Low Temperature Property Study of MMCs Used for Neutrinoless Double Beta Decay

We present a comparison of the two types of metallic magnetic calorimeters (MMCs) developed for a large-scale neutrinoless double beta decay (
$0\nu \beta \beta$
) search experiment. As high resolution micro-calorimeters, MMCs employ micro-fabricated superconducting circuits to measure the changes in magnetization of a paramagnetic sensor material. We constructed two alpha spectrometers with two MMC sensors made of Au:Er and Ag:Er (i.e., gold and silver alloys with small concentrations of Er). A piece of gold foil with a heat capacity comparable to that of 350 g CaMoO
$_4$
crystal near 10 mK was used as the absorber for each alpha spectrometer. These detector setups allowed us to investigate the temperature dependence of magnetization for the different sensor materials over a wide temperature range with various amounts of field current. The magnetization measurements for both the Au:Er and Ag:Er sensors were in good agreement with calculations for field currents of up to 100 mA. Moreover, the measured and estimated pulse heights of alpha events agreed with each other, and were compared between the different sensor materials. We discuss how the excess heat capacity from the nuclear spins in Au:Er influences the pulse size and shape in calorimetric detection. Moreover, we describe the benefit of using Ag:Er sensors in thermal calorimeters with large crystal absorbers in a
$0\nu \beta \beta$
search experiment.