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Construction and performance of 128-channel segmented pulsed μSR and μe detection system at Dai Omega
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Nuclear Instruments and Methods in Physics Research A 554 (2005) 201–211 www.elsevier.com/locate/nima
Construction and performance of 128-channel segmented pulsed mSR and me detection system at Dai Omega H.K.M. Tanakaa,b,, K. Nagaminea,b,c, H. Miyaderab,d, K. Shimomurab, Y. Ikedob, K. Nishiyamab,d, K. Ishidae a Physics Department, University of California, Riverside (UCR), CA 92521, USA Muon Science Laboratory, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan c Atomic Physics Laboratory, Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan d Department of Physics, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan e Meson Science Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan b
Received 6 June 2005; received in revised form 3 August 2005; accepted 9 August 2005 Available online 30 August 2005
Abstract In order to perform mSR and me measurements with an instantaneously intense pulsed 4-MeV muon beam currently available at Dai Omega, which is a large solid-angle-axial focusing superconducting surface muon channel at KEKMSL, the detector segmentation in the conventional digital technique was improved to accept the predicted event rate of 2.0 104 muons/spill. A new detection system was designed to collect intense me decay positrons with 128 two-fold coincidence detector elements. A major feature of the new detection system was an employment of multi-anode phototubes (MAPMT) to make the detection system be compact. We mitigated the problems related cross talks among channels by employing a special fiber holder. The performance of the detection system was confirmed by mSR and me test experiments. r 2005 Elsevier B.V. All rights reserved. PACS: 29.30.Aj; 29.27.Hj; 29.40.Mc Keywords: Pulsed muon beam; Multi-anode phototube; Muon spin rotation; Muonium
1. Introduction Corresponding author. Physics Department, University of
California, Riverside (UCR), CA 92521, USA. Tel.: 951 827 5331; fax: 951 827 4529. E-mail address: [email protected] (H.K.M. Tanaka).
In 1978, the first pulsed mSR (muon spin rotation, relaxation, and resonance) was observed. There they employed a digital method with a single telescope of two Cherenkov counters and an eightchannel segmented plastic scintillation detection
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.051
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system [1]. The mSR spectrum observed for positive muons exhibited the correct precession pattern. In 1997, a high-segmented positrondetection system, ‘‘ARGUS (Advanced Riken General-purpose mSR Spectrometer)’’, was constructed and installed at Port 2 of RIKEN-RAL (The Institute of Physical and Chemical Research—Rutherford Appleton Laboratory) [2]. In our measurements for the muon beam transported through Dai Omega, which is a large solid-angle-axial focusing superconducting surface muon channel at KEK-MSL, we faced the following basic important problems before we constructing a mSR spectrometer: (1) high instantaneous rate of 5 104 muon events/pulse, (2) high radiation background of 5 103 events/ pulse; the primary proton beam causes radiation background from the production target at the focusing point after passing through a 4 m long shield, which is made of iron, lead, concrete, and boride shields, as shown in Fig. 1. Among various background particles, beamassociated neutron background was reduced to 5 102 events/pulse by a shield of boride blocks (more than 5.0 cm thick),
(3) residual beam-related positron background; the residual positron background was discriminated by using a separation system [3], (4) leakage magnetic field from Dai Omega itself; a magnetic field of 170 G along the beam direction at the target sample region; an iron plate with a thickness of 1.0 cm was installed to reduce it to 10–20 G. The residual magnetic field was cancelled by recently constructed magnet systems [4]. At Dai omega, in order to overcome a restriction of the data-accumulation rate of multi-stop TDCs, the detection system was designed to collect a high intensity of me decay positrons with 128 coincidence detector elements. The present report intends to be a complete description of the design of the highly segmented detection system installed at Dai Omega.
2. Experimental requirements The essential features of the mSR experiments are very similar , and the approach is quite simple. Beam stopping at a sample target and detection techniques of decay positrons vary among the
Fig. 1. Side view of Dai Omega. Dai Omega consists of four large-aperture super conducting coils. Dai Omega is directly connected to the proton beam line.
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experiments, but several systematic issues were common to all. Positive muons are brought to rest in a sample target (either one at a time (DC beam) or in a small bunch (pulsed beam)), and decay within a few microseconds ðtm ¼ 2:2 msÞ. The two neutrinos escape unobserved, but the decay positron can be detected by a detection system, which surrounds the stopping target. The muon precession in a sample target can change with time the angular distribution of decay positrons. The time of the positron detection is recorded by a multistop TDC. In operation of a multistop TDC the sweep is started by a user-supplied trigger pulse. Then subsequent events detected at the stop input are recorded, each in a specific time bin corresponding to the time of arrival relative to the start pulse. Compared to non-multihit devices, the multihit TDC can accept a new stop event after a prior event. The average number of positrons striking an individual detector element per cycle must be less than the maximum depth that is restricted by the ability of a multistop TDC and is usually less than 5–6. This hurdle, coupled with the very difficult funding climate for a highly segmented mSR counting system, has discouraged us from constructing a conventional full-equipped counting system, such as ARGUS. The list below includes many of the main and general conclusions, which are important in the design.
each viewed by MAPMTs. The advantage of the scintillator is its large light output and relatively short signal duration.
3. Detector design The mSR detection system was designed with 128 independent coincident elements. Each counter consists of two plastic scintillators in the shapes of a solid rectangle with a size of 40 mm (L) 12 mm (W) 12 mm (H). UV-Absorbing (UVA) tapered clear acrylic fibers (clear fibers) were inserted manually into the inner plastic scintillator as shown in Fig. 2. Clear fibers allow the transport of scintillation light over a meter to the MAPMTs without significant attenuation. The UVA clear fibers unwant Cherenkov light from introducing an energy-dependent additional light source to this counter. The light from the scintillator is transported to a MAPMT through a solid, clear fibers whose length is 1000 mm that is sufficient to transport photons from a scintillator. The uninserted ends of the clear fibers were connected to a multi-anode phototube. The outer scintillator is coupled to the MAPMT through a UVA acrylic lightguide attached at right angles to
The detector should be designed to be as symmetric for a sample target as possible. A coincidence system should exist for each decay positron measurement. This ensures identification of decay positrons with a low beam-related background. High segmentation requires highly concentrated PMTs, such as multi-anode phototubes (MAPMTs). The light transmission through the light guides must be efficient.
