shower particles (photons and electrons) are absorbed in * Present address: Department of Physics, Tokyo Institute of Technolog3/, Meguroku, Tokyo 152, Japan. ** Present address: Institute of Plasma Physics, Nagoya University, Chikusaku, Nagoya 464, Japan. 0167-5087/82/0000-0000/$02.75
© 1982 North-Holland
the detector. However, it does not give the full energy loss when some photons following the shower escape from the detector. Such photons are detected by plastic scintillators surrounding the NaI crystal. Thus the full energy loss peak can be selected by requiring anticoincidence with the signal from the plastic scintillators. Several minor improvements in uniformity of the NaI crystal, collimators, elecronic circuit and so on [8] h~ve recently been made. The enei:gy resolution and S / N ratio for these detectors, however, are not sufficient to clearly separate g a m m a transitions to the ground state from those to low lying excited states in most of the medium heavy nuclei. We designed a new gamma ray spectrometer which greatly exceeds the previous ones in both energy resolution and S / N ratio without reducing the efficiency. The new large NaI spectrometer is composed of two parts. One is a central 6" O X 11" cylindrical crystal, the other is an I 1" O X 11" annular crystal surrounding the central one. The total energy is obtained by summing up signals from both the central and the annular crystals. This new design improved both the energy resolution and the S / N ratio very much as explained in the following. l) Improvement of the energy resolution: High energy gamma rays have to be collimated to the central region of a conventional large (-- 10"O X 10") NaI detector in order to get the full energy loss peak efficiently because g a m m a rays entering the outer region of the crystal are not likely to lose the total energy within the detector. In other words, most of the energy has to be deposited in
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the central region of the crystal. Therefore it is essential to improve the energy resolution of the central part of the detector, namely the 6 " 0 X 10" crystal in the present case. The 6 " ~ X 11" size is practically just adequate to get a good resolution crystal, 2) Improvement of the S / N ratio: Smaller detectors are better than larger detectors with respect to the S / N ratio. This is because noise signals due to cosmic rays, background gamma rays and neutrons are roughly proportional to the volume of the detector. In the present case the effective volume of the detector is that of the small 6 " 0 X 11" crystal since we record events when the central crystal fires. Thus the background counting rate is essentially that of the small central detector. Furthermore the central crystal is well shielded by the 1 1 " ~ X 11" annular crystal. Therefore a good S / N ratio is expected with the new design. The new type of high energy gamma radiation measuring system, called H E R M E S , has an energy resolution of 2.8% for 22.5 MeV gamma rays. The S / N ratio has been improved very much compared to conventional large NaI spectrometers.
2. The high energy gamma radiation measuring system (HERMES)
detectors, but the whole crystal is optically separated into a central 6" O X 11" cylindrical NaI(TI) crystal and an annular 11"O X 11" one surrounding the central crystal. The annular one is divided into four segments of a quarter cylinder. Each crystal is optically isolated by magnesium oxide (MgO) powder of 1.5 mm thick. The absorption due to the MgO layer is negligible for gamma rays higher than 50 keV. The central 6 " 0 x 11" crystal is viewed by a 5 " 0 ( R C A 4525) photomultiplier (PMT). Each segment of the annular crystal is viewed by two 2 " ~ (RCA 4523) PMTs. The whole NaI crystal is packed in a 3 m m thick aluminum can. The NaI(TI) crystal was manufactured by the Harshaw Chemical Co. according to our design and specification. The performance obtained for the present 6 " 0 X 11" crystal is satisfactory. The energy resolution of the central crystal is 7.1% for 137Cs 662 keV gamma rays. Uniformity of the pulse height along the length has been achieved by changing the reflection coefficient at the boundary of the NaI detector so as to give the same pulse height in different gamma source positions. The difference of the pulse height between front and rear side source positions is less than 1.6%. Energy resolutions for the four segments of the annular crystal are 8.4%, 8.7%, 8.9% and 9.0% for 662 keV 137Csgamma rays. The pulse heights of the four segments of the annular crystal are uniform within 3%.
2.1. A new large NaI(TI) detector for the H E R M E S A sectional layout of the H E R M E S is shown in fig. 1. The NaI(T1) is an 11"O X 11" cylindrical crystal. The volume is similar to conventional type large NaI
Fig. 1. Sectional layout of the HERMES. The 11"~ X l l " Nal crystal is divided into two parts. Each part is optically isolated by 1.5 mm MgO layers. Eight 2" ~ (RCA 4523) PMTs and the 5" ~ (RCA 4525) PMT are attached to the annular 11" ~ × I 1" crystal and the central 6"~ x I1" crystal, respectively. The whole NaI assembly is packed in a 3 mm thick aluminum can, Four and eight 2"~ (Hamamatsu R329) PMTs are attached to the front plastic scintillator disc and the annular plastic scintillator, respectively.
