# Low Temperature Microcalorimeters for Decay Energy Spectroscopy
## 3 History
### Low Energy DES Measurements
(_Bottom_) The resolving power of DES measurements is compared to benchmark measurements, which correspond to predictions based on the noise or baseline signal, measurements of other radiation such as X-rays or gamma rays, or from prior detector knowledge. In this case, some measurements with very low resolving power are predicted and achieved, but predictions of higher resolving power as is standard with low temperature microcalorimeters were frequently not achieved in these DES measurements. deficit in the energy thermalization accompanied by long time constants\" [27]. A good review of the history of rhenium absorbers is given in [27]. In the meantime, in 1997, [PERSON] et al. measured \({}^{163}\)Ho within a tin absorber coupled to an NTD Ge thermistor. The holmium source was dried onto a tin foil from a water solution and covered with epoxy. The foil was then folded on itself and pressed to form a thermal link with the epoxy. The results of this measurement were lower than expected energy values of the measured peaks, asymmetric peaks, and degraded resolution. All of these effects were attributed to inefficient energy thermalization due to the inhomogeneous absorber (epoxy and tin) [16]. In the early 2000s, a brief foray into using TESs to measure the beta spectrum of tritium was conducted [14; 28], but the detectors ended up being too slow to achieve meaningful results for neutrino mass measurements, as had been the purpose. From 2003-2008, Cuoricino used bolometers to measure double beta decay on \({}^{130}\)Te within TeO\({}_{2}\) crystals and actually achieved better energy resolution than predicted [29]. This experiment developed into the CUORE neutrinoless double beta decay experiment with a better sensitivity for neutrinoless double beta decay [30]. The bolometer research is not included in the papers analyzed in this review article due to mostly being used in macrocalorimeter research. In 2008 and 2010, [PERSON] et al. published a couple papers using the embedded source method for metrological purposes. The radionuclides of interest were dissolved in a solution and dried onto a gold foil, which was folded over, pressed, and diffusion-welded to encapsulate the material, then coupled to a Au:Er MMC [31; 32]. In the first measurement [31], the spectral resolution allowed for differentiation between the K captures and L captures of \({}^{55}\)Fe, but was not sufficient to differentiate between the electronic subshells (i.e., capture from the L1 shell versus L2). Energy loss within the source (FeCl\({}_{3}\) crystals) contributed to spectral distortions in this measurement and there was a substantial difference in the detector response to X-rays versus Auger electrons. While the resolution was far from optimal, for the purposes of measuring the total capture probabilities, the achieved resolution was sufficient. In the second paper, the same measurement technique with a diffusion-welded foil containing a dried sample was used to measure the beta spectrum of \({}^{241}\)Pu [32]. There was some question with this measurement about whether other material co-deposited with the source (specifically a substance from the chemical separation of Pu and Am) contributed to spectral changes arising from different thermalization between electrons and photons. In 2017, a second measurement of \({}^{55}\)Fe featured an electroplated sample on a gold foil that was subsequently diffusion-welded at 400 \({}^{\circ}\)C [33]. This measurement was successful in both cleaning up the spectrum and lowering the energy threshold to 50 eV to include the M captures. About this time, direct neutrino mass experiments, such as Electron Capture \({}^{163}\)Ho experiment (ECHo), Holmium neutrino experiment (HOLMES), and Neutrino Mass via \({}^{163}\)Ho Electron Capture Spectroscopy (NuMECS) explored methods of embedding \({}^{163}\)Ho in gold absorbers. For excellent reviews of neutrino mass measurements with \({}^{163}\)Ho, see [27; 34].