# Low Temperature Microcalorimeters for Decay Energy Spectroscopy
## 1 Introduction  
Low temperature detectors have been used for various applications [1] ranging from low energy measurements like X-rays to gamma-rays to high energy measurements of a particles and even fission fragments. Excellent reviews of the X-ray [2, 3, 4, 5] and gamma-ray applications [3] exist and there are few low temperature measurements of a particles [6, 7, 8, 9, 10, 11] and fission fragments [12, 13], so those papers can be easily read for an entire review of the field. Low temperature detectors have also been used in the internal measurement mode with macrocalorimeters and bolometers, but this review focuses on microcalorimeters, which measure thermalized energy on an event by event basis. Although the use of microcalorimeters for the internal or embedded technique has three decades worth of publications, a review of the technique is lacking, particularly as it pertains to the energy resolution achievable with this technique. Low temperature detectors' primary advantage is superior energy resolution as compared to semiconductor detectors. For gamma-rays, an order of magnitude resolution better than High Purity Germanium detectors is standard at 100 keV (resolving power of about 1500) and for X-rays the story is similar. For a detectors, achieving less than 1 keV FWHM on a 5 MeV peak is typical with careful consideration to source preparation and position sensitivity of the detector, which yields a resolving power of about 5000. For fission fragments, achieved resolution is now 91 keV for measurements of 20.7 MeV ions [12]. However, for internal measurements with Decay Energy Spectroscopy, the achieved resolution was falling short of the predicted achievable resolution. Decay Energy Spectroscopy (DES) is a particular measurement mode using low temperature detectors where the radioactive source is embedded within an absorber. This absorber thermalizes the decay energy and is coupled to a low temperature detector, such as a Transition Edge Sensor (TES), Metallic Magnetic Calorimeter (MMC), or other low temperature detector. When the source decays, the decay radiation (i.e., electrons, \(X\)-rays, gamma-rays, \(\alpha\) particles, recoil daughter nuclei) is thermalized within the absorber. Due to the long thermal time constants of DES detectors, the entire energy of the decay is measuredrather than the energies of individual particles involved in the decay. This is excepting the radiation that escapes the absorber, such as neutrinos and high energy gamma-rays. The detector becomes a highly efficient method of measuring radiation and can be considered a 4\(\pi\) detector if careful attention is paid to whether material is on the surfaces of the absorber or entirely contained within the volume of the absorber. As a result, these detectors can be used for very low activity measurements, often targeting 1-10 Bq. Due to the long time constants of these detectors, higher activities present a challenge of pileup. Aside from the very high energy resolution that comes from using low temperature detectors, the DES measurement technique offers another significant advantage: it is independent of decay path. For example, measuring the \(\alpha\) energy spectrum of an isotope that \(\alpha\)-decays results in peaks for each \(\alpha\) energy. Measuring the same isotope with DES results in a single peak corresponding to the \(Q\) value or reaction energy. This is because the detector is able to thermalize energy regardless of its form, such as kinetic energy in the recoil daughter nucleus or gamma-rays resulting from an \(\alpha\) decay to an excited state of the daughter nucleus. Provided the energy does not get stored in a long-lived state, the energy will be read out by the DES detector and can simplify many complicated spectra resulting from a variety of decay paths. This review aims to highlight the diversity of applications of the DES technology, giving a brief history of Decay Energy Spectroscopy over the past few decades.