- Email: [email protected]
Development of new μ–e decay counter in new multi-channel μSR spectrometer for intense pulsed muon beam
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 600 (2009) 44–46
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Development of new m–e decay counter in new multi-channel mSR spectrometer for intense pulsed muon beam D. Tomono a,, Y. Hirayama b, M. Iio a, K. Ishida a, M. Iwasaki a, H. Outa a, H. Ohnishi a, T. Matsuzaki a, Y. Matsuda a, H. Yamazaki c, J. Kasagi c, R. Klein d, S.N. Nakamura d a
RIKEN Nishina Center, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan c Laboratory of Nuclear Science (LNS), Tohoku University, 1-2-1 Mikamine, Taihaku, Sendai, Miyagi 982-0826, Japan d Department of Physics, Tohoku University, 6-3 Aoba, Aramaki, Aoba, Sendai, Miyagi 980-8576, Japan b
a r t i c l e in f o
a b s t r a c t
Available online 27 November 2008
Development of m–e decay counter for the new multi-channel mSR spectrometer at the RIKEN-RAL muon facility is presented. For the new spectrometer with an intense pulsed muon beam, finely segmented counters and compact read-out system are essential in order to increase segments and to reduce pile-up signals. In addition, to suppress background particles, the counter has an angular resolution by discriminating the light output. Feasibility test of prototype counters with the positron beam and the muon beam are performed. Based on this test, a new counter design is discussed. & 2008 Published by Elsevier B.V.
Keywords: Muon mSR Spectrometer
1. Introduction Owing to the upgradation of the proton accelerator (ISIS) at the Rutherford Appleton Laboratory (RAL) in 2008, approximately 3 104 muons are expected to be available at the RIKEN-RAL muon facility, UK [1], which is 1.5 times more intense than the present beam. Using such highly intense muon beams, we are planning to explore internal structure and dynamics in matter under various extreme conditions by the mSR (muon spin rotation, relaxation and resonance) technique. For this purpose, a new multi-channel mSR spectrometer is essential to be developed. Spectrometers are generally composed of segmented positron counters and three pairs of magnetic coils to apply the magnetic field to the sample material. The most severe source of systematic error with the highly intense beam is the distortion of the muon decay time spectrum caused by signal pile-up; more than one positron sequentially hit the counter. The straightforward solution is increasing the counter segments to reduce the incoming positron rate per counter. However, conventional counter design is not suitable for increasing counter segments. This is partly because of acrylic bulky light guides, each of which was attached to a large photomultiplier tube. They were closely spaced and occupied the magnet bore space significantly. Furthermore, in the conventional spectrometer, a pair of bulky counters is installed to trace the decay electron/positron by the coincidence method. This
Corresponding author.
E-mail address: [email protected] (D. Tomono). 0168-9002/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.nima.2008.11.067
method is another obstacle because it reduces the number of effective segments to half. Therefore, we developed a new counter to realize the compact read-out in order to increase the counter segments easily and to trace the electron/positron without the coincidence method.
2. Counter design Prototype counters were manufactured as shown in Fig. 1. The characteristics of the new counter are as follows: 1. The WSF (Kuraray, Y-11(200)MS 1 mmf) was mounted at the center of a spindle scintillator (EJ-200, square 14 14 50 mm3 or hexagon inscribed in a circle of 14 mm in diameter) as a light guide. The wavelength shifter fiber (WSF) is used as a compact light-guide of segmented counters. Only one thin WSF is enough to read out photons from the scintillator instead of the acrylic bulky light guide. Approximately 600 counters can be mounted, although the magnet bore is almost the same size as conventional spectrometers. 2. A multi-anode photomultiplier tube (MAPMT, Hamamatsu, H6558-10) is employed to increase read-out channels with small costs. Furthermore, an ultra-bialkali photo-cathode type MAPMT is used to increase the number of observed photons twice as large as the normal MAPMT in order to obtain a large pulse signal. The clear fiber intermediates between the WSF and MAPMT which, is installed far from the magnetic coils, so that the magnetic field should not deteriorate the output pulse.
ARTICLE IN PRESS D. Tomono et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 44–46
It is noted that, in the prototype counter, the WSF is directly attached to the MAPMT for simplicity so that photon collection efficiency and connection efficiency between optical fibers could be separately discussed. 3. A spindly rectangular-cylinder-shaped scintillator is employed. It enables one to observe decay positron selectively from the sample by a single counter. The spindle scintillator points to the observing sample. The decay positrons from the sample penetrate the scintillator in its longer path. On the contrary, background particles originating other than the sample penetrate it through its shorter path. Since the light out put is proportional to the traveling path length in the scintillator, these particles could be discriminated by a pulse height. If a proper threshold level for the pulse is provided, such decay positron from the target can be preferably observed. This method is used by the ARGUS spectrometer [2] with planar scintillators to have direction sensitivity of the radial direction to the beam. Based on this method, we developed twodimensional direction-sensitive counters to increase counter segments more finely.