2.2. The H E R M E S The NaI detector ig surrounded by a 10 cm thick annular plastic scintillator in order to reject evems where some photons escape from the NaI detector and enter the plastic scintillator, The annular plastic scintillator was manufactured by the CI Industrial Co. It has about a l m attenuation length. It is divided into four segments, each of which is viewed by two 2 " 0 (Hamamatsu R329) PMTs. The 7 c m thick N E 110 plastic scintillators are covered by aluminum foil and are taped tight with black tape in order to shield them from light. The pulse height of the plastic scintillators is uniform within 20% over their whole area. The effective noise level of the plastic scintillators was about 40 keV for electrons. Lithium carbonate with paraffin is packed in the 15 mm gap between the NaI and the plastic scintillators. The annular plastic scintillator is covered with a 5 cm thick lead shield. A 10 cm thick lead shield is placed in front of the plastic scintillator disc in order to reject gamma rays from the target. The lead shield has a hole of 110 mm in diameter, into which a collimator is set. The collimator defines the solid angle of the H E R M E S . All lead shields are covered by 3.2 mm thick iron plates to protect them against mechanical shock. The physical layout of the H E R M E S is shown in
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2
1 1
-
-
4
Fig. 2. Physical layout of the HERMES. The goniometer has a turntable which can rotate around the target point. (I) Scattering chamber and target. (2) 2" PMTs for the plastic scintillators. (3) Main lead shield. (4) Handle for controlling the target-point-to-detector distance. (5) Rail. (6) Drive motor. (7) Goniometer, fig. 2. The whole assembly weighs about a ton. It is mounted on a reinforced carriage on a turntable. It can be rotated around the target point from 30 ° to 150° with respect to the beam when the distance from the target to the detector face is 25 cm. The side of the lead shield and the plastic scintillator disc has been removed in order to move the HERMES close to the beam line. 2.3. Signal processing
The signal processing circuit after the PMTs is shown in fig. 3. The system deals with signals from the central 6" ~ X 11" crystal, those from the annular 11" ~ X 11"
crystal, and those from the plastic scintillators. Anode signals of the PMTs for the annular and the disc plastic scintillators are actively mixed by an active mixer A. Anode signals of the PMTs for the annular NaI crystal are also mixed by another active mixer B. Anode signals of the PMT for the central NaI crystal are amplified by a fast amplifier and distributed into three output channels through a linear fan-out (428 F). One of them is amplified by a timing filter amplifier and is fed into a high level discriminator (924). The threshold of the high level discriminator is set at several MeV in order to suppress the counting rate and to prevent pulse pile-up. The logic signal from the high level discriminator opens both the linear gates for the linear signal from the central crystal and that for the linear signal from the annular crystal. The slow rise time (60 ns) of the anode signals from the NaI detector gives rise to some time walk. In order to minimize the time walk a constant fraction discriminator (CFD) is used for the high level discriminator. The gate width of the linear gate was chosen to be 400 ns. The output signals of the linear gates are amplified by spectroscopy amplifiers. The energy resolution becomes better as the gate width becomes wider until it reaches 300 ns. Thus we set the gate width at 400 ns. This is also shown in ref. 8. Since the nearly rectangular pulse with 400 ns width is amplified charge sensitively by the spectroscopy amplifier, the time walk of the gating logic signal from the high level discriminator changes the output pulse height a little. We adjusted the threshold and the external delay time of the CFD so as to make the energy resolution of the pulser peak b6tter
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frOmNalcentral
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Fig. 3. Schematic diagram of the singles electronics used for the HERMES. Model numbers are those of Ortec, EG&G, LeCroy and Willam&Harris models.
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than 0.6%. Signals from another output of the fan-out are amplified and are used for gain stabilization. The 4.4 MeV gamma ray from 12C, the 2.2 MeV gamma ray from the H(n, "~)D reaction or the 1.84 MeV gamma ray from the 88y source were monitored to stabilize the gain of the signal from the central 6 " ~ x 11" crystal. The gain stabilizer feeds back a correction signal to the high voltage power supply for the NaI. Output signals of the central and the annular crystals from the linear gates are individually amplified so as to .give the same overall gain. Then they are summed up together. The summed signal is analyzed by a multichannel analyzer, and routed to either an accepted spectrum or a rejected spectrum. Here the router signal is generated by a fast coincidence between the logic signal of the high level discriminator for the central 6"O X 11" crystal and the logic signal of the discriminator for the plastic scintillator. The threshold for the latter is set just above the noise level. The resolving time of this coincidence is about 70 ns. Thus the summed signals accompanied by a signal from the plastic scintillator are stored in the "rejected" spectrum and those without any leakage photons into the plastic scintillators are stored in the "accepted" spectrum. We sometimes use a pulse pile-up rejection circuit [9] when the counting rate is very high. A time-of-flight measurement with a pulsed beam is useful for separating gamma rays from neutrons whicil become serious as the projectile beam energy becomes high.
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Fig. 4. The upper, middle and lower spectra are an accepted, rejected and fitted energy spectrum, respectively. They were measured by the HERMES for the liB(p, T)t2C reaction at Ep 7.4 MeV. The fitted spectrum was obtained by subtracting 90% of the rejected spectrum from the accepted spectrum. The factor 90% was chosen so that the full energy loss peak becomes symmetric and the background becomes close to zero.
25C
11B(p, ~'~2C ACCEPTED
3. Results 20(
3.1. Energy resolution 150
As mentioned in previous articles [4-8] the energy resolution depends on the quality of the NaI, the aperture of the collimator, the threshold of the discriminator for the plastic scintillator, the counting rate of the NaI and so on. The following points are important also for the HERMES. The first is the energy resolution of the central 6 " ~ X 11" crystal, and the second is the uniformity of the pulse height along its cylindrical axis. The third is the time walk of the timing signal from the high level discriminator which is used as the gate pulse for the linear gate. The fourth is the fraction of the gamma ray energy deposited in the central crystal. The energy resolution of the central crystal and the effect of the timing signal walk have already been discussed in the previous section. In what follows we discuss the fourth point. Accepted and rejected energy spectra of gamma rays from the I~B(p, 3,)12C reaction at Ep =7.4 MeV are shown in fig. 4. The energy resolution of the accepted spectrum for the 22.7 MeV peak is 2.8% fwhm. Fig. 5 shows the accepted and rejected spectra for the same
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measured by the central 6" O × 11" crystal for the 11B(p,"t)12C reaction at El, = 7.2 MeV. Events accompanied by a signal from the annular 11"O×11" crystal are routed to the rejected spectrum.
