# Low Temperature Microcalorimeters for Decay Energy Spectroscopy
## 4 Results  
Decay Energy Spectroscopy has been used with a number of different cryogenic microcalorimeters (see Figure 2), with half the research having been done with MMCs [25; 39; 40; 41; 42; 48; 49; 50; 51; 55; 60; 61; 62; 63; 64] and a quarter having been done with TESs [9; 14; 35; 39; 57; 58; 59; 44; 52; 53; 59]. NTD-Ge thermistors were prevalent in the early research measuring \({}^{187}\)Re for neutrino mass measurements. The Superconducting Tunnel Junction (STJ) and Si:P:B thermistor work were measurements of \({}^{7}\)Be [17; 46; 47]. When the resolving power is plotted as a function of detector type, it becomes clear that the energy resolution is not a function of detector type, but rather with development, a detector can achieve very good resolving power. Since most development in DES measurements has been done with MMCs and TESs, both of these technologies have achieved the high resolution results expected for both high energy and low energy applications. DES detectors were first used for neutrino mass measurements and have continued to be used for neutrino physics with half of the papers on this topic [14; 15; 16; 19; 21; 25; 35; 39; 40; 41; 46; 47; 52; 61; 63; 65; 66]. A third of the papers deal with isotopic determination for safeguards [9; 50; 51; 53; 55; 57; 58; 62; 64]. The papers presenting details on atomic theory [17; 42; 44; 60], for environmental purposes [48; 49], and medical purposes [59] comprise the remainder of the applications as can be seen in Figure 3. Some of these have overlap, most notably the low energy measurements for atomic theory and for neutrino physics. Figure 2: (_Top_) Prevalence of detector use for DES measurements. (_Bottom_) The resolving power is given as a function of detector and it is clear that high energy resolution measurements are not the sole domain of a single detector type. The nuclides measured with DES have varied widely from low energy (<2 MeV) to high energy (>2 MeV) measurements. The former represent electron capture and beta-decaying radionuclides, while the latter are \(\alpha\)-decaying nuclides. These are shown in Figure 4. As a function of nuclide, the resolving power does not appear to show any discernible trend other than for each radionuclide there appears to be at least one measurement with a low resolving power, indicating that it takes multiple attempts to get a good measurement. Figure 4: (_Top_) Nuclides measured with DES detectors. The bars in grey represent the low energy nuclides that decay primarily by electron capture or beta decay. The bars in blue represent the high energy nuclides that decay primarily by \(\alpha\) decay. (_Bottom_) As a function of nuclide, the resolving power does not appear to show any discernible trend. Figure 3: Applications of DES detectors. The majority of measurements, particularly those after the Re experiments of early 2000 s, have used Au as the absorber material as seen Figure 5. Au provides the advantage that it can be electrodeposited, which is important for encapsulation, and formed into foils, which is important for the dried drop methods of encapsulation. The Re experiments used both Re crystal and silver perhenate (AgReO\({}_{4}\)) forms, and since \({}^{187}\)Re is found in natural Re, these did not have to be enriched in the isotope for measurement. Despite 8 measurements of the silver perhenate form, the energy resolution never improved to the current limit of low energy DES measurements (resolving power of 1000). The Pt absorber measurement was performed with enriched and irradiated Pt, and it was determined that the heat capacity was too high for the measurements to produce high energy resolution [44]. The measurements with Si was of an ion-implanted \({}^{7}\)Be source and the resolving power for this measurement was quite good (689) [17]. The Sn measurement was a dried drop of \({}^{163}\)Ho in solution. This had poor energy resolution, but it is not clear if this is due to the Sn absorber or the dried droplet method [16].