3. Feasibility test and analysis
45
photon by measuring a single photon peak. When a positron penetrated a 50 mm scintillator, approximately 100 photons were observed including all efficiencies such as WSF absorption, fiber transmission and quantum efficiency at MAPMT. It is a large enough number to observe without amplifier if PMT gain of 106 is taken into account. Fig. 3 shows an angular dependence of observed photons. The vertical axis is normalized by the averaged number of photon at 01. The number of photons calculated from the shape of the scintillator is shown with a solid line. The calculated values are consistent with the experimental values, suggesting that output pulse height distribution is determined by the geometrical shape of the scintillator. In the prototype scintillator case, if a threshold level was set at 0.8 of the pulse height at 01, positron incidence angle was effectively restricted within 151. In the practical spectrometer, tighter constraint to a positron incidence angle is required. Therefore, narrower scintillators (10 10 50 mm3) will be employed. Its angular distribution, which was geometrically calculated is shown in Fig. 3 with a dotted line. The new counter restricts the positron incidence angle within 71. Fig. 4 shows m–e decay time spectra, which were measured at the RIKEN-RAL, Port-2 area. Muons were stopped at the silver target in the magnetic field of 50 G perpendicular to the muon
The feasibility studies were performed using a high-momentum positron beam (251 MeV/c) at the Laboratory of Nuclear Science, Tohoku University (LNS-Tohoku) [3] and subsequently performed by using a m–e decay positron in the same condition as the actual mSR spectrometer at RIKEN-RAL [1]. The highmomentum positron beam is suitable for the confirmation of principle since the beam penetrates the scintillator with less multiple scattering. A schematic figure of the test setup at LNS Tohoku is shown in Fig. 2. A positron beam penetrated a 16channel counter bundle and two trigger counters. Since the positron beams are parallel in 11, beam hit position and the beam injection angle could be defined by two trigger counters. The counter bundle was tilted at angles of 01, 51, 101, 171 and 901 and then the scintillation light was measured by the ADC module. Fig. 3 inset shows photon distribution in the typical counter. The vertical axis is scaled from the ADC channel to the number of
Fig. 1. Schematic figure of the prototype counter. The WSF was mounted at the center of the scintillator. The MAPMT was directly attached at the opposite end of the WSF for simplicity.
Fig. 3. Angular dependence of the observed photon yield and photon distribution of a typical counter at 01 in the inset. The vertical axis is normalized by the number of photon at 01. The observed data points are shown with open circles and their bars show rms of each ADC peak. A solid line shows the results of the geometrical calculation in the case of a 14 14 50 mm3 scintillator and a dashed line shows the result in the case of a 10 10 50 mm3.
Fig. 2. Schematic figure of the setup at the LNS Tohoku.
ARTICLE IN PRESS 46
D. Tomono et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 44–46
Table 1 Design values in prototype and practical counters. Scintillator
Prototype counter EJ-200 covered with TiO2 sheets
Practical counter (preliminary) Extruding plastic scintillator
Scintillator size (mm3) Angular resolution (calculation) Optical fiber Fiber length (m) Observed number of photons Photomultiplier tube
14 14 50
10 10 50
15
7
WSF 0.75 100
WSF+clear fiber 2.5 60 (preliminary, including connection efficiency) H6558-200-10 (ultra-bialkali type)
H6558-10
design and the preliminary practical counter design. The total number of segments is 606, which is three times larger than the present spectrometer. The counter covers approximately 26% of the sample in solid angle. It has sensitivity to view only a region in 24 mm radius around the target. Fig. 4. Muon decay time spectra in the transverse magnetic field of 50 G.
5. Summary spin. Decay positrons were observed by counters installed to the right and left sides of the target. The amplitude of the spin rotation is obtained to be approximately 24% of initial polarization. This is consistent with that observed in the ARGUS spectrometer [2]. If the threshold level of the output pulse is set at 0.7 as shown in Fig. 3, the constant background level is suppressed at least below 10 4 of the statistics at time zero in this total statistics.
