The ECHo experiment will be performed using Metallic Magnetic Calorimeters (MMCs). MMCs are energy dispersive detectors typically operated at temperatures below 50 mK.

These detectors consist of a particle absorber, where the energy is deposited, tightly connected to a temperature sensor which is weakly connected to a thermal bath. The deposition of energy in the absorber leads to an increase of the detector temperature. The temperature sensor of the MMCs is a paramagnetic alloy which resides in a small magnetic field. The change of temperature leads to a change of magnetization of the sensor which is read-out as a change of flux by a low-noise SQUID magnetometer. The sensor material, presently used for MMCs, is a dilute alloy of erbium in gold, Au:Er. The concentration of erbium ions in the sensor can be chosen to optimize the detector performance and usually varies between 200 ppm and 800 ppm.

The spectral resolving power of a state of the art MMCs for soft x-rays is above 3000. For completely micro-structured detectors, an energy resolution of ΔEFWHM = 2eV at 6keV and a signal rise-time τr = 90ns have been achieved12. Moreover the typical non-linearity at 6 keV is less than 1% and the non-linear part can be described very well by a polynomial function of second order. The achieved performance suggests that MMCs are suitable detectors for measuring the high precision and high statistics EC spectrum of 163Ho.

First MMC prototype for ECHo