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Development of the detector system for β -decay spectroscopy at the KEK Isotope Separation System
Nuclear Instruments and Methods in Physics Research B 376 (2016) 338–340
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of the detector system for b-decay spectroscopy at the KEK Isotope Separation System S. Kimura a,b,⇑, H. Ishiyama b, H. Miyatake b, Y. Hirayama b, Y.X. Watanabe b, H.S. Jung b, M. Oyaizu b, M. Mukai a,c, S.C. Jeong d, A. Ozawa a a
Department of Physics, University of Tsukuba, Ibaraki 305-8577, Japan Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), Saitama 351-0198, Japan Nishina Center for Accelerator Based Science, RIKEN, Saitama 351-0198, Japan d Rare Isotope Science Project (RISP), Institute of Basic Science (IBS), Daejeon 305-811, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 1 September 2015 Received in revised form 20 January 2016 Accepted 25 January 2016 Available online 17 February 2016 Keywords: b-Ray telescope Lifetime measurement Low background measurement
a b s t r a c t The KEK Isotope Separation System has been developed to study the b-decay properties of the neutronrich nuclei around the neutron magic number N = 126. These properties are essential for understanding the origin of the third peak in the r-process element abundance pattern. The detector system for b-decay spectroscopy at the KEK Isotope Separation System should have high detection efficiency for low-energy b-rays, and should be operated under a low-background environment. The detector system of the KEK Isotope Separation System consists of b-ray telescopes and a tape transport system. The solid angle covered by the b-ray telescopes is as large as 75% of 4p in total. The Q b -value dependence of the detection efficiency was estimated by Geant4 simulation. The background rate was 0.09 cps using a veto counter system and Pb shields. This background rate allows us to measure the lifetime of 202Os. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The KEK Isotope Separation System (KISS) [1] has been developed to study the b-decay properties of the neutron-rich nuclei around the neutron magic number N = 126. Lifetimes of these nuclei are essential for understanding the origin of the third peak in the r-process element abundance pattern [2,3]. For producing the nuclei of interest, which are difficult to be produced by highenergy fragmentation reactions, we use multinucleon transfer (MNT) reactions between heavy ion beams and targets which have large mass numbers [4]. The KISS is an apparatus that consists of a dipole magnet and an argon gas-cell with in-source laser resonance ionization which enables to stop and to transport MNT reaction products efficiently by argon gas laminar flow. Isotope separation is performed in two steps; at first, laser resonance ionization for selection of the chemical element, followed by a mass-over-charge separation in the magnetic field. The half-lives and the Q b -values of the nuclei of interest predicted by KUTY model [5,6] are tabulated in Table 1. Most of these
⇑ Corresponding author at: Department of Physics, University of Tsukuba, Ibaraki 305-8577, Japan E-mail address: [email protected] (S. Kimura). http://dx.doi.org/10.1016/j.nimb.2016.01.041 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
nuclei have relatively low Q b -values. For this condition, a detector for b-decay spectroscopy must have sensitivity for low-energy b-rays and have a large solid angle to detect such b-rays efficiently. Fig. 1 shows the expected extraction yields of nuclei produced in a MNT reaction between 136Xe and 198Pt from the KISS. The intensity and energy of the 136Xe beam was assumed to be 10 pnA and 8 MeV/A, respectively. The thickness of the 198Pt target was assumed to be 6 mg/cm2. The cross sections were evaluated by using GRAZING code [7], and the extraction efficiency for each nuclide from the KISS was assumed to be 0.1% [8]. It is necessary that the background rate of the detector system is smaller than the extraction yields to perform lifetime measurements. These conditions require that the detector system be operated with high detection efficiency to the low-energy b-rays and under the low background environment.