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reaction at Ep = 7.2 MeV by using only the central 6 " ~ X 11" NaI as the gamma ray detector and the annular l l " O × l l " NaI as the anticoincidence detector. The energy resolution for the 22.5 MeV peak is 2.7%. The two resolutions differ little. This is easily understood from the following two-dimensional measurement for the same reaction. The energy spectra of gamma rays for various energy depositions in the annular crystal are shown in fig. 6. Each bin corresponds to the energy deposited in the annular crystal. These spectra show that the energy deposited in the annular crystal is distributed in the fairly low energy region of
0.1-1 MeV. Yields of 22.5 MeV 70 peak in the accepted spectra are plotted in fig. 7 as a function of the energy [E~,(AN)] deposited in the annular crystal. The 70 yield decreases exponentially with increasing energy ET(AN). A sharp peak appears at about 500 keV. These features are explained as follows. Most of the high energy gamma rays entering the central NaI crystal give rise to electron -position pair creation. The pairs lose the kinetic energy in the central crystal and two 511 keV photons are emitted in opposite directons. The sharp 511 keV peak corresponds to the case where one of the 511 keV photons escapes from the central crystal and deposits its
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energy in the annular crystal. The probability that two 511 keV photons deposit an energy of 1022 keV in the annular crystal equals roughly the probability that one 511 keV photon penetrates through the 6 " 0 X 11" central crystal. Since this is less than 1% the 1022 keV peak is not seen in fig. 7. The exponential decrease of the Ev(AN) spectrum corresponds to the bremsstrahlung. The energy resolution of the HERMES is then given by Rs=
9Be(3He."/')12C
:ir
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Here R S is the overall energy resolution for the sum [E~(SUM)] of the E~(CEN) (central) and the E~(AN) (annular), R c and R , are the energy resolutions for the central and annular crystals, respectively.f is the fraction Er(AN)/Er(SUM) of the energy deposited in the annular crystal. As seen in fig. 5 and fig. 7 the fraction is as small as 2% for 22.5 MeV gamma rays. Thus Rs nearly equals R c provided that Ev(AN) <
200
300
400 CHANNEL NUMBER
Fig. 8. An accepted gamma ray spectrum from the bombardment of 11 MeV 3He particles on 9Be observed at 125° to the beam axis. Labeled arrows indicate the expected positions of captures to each state in 12C. The excitation energy is given in units of MeV. gamma rays. Cosmic rays (muons) enter the NaI through the plastic scintillator depositing on energy of at least 2 MeV in it. Thus they are rejected by the anticoincidence with the plastic scintillator. Cosmic rays are actually reduced to less then 2%. Pulse pile-up is a severe problem when the counting rate of the NaI detector is high. We used a pile-up rejection circuit for measuring the 9Be(3He, 7)12C reaction. A detailed description of the circuit is presented in ref. 9. Slow neutrons, being radiatively captured, contribute to the gamma ray backgrounds. Cadmium sheets wrapped around the whole assembly of the HERMES are quite effective for suppressing these. Fast neutrons are produced by nuclear reactions at the target when the energy of the projectile is high. They enter the central NaI crystal and contribute to the background in the high energy region. They are effectively suppressed by means of a time-of-flight method with a pulsed beam. This also reduces considerably the cosmic ray background. An accepted gamma ray spectrum for the 9Be(3He, 7)~2C reaction at Een° = 11 MeV is shown in fig. 8. The background level near the 9.64 MeV 3 - peak (E v = 25 MeV) is smaller by a factor of three than that of the previous work [10]. The spectrum is well interpreted by the response described in the following section. In short, the background counts in the present HERMES are almost negligible.
3.3. Efficiency and response 3.2. Signal-to-noise ratio Main sources of backgrounds at the high energy region are background gamma rays from the beam line, cosmic rays, pile-up pulses and neutrons. The small central 6 " 0 × 11" crystal much reduces all the backgrounds, as mentioned in section 1, and the annular 11"O × 11" crystal is a good shield for background
The photopeak efficiency is defined as the ratio of photopeak yields in the measured spectru m to the number of gamma rays emitted. We deduced the absolute efficiency and the response of the HERMES by measuring gamma rays in coincidence with deuterons from the l aB(3He, d7)12C reaction. The reaction excites well the 15.1 MeV level in 12C which decays 88% by emitting the
T. Kishimoto et aL / HERMES 15.1 MeV gamma ray and the 4.4 MeV level which decays 100% by emitting the 4.4 MeV gamma ray. Therefore one gets the absolute efficiency for 15.1 MeV and 4.4 MeV gamma rays. The 19 MeV 3He beam was provided by the Osaka University variable energy 110 cm cyclotron. The deuterons were detected at 40 ° to the beam axis by a counter telescope which consists of 300 /zm and 2 m m thick surface barrier type silicon detectors. The slit of the telescope defines a solid angle of 4.5 msr. The H E R M E S was set at 90 ° to the beam axis. The distance between the detector face of the H E R M E S and the target was 130 m m because sufficient coincidence events were necessary for the measurement of the response. Smaller than 5% corrections were made for the absolute efficiency from the angular distribution measurement at angles of 67 ° , 90 ° and 113 ° . Only the coincidence events were measured in the angular distribution measurement, where the H E R M E S was located far from the target at 220 mm. The 4-dimensional data which consist of particle energy ( E + A E), particle identification signal (PI), output of the time-to-pulse-height converter (TAC) and g a m m a ray energy (Ev) were recorded event by event on a magnetic tape through the interface [11] between the computer (PDP 11/34) and ADCs. The data were analyzed after the experiment by using the computer.
