Proc. 12th Int. Conf. Low Energy Antiproton Physics (LEAP2016) https://doi.org/10.7566/jpscp.18.011023 Development of Three Dimensional Muon Beam Profile Monitor for New Spectroscopy of Muonium Y. Ueno 1, M. Aoki 2, M. Fukao 3, Y. Higashi 1, T. Higuchi 1, H. Iinuma 3, Y. Ikedo 3, K. Ishida 4, T. U. Ito 5, M. Iwasaki 4, R. Kadono 3, O. Kamigaito 4, S. Kanda 5, D. Kawall 6, N. Kawamura 3, A. Koda 3, K. M. Kojima 3, M. K. Kubo 7, Y. Matsuda 1, T. Matsudate 1, T. Mibe 3, Y. Miyake 3, T. Mizutani 1, K. Nagamine 3, S. Nishimura 5, K. Nishiyama 3,, T. Ogitsu 3, R. Okubo 3, N. Saito 3, K. Sasaki 3, K. Shimomura 3, 8, P. Strasser 3, M. Sugano 3, M. Tajima 1, T. Tanaka 1, K. S. Tanaka 1, 4, D. Tomono 8, H. A. Torii 1, E. Torikai 9, A. Toyoda 3, K. Ueno 3, A. Yamamoto 3 and M. Yoshida 3 1 Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan 2 Department of Physics, Osaka University, Toyonaka, Osaka, Japan 3 KEK, Oho, Tsukuba, Ibaraki, Japan 4 RIKEN, Wako, Saitama, Japan 5 Department of Physics, University of Tokyo, Hongo, Bunkyo, Tokyo, Japan 6 University of Massachusetts Amherst, Amherst, MA, USA 7 International Christian University, Osawa, Mitaka, Tokyo, Japan 8 RCNP, Osaka University, Toyonaka, Osaka, Japan 9 University of Yamanashi, Kofu, Yamanashi, Japan deceased E-mail: yueno@radphys4.c.u-tokyo.ac.jp (Received June 28, 2016) We have developed a three dimensional muon beam profile monitoring system for the new spectroscopy of muonium planned at the Japan Proton Accelerator Research Complex (J-PARC). The monitoring system measures the muon profile in the krypton gas target used for mass production of muonium. The beam profile is crucial for suppressing the systematic uncertainty in the measurement of the muonium hyperfine structure. A recent pilot measurement of the muon beam profile at J-PARC evaluated the performance of the monitoring system. We also discuss the future development plan of the beam monitoring system. KEYWORDS: Muonium, Spectroscopy, Beam Monitor 1. Introduction Muonium is a hydrogen-like bound state of an electron and a positive muon. Since muonium is a purely leptonic state and free from the finite size effect of nucleon, the muonium hyperfine structure (MuHFS) is a good probe for testing bound-state quantum electrodynamics. In addition to the MuHFS interval, the ratio of the muon mass to the proton mass can also be determined from the measurement of the MuHFS under a strong magnetic field. Moreover, precision measurement of MuHFS can test the Lorentz or CPT symmetries, based on the standard model extension [1]. The MuSEUM (Muonium Spectroscopy Experiment Using Microwave) collaboration aims at a ten-fold improvement of the experimental value of MuHFS. The latest experimental value is determined by the LAMPF (Los Alamos Meson Physics Facility) group [2]. The LAMPF value was mainly limited by statistical uncertainty. The MuSEUM collaboration can obtain a larger statistics and thus 1 011023-1 2017 The Author(s) This article is available under the terms of the Creative Commons Attribution 4.0 License. Any further distribution of this work must maintain attribution to the author(s) and the title of the article, journal citation, and DOI.
