Properties of vacuum-evaporated bismuth absorber

Bi films that we fabricated with vacuum-evaporation deposition show a crystal structure very similar to that of bulk Bi. However they have granular structure on sub-micron scales, and the electrical resistance increases at low temperatures, which is opposite to bulk Bi. A TES microcalorimeter with a Bi absorber shows an energy resolution significantly degraded compared to the baseline resolution. A thermal model of the calorimeter based on the present experimental results indicates that a position dependence of the pulse height and rise time degrades the energy resolution. The position dependence is further considered to be due to mismatch between the rise time of pulse which is mostly determined by the thermal conductance between Bi and TES and the thermal diffusion time in the Bi absorber.