Large area bismuth absorbers for X-ray microcalorimeters

Two challenges facing the use of large area (2 mm x 2 mm) bismuth absorbers for microcalorimetry are uncertainties in the heat capacity of bismuth and the effects of lateral heat conduction and position dependence due to the absorber's large size. We have measured the heat capacity of three Bi samples to be 0.3 - 0.6 J K-1 m(-3) at 100 mK. These absorbers also exhibit response variations as phonons created by an X-ray event at an absorber edge will take longer to propagate to the thermometer attachment point than those at the absorber center. This effect may degrade the detector's energy resolution if the propagation time is not very short compared to the thermometer time constant. We show that the response of the largest absorber varies by similar to4% across its area. (C) 2003 Elsevier B.V. All rights reserved.