Metallic magnetic calorimeters for particle detection

The principles and theory of operation of a magnetic calorimeter, made of a dilute concentration of paramagnetic ions in a metallic host, is discussed in relation to the use of such a device as a detector of x-rays. The response of a calorimeter to the absorption of energy depends upon size, heat capacity, temperature, magnetic field, concentration of spins and interactions among them. The conditions that optimize the performance of a calorimeter are derived. Noise sources, especially that due to thermodynamic fluctuations of the electrons in the metal, are analyzed. Measurements have been made on detectors in which Er serves as the paramagnetic ion and Au as the host metal. The measured resolution of a detector with a heat capacity of 10(-12) J/K was 12 eV at 6 keV. In a detector suitable for use with hard x-rays up to 200 keV a resolution of 120 eV was obtained. Calculations indicate that the performance of both detectors can be improved by an order of magnitude. At temperatures below 50 mk, the time response of the Au: Er calorimeters to an energy deposition indicates the presence of an additional heat capacity, which we interpret as arising from the quadruple splitting of the Au nuclei in the electric field gradients introduced by the presence of the Er ions.