2. Design of the detector system The detector system consists of b-ray telescopes and a tape transport system. A schematic view of the detector system is shown in Fig. 2. The b-ray telescopes are composed of three double-layered thin plastic scintillators; the thicknesses of the first and second layers are 1 mm and 2 mm, respectively. The solid
339
S. Kimura et al. / Nuclear Instruments and Methods in Physics Research B 376 (2016) 338–340 Table 1 Expected half-lives and Q b -values 200
Half life (s) Q b (MeV)
W
0.80 4.54
201
202
1.09 5.73
6.28 2.96
Re
Os
203
Ir
8.70 4.19
204
Pt
10.3 1.68
The Q b -value dependence of the detection efficiencies was estimated with the Geant4 simulation tool kit [9] and the results are shown in Fig. 3. The energy threshold of the first and the second layer were assumed to be 20 keVee and 40 keVee, respectively. Detection efficiencies were saturated around Q b 4 MeV, and became to be almost zero at less than Q b 0.7 MeV because most of the b-rays were stopped in the first layer. The black circle in Fig. 3 is a preliminary result of a detection efficiency measurement using b-rays fed from a 90Sr/90Y source, which is consistent with the estimated value. 3. Background of the detector system
Fig. 1. Expected extraction yields from the KISS. Details are given in the text.
The origins of the background were considered to be cosmic rays and electrons scattered by c-rays from natural radioactivity. To reduce the background originated from cosmic rays, a veto counter system was installed, which consisted of thirteen plastic scintillator slabs of 1500 mm length, 150 mm thickness and 150 mm width, and four 50-mm-wide slabs with the same length and thickness. The configuration of the plastic scintillators was designed based on the estimation with Geant4, and the vetoefficiency was estimated to be 94% for 1 GeV muon which obey a typical zenith angle distribution:
JðhÞ ¼ J0 cos2 h:
ð1Þ
In addition, to reduce the room background c-rays from natural radioactivity, shields with low-activity Pb blocks were installed surrounding the b-ray telescopes. We performed background measurements of the b-ray telescopes under three conditions; without any veto counters and Pb shields, with Pb shields, and with veto counters and Pb shields. The energy thresholds of the first and the second layers of the b-ray telescopes were set to be 20 keVee and 30 keVee, respectively. Results of the background measurements are shown in Fig. 4. The background rate without veto counters and Pb shields was measured to be 0.63 cps. With Pb shields, the background rate became 0.38 cps. Finally, we reduced the background rate to be 0.09 cps by using veto counters and Pb shields. In Fig. 4, the background energy spectra displays two main contributions, a low energy tail up to 200 keV and a broad peak located at around 400 keV. The origin of these parts are considered as environmental c-rays and cosmic rays, respectively. The Pb shields take an
angle of the b-ray telescopes is 75% of 4p. Reaction products of interest from KISS are implanted on this tape and are measured in a certain time sequence. Then those reaction products are moved away by the tape transport system for reduction of the background radiation events from daughter and granddaughter nuclei. In addition, two Ge detectors can be installed. Then the b–c spectroscopy and the particle identification by detecting the characteristic X-rays of the nuclei of interest can be available. All plastic scintillators of the b-ray telescopes are connected to photomultiplier tubes (HAMAMATSU H3178-51) installed outside of the detector chamber and coupled via light guides. All plastic scintillators and light guides were wrapped by reflection sheets which are called ‘‘LUIREMIRROR” [10] for suppressing light attenuation effects of the thin plastic scintillators. The reflectance of the ‘‘LUIREMIRROR” is 97.5%.
Detection Efficiency (%)
100 Fig. 2. Configuration of the b-ray telescopes. (a) downstream dE1st counter, (b) downstream dE2nd counter, (c) upstream dE1st counter, (d) upstream dE2nd counter, (e) tape transport system. All components indicated in here are installed in a detector chamber and are operated under vacuum.
measurement,
90
Sr /
90
Y
80
60
40
20
0 0
1
2
3
4 5 Qβ- (MeV)
6
7
8
Fig. 3. Expected detection efficiency of the b-ray telescopes. Energy thresholds of the first and the second layer were assumed to be 20 keVee and 40 keVee, respectively. A black circle indicates the result measured using b-rays from a 90 Sr/90Y source and the error is within the circle size. Detection efficiencies are limited by the solid angle of the b-ray telescopes.