We have considered several detector schemes and geometries to address these demands, and have settled on a design, which includes coincident timing elements made of fast plastic scintillators,
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Fig. 2. Closer view of a counter system.
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one of the 2.0 cm long triangular edges. The end of the light guide has a 451bevel, which is mirrored in order to reflect scintillation light into the clear fibers as shown in Fig. 2. Each light-guide end is held against the face of the multi-anode phototube by a black-color fiber holder in order to illuminate one of 16 pixels. The fiber-phototube junction was covered with a black silicon rubber to reduce the light leakage level to below 10 events/min. Fig. 3 shows the overall geometrical structure containing 128 coincident elements. The area of each counter is 4.8 cm2. The inner radius is 10 cm. The outer radius is 12.6 cm. The 128 counter pairs therefore occupy approximately 5% of all the solid angle. An overall mechanical support system holds the MAPMT housings, the clear fibers, and 128 coincident elements. 3.1. PMT and scintillators The choice of the PMT is motivated by a desire to find a compact and highly concentrated tube with good gain and minimum time jitter. An excellent candidate PMT is the Hamamatsu R6427 multi-anode phototube. It is a 25 25 mm2 square 12-stage head-on multi-anode tube with a gain of 3 106 and a maximum quantum efficiency of
approximately 25%. This tube contains 16 individual anodes in a single vacuum envelope. The anodes are arranged in a 4 4 array and have an effective area of 4 4 mm2 on a 4.5 mm pitch as shown in Fig. 4. The 10–90% rise time is 1.7 ns and the transit time jitter for single photoelectrons has a standard deviation of about 200 ps. The cost of the tube is around $100/channel. The MAPMTs allow a compact design of the spectrometer, as sown in Fig. 4. The phototubes (with a size of 25.7 (L) 25.7 (W) 45 (H) mm3) were placed at the corners of the detector holder. The typical yield from the clear fibers was around three-times larger than the single photoelectron level. We measured the magnitude of the light attenuation through clear fibers with a checking source of 106Ru. The observed pulse height was Eb100–200 mV for b-ray signals and ETh 20 mV for thermal noise at an operation voltage of 800 V. A uniformity of the response of each counter was also measured. The counting rate varied by 20% among each counter. An aluminum ring of approximately 6 mm in thickness was inserted between the inner and outer counters to eliminate the low-energy backgrounds. The decay positrons scattered through the aluminum ring are rejected by the coincidence. A Geant
Fig. 3. Schematic view of a multi-anode phototube.
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Fig. 4. Overall geometrical structure of mSR detection system; drawing (left); photograph (right).
3.2. Crosstalk
2x104 None Acryl 1.0 cm Acryl 1.0 cm + Al 1.0 cm Acryl 1.0 cm + Al 2.0 cm
Event
1.5x104
1x104
5000
0 0
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Fig. 5. Geant-4 Monte Carlo simulation results, showing the energy cutoff of the decay positrons passing through aluminium plates with a thickness of 1.0 and 2.0 cm.
4 detector simulation shows that the positrons with energies less than 20 MeV are practically cut as shown in Fig. 5. The phototubes were placed at the corners of the detector holder. In order to reduce the gain loss of the MAPMT by the leakage magnetic field from Dai Omega, each phototube was placed in an iron cylinder (10 mm in thickness). We measured the magnetic field of less than 1.0 G in the cylinder. The longer axis of phototubes was aligned with the external field.
There are two types of crosstalk among channels in a multi-anode phototube. (a) External crosstalk; optical crosstalk occurs due to the divergence of light as it passes through the fibers, and (b) intrinsic crosstalk; electronic crosstalk occurs after the photocathode has produced an electron via the photoelectric effect, and the electron is being steered toward a dynode chain. The electron then misses dynode slit and hits the metal plate around the dynode causing it to hop a distance less than, or equal to, the cathode to the first dynode separation (4 mm). The electron then begins the multiplication process in an incorrect dynode chain, leading to a signal in an incorrect anode. The latter type of the crosstalk can be reduced by adjusting the discrimination level. The typical pulse height of this type of the crosstalk is ten times smaller than the usual signal pulse height. A special fiber holder was designed to absorb the divergence of light to reduce the former type of corsstalk, as shown in Fig. 6. The effectiveness of this fiber holder was tested and found to reduce the former type of the crosstalk to less than 5%. 3.3. Electronics and data acquisition In order to perform a high-density time to digital conversion, a multi-stop TDC (Le Croy; model 3377) was employed. The on-line
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4. lSR test experiment
Fig. 6. Special lightguide holder designed to reduce optical cross talks.
monitoring/controlling system on the PC was accomplished by the CAMAC data acquisition system EXP 2000 [5], which controls the auxiliary crate controller (ACC; Kinetics K 3976), the CAMAC-SCSI interface (CC; Kinetics K 3929) and the PAW++ data analysis software [6]. The task of the front-end electronics is to provide timing information for signals produced by decay positrons in the coincident elements. We require that our system be able to read out 256 channels of phototube signals, one signal from each of the 128 coincident elements. Block diagrams of the electronics are shown in Fig. 7. An analog waveform from the MAPMT is converted into an NIM signal by a discriminator (KAIZU KN246 Octal Discriminator). Coincidence signals from a counter pair are sent to a multi-stop TDC by converting from a NIM pulse to an emitter-coupled logic (ECL) pulse. A proper equipment (Technoland; N-TM 317) was available to make this conversion. We used the signal from the pulsed kicker magnet to trigger the data acquisition at a sequence of times (2.0 ms) before the muon injection. We measured the total data rate less than 32,000 bits/ spill. This amount of data can be transferred through an ordinary SCSI BUS to the Linux operating system. The data analysis software PAW++ creates a set of histogram for the timing of decay positrons. The counting rate of the decay positron from the sample target was 10,000/spill before the coincidence and 600/spill after the coincidence.
