Noise and energy resolution of X-ray microcalorimeters

Two type of sensor geometries of voltage-biased X-ray microcalorimeters with a phase-transition thermometer have been built and tested. Both devices show, in addition to the well-known noise sources of thermal fluctuation noise 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 sensor, 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 sensor, 6.8+/-0.3 eV FWHM @ 5.9 keV, is limited by excess noise at frequencies below 1000 Hz. The origin of this noise component is unknown so far.