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Metallic magnetic calorimeters have been used for several years for the determination of the shapes of beta spectra. Experimental spectra of the beta decays of $$^{63}$$ Ni and $$^{241}$$ Pu exhibit, at very low energy, strong deviations from standard theoretical calculations including corrections for screening, radiative, and nuclear size effects. These discrepancies can be attributed to the atomic exchange effect. In particular the spectrum of $$^{63}$$ Ni measured with an electroplated $$^{63}$$ Ni source is in excellent agreement with theory including the exchange effect. This underlines the quality of the experimental spectrum as well as the correctness of the calculation of the exchange effect. [ABSTRACT FROM AUTHOR]

We are developing metallic magnetic calorimeters for beta spectrometry with the aim of determining the shape factors of beta spectra. The beta emitter is enclosed inside the detector absorber and the absorber size is chosen such that the detection efficiency is close to 100% for the entire spectrum. The nature of the deposit of the beta emitter is critical for precise measurement. A dependence of the shape of the spectrum on the quality of the source has been observed, supposedly due to interactions between the electrons and the deposit itself. We chose for this comparative study a nuclide with an allowed transition whose theoretical spectrum is relatively well-known, Ni, a pure beta emitter with an endpoint energy of 66.98 keV. Results of measurements of Ni deposited with different techniques are presented and compared to each other and to the theoretical spectrum. [ABSTRACT FROM AUTHOR]

A novel type of absorber, dedicated to cryogenic detectors, was conceived to reach a high and constant intrinsic detection efficiency (>98%) for up to 25 keV X-ray photons. The absorber consists of two layers having a different atomic number Z. The role of the first layer (large Z) is to make negligible the transmission through the absorber; while the second layer (medium Z) has to reabsorb the escape photons from the first layer. A metallic magnetic calorimeter was realized with an Au-Ag absorber. The required thicknesses of both layers were determined using Monte Carlo simulation of the efficiency. To show the advantages of such a detector, its efficiency and its energy resolution are compared to the efficiency and energy resolution of a gold absorber using a Am source. [ABSTRACT FROM AUTHOR]