# Low Temperature Microcalorimeters for Decay Energy Spectroscopy
## 3 History
### Low Energy DES Measurements  
DES measurements began with low energy measurements (<2 MeV) of radionuclides decaying primarily by electron capture and beta decay. Beta decay measurements with embedded \({}^{187}\)Re for the purpose of neutrino mass measurements were the first explorations of this type of measurement. The advantage here being that Re is a superconducting material at around 1.7 K [18], with the isotope of interest, \({}^{187}\)Re, naturally occurring in metallic Re at 63%. After initial work on understanding the metallic rhenium crystal properties, the MANU Re beta experiments in Genoa reported the first embedded spectrum in 1992 as measured with a Neutron Transmutation Doped (NTD) thermistor [15]. MANU discovered the Beta Environmental Fine Structure (BEFS), a result of self-interference of the outgoing beta-waves with the reflected beta-waves from surrounding atomic shells [18]. This manifested as an undulating modulation on the spectrum. In 1992, the MIBETA Re beta experiment in Milan was begun to work on the \({}^{187}\)Re experiments, employing eight silicon-implanted thermistors coupled to four crystals of silver perrhenate (AgReO\({}_{4}\)) [19; 20; 21]. MIBETA took data between 2002 and 2003. Both MANU and MIBETA were able to provide bounds on the direct anti-neutrino mass of about 26 eV at 95% CL and 15 eV at 90% CL, respectively. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Ref.** & **First Author** & **Year** & **Application** & **Detector** & **Absorber Material** & **Nuclide** & **Deposition** & **Attachment** \\ \hline \({}^{15}\) & Cosulich & 1992 & Neutrino mass & NTD-Ge & Re & \({}^{187}\)Re & natural & \\ \({}^{16}\) & Gatti & 1997 & Neutrino mass & NTD-Ge & Sn & \({}^{163}\)Ho & dried drop & epoxy \\ \({}^{14}\) & Deptuck & 2000 & Neutrino mass & TES & Cu & \({}^{3}\)H & ion implantation & \\ \({}^{17}\) & Voytas & 2002 & Atomic Theory & Si:P:B & Si & \({}^{7}\)Be & ion implantation & epoxy \\ \hline \end{tabular}
\end{table} 
Table 1: Meta data for four papers. These data were derived from information within the paper where possible. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Ref.** & **Radiation Source** & **FWHM** & **E** & **Power** & **Comparison Source** & **FWHM** & **E** & **Power** \\ \hline \({}^{15}\) & Beta & 0.05 & 1.5 & 30 & X-ray & 0.565 & 30 & 53 \\ \({}^{16}\) & Beta & 0.08 & 6 & 75 & & & & \\ \({}^{14}\) & EC & 0.0085 & 0.12 & 14 & tests & 0.03 & 6 & 200 \\ \({}^{17}\) & Beta & 0.09 & 62 & 689 & X-ray & 0.006 & 5.9 & 983 \\ \hline \end{tabular}
\end{table} 
Table 2: Energy resolution values for four papers. The resolving power is determined by dividing the energy by the FWHM. All energies are in units of keV. The Microcalorimeter Arrays for a Neutrino Mass Experiment (MARE) was formed toward the end of these two experiments in 2006, with a mandate to explore \({}^{187}\)Re as a potential source and various low temperature microcalorimeters for a direct neutrino mass measurement [22; 23; 24]. There were early indications that the Re absorber was not behaving as expected for a superconducting absorber [25], with measured heat capacity of the absorbers several orders of magnitude higher than superconducting Re and long thermal tails on the pulses. After a decade of attempts to make Re work as an absorber, the project eventually turned to \({}^{163}\)Ho as the best candidate, having never succeeded in improving the energy resolution sufficiently [26]. The results of MARE were similar to those of its predecessors: \"Rhenium absorbers behave inconsistently, showing a large  
Figure 1: (_Top_) Detector resolving power versus publication date. The first high energy (>2 MeV) DES results occur in 2007 and achieve results exceeding a resolving power of 5000 by 2015. For the low energy (<2 MeV) measurements, the best measurements achieve resolving powers less than 1000.