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.
