Development of a metallic magnetic calorimeter with integrated SQUID readout

Abstract

This thesis describes the development of a high-resolution soft X-ray detector based on metallic magnetic calorimeters (MMCs). MMCs are cryogenic, energy dispersive particle detectors which consist of a particle absorber that is thermally coupled to a paramagnetic temperature sensor. The latter is placed in a weak magnetic field, hence exhibiting a temperature dependent magnetization M(T). Upon X-ray photon absorption, the rise of detector temperature causes a change of sensor magnetization, which is usually read out with a current-sensing dc-SQUID via a superconducting flux transformer. Here, an imperfect transformer matching, as well as a transformer intrinsic energy coupling losses, limit the achievable energy resolution. To challenge this limit, a novel integrated detector was developed, in which the temperature sensor is integrated into a custom-designed dc-SQUID to maximize signal coupling. A major challenge of this configuration is the Joule heating of the SQUID, since heating effects prevent cooling of the detector and thus limit its performance. For this reason, the developed 32 pixel detector makes use of a newly developed thermalization scheme for the SQUID’s shunt resistors, resulting in operation temperatures below 20 mK for the detector. With this kind of detector, a baseline energy resolution of dE = 1.3 eV, and dE = 1.8 eV at 5.9 keV was achieved.