Planned Research
A01: Precision Measurements in Atomic and Molecular Physics using a Negative Muon Beam and its Application to Observations in Astrophysics

Muonic atom formed by capture of a negative muon to a nucleus, and emitted muonic X-rays during de-excitation
Looking at the process of capturing negative muons in detail, the negative muon is captured into a highly excited state of an atom, and then it steps down to the next lower energy level one after another in a much shorter time than the intrinsic lifetime of the negative muon. Initially, all (or many) of the bound electrons originally bound by the atom are peeled off and emitted as Auger electrons, and a muonic atom composed of a negative muon and an atomic nucleus, that is, an exotic hydrogen-like multiply-charged ion produced in vacuum. From this state, muon characteristic X-rays are further emitted along with the transition between the levels, and the negative muon finally reaches the ground level.

Research aiming to the high scientific goal to answer the fundamental questions of space, such as elementary particle physics experiments and space observations, are often results of the uninterrupted pursuit of sensitivity and resolution. By the advanced detector with the extreme performance, the x-ray spectroscopy of the muonic atom, one of the most clean atomic systems, is performed in the precision far exceeding the conventional accuracy over a wide band. This is expected to provide the benchmark data suitable for the model validation and precise understanding of the electronic recombination process in X-ray astrophysics.

In this research, we prepare isolated muonic atoms in vacuum by stopping the negative muon in a dilute gas target using the low-velocity negative muon beam of the world's highest intensity. The stopping position of the muon atom at an accuracy of 0.1 mm is tracked by a CdTe hard X-ray imager developed for space observation. By a multi-pixel superconducting transition edge microcalorimeter (TES) detector, the energy of emitted muonic X-rays of several keV (electron volt) will be measured at high resolution of several eV (ΔE/E ∼ 0.001). In this research, we aim to simulate the radiation spectrum obtained by actual astrophysical observation utilizing the code calculation of the refined de-excitation and recombination process, and to develop the results widely to X-ray astrophysics.


Principal Investigator AZUMA, Toshiyuki
(RIKEN Cluster for Pioneering Research)
Co-Investigators WATANABE, Shin (JAXA)  
YAMADA, Shinya (Tokyo Metropolitan University)  
ICHINOHE, Yuto (Rikkyo University)  
BAMBA, Aya (The University of Tokyo)  
INOUE, Yoshiyuki (RIKEN)  
Research Collaborators TAKAHASHI, Tadayuki (The University of Tokyo)  
OKADA, Shinji (Chubu University)  
NINOMIYA, Kazuhiko (Osaka University)  
KINO, Yasushi (Tohoku University)  

Reference Materials

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  • H. Tatsuno, S. Okada et al., “Absolute energy calibration of X-ray TESs with 0.04 eV uncertainty at 6.4 keV in a hadron-beam environment,” J. Low Temp. Phys 184, 930-937 (2016), DOI: 10.1007/s10909-016-1491-2 .