340
S. Kimura et al. / Nuclear Instruments and Methods in Physics Research B 376 (2016) 338–340
particles and/or c-rays scattered by cosmic rays. However, further investigation to understand the origin of background events is required. 4. Summary and perspective
Fig. 4. Energy spectra for background events detected by an upstream dE2nd counter, without Pb shields and veto counters (magenta bars), with Pb shields (yellow bars) and with Pb shields and veto counters (light-blue bars), respectively. Each spectrum was normalized with the measuring time. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
essential role to decrease the background events of the first part, although the background events appearing in the region below 70 keV were not reduced. The background events in the region below 70 keV were significantly reduced by using the veto counters which can mainly reject cosmic rays. Therefore, those background events lower than 70 keV might be originated from
The detector system of the KISS has been installed and operated for b-decay spectroscopy. The detector system consists of three b-ray telescopes and a tape transport system. Maximum detection efficiency of the telescopes is estimated to become a geometrical acceptance of 75% at higher Q b -values than 4 MeV. The detection efficiency measured using b-rays from a 90Sr/90Y source was consistent with the estimated value. Using the veto counter system and Pb shields, the background rate for b-ray telescopes was reduced down to 0.09 cps, which satisfies the experimental condition for, e.g., the lifetime measurement of 202Os. For further reduction of the background, the investigation to understand the origins of background events is underway. In addition, we have started to develop a gas counter instead of a dE1st plastic counter, since electrons scattered by c-rays originated from natural radio activities can be significantly reduced due to a smaller amount of material [11]. Our next goal for the background rate is to reach 0.01 cps. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S.C. Jeong, et al. KEK Report 2010–2. A.I. Morales et al., Phys. Rev. Lett. 113 (2014) 022702. G. Lorusso et al., Phys. Rev. Lett. 114 (2015) 192501. Y.X. Watanabe et al., Phys. Rev. Lett. 115 (2015) 172503. H. Kokura et al., Prog. Theor. Phys. 113 (2005) 305.. A. Winther, Nucl. Phys. A 572 (191) (1994). 594, 203, (1995). Y. Hirayama, et al. these proceedings. S. Agostinelli et al., Nucl. Instrum. Meth. A 506 (2003) 250. REIKO Co:; Ltd, . E. Tanaka, Nucl. Instrum. Meth. 13 (1961) 43.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Development of the detector system for b-decay spectroscopy at the KEK Isotope Separation System S. Kimura a,b,⇑, H. Ishiyama b, H. Miyatake b, Y. Hirayama b, Y.X. Watanabe b, H.S. Jung b, M. Oyaizu b, M. Mukai a,c, S.C. Jeong d, A. Ozawa a a
Department of Physics, University of Tsukuba, Ibaraki 305-8577, Japan Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), Saitama 351-0198, Japan Nishina Center for Accelerator Based Science, RIKEN, Saitama 351-0198, Japan d Rare Isotope Science Project (RISP), Institute of Basic Science (IBS), Daejeon 305-811, Republic of Korea b c
a r t i c l e
i n f o
Article history: Received 1 September 2015 Received in revised form 20 January 2016 Accepted 25 January 2016 Available online 17 February 2016 Keywords: b-Ray telescope Lifetime measurement Low background measurement
