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 respo nse of a calorimeter to the absorption of energy depends upon size, heat ca pacity, temperature, magnetic field, concentration of spins and interaction s among them. The conditions that optimize the performance of a calorimeter are derived. Noise sources, especially that due to thermodynamic fluctuati ons of the electrons in the metal, are analyzed. Measurements have been mad e on detectors in which Er serves as the paramagnetic ion and Au as the hos t 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 t hat the performance of both detectors can be improved by an order of magnit ude. At temperatures below 50 mk, the time response of the Au: Er calorimet ers to an energy deposition indicates the presence of an additional heat ca pacity, which we interpret as arising from the quadruple splitting of the A u nuclei in the electric field gradients introduced by the presence of the Er ions.