118( 3He ' d "r)12C 120
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0.8 "~---._
275
FITTED AREA TO TOTAL AREA RATIO
0.E
0.4
FITTED AREA EFFICIENCY
O~
04
Fig. 10. Efficiency of the HERMES. Experimentally determined ratio of fitted area to total area (upper) and fitted area efficiency to the geometrical solid angle of the collimator.The total area is composed of a full energy loss peak and a tail. We calculated the total area of the 15. I MeV gamma ray in the spectrum by summing up yields above l l MeV. Yields below l l MeV were estimated by exponentially extrapolating the tail to zero energy. However, it contributes less than 6% to the total area. Coincident gamma ray spectra are shown in fig. 9, The photopeak looks quite symmetric compared to the conventional detector. The photopeak (fitted area) efficiency to the geometrical solid angle derived from the measurement is shown in fig. 10. The attenuation due to the various materials which sit between target and NaI crystal, such as paraffin and the aluminum metal, was corrected for. The value for the H E R M E S is close to efficiencies of conventional detectors.
40
4. Concluding remarks
8
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-
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200 300 CHANNEL NUMBER
400
Fig. 9. An accepted (upper), a rejected (middle) and a fitted (lower) spectrum for 15.1 MeV gamma rays following the IJB(3He, dy)12C reaction. The 15.1 MeV gamma rays were obtained by gating deuterons feeding the 15.1 MeV level in ~2C. The fitted spectrum was obtained by subtracting 90% of the rejected spectrum from the accepted spectrum.
A new high energy gamma radiation measuring system ( H E R M E S ) has been successfully constructed. It is much superior to conventional detectors in both the energy resolution and the S / N ratio because of the new design of the large l l " O X l l " NaI(Tl) crystal. Essential is the optical separation of the central 6 " O × l l " crystal from the annular I 1"O X I l " crystal. Because of this new design the energy resolution of the H E R M E S becomes as good as that of the central 6" ~ X l l" crystal. Important is that the good energy resolution is achieved without reducing the overall efficiency much. The separation has much improved the S / N ratio. This is because we select only events accompanied by large signals from the small central crystal which is shielded well by the annular crystal. The energy resolution and the S / N ratio may be improved more by selecting events where the 511 keV energy is deposited in the annular crystal as seen in fig. 6.
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The optical division of the N a I crystal makes it possible to use the H E R M E S for Compton suppression and linear polarization detection. The C o m p t o n suppression is easily made by using the annular NaI crystal as the anticoincidence counter. The Compton suppression is useful for studying continuum gamma rays of several MeV because of its large photopeak efficiency. Here we note that the photopeak efficiency is also much improved by taking the coincidence between the central and the annular NaI crystals. For the linear polarization measurement the central NaI is used as a scatterer and the annular detector divided into four segments is used for detecting the Compton-scattered gamma rays. As the projectile energy becomes high neutron backgrounds from the target and beam line become severe. These can be effectively rejected by the time-of-flight measurement. The time resolution of the H E R M E S , however, is not very good at the moment since the 5 " ~ P M T used is not adequate for fast timing. It will soon be replaced by a P M T with a faster rise time. The H E R M E S with its fairly good energy resolution and good S / N ratio may open up research for gamma transitions from higher excited states to various low lying excited states in a wide range of nuclei. The authors wish to thank Mr. K. Tsujita for his kind help in measurement and data analysis. They are
grateful to Dr. Nagai and Dr. Okada for valuable discussions during the development of the system. Thanks are also due to Mr. Matsuoka and Mr. Higa for the cyclotron operation.
References [1] Berman, Proc. Int, Conf. on Photonuclear reactions and applications, Alismore (1973). [2] S.S. Hanna, Proc. Int. Conf. on Nuclear structure and spectroscopy, vol. 2 (1974) p. 249. [3] G.R. Satchler, Phys. Rep. 14 (1974) 97. [4] M. Suffert, W. Feldman, T. Makieux and S,S. Hanna, Nucl. Instr. and Meth. 63 (1968) I. [5] E.M. Diener, J.F. Amann, S.L. Blatt and P. Paul, Nucl. Instr. and Meth. 83 (1970) 115. [6] G. Kernel, W.W. Mason and N.W. Tanner, Nucl. Instr. and Meth. 89 (1970) 1. [7] W.F. Davidson, J.L. Black and M.R. Najam, Nucl. Phys. A168 (1971) 339. [8] M. Hashinoff, S.T. Lim, D.E. Meusday and T.L. Mulligan, Nucl. Instr. and Meth. 117 (1974) 375. [9] S. Blatt, J. Mahieux and D. Kohler, Nucl. Instr. and Meth. 60 (1969) 221. [10] H.D. Shay, R.E. Peschel, J.M. Long and D.A. Bromley, Phys. Rev. C 9 (1974) 76. [ I I] M. Noumach, K. Okada and T. Shibata, Annual Report of Laboratory of Nuclear Study Osaka University 22 (1979).
269
A NEW HIGH ENERGY GAMMA RADIATION MEASURING SYSTEM (HERMES) Tadafumi KISHIMOTO Hiroyasu EJIRI
*, T o k u s h i S H I B A T A , M a m i k o S A S A O **, M a s a h a r u N O U M A C H I
and
Department of Physics, Osaka University, Toyonaka, Osaka 560, Japan Received 6 July 1981 and in revised form 27 November 1981
A high energy gamma radiation measuring system (HERMES) for 10-50 MeV high energy gamma rays was designed and was constructed successfully. It is very useful for high resolution study of radiative hadron capture reactions. It consists of an 1I" ~ × 11" NaI(T1) detector surrounded by a plastic scintillator for anticoincidence requirement. The large NaI(TI) detector is composed of a 6"~ X 1I" central NaI(TI) crystal and an 11"~ × 11" annular NaI(TI) crystal. These NaI(T1) crystals are optically isolated from each other. Total energy of a gamma ray is obtained by summing up signals from the central 6"~ x 11" crystal and the annular 1I " ~ X I 1" one. The central crystal has the best energy resolution (7.1% for 662 keV gamma rays) among conventional large NaI detectors. It leads to the good energy resolution of the HERMES (2.8% for 22.5 MeV gamma rays) because high energy gamma rays collimated to the central crystal deposit most of their energy in the central crystal. Also the signal-to-noise ratio has been improved very much compared to conventional large NaI detectors mainly due to the optical separation of the large NaI detector into the central and annular parts.