a b s t r a c t The KEK Isotope Separation System has been developed to study the b-decay properties of the neutronrich nuclei around the neutron magic number N = 126. These properties are essential for understanding the origin of the third peak in the r-process element abundance pattern. The detector system for b-decay spectroscopy at the KEK Isotope Separation System should have high detection efficiency for low-energy b-rays, and should be operated under a low-background environment. The detector system of the KEK Isotope Separation System consists of b-ray telescopes and a tape transport system. The solid angle covered by the b-ray telescopes is as large as 75% of 4p in total. The Q b -value dependence of the detection efficiency was estimated by Geant4 simulation. The background rate was 0.09 cps using a veto counter system and Pb shields. This background rate allows us to measure the lifetime of 202Os. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The KEK Isotope Separation System (KISS) [1] has been developed to study the b-decay properties of the neutron-rich nuclei around the neutron magic number N = 126. Lifetimes of these nuclei are essential for understanding the origin of the third peak in the r-process element abundance pattern [2,3]. For producing the nuclei of interest, which are difficult to be produced by highenergy fragmentation reactions, we use multinucleon transfer (MNT) reactions between heavy ion beams and targets which have large mass numbers [4]. The KISS is an apparatus that consists of a dipole magnet and an argon gas-cell with in-source laser resonance ionization which enables to stop and to transport MNT reaction products efficiently by argon gas laminar flow. Isotope separation is performed in two steps; at first, laser resonance ionization for selection of the chemical element, followed by a mass-over-charge separation in the magnetic field. The half-lives and the Q b -values of the nuclei of interest predicted by KUTY model [5,6] are tabulated in Table 1. Most of these
⇑ Corresponding author at: Department of Physics, University of Tsukuba, Ibaraki 305-8577, Japan E-mail address: [email protected] (S. Kimura). http://dx.doi.org/10.1016/j.nimb.2016.01.041 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
nuclei have relatively low Q b -values. For this condition, a detector for b-decay spectroscopy must have sensitivity for low-energy b-rays and have a large solid angle to detect such b-rays efficiently. Fig. 1 shows the expected extraction yields of nuclei produced in a MNT reaction between 136Xe and 198Pt from the KISS. The intensity and energy of the 136Xe beam was assumed to be 10 pnA and 8 MeV/A, respectively. The thickness of the 198Pt target was assumed to be 6 mg/cm2. The cross sections were evaluated by using GRAZING code [7], and the extraction efficiency for each nuclide from the KISS was assumed to be 0.1% [8]. It is necessary that the background rate of the detector system is smaller than the extraction yields to perform lifetime measurements. These conditions require that the detector system be operated with high detection efficiency to the low-energy b-rays and under the low background environment.
2. Design of the detector system The detector system consists of b-ray telescopes and a tape transport system. A schematic view of the detector system is shown in Fig. 2. The b-ray telescopes are composed of three double-layered thin plastic scintillators; the thicknesses of the first and second layers are 1 mm and 2 mm, respectively. The solid
339
S. Kimura et al. / Nuclear Instruments and Methods in Physics Research B 376 (2016) 338–340 Table 1 Expected half-lives and Q b -values 200
Half life (s) Q b (MeV)
W
0.80 4.54
201
202
1.09 5.73
6.28 2.96
Re
Os
203
Ir
8.70 4.19
204
Pt
10.3 1.68
The Q b -value dependence of the detection efficiencies was estimated with the Geant4 simulation tool kit [9] and the results are shown in Fig. 3. The energy threshold of the first and the second layer were assumed to be 20 keVee and 40 keVee, respectively. Detection efficiencies were saturated around Q b 4 MeV, and became to be almost zero at less than Q b 0.7 MeV because most of the b-rays were stopped in the first layer. The black circle in Fig. 3 is a preliminary result of a detection efficiency measurement using b-rays fed from a 90Sr/90Y source, which is consistent with the estimated value. 3. Background of the detector system
Fig. 1. Expected extraction yields from the KISS. Details are given in the text.