The surface muons, which are highly polarized with their spin anti-parallel to their momentum direction, arrive at a target. If some net average polarization (S) remains after the muons have stopped in the target, the spin will precess with angular frequency o along an axis parallel to any existing magnetic field B. The precession frequency is proportional to B and to the muon’s g-factor. For a free muon, the precession rate is o/ B ¼ 8.5 108 rad/sT, which corresponds to a revolution frequency of 13.5 kHz/G. If S and o are both non-zero, the precession of the muon spins will produce a change in the angular distribution of the decay positrons and, consequently, a change with time in the number of counts registered in any element of the detection system. The muon spin precession therefore alters the expected pure exponential shape of the decay, since muons stopped in the sample target with their spin vectors initially aligned anti-parallel to the beam direction. To understand how the amplitude of spin precession is affected, we divide the contribution according to two major items, which will influence the result. They are: 1. polarization of the muon beam, 2. The residual decay positrons. It is well-known that m+ depolarization rate is below 0.01 m/s in an aluminum target at room temperature. We measured the muon spin precession for a certain magnetic field strength in an aluminum target. The obtained mSR time spectrum is shown in Fig. 8 for a transverse magnetic field strength of 2071 G. The spectrum in Fig. 8 was obtained by taking the ratio between the decay spectrum as obtained with the upstream detectors and that with the downstream detectors. This ratio defines an asymmetry of the mSR spectra. The correction causes large statistical fluctuations in the third and forth peaks. An asymmetry of 20% reflects the S/N ratio that is consistent with the conventional mSR spectra [7]. The frequency of the muon spin precession is 3.570.2 ms. The result is consistent with the applied magnetic field and the muon’s g-factor. The observed S/N ratio was 106
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Fig. 7. Diagram of the electronics and computer system used for the present detection system.
at time zero and 102 at t ¼ 10 ms. Magnetism of the microorganism originated sedimentary rock laid down around 3.0 billion years ago was investigated with the present spectrometer [8].
5. le test experiment and its application to the thermal Mu production in mesoporous silica The generation of thermal Muonium (Mu) is a basic technique to produce ultra slow muon [9,10] for future muon accelerators. In the first observation of muonium by Hughes et al. [11], polarized muons were stopped in a target of argon gas at 50atm pressure, and identification was made by observing the characteristic Larmor precession frequency of muonium. Although this technique
produces a large yield of muonium, (8579%)of incident muons [12], it is recognized that for the purpose of the production of ultra slow muons, muonium have to be observed in the vacuum. In order to produce muonium in the vacuum, muonium production has been studied with a fine powder of fumed silica [13,14]. The formation of muonium in fumed silica is interpreted as a three stage process [15]: (1) Positive muons stop in the silica particles forming muonium; (2) The fraction of muos which form muonium inside the silica powder is assumed to be the same as the muonium production fraction in bulk silica of 6173% [16,17]; and (3) The muonium atoms diffuse to the surface of the particles and emerge into the vacuum. Thus, it is recognized that a specific surface area is the key factor to determine the
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+ in Al H = 20G
Asymmetry
1.2
1
Fig. 9. Conceptual view of MCM-48 meorposous silica. A thin amorphous wall followed a gyroid surface. 0.8
2000
4000
6000
8000
10000
12000
14000
time (ns) Fig. 8. Muon spin rotation time spectrum observed at Dai Omega. The external transverse field was 20 G at room temperature (bin width 16 ns). A solid line is a sinusoidal curve corresponding to the frequency of the muon spin precession is 3.5 ms with a baseline located at an asymmetry of 1.05.
amount of the muonium emitted to the vacuum. The typical fumed silica has a specific surface area of between 200 and 400 m2/g depending on the grade. For this experiment, a new type of mesoporous silica MCM-48 with a specific surface area of 1500 m2/g was used. An improvement on the muonium yield was expected. The MCM-48 ordered mesoporous silica (The porous silica having an ordering on submicron pores) is a member of the M41S silica family first synthesized by Kresge et al. [18]. MCM-48 is a large pore (20 A˚) molecular sieve with two independent interwoven three-dimensional channels as shown in Fig. 9. All structural observations of the MCM-48 silica are explained by assuming that the thin amorphous wall followed by the gyroid (G) surface. Owing to its pore structure, the cubic mesophase, known as a space group of Ia3d has a large surface area of 1500 m2. The sample solution is a mixture of Cabo-Sil silica, trimethyleammonium hydrooxyde (TMAOH), cetyltrimethylammonium bromide (CTAB), and water. The samples were synthesized in an autoclave at
130 1C for 40 h [19]. The obtained materials were filtered, washed, and calcined at 540 1C. The nitrogen gas absorption spectrum and powder Xray diffraction image showed a typical feature of MCM-48. Twenty milligram of MCM-48 powder was dissolved in acetone allowing a natural sedimentation process to form a uniform sample layer of 10 mm in thickness on an aluminum foil of 15 mm in thickness. The sample was then attached to a tungsten degrader of 50 mm in thickness. The thickness of the tungsten foil was optimized for 4-MeV muon penetration through the tungsten degrader and stopping in the MCM-48 sample layer [3]. Geant 4 simulations were carried out to estimate the stopping distribution in the target. 4.22% of the incident muons stop in the MCM-48 target. Likewise, 250 mg of MCM-48 powder was used to form a uniform sample layer of 125 mm in thickness on an aluminum foil of 45 mm in thickness. The sample was then attached to a tungsten degrader of 25 mm in thickness. Geant 4 simulations were carried out to estimate the stopping distribution in the target. The results are shown in Fig. 10. This target is a poor producer of muonium at a beam momentum of 29.7 MeV/c because the muon stopped in the sample per incident muon is only 0.55%. However, 15.3% of the incident muon stops when the incident momentum is lowered by 5%. A muonium observation setup was constructed at Dai Omega. As shown in Fig. 11, the key feature of the setup is using mSR detector for the
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Stopping Distribution 160 100%
140
95%
70 Number of Muon (counts)
Number of Muon (counts)
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0 0.5
-0 (B)
15.3% 0.4
0.3
0.2
0.1
-0
MCM-48 Sample Thickness (mm)
Fig. 10. Simulation results of the stopping distributions of the muon for different incident momenta in the MCM-48 sample layer after passing thorugh the sample substrate with; (A) 100% of the incident momentum (29.7 MeV/c); (B) 95% of the incident momentum (28.2 MeV/c).