1. Introduction The study of nuclear gamma rays is one of the powerful probes to investigate nuclear properties because the electro-magnetic interaction responsible for a g a m m a transition is well known. The long wavelength approximation for the gamma emission greatly simplifies the multipolarity of the transition operator and the selection rule. Recent investigations of the giant multipole resonances gave a momentous impact to the study of high energy gamma rays [1-3]. It demands a good detector for high energy gamma rays in the 10-50 MeV range. This paper deals with a new type of large N a I spectrometer for high energy gamma rays. The energy resolution and the signal-to-noise ( S / N ) ratio of the new spectrometer are much better than those previously reported. Several large NaI spectrometers have been developed previously [4-8]. Characteristics for some of them are listed in ref. 8. The basic idea of these detectors is essentially the same. They consist of a large NaI(T1) crystal of typically 1 0 " ~ x 10". High energy g a m m a rays entering the NaI crystal give rise to a shower of photons and electrons in the crystal. The total energy of the gamma ray is deposited in the detector if all of the shower particles (photons and electrons) are absorbed in * Present address: Department of Physics, Tokyo Institute of Technolog3/, Meguroku, Tokyo 152, Japan. ** Present address: Institute of Plasma Physics, Nagoya University, Chikusaku, Nagoya 464, Japan. 0167-5087/82/0000-0000/$02.75
© 1982 North-Holland
the detector. However, it does not give the full energy loss when some photons following the shower escape from the detector. Such photons are detected by plastic scintillators surrounding the NaI crystal. Thus the full energy loss peak can be selected by requiring anticoincidence with the signal from the plastic scintillators. Several minor improvements in uniformity of the NaI crystal, collimators, elecronic circuit and so on [8] h~ve recently been made. The enei:gy resolution and S / N ratio for these detectors, however, are not sufficient to clearly separate g a m m a transitions to the ground state from those to low lying excited states in most of the medium heavy nuclei. We designed a new gamma ray spectrometer which greatly exceeds the previous ones in both energy resolution and S / N ratio without reducing the efficiency. The new large NaI spectrometer is composed of two parts. One is a central 6" O X 11" cylindrical crystal, the other is an I 1" O X 11" annular crystal surrounding the central one. The total energy is obtained by summing up signals from both the central and the annular crystals. This new design improved both the energy resolution and the S / N ratio very much as explained in the following. l) Improvement of the energy resolution: High energy gamma rays have to be collimated to the central region of a conventional large (-- 10"O X 10") NaI detector in order to get the full energy loss peak efficiently because g a m m a rays entering the outer region of the crystal are not likely to lose the total energy within the detector. In other words, most of the energy has to be deposited in
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the central region of the crystal. Therefore it is essential to improve the energy resolution of the central part of the detector, namely the 6 " 0 X 10" crystal in the present case. The 6 " ~ X 11" size is practically just adequate to get a good resolution crystal, 2) Improvement of the S / N ratio: Smaller detectors are better than larger detectors with respect to the S / N ratio. This is because noise signals due to cosmic rays, background gamma rays and neutrons are roughly proportional to the volume of the detector. In the present case the effective volume of the detector is that of the small 6 " 0 X 11" crystal since we record events when the central crystal fires. Thus the background counting rate is essentially that of the small central detector. Furthermore the central crystal is well shielded by the 1 1 " ~ X 11" annular crystal. Therefore a good S / N ratio is expected with the new design. The new type of high energy gamma radiation measuring system, called H E R M E S , has an energy resolution of 2.8% for 22.5 MeV gamma rays. The S / N ratio has been improved very much compared to conventional large NaI spectrometers.
2. The high energy gamma radiation measuring system (HERMES)
detectors, but the whole crystal is optically separated into a central 6" O X 11" cylindrical NaI(TI) crystal and an annular 11"O X 11" one surrounding the central crystal. The annular one is divided into four segments of a quarter cylinder. Each crystal is optically isolated by magnesium oxide (MgO) powder of 1.5 mm thick. The absorption due to the MgO layer is negligible for gamma rays higher than 50 keV. The central 6 " 0 x 11" crystal is viewed by a 5 " 0 ( R C A 4525) photomultiplier (PMT). Each segment of the annular crystal is viewed by two 2 " ~ (RCA 4523) PMTs. The whole NaI crystal is packed in a 3 m m thick aluminum can. The NaI(TI) crystal was manufactured by the Harshaw Chemical Co. according to our design and specification. The performance obtained for the present 6 " 0 X 11" crystal is satisfactory. The energy resolution of the central crystal is 7.1% for 137Cs 662 keV gamma rays. Uniformity of the pulse height along the length has been achieved by changing the reflection coefficient at the boundary of the NaI detector so as to give the same pulse height in different gamma source positions. The difference of the pulse height between front and rear side source positions is less than 1.6%. Energy resolutions for the four segments of the annular crystal are 8.4%, 8.7%, 8.9% and 9.0% for 662 keV 137Csgamma rays. The pulse heights of the four segments of the annular crystal are uniform within 3%.
