Quantum Efficiency Study and Reflectivity Enhancement of Au/Bi Absorbers

Abstract

X-ray absorbers of the X-ray Integral Field Unit (X-IFU) microcalorimeters are required to provide high quantum efficiency (QE) for incident X-rays and high reflectivity to longer wavelength radiation. The thickness of the electroplated Au and Bi layers of the absorber is tuned to provide the desired pixel heat capacity and the QE. To calculate the QE precisely, in addition to filling factor, we have included the effects of surface roughness, edge profile of the absorbers and the effects of the different angles of incidence of the incoming X-rays from the X-IFU optic. Based on this analysis, it is found that thickness of the Bi layer needs to be adjusted by 4.3% to achieve the X-IFU QE requirements. To enhance the absorber's rejection of low-energy radiation, a second thin layer of Au is sputter-deposited on top of the Bi layer. Optical measurements in the wavelength range 0.3-20 μm show a significant increase in reflectivity compared to a bare Bi layer.

A Review of X-ray Microcalorimeters Based on Superconducting Transition Edge Sensors for Astrophysics and Particle Physics

“…For an Au/Bi absorber, this can be done for example by adding a Au capping layer on top of the Bi, with a thin Ti adhesion layer. A quantum efficiency of 90% has been reported for TES-X-ray calorimeters designed for Athena/X-IFU [106] with an increase of reflectivity, measured at room temperature, from 45% to up to 80% after adding a 40 nm Au capping layer.…”

“…To achieve the required high QE however, a careful study needs to be performed on the thickness and the quality of the Au/Bi bilayer. It has been recently shown [106] that the roughness, the edge profile, the angles of incident of the X-ray photons, and the filling factor needs to be known well to obtain an accurate estimate of the QE. To minimize the effect of the absorption of the stray light in the detector, it is also desirable to increase the reflectivity of the absorber for long-wavelength infrared radiation.…”

The 511-CAM Mission: A Pointed 511 keV Gamma-Ray Telescope with a Focal Plane Detector Made of Stacked Transition Edge Sensor Microcalorimeter Arrays

“…These two decays power much or all of the emission from SNe Ia, 30 and a NUSTAR followup mission could detect several tens of these objects per year. Diehl et al list prominent γ-ray lines at 68 keV and 78 keV (from the radioactive decay of 44 Ti in core-collapse supernovae), at 122 keV and 158 keV (from decays of 57 Ni and 56 Ni, respectively, in supernova nucleosynthesis), at 478 keV (from the decay of 7 Be in nova nucleosynthesis), and at 812 keV and 847 keV (from the decays of 56 Ni and 56 Co, respectively, in supernova nucleosynthesis) as important diagnostics. 31 A deeper understanding of SNe Ia explosion mechanisms would greatly benefit their use as standard candles in astronomy.…”

Electroplating Deposition of Bismuth Absorbers for X-ray Superconducting Transition Edge Sensors

“…TES has been widely used in the field of X-ray detection [ 1 , 2 ]. X-ray TES is a very sensitive thermometer based on the transition between the superconducting and normal state of a superconducting metal film due to the thermal transfer from the absorber, which absorbs the energy of incident X-ray photons [ 3 , 4 , 5 ]. The energy resolution (Δ E ) in the strong electro-thermal feedback is related to ~ C 1/2 , C is the heat capacity of superconducting thin films and the absorber [ 6 , 7 , 8 ].…”

“…For the best Δ E , the C of the absorber should be as low as possible. Moreover, the absorber should show a high absorbing efficiency for X-ray radiation to realize high quantum efficiency [ 4 ]. Several kinds of metals, such as superconducting tantalum (Ta) [ 9 , 10 ] and tin (Sn) [ 11 , 12 ], normal metal Au [ 13 , 14 ], and semimetal Bi [ 15 , 16 ], have been used as absorbers.…”