Metallic Magnetic Calorimeters

Abstract

Magnetic calorimeters employ the magnetization of a paramagnetic sensor to detect temperature changes produced by the absorption of X-rays or other energetic particles. For typical applications, the detector consists of a metallic absorber and a paramagnetic sensor, which are in strong thermal contact with each other but have a rather weak coupling to a thermal reservoir. The absorption of energy in the calorimeter leads to a rise in temperature and a decrease in magnetization of the magnetic sensor, which can be measured accurately using a low noise, high bandwidth dc-SQUID magnetometer. Fast thermal response can be achieved by using a dilute concentration of paramagnetic ions in a metallic host as sensor material. The sensitivity of the calorimeter to the absorption of energy depends upon size, heat capacity, temperature, magnetic field, concentration of magnetic ions and the interactions among them. Theoretical models, which describe the thermodynamic properties of the calorimeter are discussed, and the conditions that optimize the performance of the detector are derived. Noise sources, especially magnetic Johnson noise and thermodynamic fluctuations of energy between the sub-systems of the calorimeter are analyzed. We discuss the demands placed on the SQUID magnetometer and present a theoretical analysis of the energy resolution. The performance of detector prototypes, including count rate, linearity and energy resolution are described. The measured resolution of devices which were designed for the detection of soft X-rays is E_FWHM= 3.4 eV at an X-ray energy of 6 keV. Calculations indicate that fully optimized magnetic calorimeters will reach energy resolutions of the order of 1 eV under realistic experimental conditions.