2.1. A new large NaI(TI) detector for the H E R M E S A sectional layout of the H E R M E S is shown in fig. 1. The NaI(T1) is an 11"O X 11" cylindrical crystal. The volume is similar to conventional type large NaI
Fig. 1. Sectional layout of the HERMES. The 11"~ X l l " Nal crystal is divided into two parts. Each part is optically isolated by 1.5 mm MgO layers. Eight 2" ~ (RCA 4523) PMTs and the 5" ~ (RCA 4525) PMT are attached to the annular 11" ~ × I 1" crystal and the central 6"~ x I1" crystal, respectively. The whole NaI assembly is packed in a 3 mm thick aluminum can, Four and eight 2"~ (Hamamatsu R329) PMTs are attached to the front plastic scintillator disc and the annular plastic scintillator, respectively.
2.2. The H E R M E S The NaI detector ig surrounded by a 10 cm thick annular plastic scintillator in order to reject evems where some photons escape from the NaI detector and enter the plastic scintillator, The annular plastic scintillator was manufactured by the CI Industrial Co. It has about a l m attenuation length. It is divided into four segments, each of which is viewed by two 2 " 0 (Hamamatsu R329) PMTs. The 7 c m thick N E 110 plastic scintillators are covered by aluminum foil and are taped tight with black tape in order to shield them from light. The pulse height of the plastic scintillators is uniform within 20% over their whole area. The effective noise level of the plastic scintillators was about 40 keV for electrons. Lithium carbonate with paraffin is packed in the 15 mm gap between the NaI and the plastic scintillators. The annular plastic scintillator is covered with a 5 cm thick lead shield. A 10 cm thick lead shield is placed in front of the plastic scintillator disc in order to reject gamma rays from the target. The lead shield has a hole of 110 mm in diameter, into which a collimator is set. The collimator defines the solid angle of the H E R M E S . All lead shields are covered by 3.2 mm thick iron plates to protect them against mechanical shock. The physical layout of the H E R M E S is shown in
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2
1 1
-
-
4
Fig. 2. Physical layout of the HERMES. The goniometer has a turntable which can rotate around the target point. (I) Scattering chamber and target. (2) 2" PMTs for the plastic scintillators. (3) Main lead shield. (4) Handle for controlling the target-point-to-detector distance. (5) Rail. (6) Drive motor. (7) Goniometer, fig. 2. The whole assembly weighs about a ton. It is mounted on a reinforced carriage on a turntable. It can be rotated around the target point from 30 ° to 150° with respect to the beam when the distance from the target to the detector face is 25 cm. The side of the lead shield and the plastic scintillator disc has been removed in order to move the HERMES close to the beam line. 2.3. Signal processing
The signal processing circuit after the PMTs is shown in fig. 3. The system deals with signals from the central 6" ~ X 11" crystal, those from the annular 11" ~ X 11"
crystal, and those from the plastic scintillators. Anode signals of the PMTs for the annular and the disc plastic scintillators are actively mixed by an active mixer A. Anode signals of the PMTs for the annular NaI crystal are also mixed by another active mixer B. Anode signals of the PMT for the central NaI crystal are amplified by a fast amplifier and distributed into three output channels through a linear fan-out (428 F). One of them is amplified by a timing filter amplifier and is fed into a high level discriminator (924). The threshold of the high level discriminator is set at several MeV in order to suppress the counting rate and to prevent pulse pile-up. The logic signal from the high level discriminator opens both the linear gates for the linear signal from the central crystal and that for the linear signal from the annular crystal. The slow rise time (60 ns) of the anode signals from the NaI detector gives rise to some time walk. In order to minimize the time walk a constant fraction discriminator (CFD) is used for the high level discriminator. The gate width of the linear gate was chosen to be 400 ns. The output signals of the linear gates are amplified by spectroscopy amplifiers. The energy resolution becomes better as the gate width becomes wider until it reaches 300 ns. Thus we set the gate width at 400 ns. This is also shown in ref. 8. Since the nearly rectangular pulse with 400 ns width is amplified charge sensitively by the spectroscopy amplifier, the time walk of the gating logic signal from the high level discriminator changes the output pulse height a little. We adjusted the threshold and the external delay time of the CFD so as to make the energy resolution of the pulser peak b6tter
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Fig. 3. Schematic diagram of the singles electronics used for the HERMES. Model numbers are those of Ortec, EG&G, LeCroy and Willam&Harris models.
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than 0.6%. Signals from another output of the fan-out are amplified and are used for gain stabilization. The 4.4 MeV gamma ray from 12C, the 2.2 MeV gamma ray from the H(n, "~)D reaction or the 1.84 MeV gamma ray from the 88y source were monitored to stabilize the gain of the signal from the central 6 " ~ x 11" crystal. The gain stabilizer feeds back a correction signal to the high voltage power supply for the NaI. Output signals of the central and the annular crystals from the linear gates are individually amplified so as to .give the same overall gain. Then they are summed up together. The summed signal is analyzed by a multichannel analyzer, and routed to either an accepted spectrum or a rejected spectrum. Here the router signal is generated by a fast coincidence between the logic signal of the high level discriminator for the central 6"O X 11" crystal and the logic signal of the discriminator for the plastic scintillator. The threshold for the latter is set just above the noise level. The resolving time of this coincidence is about 70 ns. Thus the summed signals accompanied by a signal from the plastic scintillator are stored in the "rejected" spectrum and those without any leakage photons into the plastic scintillators are stored in the "accepted" spectrum. We sometimes use a pulse pile-up rejection circuit [9] when the counting rate is very high. A time-of-flight measurement with a pulsed beam is useful for separating gamma rays from neutrons whicil become serious as the projectile beam energy becomes high.
