Microstructure analysis of bismuth absorbers for transition-edge sensor X-ray microcalorimeters

TL;DR

This study compares the microstructure and spectral response of bismuth absorbers fabricated by evaporation and electroplating for transition-edge sensor X-ray microcalorimeters, revealing grain size differences as a key factor.

Contribution

It provides a detailed microstructure analysis of Bi absorbers made by two fabrication methods, linking grain size to spectral response differences.

Findings

Evaporated Bi has much smaller grains than electroplated Bi.

Both Bi types share the same crystallographic structure.

Grain size influences spectral response characteristics.

Abstract

Transition-edge sensors (TESs) as microcalorimeters offer high resolving power, owning to their sharp response and low operating temperature. In the hard X-ray regime and above, the demand for high quantum-efficiency requires the use of absorbers. Bismuth (Bi), owing to its low heat carrier density and high X-ray stopping power, has been widely used as an absorber material for TESs. However, distinct spectral responses have been observed for Bi absorbers deposited via evaporation versus electroplating. Evaporated Bi absorbers are widely observed to have a non-Gaussian tail on the low energy side of measured spectra. In this study, we fabricated Bi absorbers via these two methods, and performed microstructure analysis using scanning electron microscopy (SEM) and X-ray diffraction microscopy. The two types of material showed the same crystallographic structure, but the grain size of the evaporated Bi was about 40 times smaller than that of the electroplated Bi. This distinction in grain size is likely to be the cause of their different spectral responses.