efficient detection of decay positron from muonium. The up-stream and down-stream detector of the mSR spectrometer was set to observe 5–15 and 25–35 cm region from the sample target, respectively. The vacuum chamber was made of stainless steal (SUS 304) and the inside surface of the chamber was electrolytically polished to reduce the surface area. A turbo molecular vacuum pump was connected directly to the vacuum chamber. Two ports, ICF70, were used for the oxygen leak and for a vacuum gage. The design of the vacuum chamber is shown in Fig. 11. The pressure in the vacuum chamber was 107–108 Torr during the measurements. The radiation shieldings furnished on the vacuum chamber were made of lead. Two beam collimators were installed inside the vacuum chamber, one at the up-stream and another at the down-stream of the sample target. Both collimators were made of heat-resistant stainless steel (SUS 310). The up-stream collimator collimated the beam size to f ¼ 35 mm in diameter. The down-stream collimator cut the scattered energetic muons so as not to stop them near the up-stream detectors. The yield of the emission of the muonium from the sample target in the vacuum
was normalized to that in the 1-Torr oxygen gas. The diffusion length of the muonium atom at 300 K in the oxygen gas of 1 Torr at 300 K within the lifetime of muonium of 2.2 106 s is 9.95 104 m. Decay positrons from the sample region, are shielded by a lead collimator, as shown in Fig. 11. Fig. 12 shows the obtained muonium yield for a sample thickness of 125 and 10 mm. The Mu-TOF spectra for the 125-mm thick sample were obtained by lowering the momentum of the incident muon beam by 5%. Subtractions of the data in the vacuum from the data in the oxygen gas were taken. Since muonium is a neutral particle, a particle selection of the emitted particles can be easily performed, by applying a certain electric field. The electric potential of the tungsten foil was switched by data acquisition system so as to be 0 and +6 V in order to confirm whether or not the emitted particle is a charged particle. The TOF signals of the thermal muonium emitted from a hot tungsten foil (T ¼ 2000 K) [3] are also plotted in Fig. 12. The muonium emitted from MCM-48 arrives the detection region 2.6 times slower than the one from a tungsten foil. The result is
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Fig. 11. Experimental setup taken for the present thermal muonium production experiment.
50 Number of Mu (/1M muon microses)
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0-10 micro sec Hot W Thick MCM-48 Thin MCM-48
40
908ppm/ incident muon 2632ppm/ incident muon 405ppm/ incident muon
0.18%/stopped muon 1.72%/stopped muon 0.96%/stopped muon
30 Thick MCM-48
20 Hot W
10 0 0 -10
1
2
3
4
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6
7
8
9
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Thin MCM-48
-20 Time (microsec) Fig. 12. Time spectra of the absolute values of the thermal muonium produced from different materials.
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consistent with the temperature of MCM-48 (T ¼ 300 K). The production rate of the thermal muonium was estimated within the time range between 0 and 10 ms. The obtained values are 405 ppm/(incident muon) and 2632 ppm/(incident muon) (0.96% /stopped muon and 1.72%/stopped muon) for 10-mm thick sample and 125-mm thick sample, respectively. Our yield is consistent with the previously reported value [15] measured with the different target (fumed silica) and beam parameters.
6. Discussion As a conclusion of the mSR and me measurements using the highly segmented detection system at Dai Omega, we found several unsolved problems that should be answered in the nearest future.
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Acknowledgements This work was supported by a Grant in Aid from the Japanese Society of Promotion of Science (JSPS) and Core to Core Program of JSPS. We are indebted to Mr. K. Fukuchi for his contributions to the construction and operation of Dai omega. We thank Professor Y. Hamano for his meaningful discussions about detrital remnant magnetization in banded iron formation. Professor A.P. Mills Jr. is acknowledged for valuable discussions focused on the thermal muonium production in mesoporous silica. Professor EL. Chronister is acknowledged for valuable suggestions on the chemistry and sysnthesis of porous silica materials. Dr. M. McIntire who kindly helped us to prepare mesoporous silica samples is also acknowledged. Mr. R. Castillo is acknowledged for valuable discussions on mesoporous silica. References
1. High radiation background: We installed a new particle separation system to eliminate the decay positrons from negative muons that contaminate the mSR spectrum. Installation of this system led to progress in the mSR experiment at Dai Omega. 2. Leakage magnetic flux: An iron plate with a thickness of 10 cm was installed between the detection system and the flange of the beam channel in order to clamp down on the leakage magnetic field. It was effective, reducing the leakage magnetic flux from 170 to 10–20 G. The reduction of the leakage magnetic flux reduces the field gradient, and thus we can perform a zero-field experiment. However, to measure weak remnant magnetism in sedimentary rocks, further progress is required. 3. This experiment may provide the useful information for a detector development at a further intense muon facility at J-PARC (Japan Proton Accelerator Research Complex).
[1] K. Nagamine, Hyperfine Interact. 8 (1981) 787. [2] R. Kadono, et al. RIKEN-RAL Muon Facility Report 1, 1997, P. 27. [3] H. Miyadera, Ph.D. Thesis, Tokyo University, 2005. [4] Y. Ikedo, et al., in preparation. [5] S.N. Nakamura, M. Iwasaki, Nucl. Instr. and Meth A388 (1997) 220. [6] CERN Application Software Group, CERN Program Library Q121, 1995. [7] T. Yamazaki, et al., Nucl. Instr. and Meth. 196 (1982) 289. [8] H.K.M. Tanaka, et al. in preparation. [9] K. Nagamine, A.P. Mills Jr., Los Alamos Report, LA10714C, 1986, p. 21. [10] K. Nagamine, et al., Phys. Rev. Lett. 74 (1995) 4814. [11] V.W. Hughes, et al., Phys. Rev. A 5 (1960) 63. [12] B.A. Barnett, et al., Phys. Rev. A 11 (1960) 39. [13] G.M. Marshall, et al., Phys. Lett. 65A (1978) 351. [14] G.A. Beer, et al., Phys. Rev. Lett. 57 (1995) 671. [15] A.C. Janissen, et al., Phys. Rev. A 42 (1990) 161. [16] R. Kiefl, et al., Hyperfine Interact. 6 (1979) 185. [17] G.M. Marshall, et al., Phys. Rev. D 25 (1982) 1174. [18] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [19] A. Sayari, J. Am. Chem. Soc. 122 (2000) 6504.
