Simulation of the Cosmic Ray Impact on the TES Detectors of SPICA/SAFARI

The data from the Planck and Herschel space observatories revealed that the cosmic rays at L2 orbit can have a significant impact on the performance of scientific instruments. In this paper, we present our simulation results of such impacts on SAFARI/SPICA, a far-infrared spectrometer equipped with transition-edge sensors (TESs). These TESs are fabricated on SiN membranes and suspended by long and thin SiN legs that thermally isolate them from the surrounding silicon structure (wafer). Cosmic rays that pass through this surrounding structure deposit a portion of their energy, leading to temperature fluctuations in the wafer. These temperature fluctuations are sensed by the TES detectors as an effective bath temperature and result in additional noise. To simulate the impact, we generate a 2D model of the wafer and the suspended TESs in COMSOL 5.4. This 2D model is bombarded with 128 randomly generated cosmic rays according to the observed energy distributions at L2. Subsequently, the temperature fluctuations at different points on the wafer are estimated. Our results show that these thermal fluctuations, as well as the calculated additional TES noise caused by them, depend strongly on the heat-sink design of the wafer. We study the impact of the different heat sink designs on the noise profile of the system. Later, these results are compared to the SAFARI instrument noise requirements.