Noise and energy resolution of X-ray microcalorimeters

Two type of sensor geometries of voltage-biased Xray microcalorimeters with a phase-transition thermometer have been built and tested. Both devices sh ow, in addition to the well-known noise sources of thermal fluctuation nois e or phonon noise from the heatlink to the bath and Johnson noise from the thermometer resistance, also thermal fluctuation noise from the thermometer itself. In both cases however the measured energy resolution is limited by other sources. The energy resolution of the asymmetric lateral type of sen sor, 12 to 15 eV @ 5.9 keV, is limited by a position dependent heatleak of the absorber to the bath. The energy resolution of the symmetric lateral se nsor, 6.8 +/- 0.3 eV FWHM @ 5.9 keV, is limited by excess noise at frequenc ies below 1000 Hz. The origin of this noise component is unknown sofar.