Absorber Materials for Transition-Edge Sensor X-ray Microcalorimeters

Abstract

Absorber Materials for Transition-Edge Sensor X-ray MicrocalorimetersArrays of superconducting transition-edge sensors (TES) can provide high spatial and energy resolution necessary for x-ray astronomy. High quantum efficiency and uniformity of response can be achieved with a suitable absorber material, in which absorber x-ray stopping power, heat capacity, and thermal conductivity are relevant parameters. Here we compare these parameters for bismuth and gold. We have fabricated electroplated gold, electroplated gold/electroplated bismuth, and evaporated gold/evaporated bismuth 8x8 absorber arrays and find that a correlation exists between the residual resistance ratio (RRR) and thin film microstructure. This finding indicates that we can tailor absorber material conductivity via microstructure alteration, so as to permit absorber thermalization on timescales suitable for high energy resolution x-ray microcalorimetry. We show that by incorporating absorbers possessing large grain size, including electroplated gold and electroplated gold/electroplated bismuth, into our current MoIAu TES, devices with tunable heat capacity and energy resolution of 2.3 eV (gold) and 2.1 eV (goldhismuth) FWHM at 6 keV have been fabricated.

Development of Cryogenic X-Ray Detectors Based on Mo/Au Transition Edge Sensors

“…Our best films display RRR~0.84-0.9 and (300 K) ~ 0.2-0.3 mcm. These values are close to the best ones reported to date for other Bi electrodeposited films of similar thickness [7] and evidence that the addition of an Au layer to the absorber might be required to improve its thermal conductance.…”

“…Zaragoza 50018, Spain detected. For X-rays, layers several micrometers thick of a material with high stopping power and suitable heat capacity, thermal conductivity and electrical conductivity are required; Bi and Au/Bi layers are most convenient [7], [2].…”

Optimization of thermal kinetic inductance detectors for x-ray spectroscopy

“…We started with a T C of 1 K, which is the lowest T C among the materials commonly used for KIDs (Al or 1 K TiN) and sets the operating temperature at 0.2 K. Next we selected a heat capacity of 1 pJ/K, which corresponds to a Bi absorber of 1 mm × 1 mm × 25 μm [12]. 25 μm thick Bi provides near unity absorption at 6 keV.…”

Towards Mo/Au based TES detectors for Athena/X-IFU

“…Although bulk Bi is a semimetal with very weak dR/dT, usually 15,16,17 Bi films display semiconducting behavior as that shown in this figure. If we define RRR=R(300K)/R(4.2K), the goal would be to obtain an RRR value as close to 1 as possible 18,19 . Values of RRR around 0.7 are measured, but strategies to further reduce this and the room temperature resistance are also being addressed.…”