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25C
11B(p, ~'~2C ACCEPTED
3. Results 20(
3.1. Energy resolution 150
As mentioned in previous articles [4-8] the energy resolution depends on the quality of the NaI, the aperture of the collimator, the threshold of the discriminator for the plastic scintillator, the counting rate of the NaI and so on. The following points are important also for the HERMES. The first is the energy resolution of the central 6 " ~ X 11" crystal, and the second is the uniformity of the pulse height along its cylindrical axis. The third is the time walk of the timing signal from the high level discriminator which is used as the gate pulse for the linear gate. The fourth is the fraction of the gamma ray energy deposited in the central crystal. The energy resolution of the central crystal and the effect of the timing signal walk have already been discussed in the previous section. In what follows we discuss the fourth point. Accepted and rejected energy spectra of gamma rays from the I~B(p, 3,)12C reaction at Ep =7.4 MeV are shown in fig. 4. The energy resolution of the accepted spectrum for the 22.7 MeV peak is 2.8% fwhm. Fig. 5 shows the accepted and rejected spectra for the same
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measured by the central 6" O × 11" crystal for the 11B(p,"t)12C reaction at El, = 7.2 MeV. Events accompanied by a signal from the annular 11"O×11" crystal are routed to the rejected spectrum.
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reaction at Ep = 7.2 MeV by using only the central 6 " ~ X 11" NaI as the gamma ray detector and the annular l l " O × l l " NaI as the anticoincidence detector. The energy resolution for the 22.5 MeV peak is 2.7%. The two resolutions differ little. This is easily understood from the following two-dimensional measurement for the same reaction. The energy spectra of gamma rays for various energy depositions in the annular crystal are shown in fig. 6. Each bin corresponds to the energy deposited in the annular crystal. These spectra show that the energy deposited in the annular crystal is distributed in the fairly low energy region of
0.1-1 MeV. Yields of 22.5 MeV 70 peak in the accepted spectra are plotted in fig. 7 as a function of the energy [E~,(AN)] deposited in the annular crystal. The 70 yield decreases exponentially with increasing energy ET(AN). A sharp peak appears at about 500 keV. These features are explained as follows. Most of the high energy gamma rays entering the central NaI crystal give rise to electron -position pair creation. The pairs lose the kinetic energy in the central crystal and two 511 keV photons are emitted in opposite directons. The sharp 511 keV peak corresponds to the case where one of the 511 keV photons escapes from the central crystal and deposits its
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energy in the annular crystal. The probability that two 511 keV photons deposit an energy of 1022 keV in the annular crystal equals roughly the probability that one 511 keV photon penetrates through the 6 " 0 X 11" central crystal. Since this is less than 1% the 1022 keV peak is not seen in fig. 7. The exponential decrease of the Ev(AN) spectrum corresponds to the bremsstrahlung. The energy resolution of the HERMES is then given by Rs=
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(1)
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200
300
400 CHANNEL NUMBER
Fig. 8. An accepted gamma ray spectrum from the bombardment of 11 MeV 3He particles on 9Be observed at 125° to the beam axis. Labeled arrows indicate the expected positions of captures to each state in 12C. The excitation energy is given in units of MeV. gamma rays. Cosmic rays (muons) enter the NaI through the plastic scintillator depositing on energy of at least 2 MeV in it. Thus they are rejected by the anticoincidence with the plastic scintillator. Cosmic rays are actually reduced to less then 2%. Pulse pile-up is a severe problem when the counting rate of the NaI detector is high. We used a pile-up rejection circuit for measuring the 9Be(3He, 7)12C reaction. A detailed description of the circuit is presented in ref. 9. Slow neutrons, being radiatively captured, contribute to the gamma ray backgrounds. Cadmium sheets wrapped around the whole assembly of the HERMES are quite effective for suppressing these. Fast neutrons are produced by nuclear reactions at the target when the energy of the projectile is high. They enter the central NaI crystal and contribute to the background in the high energy region. They are effectively suppressed by means of a time-of-flight method with a pulsed beam. This also reduces considerably the cosmic ray background. An accepted gamma ray spectrum for the 9Be(3He, 7)~2C reaction at Een° = 11 MeV is shown in fig. 8. The background level near the 9.64 MeV 3 - peak (E v = 25 MeV) is smaller by a factor of three than that of the previous work [10]. The spectrum is well interpreted by the response described in the following section. In short, the background counts in the present HERMES are almost negligible.
3.3. Efficiency and response 3.2. Signal-to-noise ratio Main sources of backgrounds at the high energy region are background gamma rays from the beam line, cosmic rays, pile-up pulses and neutrons. The small central 6 " 0 × 11" crystal much reduces all the backgrounds, as mentioned in section 1, and the annular 11"O × 11" crystal is a good shield for background
The photopeak efficiency is defined as the ratio of photopeak yields in the measured spectru m to the number of gamma rays emitted. We deduced the absolute efficiency and the response of the HERMES by measuring gamma rays in coincidence with deuterons from the l aB(3He, d7)12C reaction. The reaction excites well the 15.1 MeV level in 12C which decays 88% by emitting the
T. Kishimoto et aL / HERMES 15.1 MeV gamma ray and the 4.4 MeV level which decays 100% by emitting the 4.4 MeV gamma ray. Therefore one gets the absolute efficiency for 15.1 MeV and 4.4 MeV gamma rays. The 19 MeV 3He beam was provided by the Osaka University variable energy 110 cm cyclotron. The deuterons were detected at 40 ° to the beam axis by a counter telescope which consists of 300 /zm and 2 m m thick surface barrier type silicon detectors. The slit of the telescope defines a solid angle of 4.5 msr. The H E R M E S was set at 90 ° to the beam axis. The distance between the detector face of the H E R M E S and the target was 130 m m because sufficient coincidence events were necessary for the measurement of the response. Smaller than 5% corrections were made for the absolute efficiency from the angular distribution measurement at angles of 67 ° , 90 ° and 113 ° . Only the coincidence events were measured in the angular distribution measurement, where the H E R M E S was located far from the target at 220 mm. The 4-dimensional data which consist of particle energy ( E + A E), particle identification signal (PI), output of the time-to-pulse-height converter (TAC) and g a m m a ray energy (Ev) were recorded event by event on a magnetic tape through the interface [11] between the computer (PDP 11/34) and ADCs. The data were analyzed after the experiment by using the computer.