4. New spectrometer Based on these results, the counter of the new spectrometer is designed. Table 1 shows the summary of the prototype counter
The counter performance is as we expected and the time spectrum of decay positron is successfully observed without severe distortion. At present, construction of the new spectrometer is under way at RIKEN-RAL. This technique will also be applicable to a spectrometer with the same kind of pulsed muon beam. We thank the staff of LNS-Tohoku for experimental assistance. References [1] T. Matsuzaki, et al., Nucl. Instr. and Meth. A 465 (2001) 365. [2] R. Kadono, et al., RIKEN Accel. Prog. Rep. 29 (1996) 196. [3] H. Yamazaki, et al., Nucl. Inst. and Meth. A 536 (2005) 70.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Development of new m–e decay counter in new multi-channel mSR spectrometer for intense pulsed muon beam D. Tomono a,, Y. Hirayama b, M. Iio a, K. Ishida a, M. Iwasaki a, H. Outa a, H. Ohnishi a, T. Matsuzaki a, Y. Matsuda a, H. Yamazaki c, J. Kasagi c, R. Klein d, S.N. Nakamura d a
RIKEN Nishina Center, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan c Laboratory of Nuclear Science (LNS), Tohoku University, 1-2-1 Mikamine, Taihaku, Sendai, Miyagi 982-0826, Japan d Department of Physics, Tohoku University, 6-3 Aoba, Aramaki, Aoba, Sendai, Miyagi 980-8576, Japan b
a r t i c l e in f o
a b s t r a c t
Available online 27 November 2008
Development of m–e decay counter for the new multi-channel mSR spectrometer at the RIKEN-RAL muon facility is presented. For the new spectrometer with an intense pulsed muon beam, finely segmented counters and compact read-out system are essential in order to increase segments and to reduce pile-up signals. In addition, to suppress background particles, the counter has an angular resolution by discriminating the light output. Feasibility test of prototype counters with the positron beam and the muon beam are performed. Based on this test, a new counter design is discussed. & 2008 Published by Elsevier B.V.
Keywords: Muon mSR Spectrometer
1. Introduction Owing to the upgradation of the proton accelerator (ISIS) at the Rutherford Appleton Laboratory (RAL) in 2008, approximately 3 104 muons are expected to be available at the RIKEN-RAL muon facility, UK [1], which is 1.5 times more intense than the present beam. Using such highly intense muon beams, we are planning to explore internal structure and dynamics in matter under various extreme conditions by the mSR (muon spin rotation, relaxation and resonance) technique. For this purpose, a new multi-channel mSR spectrometer is essential to be developed. Spectrometers are generally composed of segmented positron counters and three pairs of magnetic coils to apply the magnetic field to the sample material. The most severe source of systematic error with the highly intense beam is the distortion of the muon decay time spectrum caused by signal pile-up; more than one positron sequentially hit the counter. The straightforward solution is increasing the counter segments to reduce the incoming positron rate per counter. However, conventional counter design is not suitable for increasing counter segments. This is partly because of acrylic bulky light guides, each of which was attached to a large photomultiplier tube. They were closely spaced and occupied the magnet bore space significantly. Furthermore, in the conventional spectrometer, a pair of bulky counters is installed to trace the decay electron/positron by the coincidence method. This
Corresponding author.
E-mail address: [email protected] (D. Tomono). 0168-9002/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.nima.2008.11.067
method is another obstacle because it reduces the number of effective segments to half. Therefore, we developed a new counter to realize the compact read-out in order to increase the counter segments easily and to trace the electron/positron without the coincidence method.
2. Counter design Prototype counters were manufactured as shown in Fig. 1. The characteristics of the new counter are as follows: 1. The WSF (Kuraray, Y-11(200)MS 1 mmf) was mounted at the center of a spindle scintillator (EJ-200, square 14 14 50 mm3 or hexagon inscribed in a circle of 14 mm in diameter) as a light guide. The wavelength shifter fiber (WSF) is used as a compact light-guide of segmented counters. Only one thin WSF is enough to read out photons from the scintillator instead of the acrylic bulky light guide. Approximately 600 counters can be mounted, although the magnet bore is almost the same size as conventional spectrometers. 2. A multi-anode photomultiplier tube (MAPMT, Hamamatsu, H6558-10) is employed to increase read-out channels with small costs. Furthermore, an ultra-bialkali photo-cathode type MAPMT is used to increase the number of observed photons twice as large as the normal MAPMT in order to obtain a large pulse signal. The clear fiber intermediates between the WSF and MAPMT which, is installed far from the magnetic coils, so that the magnetic field should not deteriorate the output pulse.
ARTICLE IN PRESS D. Tomono et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 44–46
It is noted that, in the prototype counter, the WSF is directly attached to the MAPMT for simplicity so that photon collection efficiency and connection efficiency between optical fibers could be separately discussed. 3. A spindly rectangular-cylinder-shaped scintillator is employed. It enables one to observe decay positron selectively from the sample by a single counter. The spindle scintillator points to the observing sample. The decay positrons from the sample penetrate the scintillator in its longer path. On the contrary, background particles originating other than the sample penetrate it through its shorter path. Since the light out put is proportional to the traveling path length in the scintillator, these particles could be discriminated by a pulse height. If a proper threshold level for the pulse is provided, such decay positron from the target can be preferably observed. This method is used by the ARGUS spectrometer [2] with planar scintillators to have direction sensitivity of the radial direction to the beam. Based on this method, we developed twodimensional direction-sensitive counters to increase counter segments more finely.
