Microcalorimeters

Microcalorimeters are cryogenic detectors that measure the energy of individual photons or particles by detecting the small temperature rise produced when the energy is absorbed. They consist of a very small absorber bonded to a sensitive thermometer, all cooled to cryogenic temperatures so the total heat capacity is extremely small. When energy E is deposited, the resulting temperature increase ΔT is approximately E divided by the heat capacity C. The thermometer converts this temperature change into an electrical signal, which is read out by low-noise electronics. The signal amplitude is proportional to the deposited energy, enabling high-resolution spectroscopy.

Two common types are transition-edge sensor (TES) microcalorimeters and magnetic microcalorimeters (MMCs). TES calorimeters use a superconducting film biased in its transition region; a small temperature change causes a large resistance change, read out with a SQUID. MMCs employ a paramagnetic sensor in a weak magnetic field, where changes in magnetization produce a measurable magnetic flux, also read by a SQUID. Both designs aim for excellent energy resolution across a broad energy range and can be operated in arrays to achieve imaging and higher throughput.

Arrays of microcalorimeters require multiplexed readout to manage wiring and thermal load. Readout schemes include time-division, frequency-division, and microwave multiplexing. Applications span X-ray astronomy (high-resolution spectroscopy with instruments such as TES-based detectors), laboratory gamma-ray and X-ray spectroscopy, materials analysis, and some areas of particle physics. Challenges include maintaining sub-kelvin temperatures, cryogenic infrastructure, fabrication precision, and achieving linear, rapid response across large dynamic ranges. The combination of high energy resolution and broad spectral coverage makes microcalorimeters a key tool in precision spectroscopy at cryogenic performance.