The origins of the background were considered to be cosmic rays and electrons scattered by c-rays from natural radioactivity. To reduce the background originated from cosmic rays, a veto counter system was installed, which consisted of thirteen plastic scintillator slabs of 1500 mm length, 150 mm thickness and 150 mm width, and four 50-mm-wide slabs with the same length and thickness. The configuration of the plastic scintillators was designed based on the estimation with Geant4, and the vetoefficiency was estimated to be 94% for 1 GeV muon which obey a typical zenith angle distribution:
JðhÞ ¼ J0 cos2 h:
ð1Þ
In addition, to reduce the room background c-rays from natural radioactivity, shields with low-activity Pb blocks were installed surrounding the b-ray telescopes. We performed background measurements of the b-ray telescopes under three conditions; without any veto counters and Pb shields, with Pb shields, and with veto counters and Pb shields. The energy thresholds of the first and the second layers of the b-ray telescopes were set to be 20 keVee and 30 keVee, respectively. Results of the background measurements are shown in Fig. 4. The background rate without veto counters and Pb shields was measured to be 0.63 cps. With Pb shields, the background rate became 0.38 cps. Finally, we reduced the background rate to be 0.09 cps by using veto counters and Pb shields. In Fig. 4, the background energy spectra displays two main contributions, a low energy tail up to 200 keV and a broad peak located at around 400 keV. The origin of these parts are considered as environmental c-rays and cosmic rays, respectively. The Pb shields take an
angle of the b-ray telescopes is 75% of 4p. Reaction products of interest from KISS are implanted on this tape and are measured in a certain time sequence. Then those reaction products are moved away by the tape transport system for reduction of the background radiation events from daughter and granddaughter nuclei. In addition, two Ge detectors can be installed. Then the b–c spectroscopy and the particle identification by detecting the characteristic X-rays of the nuclei of interest can be available. All plastic scintillators of the b-ray telescopes are connected to photomultiplier tubes (HAMAMATSU H3178-51) installed outside of the detector chamber and coupled via light guides. All plastic scintillators and light guides were wrapped by reflection sheets which are called ‘‘LUIREMIRROR” [10] for suppressing light attenuation effects of the thin plastic scintillators. The reflectance of the ‘‘LUIREMIRROR” is 97.5%.
Detection Efficiency (%)
100 Fig. 2. Configuration of the b-ray telescopes. (a) downstream dE1st counter, (b) downstream dE2nd counter, (c) upstream dE1st counter, (d) upstream dE2nd counter, (e) tape transport system. All components indicated in here are installed in a detector chamber and are operated under vacuum.
measurement,
90
Sr /
90
Y
80
60
40
20
0 0
1
2
3
4 5 Qβ- (MeV)
6
7
8
Fig. 3. Expected detection efficiency of the b-ray telescopes. Energy thresholds of the first and the second layer were assumed to be 20 keVee and 40 keVee, respectively. A black circle indicates the result measured using b-rays from a 90 Sr/90Y source and the error is within the circle size. Detection efficiencies are limited by the solid angle of the b-ray telescopes.
340
S. Kimura et al. / Nuclear Instruments and Methods in Physics Research B 376 (2016) 338–340
particles and/or c-rays scattered by cosmic rays. However, further investigation to understand the origin of background events is required. 4. Summary and perspective
Fig. 4. Energy spectra for background events detected by an upstream dE2nd counter, without Pb shields and veto counters (magenta bars), with Pb shields (yellow bars) and with Pb shields and veto counters (light-blue bars), respectively. Each spectrum was normalized with the measuring time. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
essential role to decrease the background events of the first part, although the background events appearing in the region below 70 keV were not reduced. The background events in the region below 70 keV were significantly reduced by using the veto counters which can mainly reject cosmic rays. Therefore, those background events lower than 70 keV might be originated from
The detector system of the KISS has been installed and operated for b-decay spectroscopy. The detector system consists of three b-ray telescopes and a tape transport system. Maximum detection efficiency of the telescopes is estimated to become a geometrical acceptance of 75% at higher Q b -values than 4 MeV. The detection efficiency measured using b-rays from a 90Sr/90Y source was consistent with the estimated value. Using the veto counter system and Pb shields, the background rate for b-ray telescopes was reduced down to 0.09 cps, which satisfies the experimental condition for, e.g., the lifetime measurement of 202Os. For further reduction of the background, the investigation to understand the origins of background events is underway. In addition, we have started to develop a gas counter instead of a dE1st plastic counter, since electrons scattered by c-rays originated from natural radio activities can be significantly reduced due to a smaller amount of material [11]. Our next goal for the background rate is to reach 0.01 cps. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S.C. Jeong, et al. KEK Report 2010–2. A.I. Morales et al., Phys. Rev. Lett. 113 (2014) 022702. G. Lorusso et al., Phys. Rev. Lett. 114 (2015) 192501. Y.X. Watanabe et al., Phys. Rev. Lett. 115 (2015) 172503. H. Kokura et al., Prog. Theor. Phys. 113 (2005) 305.