Multiplexed microcalorimeter arrays for precision measurements from microwave to gamma-ray wavelengths

Cryogenic microcalorimeters are a promising technology for ultrasensitive measurements of electromagnetic radiation from microwave to gamma-ray wavelengths. Cryogenic microcalorimeters derive their exquisite sensitivity from the minimal thermal noise at typical operating temperatures near 0.1 K. The core technology of the microcalorimeters under development at NIST is independent of the application wavelength: thin-film thermometers whose temperature and resistance change in response to absorbed energy. However, the absorbing structures used to couple radiation into the thermometers depend strongly on the application wavelength. Here, we describe microcalorimeter technology and its application to microwave, X-ray, and gamma-ray measurements. In particular, we present results from a 13 pixel gamma-ray microcalorimeter array with a coadded energy resolution of 51 eV FWHM at 103 keV and a single pixel with resolution of 27 eV FWHM at 103 keV. One application for gamma-ray microcalorimeters is to deconvolve the complex spectrum of a mixture of Pu isotopes near 100 keV.