118( 3He ' d "r)12C 120
:~ :E
80
0.8 "~---._
275
FITTED AREA TO TOTAL AREA RATIO
0.E
0.4
FITTED AREA EFFICIENCY
O~
04
Fig. 10. Efficiency of the HERMES. Experimentally determined ratio of fitted area to total area (upper) and fitted area efficiency to the geometrical solid angle of the collimator.The total area is composed of a full energy loss peak and a tail. We calculated the total area of the 15. I MeV gamma ray in the spectrum by summing up yields above l l MeV. Yields below l l MeV were estimated by exponentially extrapolating the tail to zero energy. However, it contributes less than 6% to the total area. Coincident gamma ray spectra are shown in fig. 9, The photopeak looks quite symmetric compared to the conventional detector. The photopeak (fitted area) efficiency to the geometrical solid angle derived from the measurement is shown in fig. 10. The attenuation due to the various materials which sit between target and NaI crystal, such as paraffin and the aluminum metal, was corrected for. The value for the H E R M E S is close to efficiencies of conventional detectors.
40
4. Concluding remarks
8
Z
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0
-
i00
'
I
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200 300 CHANNEL NUMBER
400
Fig. 9. An accepted (upper), a rejected (middle) and a fitted (lower) spectrum for 15.1 MeV gamma rays following the IJB(3He, dy)12C reaction. The 15.1 MeV gamma rays were obtained by gating deuterons feeding the 15.1 MeV level in ~2C. The fitted spectrum was obtained by subtracting 90% of the rejected spectrum from the accepted spectrum.
A new high energy gamma radiation measuring system ( H E R M E S ) has been successfully constructed. It is much superior to conventional detectors in both the energy resolution and the S / N ratio because of the new design of the large l l " O X l l " NaI(Tl) crystal. Essential is the optical separation of the central 6 " O × l l " crystal from the annular I 1"O X I l " crystal. Because of this new design the energy resolution of the H E R M E S becomes as good as that of the central 6" ~ X l l" crystal. Important is that the good energy resolution is achieved without reducing the overall efficiency much. The separation has much improved the S / N ratio. This is because we select only events accompanied by large signals from the small central crystal which is shielded well by the annular crystal. The energy resolution and the S / N ratio may be improved more by selecting events where the 511 keV energy is deposited in the annular crystal as seen in fig. 6.
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The optical division of the N a I crystal makes it possible to use the H E R M E S for Compton suppression and linear polarization detection. The C o m p t o n suppression is easily made by using the annular NaI crystal as the anticoincidence counter. The Compton suppression is useful for studying continuum gamma rays of several MeV because of its large photopeak efficiency. Here we note that the photopeak efficiency is also much improved by taking the coincidence between the central and the annular NaI crystals. For the linear polarization measurement the central NaI is used as a scatterer and the annular detector divided into four segments is used for detecting the Compton-scattered gamma rays. As the projectile energy becomes high neutron backgrounds from the target and beam line become severe. These can be effectively rejected by the time-of-flight measurement. The time resolution of the H E R M E S , however, is not very good at the moment since the 5 " ~ P M T used is not adequate for fast timing. It will soon be replaced by a P M T with a faster rise time. The H E R M E S with its fairly good energy resolution and good S / N ratio may open up research for gamma transitions from higher excited states to various low lying excited states in a wide range of nuclei. The authors wish to thank Mr. K. Tsujita for his kind help in measurement and data analysis. They are
grateful to Dr. Nagai and Dr. Okada for valuable discussions during the development of the system. Thanks are also due to Mr. Matsuoka and Mr. Higa for the cyclotron operation.
References [1] Berman, Proc. Int, Conf. on Photonuclear reactions and applications, Alismore (1973). [2] S.S. Hanna, Proc. Int. Conf. on Nuclear structure and spectroscopy, vol. 2 (1974) p. 249. [3] G.R. Satchler, Phys. Rep. 14 (1974) 97. [4] M. Suffert, W. Feldman, T. Makieux and S,S. Hanna, Nucl. Instr. and Meth. 63 (1968) I. [5] E.M. Diener, J.F. Amann, S.L. Blatt and P. Paul, Nucl. Instr. and Meth. 83 (1970) 115. [6] G. Kernel, W.W. Mason and N.W. Tanner, Nucl. Instr. and Meth. 89 (1970) 1. [7] W.F. Davidson, J.L. Black and M.R. Najam, Nucl. Phys. A168 (1971) 339. [8] M. Hashinoff, S.T. Lim, D.E. Meusday and T.L. Mulligan, Nucl. Instr. and Meth. 117 (1974) 375. [9] S. Blatt, J. Mahieux and D. Kohler, Nucl. Instr. and Meth. 60 (1969) 221. [10] H.D. Shay, R.E. Peschel, J.M. Long and D.A. Bromley, Phys. Rev. C 9 (1974) 76. [ I I] M. Noumach, K. Okada and T. Shibata, Annual Report of Laboratory of Nuclear Study Osaka University 22 (1979).