High Resolution Magnetic Micro-calorimeters: Thermodynamics, Cooling Requirements, and Noise

Magnetic micro-calorimeters (MMCs) are cryogenic particle detectors well suited for high-precision X-ray spectroscopy. They measure the temperature rise caused by an X-ray impact via the change in magnetization of a paramagnetic temperature sensor. Until now, MMCs have been designed to operate at around 20mK, requiring sophisticated cooling, which limits their application. In this work, we show that magnetic micro-calorimetry is possible at significantly higher temperatures, by developing two novel MMCs with reduced cooling requirements. The first illustrates a new application for MMCs in the field of particle induced X-ray emission spectroscopy. At an operating temperature of 85mK, this detector has a FWHM energy resolution of 19eV at 5.9keV, outperforming current alternatives. Our second MMC is a proof-of-principle detector, which demonstrates that operating temperatures of up to 300mK are feasible. With a third, stand-alone device, we analyze noise sources affecting superconducting microstructures, such as MMCs. By comparing results from three different operation modes, we are able to disentangle noise components, in particular magnetic flux noise. High-precision measurements of noise originating from the sensor show a previously unobserved Johnson noise component and unexpected variations in the magnetic flux noise, which we relate to the dynamics of the magnetic moments in the sensor. Overall, our results broaden the application range of MMCs, and illustrate how noise analysis can improve the performance of superconducting devices.