011023-2 improve the statistical uncertainty by utilizing the intense pulsed muon beam which is available at J-PARC (Japan Proton Accelerator Research Complex). As we reduce the statistical uncertainty, suppression of the systematic uncertainty becomes relatively important. We briefly show how to measure MuHFS. More detailed information can be found in other references [3] [4]. As shown in Fig. 1, polarized muons are injected into a target gas cell filled with krypton (Kr). Each muon is decelerated in the gas target and forms a muonium atom by capturing an electron from one of the surrounding Kr gas atoms. Muon decays with a lifetime of about 2.2 µs and emits a positron preferentially in the direction of the muon spin. By controlling the frequency of the induced microwave inside a microwave cavity placed in the gas target, one can make the MuHFS transition occurs, i.e., flip the muon spin. Scintillation counters around the gas target talliy the number of positrons, and then determine weather the transition has occurred or not. Fig. 1. Experimantal procedure. The spectroscopy is conducted in a krypton gas target used for muonium production. 2. Three Dimensional Muon Beam Profile Monitor As shown above, the spectroscopy of muonium is performed in a Kr gas target. The gas target is vital for the mass production of muonium and the suppression of statistical uncertainty. However, suppressing the systematic uncertainty is difficult due to the broad muon distribution because of their initial energy difference and the multiple scattering with target gas atoms. MuSEUM recently developed a new three dimensional muon beam profile monitor based on the muon profile monitor for µsr (MUon Spin Rotation) technique [5]. It is called Target Beam Profile Monitor (TBPM). The schematic view of the TBPM is shown in Fig. 2. The muons are stopped in a scintillation disk, and the disk emits scintillation light. A CCD (Charge-Coupled Device) camera (BU-50LN by BITRAN) placed downstream captures the image of the scintillation light thus obtaining the cross-sectional distribution of the muon beam. By moving the scintillation disk in the gas target, the CCD camera obtains several cross-sectional beam distribution and based on them it can reconstruct the three-dimensional distribution of the muon beam. The scintillator and the CCD camera are remotely moved by an actuator and a stepping motor, as shown in Fig. 3. The step size of the actuator is a few µm. The TBPM has an Image-Intensifier (IIF, C9016-23EXP by Hamamatsu Photonics) in front of the CCD camera, and the IIF has a time gating system. By matching the gate timing with the muon beam pulse, we can suppress the scintillation light caused by background particles, mainly positrons. 2
Proceedings of the 12th International Conference on Low Energy Antiproton Physics (LEAP2016) 011023-3 Fig. 2. Schematic view of the three dimensional muon beam profile monitor. A typical cross-sectional image of muon beam is also shown. Fig. 3. An actuator with a stepping motor is used for the scintillator control. The scintillator in the gas chamber is driven by the actuator on the outer side of the acrylic flange via a feedthrough screw. The relative light yield with different gate timing is shown in Fig. 4. One can see two muon pulses with one preceding pulse each, which corresponds to the positrons produced at the muon production target. The time difference of the pulses can be explained by the proton beam structure for muon production and the mass difference between muon and positron. The double pulsed beam structure from J-PARC was clearly reconstructed by the data. The measurements of the beam profile shown in Fig. 5 and Fig. 6 were conducted with the gating time adjusted with the timing of the first muon pulse. Utilizing the time structure of the beam, the TBPM can suppress the effect from background 3
011023-4 positrons. Fig. 4. The relative light yield of the TBPM with different gating time. The double pulsed structure of the muon beam is clearly obtained by the monitor. In the measurements of the center and width of the muon beam the gate timing is tuned to the first muon pulse. The cross-sectional images with different scintillator position are analyzed and fitted by a twodimensional gaussian function. The beam width is defined as one sigma of the gaussian. Beam center and width are shown in Fig. 5. The zero of the scintillator position corresponds to the center of the gas target chamber. The beam center is plotted with pale colored bands which corresponds to the required precision of the beam center (± 2 mm which corresponds to less than 1 Hz of the uncertainty for the spectroscopy result). The required precision is determined based on a Monte Carlo (MC) simulation which calculates resonance line from muonium distribution, microwave distribution and detection efficiency [6]. The beam width was also measured and compared with the width calculated by a MC simulation based on the Molière formula. The beam width and the simulated width are in good agreement. Combined with the result of the beam center measurement, the beam profile monitor successfully determined the cross-sectional profile of the muon beam. In addition to the cross-sectional distribution, we have also analyzed the longitudinal distribution of the muon beam. One can expect that the longitudinal light yield should be monotonically decreasing as the scintillator is moved downstream due to the loss of muons by multiple scattering or energy loss from collision with Kr atoms. The obtained data set shown in Fig. 6, however, does not match such expectation. This is mainly due to light reflection from the inner wall of the gas chamber, which is made of aluminum. The reflected light produces irregular background and it is difficult to calibrate the background level and analyze the data properly. 4
011023-5 Fig. 5. Left: The beam center determined by the beam monitor. The pale colored bands correspond to the required precision of the beam center determination. Right: The beam width measured by the beam monitor (circles) and the width expected from a Monte Carlo (MC) simulation (squares). The measured widths and MC widths are in good agreement. The zero of the scintillator position corresponds to the center of the gas target chamber. Fig. 6. The light yield of the scintillator with different scintillator position. The light yield should be decreasing as the scintillator move towards the back of the gas chamber (positive direction), but the data set has a kink. 3. Conclusion and Future Prospects In this paper, we have reported the development of a three dimensional muon beam profile monitor for precise spectroscopy of the muonium hyperfine structure. The monitor successfully measured the cross-sectional distribution of the muon beam. The longitudinal distribution could not be determined accurately due to reflected light in the gas chamber. We are planning to upgrade the beam profile monitor by placing a sheet of very thin ( 100 µm) black paper on the surface of the scintillator to suppress the reflected light. The upgraded monitor will be tested in June 2016. 5
011023-6 References [1] V. A. Kostelecký and A. J. Vargas, Physical Review D, 92, 056002 (2015) [2] W. Liu, et al., Physical Review Letters, 82, 711 (1999) [3] S. Kanda, et al., JPS Conference Proceedings, 8, 025006 (2015) [4] K. S. Tanaka, et al., JPS Conference Proceedings, 2, 010405 (2014) [5] T. U. Ito, et al., Nuclear Instruments and Methods in Physics Research A, 754, 1-9 (2014) [6] H. A. Torii, et al., JPS Conference Proceedings, 8, 025018 (2015) 6