Optimization of magnetic calorimeters

The properties and performance of magnetic calorimeters based on Au containing a few hundred ppm of Er can be fully described by equilibrium thermodynamics. As a consequence, the magnitude of the change of magnetization of the sensor, resulting from the absorption of a particle, can be calculated with confidence. The magnetization change depends upon a number of parameters such as the external magnetic field, the temperature and the concentration of the Er ions. This theoretical understanding of the calorimeter also allows us to calculate the flux signal detected by a SQUID and how that signal depends on the detector geometry and other relevant parameters. To date we have measured only cylindrically shaped sensors, which are located directly in a circular SQUID loop. However, a sensor having the shape of a thin strip, possibly in form of a meander pattern, enclosed by a loop of the same geometry, has the potential of providing enhanced flux coupling to the SQUID. We discuss the relation of sensor geometry to other parameters such as the dimensions and heat capacity of an attached particle absorber. The values of the adjustable parameters that optimize the performance of a magnetic calorimeter are investigated under a number of different experimental constraints.