Development of TES Microcalorimeters with Solar-Axion Converter

Several issues in developing transition edge sensor (TES) microcalorimeters for the search of monochromatic solar axions expected at 14.4 keV are studied. The potential problem of developing TES’s of this purpose is in that an axion absorber of \(^{57}\)Fe must be placed in close vicinity of a TES. We estimated the minimum distance to avoid magnetic interference from iron using magnetic FEM simulations, and found it 30 \(\upmu \)m for an iron of 5 \(\upmu \)m thickness. We fabricated a TES with a 10 \(\upmu \)m thick iron membrane separated by 60 \(\upmu \)m. We confirmed the superconducting transition for this TES. However, both the residual normal resistance and the transition temperature was different from those of TES without iron. We also estimated the low-temperature thermal conductivity of an iron membrane by measuring the low-temperature electrical resistance and by applying the Wiedemann–Franz law. We estimated the pulse-shape dependency on the interaction position within the \(^{57}\)Fe converted using thermal FEM simulations. We found that the pulse-shape variations will limit the energy resolution to about 30 eV FWHM.