Application of Hard X-Ray and Gamma-Ray TES Microcalorimeter at Accelerator Facility

The x-ray spectroscopy of the muonic atom has attracted atomic, nuclear, and particle physicists since its discovery. The properties of a muonic atom, such as its binding energy or atomic radius, are different from an ordinary atom because of the difference in the mass between the muon and electron. Our collaboration has employed superconductor transition-edge sensor (TES) microcalorimeters for the x-ray spectroscopy of the muonic atom. Thanks to the recent detector development, the 44-keV lines from muonic Ar, which is important for the precision test of bound-state quantum electrodynamics, and the 76-keV lines from muonic Si, which is of interest from the viewpoint of the measurement of nuclear radii, have been reached by the dynamic range of the state-of-art TES microcalorimeters. An accelerator facility that can produce a high-intensity muon beam is necessary for such spectroscopic experiments. We performed a commissioning experiment of the hard x-ray and gamma-ray TES microcalorimeter at the J-PARC MLF MUSE muon beam line. The energy resolution, gain stability, and performance of timing selection of the pulses were evaluated in the environment of a large-scale accelerator facility.