# 18th International Workshop on Low Temperature Detectors (LTD-18)

22-26 July 2019
Milano
Europe/Rome timezone

## Temperature and concentration dependence of the heat capacity contribution of holmium ions embedded in metallic absorbers of MMC detectors developed for the ECHo experiment

25 Jul 2019, 17:45
1h 15m
Piazza Città di Lombardia (Milano)

### Piazza Città di Lombardia

#### Milano

Piazza Città di Lombardia, 1, 20124 Milano MI
Poster Low Temperature Detector Development and Physics

### Speaker

Andreas Reifenberger (Kirchhoff Institute for Physics, Heidelberg University, Germany)

### Description

We have investigated dilute alloys of small amounts of holmium in gold and silver in order to determine the impact of their heat capacity contribution on the performance of the microcalorimeters in the neutrino mass experiment ECHo. In particular, we focus on alloys with atomic concentrations of $x_\textrm{Ho}=0.01\,\mathrm{\%} \text{ -} \,3\,\%$ at temperatures between $10\,\textrm{mK}$ and $800\,\textrm{mK}$. Due to the large total angular momentum $J=8$ and nuclear spin $I=7/2$ of holmium, the specific heat of $\underline{\mathrm{Au}}$:Ho and $\underline{\mathrm{Ag}}$:Ho depends on the detailed interplay of various interactions. This makes it rather difficult to accurately determine the specific heat of these materials numerically.
We have measured the specific heat of the materials in question using three different experimental set-ups, two of which were optimized for different temperature ranges and are based on the well-established relaxation method, where the thermal relaxation following a well-defined heat pulse is monitored to extract the specific heat. The third set-up relies on the temperature response of two pixels of the same double meander MMC detector, one with a Ho-doped and one with a Ho-free absorber, after being hit by a K\alpha x-ray from an $^{55}$Fe-source. The results obtained with the three set-ups agree within the expected error margin. We will discuss the temperature- and concentration-dependent measurements as well as the qualitative understanding of the underlying physics.

Student (Ph.D., M.Sc. or B.Sc.) N Y

### Primary author

Andreas Reifenberger (Kirchhoff Institute for Physics, Heidelberg University, Germany)

### Co-authors

Christian Enss (Kirchhoff Institute for Physics, Heidelberg University, Germany) Andreas Fleischmann (Kirchhoff Institute for Physics, Heidelberg University, Germany) Loredana Gastaldo (Kirchhoff Institute for Physics, Heidelberg University, Germany) Matthew Herbst (Kirchhoff Institute for Physics, Heidelberg University, Germany) Sebastian Kempf (Kirchhoff Institute for Physics, Heidelberg University, Germany) Federica Mantegazzini (Kirchhoff Institute for Physics, Heidelberg University, Germany) Clemens Velte (Kirchhoff Institute for Physics, Heidelberg University, Germany) Holger Dorrer (Johannes Gutenberg University, Mainz, Germany) Christoph Düllmann (Johannes Gutenberg University, Mainz, Germany) Tom Kieck (Johannes Gutenberg University, Mainz, Germany) Klaus Wendt (Johannes Gutenberg University, Mainz, Germany) Ulli Köster (Institut Laue-Langevin, Grenoble, France)