Optimization of the TES-Bias Circuit for a Multiplexed Microcalorimeter Array.

A transition-edge-sensor (TES) microcalorimeter's shunt resistor ( R) and thermal conductance to the cryogenic bath ( G) are often considered to be interchangeable knobs with which to control detector speed. Indeed, for otherwise-identical TES-parameter models, there are many combinations of R and G that give the same decay time-constant ( τ). However, our previous work showed that with time- or code-division-multiplexed readout, the distribution of signal-to-noise ratio with frequency, which depends strongly on R and G, is just as important as τ. Here, we present a set of calculations to select the optimal values of R and G, given a linear TES model and count-rate and energy-resolution requirements. Lower G and lower R make multiplexing easier. Our calculations also determine the allowed combination of SQUID-readout noise ( S) and multiplexer row-period ( t) and row-count ( N). Recent improvements to S and t in the NIST time-division-multiplexing architecture have allowed a NIST eight-pixel TES array to be read out with 2.70 eV (full-width at half-maximum) average energy resolution at 6 keV. The improvements make the X-ray Microcalorimeter Spectrometer co-proposed by NASA and NIST for ESA's Athena X-ray observatory straightforwardly achievable, including engineering margin, with N=16. [ABSTRACT FROM AUTHOR]