Eliminating the non-Gaussian spectral response of X-ray absorbers for transition-edge sensors

TL;DR

This paper presents a new fabrication method for bismuth absorbers in transition-edge sensors, eliminating spectral response issues caused by non-Gaussian tails and improving X-ray spectroscopy performance.

Contribution

It introduces electroplated bismuth absorbers for TESs, addressing spectral tail problems seen with evaporated bismuth, and investigates the role of bismuth morphology.

Findings

Electroplated bismuth absorbers produce cleaner spectral responses.

Bismuth morphology influences the spectral response of TES microcalorimeters.

The new fabrication method retains the advantages of bismuth as an absorber material.

Abstract

Transition-edge sensors (TES) as microcalorimeters for high-energy-resolution X-ray spectroscopy are often fabricated with an absorber made of materials with high Z (for X-ray stopping power) and low heat capacity (for high resolving power). Bismuth represents one of the most compelling options. TESs with evaporated bismuth absorbers have shown spectra with undesirable and unexplained low-energy tails. We have developed TESs with electroplated bismuth absorbers over a gold layer that are not afflicted by this problem and that retain the other positive aspects of this material. To better understand these phenomena, we have studied a series of TESs with gold, gold/evaporated bismuth, and gold/electroplated bismuth absorbers, fabricated on the same die with identical thermal coupling. We show that bismuth morphology is linked to the spectral response of X-ray TES microcalorimeters.