Speaker
Description
Axion is a hypothetical elementary particle proposed to solve the strong CP problem in QCD and is one of dark-matter candidates. The sun is considered to emit axions of a continuum spectrum similar to that of blackbody emission with kT~1.3 keV by the Primakoff effect. In addition, line emission is expected through M1 transitions of nuclei; an example is 14.4 keV from $^{57}$Fe (Moriyama 1995). Namba et al., 2007 searched for a 14.4 keV gamma ray from $^{57}$Fe axion absorber with a semiconductor detector and placed on a mass upper limit of QCD axions.
However, the branching ratio from an excited $^{57}$Fe to a 14.4-keV γ-ray is only 9%, and remaining 91% of energy is self-absorbed. The total efficiency is only 1%, if we take the solid angle of the detector into account. Microcalorimeters can detect almost all energy converted from an axion, and thus can improve the detection sensitivity dramatically. Although the mass of the axion absorber of one calorimeter may be much smaller than that of semiconductor experiment, we can increase it by using an array device.
We are thus developing TES microcalorimeters for solar axion exploration. One of the challenge of this research is in ferromagnetism of the Fe axion absorber. We decided to place the absorber at a certain distance from the TES and to connect them with a thermal strap. We have studied the minimum distance required between them with simulations and experimentally measured the R-T curve of an TES with Fe placed at the distance. We then studied the thermal strap design with simulations. The results depend on the thermal conductance of ion at low temperatures. We thus measured the electrical resistivity of Fe films fabricated with electro-deposition to estimate the thermal conductivity. In this paper, we report the optimization of the design of TES microcalorimeter for a solar axion search basing on those simulations and experiments, and the current status of the TES microcalorimeter fabrication.
| Student (Ph.D., M.Sc. or B.Sc.) | Y |
|---|---|
| Less than 5 years of experience since completion of Ph.D | Y |