Nuclear Instruments and Methods in Physics Research A 554 (2005) 201–211 www.elsevier.com/locate/nima
Construction and performance of 128-channel segmented pulsed mSR and me detection system at Dai Omega H.K.M. Tanakaa,b,, K. Nagaminea,b,c, H. Miyaderab,d, K. Shimomurab, Y. Ikedob, K. Nishiyamab,d, K. Ishidae a Physics Department, University of California, Riverside (UCR), CA 92521, USA Muon Science Laboratory, High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan c Atomic Physics Laboratory, Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan d Department of Physics, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan e Meson Science Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan b
Received 6 June 2005; received in revised form 3 August 2005; accepted 9 August 2005 Available online 30 August 2005
Abstract In order to perform mSR and me measurements with an instantaneously intense pulsed 4-MeV muon beam currently available at Dai Omega, which is a large solid-angle-axial focusing superconducting surface muon channel at KEKMSL, the detector segmentation in the conventional digital technique was improved to accept the predicted event rate of 2.0 104 muons/spill. A new detection system was designed to collect intense me decay positrons with 128 two-fold coincidence detector elements. A major feature of the new detection system was an employment of multi-anode phototubes (MAPMT) to make the detection system be compact. We mitigated the problems related cross talks among channels by employing a special fiber holder. The performance of the detection system was confirmed by mSR and me test experiments. r 2005 Elsevier B.V. All rights reserved. PACS: 29.30.Aj; 29.27.Hj; 29.40.Mc Keywords: Pulsed muon beam; Multi-anode phototube; Muon spin rotation; Muonium
1. Introduction Corresponding author. Physics Department, University of
California, Riverside (UCR), CA 92521, USA. Tel.: 951 827 5331; fax: 951 827 4529. E-mail address: [email protected] (H.K.M. Tanaka).
In 1978, the first pulsed mSR (muon spin rotation, relaxation, and resonance) was observed. There they employed a digital method with a single telescope of two Cherenkov counters and an eightchannel segmented plastic scintillation detection
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.08.051
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system [1]. The mSR spectrum observed for positive muons exhibited the correct precession pattern. In 1997, a high-segmented positrondetection system, ‘‘ARGUS (Advanced Riken General-purpose mSR Spectrometer)’’, was constructed and installed at Port 2 of RIKEN-RAL (The Institute of Physical and Chemical Research—Rutherford Appleton Laboratory) [2]. In our measurements for the muon beam transported through Dai Omega, which is a large solid-angle-axial focusing superconducting surface muon channel at KEK-MSL, we faced the following basic important problems before we constructing a mSR spectrometer: (1) high instantaneous rate of 5 104 muon events/pulse, (2) high radiation background of 5 103 events/ pulse; the primary proton beam causes radiation background from the production target at the focusing point after passing through a 4 m long shield, which is made of iron, lead, concrete, and boride shields, as shown in Fig. 1. Among various background particles, beamassociated neutron background was reduced to 5 102 events/pulse by a shield of boride blocks (more than 5.0 cm thick),
(3) residual beam-related positron background; the residual positron background was discriminated by using a separation system [3], (4) leakage magnetic field from Dai Omega itself; a magnetic field of 170 G along the beam direction at the target sample region; an iron plate with a thickness of 1.0 cm was installed to reduce it to 10–20 G. The residual magnetic field was cancelled by recently constructed magnet systems [4]. At Dai omega, in order to overcome a restriction of the data-accumulation rate of multi-stop TDCs, the detection system was designed to collect a high intensity of me decay positrons with 128 coincidence detector elements. The present report intends to be a complete description of the design of the highly segmented detection system installed at Dai Omega.
2. Experimental requirements The essential features of the mSR experiments are very similar , and the approach is quite simple. Beam stopping at a sample target and detection techniques of decay positrons vary among the
Fig. 1. Side view of Dai Omega. Dai Omega consists of four large-aperture super conducting coils. Dai Omega is directly connected to the proton beam line.
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experiments, but several systematic issues were common to all. Positive muons are brought to rest in a sample target (either one at a time (DC beam) or in a small bunch (pulsed beam)), and decay within a few microseconds ðtm ¼ 2:2 msÞ. The two neutrinos escape unobserved, but the decay positron can be detected by a detection system, which surrounds the stopping target. The muon precession in a sample target can change with time the angular distribution of decay positrons. The time of the positron detection is recorded by a multistop TDC. In operation of a multistop TDC the sweep is started by a user-supplied trigger pulse. Then subsequent events detected at the stop input are recorded, each in a specific time bin corresponding to the time of arrival relative to the start pulse. Compared to non-multihit devices, the multihit TDC can accept a new stop event after a prior event. The average number of positrons striking an individual detector element per cycle must be less than the maximum depth that is restricted by the ability of a multistop TDC and is usually less than 5–6. This hurdle, coupled with the very difficult funding climate for a highly segmented mSR counting system, has discouraged us from constructing a conventional full-equipped counting system, such as ARGUS. The list below includes many of the main and general conclusions, which are important in the design.
each viewed by MAPMTs. The advantage of the scintillator is its large light output and relatively short signal duration.
3. Detector design The mSR detection system was designed with 128 independent coincident elements. Each counter consists of two plastic scintillators in the shapes of a solid rectangle with a size of 40 mm (L) 12 mm (W) 12 mm (H). UV-Absorbing (UVA) tapered clear acrylic fibers (clear fibers) were inserted manually into the inner plastic scintillator as shown in Fig. 2. Clear fibers allow the transport of scintillation light over a meter to the MAPMTs without significant attenuation. The UVA clear fibers unwant Cherenkov light from introducing an energy-dependent additional light source to this counter. The light from the scintillator is transported to a MAPMT through a solid, clear fibers whose length is 1000 mm that is sufficient to transport photons from a scintillator. The uninserted ends of the clear fibers were connected to a multi-anode phototube. The outer scintillator is coupled to the MAPMT through a UVA acrylic lightguide attached at right angles to
The detector should be designed to be as symmetric for a sample target as possible. A coincidence system should exist for each decay positron measurement. This ensures identification of decay positrons with a low beam-related background. High segmentation requires highly concentrated PMTs, such as multi-anode phototubes (MAPMTs). The light transmission through the light guides must be efficient.