3. Feasibility test and analysis
45
photon by measuring a single photon peak. When a positron penetrated a 50 mm scintillator, approximately 100 photons were observed including all efficiencies such as WSF absorption, fiber transmission and quantum efficiency at MAPMT. It is a large enough number to observe without amplifier if PMT gain of 106 is taken into account. Fig. 3 shows an angular dependence of observed photons. The vertical axis is normalized by the averaged number of photon at 01. The number of photons calculated from the shape of the scintillator is shown with a solid line. The calculated values are consistent with the experimental values, suggesting that output pulse height distribution is determined by the geometrical shape of the scintillator. In the prototype scintillator case, if a threshold level was set at 0.8 of the pulse height at 01, positron incidence angle was effectively restricted within 151. In the practical spectrometer, tighter constraint to a positron incidence angle is required. Therefore, narrower scintillators (10 10 50 mm3) will be employed. Its angular distribution, which was geometrically calculated is shown in Fig. 3 with a dotted line. The new counter restricts the positron incidence angle within 71. Fig. 4 shows m–e decay time spectra, which were measured at the RIKEN-RAL, Port-2 area. Muons were stopped at the silver target in the magnetic field of 50 G perpendicular to the muon
The feasibility studies were performed using a high-momentum positron beam (251 MeV/c) at the Laboratory of Nuclear Science, Tohoku University (LNS-Tohoku) [3] and subsequently performed by using a m–e decay positron in the same condition as the actual mSR spectrometer at RIKEN-RAL [1]. The highmomentum positron beam is suitable for the confirmation of principle since the beam penetrates the scintillator with less multiple scattering. A schematic figure of the test setup at LNS Tohoku is shown in Fig. 2. A positron beam penetrated a 16channel counter bundle and two trigger counters. Since the positron beams are parallel in 11, beam hit position and the beam injection angle could be defined by two trigger counters. The counter bundle was tilted at angles of 01, 51, 101, 171 and 901 and then the scintillation light was measured by the ADC module. Fig. 3 inset shows photon distribution in the typical counter. The vertical axis is scaled from the ADC channel to the number of
Fig. 1. Schematic figure of the prototype counter. The WSF was mounted at the center of the scintillator. The MAPMT was directly attached at the opposite end of the WSF for simplicity.
Fig. 3. Angular dependence of the observed photon yield and photon distribution of a typical counter at 01 in the inset. The vertical axis is normalized by the number of photon at 01. The observed data points are shown with open circles and their bars show rms of each ADC peak. A solid line shows the results of the geometrical calculation in the case of a 14 14 50 mm3 scintillator and a dashed line shows the result in the case of a 10 10 50 mm3.
Fig. 2. Schematic figure of the setup at the LNS Tohoku.
ARTICLE IN PRESS 46
D. Tomono et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 44–46
Table 1 Design values in prototype and practical counters. Scintillator
Prototype counter EJ-200 covered with TiO2 sheets
Practical counter (preliminary) Extruding plastic scintillator
Scintillator size (mm3) Angular resolution (calculation) Optical fiber Fiber length (m) Observed number of photons Photomultiplier tube
14 14 50
10 10 50
15
7
WSF 0.75 100
WSF+clear fiber 2.5 60 (preliminary, including connection efficiency) H6558-200-10 (ultra-bialkali type)
H6558-10
design and the preliminary practical counter design. The total number of segments is 606, which is three times larger than the present spectrometer. The counter covers approximately 26% of the sample in solid angle. It has sensitivity to view only a region in 24 mm radius around the target. Fig. 4. Muon decay time spectra in the transverse magnetic field of 50 G.
5. Summary spin. Decay positrons were observed by counters installed to the right and left sides of the target. The amplitude of the spin rotation is obtained to be approximately 24% of initial polarization. This is consistent with that observed in the ARGUS spectrometer [2]. If the threshold level of the output pulse is set at 0.7 as shown in Fig. 3, the constant background level is suppressed at least below 10 4 of the statistics at time zero in this total statistics.
4. New spectrometer Based on these results, the counter of the new spectrometer is designed. Table 1 shows the summary of the prototype counter
The counter performance is as we expected and the time spectrum of decay positron is successfully observed without severe distortion. At present, construction of the new spectrometer is under way at RIKEN-RAL. This technique will also be applicable to a spectrometer with the same kind of pulsed muon beam. We thank the staff of LNS-Tohoku for experimental assistance. References [1] T. Matsuzaki, et al., Nucl. Instr. and Meth. A 465 (2001) 365. [2] R. Kadono, et al., RIKEN Accel. Prog. Rep. 29 (1996) 196. [3] H. Yamazaki, et al., Nucl. Inst. and Meth. A 536 (2005) 70.