Time Domain Multiplexed Readout of Magnetically Coupled Calorimeters

Magnetically coupled calorimeters (MCC) have extremely high potential for X-ray applications. Although very high energy-resolution has been demonstrated, until now there has been no multiplexed read-out of MCCs. We report on the first realization of a time domain multiplexed read-out of MCCs. Although this has many similarities with time domain multiplexing of transition-edge sensors, for MCCs the energy resolution is limited by the SQUID read-out noise, and requires the well established scheme to be altered in order to minimize degradation due to noise aliasing effects. In our approach, each pixel is read out by a single first-stage SQUID that is operated in open loop. The outputs of the SQ1s are low-pass filtered with an array of low cross-talk inductors, then fed into a single-stage SQUID multiplexer. The multiplexer is addressed from room temperature and read out through a single amplifier channel. We present the noise performance and compare to expectations. We have demonstrated multiplexed X-ray spectroscopy at 5.9 keV and for improved readout noise achieved an energy resolution ΔE_FWHM <; 6 eV for emulated multiplexing. We show that in an optimized setup, it is possible to multiplex 32 detectors without significantly degrading the intrinsic detector resolution.