We have considered several detector schemes and geometries to address these demands, and have settled on a design, which includes coincident timing elements made of fast plastic scintillators,
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Fig. 2. Closer view of a counter system.
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one of the 2.0 cm long triangular edges. The end of the light guide has a 451bevel, which is mirrored in order to reflect scintillation light into the clear fibers as shown in Fig. 2. Each light-guide end is held against the face of the multi-anode phototube by a black-color fiber holder in order to illuminate one of 16 pixels. The fiber-phototube junction was covered with a black silicon rubber to reduce the light leakage level to below 10 events/min. Fig. 3 shows the overall geometrical structure containing 128 coincident elements. The area of each counter is 4.8 cm2. The inner radius is 10 cm. The outer radius is 12.6 cm. The 128 counter pairs therefore occupy approximately 5% of all the solid angle. An overall mechanical support system holds the MAPMT housings, the clear fibers, and 128 coincident elements. 3.1. PMT and scintillators The choice of the PMT is motivated by a desire to find a compact and highly concentrated tube with good gain and minimum time jitter. An excellent candidate PMT is the Hamamatsu R6427 multi-anode phototube. It is a 25 25 mm2 square 12-stage head-on multi-anode tube with a gain of 3 106 and a maximum quantum efficiency of
approximately 25%. This tube contains 16 individual anodes in a single vacuum envelope. The anodes are arranged in a 4 4 array and have an effective area of 4 4 mm2 on a 4.5 mm pitch as shown in Fig. 4. The 10–90% rise time is 1.7 ns and the transit time jitter for single photoelectrons has a standard deviation of about 200 ps. The cost of the tube is around $100/channel. The MAPMTs allow a compact design of the spectrometer, as sown in Fig. 4. The phototubes (with a size of 25.7 (L) 25.7 (W) 45 (H) mm3) were placed at the corners of the detector holder. The typical yield from the clear fibers was around three-times larger than the single photoelectron level. We measured the magnitude of the light attenuation through clear fibers with a checking source of 106Ru. The observed pulse height was Eb100–200 mV for b-ray signals and ETh 20 mV for thermal noise at an operation voltage of 800 V. A uniformity of the response of each counter was also measured. The counting rate varied by 20% among each counter. An aluminum ring of approximately 6 mm in thickness was inserted between the inner and outer counters to eliminate the low-energy backgrounds. The decay positrons scattered through the aluminum ring are rejected by the coincidence. A Geant
Fig. 3. Schematic view of a multi-anode phototube.
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Fig. 4. Overall geometrical structure of mSR detection system; drawing (left); photograph (right).
3.2. Crosstalk
2x104 None Acryl 1.0 cm Acryl 1.0 cm + Al 1.0 cm Acryl 1.0 cm + Al 2.0 cm
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Fig. 5. Geant-4 Monte Carlo simulation results, showing the energy cutoff of the decay positrons passing through aluminium plates with a thickness of 1.0 and 2.0 cm.
4 detector simulation shows that the positrons with energies less than 20 MeV are practically cut as shown in Fig. 5. The phototubes were placed at the corners of the detector holder. In order to reduce the gain loss of the MAPMT by the leakage magnetic field from Dai Omega, each phototube was placed in an iron cylinder (10 mm in thickness). We measured the magnetic field of less than 1.0 G in the cylinder. The longer axis of phototubes was aligned with the external field.
There are two types of crosstalk among channels in a multi-anode phototube. (a) External crosstalk; optical crosstalk occurs due to the divergence of light as it passes through the fibers, and (b) intrinsic crosstalk; electronic crosstalk occurs after the photocathode has produced an electron via the photoelectric effect, and the electron is being steered toward a dynode chain. The electron then misses dynode slit and hits the metal plate around the dynode causing it to hop a distance less than, or equal to, the cathode to the first dynode separation (4 mm). The electron then begins the multiplication process in an incorrect dynode chain, leading to a signal in an incorrect anode. The latter type of the crosstalk can be reduced by adjusting the discrimination level. The typical pulse height of this type of the crosstalk is ten times smaller than the usual signal pulse height. A special fiber holder was designed to absorb the divergence of light to reduce the former type of corsstalk, as shown in Fig. 6. The effectiveness of this fiber holder was tested and found to reduce the former type of the crosstalk to less than 5%. 3.3. Electronics and data acquisition In order to perform a high-density time to digital conversion, a multi-stop TDC (Le Croy; model 3377) was employed. The on-line
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4. lSR test experiment
Fig. 6. Special lightguide holder designed to reduce optical cross talks.
monitoring/controlling system on the PC was accomplished by the CAMAC data acquisition system EXP 2000 [5], which controls the auxiliary crate controller (ACC; Kinetics K 3976), the CAMAC-SCSI interface (CC; Kinetics K 3929) and the PAW++ data analysis software [6]. The task of the front-end electronics is to provide timing information for signals produced by decay positrons in the coincident elements. We require that our system be able to read out 256 channels of phototube signals, one signal from each of the 128 coincident elements. Block diagrams of the electronics are shown in Fig. 7. An analog waveform from the MAPMT is converted into an NIM signal by a discriminator (KAIZU KN246 Octal Discriminator). Coincidence signals from a counter pair are sent to a multi-stop TDC by converting from a NIM pulse to an emitter-coupled logic (ECL) pulse. A proper equipment (Technoland; N-TM 317) was available to make this conversion. We used the signal from the pulsed kicker magnet to trigger the data acquisition at a sequence of times (2.0 ms) before the muon injection. We measured the total data rate less than 32,000 bits/ spill. This amount of data can be transferred through an ordinary SCSI BUS to the Linux operating system. The data analysis software PAW++ creates a set of histogram for the timing of decay positrons. The counting rate of the decay positron from the sample target was 10,000/spill before the coincidence and 600/spill after the coincidence.
