X-ray transition-edge sensors (TESs) are superconducting microcalorimeter devices that measure X-ray photon energy. The photon-induced temperature change of the sensor results in a sharp and measurable resistance variation. The shape of the spectral response is a key factor in achieving efficient, high-energy-resolution X-ray detection with these TESs, and is limited by spectra with unexplainable and undesirably low-energy (LE) tails. Addressing this issue, experimental physicists report in Applied Physics Letters development of bismuth TESs that do not suffer from this problem.

Non-Gaussian responses, which are commonly observed in evaporated bismuth absorbers, have LE tails in X-ray emission lines, complicating X-ray line shape analysis and eroding detectability in trace element mapping. The new electroplated bismuth absorbers do not exhibit the same troublesome features, highlighting the importance of the absorber material’s microstructure to the spectral response of X-ray TESs.

The authors compared the Mn Kα X-ray energy spectra of three absorber and pixel types: gold (Au), gold/evaporated bismuth (Au/evap-Bi), and gold/electroplated bismuth (Au/elp-Bi). They found similar energy resolution for each pixel type, but the Au/evap-Bi spectrum showed a clear LE tail.

A closer examination for subtle LE tails, by fitting the observed spectra to the Gaussian detector response function and natural line shape, confirmed the absence of LE tails in the Au and Au/elp-Bi spectral responses. The evaporated bismuth absorber offered the Gaussian spectra without compromising the benefits associated with bismuth’s high atomic number, quantum efficiency and low heat capacity.

Studying the microstructures of the evap-Bi and elp-Bi absorbers, whose differences were clear in scanning electron micrographs, implied that the different LE tail behavior is attributed to their distinct grain sizes.

Source: “Eliminating the non-Gaussian spectral response of X-ray absorbers for transition-edge sensors,” by Daikang Yan, Ralu Divan, Lisa M. Gades, Peter Kenesei, Timothy J. Madden, Antonino Miceli, Jun-Sang Park, Umeshkumar M. Patel, Orlando Quaranta, Hemant Sharma, Douglas A. Bennett, William B. Doriese, Joseph W. Fowler, Johnathon D. Gard, James Hays-Wehle, Kelsey M. Morgan, Daniel R. Schmidt, Daniel S. Swetz, and Joel N. Ullom, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.5001198.