Delayed thermalization of X-rays absorbed in tin films far below the superconducting transition temperature

Abstract As part of a program for developing high resolution X-ray detectors, we have deposited 4 μm thick Sn films on silicon calorimeters to serve as X ray absorbers. Thermistors in the silicon measured the temperature increase resulting from the thermalization of an X-ray photon. Silver-filled epoxy was placed on the Sn to establish a complete thermalization reference. The devices were operated near 0.1 K and below. Signal pulses resulting from the absorption of 6 keV photons in the Ag epoxy rose faster, peaked higher, and decayed to baseline more quickly than those resulting from absorption in the Sn. Taking the Ag pulse shape to be the impulse response of the detector, we deconvolved it from the average Sn pulse shape to obtain the thermalization function in the Sn. The result contained a slow component with a thermalization rate on the scale of several milliseconds. We consider this an indication of the quasiparticle recombination time in the Sn during the non-equilibrium condition following X-ray absorption. We were able to manipulate the thermalization by heating the device above the Sn transition temperature and varying the amount of magnetic field present upon cooling.