The surface muons, which are highly polarized with their spin anti-parallel to their momentum direction, arrive at a target. If some net average polarization (S) remains after the muons have stopped in the target, the spin will precess with angular frequency o along an axis parallel to any existing magnetic field B. The precession frequency is proportional to B and to the muon’s g-factor. For a free muon, the precession rate is o/ B ¼ 8.5 108 rad/sT, which corresponds to a revolution frequency of 13.5 kHz/G. If S and o are both non-zero, the precession of the muon spins will produce a change in the angular distribution of the decay positrons and, consequently, a change with time in the number of counts registered in any element of the detection system. The muon spin precession therefore alters the expected pure exponential shape of the decay, since muons stopped in the sample target with their spin vectors initially aligned anti-parallel to the beam direction. To understand how the amplitude of spin precession is affected, we divide the contribution according to two major items, which will influence the result. They are: 1. polarization of the muon beam, 2. The residual decay positrons. It is well-known that m+ depolarization rate is below 0.01 m/s in an aluminum target at room temperature. We measured the muon spin precession for a certain magnetic field strength in an aluminum target. The obtained mSR time spectrum is shown in Fig. 8 for a transverse magnetic field strength of 2071 G. The spectrum in Fig. 8 was obtained by taking the ratio between the decay spectrum as obtained with the upstream detectors and that with the downstream detectors. This ratio defines an asymmetry of the mSR spectra. The correction causes large statistical fluctuations in the third and forth peaks. An asymmetry of 20% reflects the S/N ratio that is consistent with the conventional mSR spectra [7]. The frequency of the muon spin precession is 3.570.2 ms. The result is consistent with the applied magnetic field and the muon’s g-factor. The observed S/N ratio was 106
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Fig. 7. Diagram of the electronics and computer system used for the present detection system.
at time zero and 102 at t ¼ 10 ms. Magnetism of the microorganism originated sedimentary rock laid down around 3.0 billion years ago was investigated with the present spectrometer [8].
5. le test experiment and its application to the thermal Mu production in mesoporous silica The generation of thermal Muonium (Mu) is a basic technique to produce ultra slow muon [9,10] for future muon accelerators. In the first observation of muonium by Hughes et al. [11], polarized muons were stopped in a target of argon gas at 50atm pressure, and identification was made by observing the characteristic Larmor precession frequency of muonium. Although this technique
produces a large yield of muonium, (8579%)of incident muons [12], it is recognized that for the purpose of the production of ultra slow muons, muonium have to be observed in the vacuum. In order to produce muonium in the vacuum, muonium production has been studied with a fine powder of fumed silica [13,14]. The formation of muonium in fumed silica is interpreted as a three stage process [15]: (1) Positive muons stop in the silica particles forming muonium; (2) The fraction of muos which form muonium inside the silica powder is assumed to be the same as the muonium production fraction in bulk silica of 6173% [16,17]; and (3) The muonium atoms diffuse to the surface of the particles and emerge into the vacuum. Thus, it is recognized that a specific surface area is the key factor to determine the
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+ in Al H = 20G
Asymmetry
1.2
1
Fig. 9. Conceptual view of MCM-48 meorposous silica. A thin amorphous wall followed a gyroid surface. 0.8
2000
4000
6000
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10000
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14000
time (ns) Fig. 8. Muon spin rotation time spectrum observed at Dai Omega. The external transverse field was 20 G at room temperature (bin width 16 ns). A solid line is a sinusoidal curve corresponding to the frequency of the muon spin precession is 3.5 ms with a baseline located at an asymmetry of 1.05.
amount of the muonium emitted to the vacuum. The typical fumed silica has a specific surface area of between 200 and 400 m2/g depending on the grade. For this experiment, a new type of mesoporous silica MCM-48 with a specific surface area of 1500 m2/g was used. An improvement on the muonium yield was expected. The MCM-48 ordered mesoporous silica (The porous silica having an ordering on submicron pores) is a member of the M41S silica family first synthesized by Kresge et al. [18]. MCM-48 is a large pore (20 A˚) molecular sieve with two independent interwoven three-dimensional channels as shown in Fig. 9. All structural observations of the MCM-48 silica are explained by assuming that the thin amorphous wall followed by the gyroid (G) surface. Owing to its pore structure, the cubic mesophase, known as a space group of Ia3d has a large surface area of 1500 m2. The sample solution is a mixture of Cabo-Sil silica, trimethyleammonium hydrooxyde (TMAOH), cetyltrimethylammonium bromide (CTAB), and water. The samples were synthesized in an autoclave at
130 1C for 40 h [19]. The obtained materials were filtered, washed, and calcined at 540 1C. The nitrogen gas absorption spectrum and powder Xray diffraction image showed a typical feature of MCM-48. Twenty milligram of MCM-48 powder was dissolved in acetone allowing a natural sedimentation process to form a uniform sample layer of 10 mm in thickness on an aluminum foil of 15 mm in thickness. The sample was then attached to a tungsten degrader of 50 mm in thickness. The thickness of the tungsten foil was optimized for 4-MeV muon penetration through the tungsten degrader and stopping in the MCM-48 sample layer [3]. Geant 4 simulations were carried out to estimate the stopping distribution in the target. 4.22% of the incident muons stop in the MCM-48 target. Likewise, 250 mg of MCM-48 powder was used to form a uniform sample layer of 125 mm in thickness on an aluminum foil of 45 mm in thickness. The sample was then attached to a tungsten degrader of 25 mm in thickness. Geant 4 simulations were carried out to estimate the stopping distribution in the target. The results are shown in Fig. 10. This target is a poor producer of muonium at a beam momentum of 29.7 MeV/c because the muon stopped in the sample per incident muon is only 0.55%. However, 15.3% of the incident muon stops when the incident momentum is lowered by 5%. A muonium observation setup was constructed at Dai Omega. As shown in Fig. 11, the key feature of the setup is using mSR detector for the
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15.3% 0.4
0.3
0.2
0.1
-0
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Fig. 10. Simulation results of the stopping distributions of the muon for different incident momenta in the MCM-48 sample layer after passing thorugh the sample substrate with; (A) 100% of the incident momentum (29.7 MeV/c); (B) 95% of the incident momentum (28.2 MeV/c).
