Current dependence of performance of TES microcalorimeters and characteristics of excess noise

We are developing transition edge sensor (TES) microcalorimeter arrays for future Japanese astronomy missions. Although <f>6.6 eV</f> energy resolution for <f>5.9 keV</f> and <f>74 μs</f> decay time are achieved, our energy resolution was limited by the excess noise of unknown origin. The dominant noise showed <f>1/R</f> dependence and it only appeared when the current through the TES, <f>I</f>, is larger than <f>10 μA</f>. This is explained if we assume a constant voltage noise source with a level of <f>∼2(4kTCRn)1/2</f>. We also investigated the influence of <f>I</f> on the TES performance and found the TES sensitivity <f>α</f> is reduced when the ratio of current to the critical current <f>I/IC</f> is large. It can explain the reduction of <f>α</f> observed when operated without a magnetic shield. We also found a strong correlation between <f>IC</f> and <f>α</f> or the baseline width. Thus, we conclude that <f>IC</f> is an essential parameter for the TES performance. [Copyright &y& Elsevier]