efficient detection of decay positron from muonium. The up-stream and down-stream detector of the mSR spectrometer was set to observe 5–15 and 25–35 cm region from the sample target, respectively. The vacuum chamber was made of stainless steal (SUS 304) and the inside surface of the chamber was electrolytically polished to reduce the surface area. A turbo molecular vacuum pump was connected directly to the vacuum chamber. Two ports, ICF70, were used for the oxygen leak and for a vacuum gage. The design of the vacuum chamber is shown in Fig. 11. The pressure in the vacuum chamber was 107–108 Torr during the measurements. The radiation shieldings furnished on the vacuum chamber were made of lead. Two beam collimators were installed inside the vacuum chamber, one at the up-stream and another at the down-stream of the sample target. Both collimators were made of heat-resistant stainless steel (SUS 310). The up-stream collimator collimated the beam size to f ¼ 35 mm in diameter. The down-stream collimator cut the scattered energetic muons so as not to stop them near the up-stream detectors. The yield of the emission of the muonium from the sample target in the vacuum
was normalized to that in the 1-Torr oxygen gas. The diffusion length of the muonium atom at 300 K in the oxygen gas of 1 Torr at 300 K within the lifetime of muonium of 2.2 106 s is 9.95 104 m. Decay positrons from the sample region, are shielded by a lead collimator, as shown in Fig. 11. Fig. 12 shows the obtained muonium yield for a sample thickness of 125 and 10 mm. The Mu-TOF spectra for the 125-mm thick sample were obtained by lowering the momentum of the incident muon beam by 5%. Subtractions of the data in the vacuum from the data in the oxygen gas were taken. Since muonium is a neutral particle, a particle selection of the emitted particles can be easily performed, by applying a certain electric field. The electric potential of the tungsten foil was switched by data acquisition system so as to be 0 and +6 V in order to confirm whether or not the emitted particle is a charged particle. The TOF signals of the thermal muonium emitted from a hot tungsten foil (T ¼ 2000 K) [3] are also plotted in Fig. 12. The muonium emitted from MCM-48 arrives the detection region 2.6 times slower than the one from a tungsten foil. The result is
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Fig. 11. Experimental setup taken for the present thermal muonium production experiment.
50 Number of Mu (/1M muon microses)
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908ppm/ incident muon 2632ppm/ incident muon 405ppm/ incident muon
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Thin MCM-48
-20 Time (microsec) Fig. 12. Time spectra of the absolute values of the thermal muonium produced from different materials.
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consistent with the temperature of MCM-48 (T ¼ 300 K). The production rate of the thermal muonium was estimated within the time range between 0 and 10 ms. The obtained values are 405 ppm/(incident muon) and 2632 ppm/(incident muon) (0.96% /stopped muon and 1.72%/stopped muon) for 10-mm thick sample and 125-mm thick sample, respectively. Our yield is consistent with the previously reported value [15] measured with the different target (fumed silica) and beam parameters.
6. Discussion As a conclusion of the mSR and me measurements using the highly segmented detection system at Dai Omega, we found several unsolved problems that should be answered in the nearest future.
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Acknowledgements This work was supported by a Grant in Aid from the Japanese Society of Promotion of Science (JSPS) and Core to Core Program of JSPS. We are indebted to Mr. K. Fukuchi for his contributions to the construction and operation of Dai omega. We thank Professor Y. Hamano for his meaningful discussions about detrital remnant magnetization in banded iron formation. Professor A.P. Mills Jr. is acknowledged for valuable discussions focused on the thermal muonium production in mesoporous silica. Professor EL. Chronister is acknowledged for valuable suggestions on the chemistry and sysnthesis of porous silica materials. Dr. M. McIntire who kindly helped us to prepare mesoporous silica samples is also acknowledged. Mr. R. Castillo is acknowledged for valuable discussions on mesoporous silica. References
1. High radiation background: We installed a new particle separation system to eliminate the decay positrons from negative muons that contaminate the mSR spectrum. Installation of this system led to progress in the mSR experiment at Dai Omega. 2. Leakage magnetic flux: An iron plate with a thickness of 10 cm was installed between the detection system and the flange of the beam channel in order to clamp down on the leakage magnetic field. It was effective, reducing the leakage magnetic flux from 170 to 10–20 G. The reduction of the leakage magnetic flux reduces the field gradient, and thus we can perform a zero-field experiment. However, to measure weak remnant magnetism in sedimentary rocks, further progress is required. 3. This experiment may provide the useful information for a detector development at a further intense muon facility at J-PARC (Japan Proton Accelerator Research Complex).
[1] K. Nagamine, Hyperfine Interact. 8 (1981) 787. [2] R. Kadono, et al. RIKEN-RAL Muon Facility Report 1, 1997, P. 27. [3] H. Miyadera, Ph.D. Thesis, Tokyo University, 2005. [4] Y. Ikedo, et al., in preparation. [5] S.N. Nakamura, M. Iwasaki, Nucl. Instr. and Meth A388 (1997) 220. [6] CERN Application Software Group, CERN Program Library Q121, 1995. [7] T. Yamazaki, et al., Nucl. Instr. and Meth. 196 (1982) 289. [8] H.K.M. Tanaka, et al. in preparation. [9] K. Nagamine, A.P. Mills Jr., Los Alamos Report, LA10714C, 1986, p. 21. [10] K. Nagamine, et al., Phys. Rev. Lett. 74 (1995) 4814. [11] V.W. Hughes, et al., Phys. Rev. A 5 (1960) 63. [12] B.A. Barnett, et al., Phys. Rev. A 11 (1960) 39. [13] G.M. Marshall, et al., Phys. Lett. 65A (1978) 351. [14] G.A. Beer, et al., Phys. Rev. Lett. 57 (1995) 671. [15] A.C. Janissen, et al., Phys. Rev. A 42 (1990) 161. [16] R. Kiefl, et al., Hyperfine Interact. 6 (1979) 185. [17] G.M. Marshall, et al., Phys. Rev. D 25 (1982) 1174. [18] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [19] A. Sayari, J. Am. Chem. Soc. 122 (2000) 6504.