Thermal Simulations of Temperature Excursions on the Athena X-IFU Detector Wafer from Impacts by Cosmic Rays

Abstract

We present the design and implementation of a thermal model, developed in COMSOL, aiming to probe the wafer-scale thermal response arising from realistic rates and energies of cosmic rays at L2 impacting the detector wafer of Athena X-IFU. The wafer thermal model is a four-layer 2D model, where 2 layers represent the constituent materials (Si bulk and Si 3 N 4 membrane), and 2 layers represent the Au metallization layer's phonon and electron temperatures. We base the simulation geometry on the current specifications for the X-IFU detector wafer, and simulate cosmic ray impacts using a simple power injection into the Si bulk. We measure the temperature at the point of the instrument's most central TES detector. By probing the response of the system and pulse characteristics as a function of the thermal input energy and location, we reconstruct cosmic ray pulses in Python. By utilizing this code, along with the results of the GEANT4 simulations produced for X-IFU, we produce realistic time-ordered data (TOD) of the temperature seen by the central TES, which we use to simulate the degradation of the energy resolution of the instrument in space-like conditions on this wafer. We find a degradation to the energy resolution of 7 keV X-rays of ≈0.04 eV. By modifying wafer parameters and comparing the simulated TOD, this study is a valuable tool for probing design changes on the thermal background seen by the detectors.

Simulations of systematic effects arising from cosmic rays in the LiteBIRD space telescope, and effects on the measurements of CMB $B$-modes

“…The first and most important task we plan to carry out is to investigate the addition of wirebonds to increase the G of the detector wafer, and therefore the speed with which thermal fluctuations arising from the CR impacts in the wafer are evacuated to the thermal bath. This will have an overall effect on the level of the thermal noise from indirect CR impacts, as has been found in other studies [20]. Due to the large size of the LiteBIRD detector wafers, as well as their uncharacteristic thickness compared with other space missions, efficient thermal evacuation to the thermal bath is of crucial importance.…”

“…The thermal model is a two-dimensional finite-element model with a virtual z-axis (using the same process as described in prior work [20]. However, unlike the X-IFU case, the LiteBIRD model consists only of one thermal layer due to the lack of a metallisation on the surface, i.e.…”

“…To facilitate this scaling, we calculate the relationship between deposited energy and pulse amplitude, and use the interpolated energy-amplitude relationships at the distance of each deposition to the thermal probe to predict the pulse height for any deposited energy, using the same procedure described in refs. [19,20]. An example of this is shown in figure 5 (solid lines) in which 0.5 MeV is deposited at various distances from the wafer centre and temperature is measured at the locations of TES 9 (left) and TES 2 (right).…”

Simulations of systematic effects arising from cosmic rays in the LiteBIRD space telescope, and effects on the measurements of CMB B-modes

“…COMSOL uses finite-element modelling methods, and we use the Heat Transfer Module to assess the propagation of heat across the wafer at varying energies and energy deposition locations. Studies of this nature have been successfully performed for CR studies in other space instruments, notably Athena X-IFU, in which the impact of thermal fluctuations in the detector wafer on instrument energy resolution were assessed [19][20][21].…”

“…The thermal model is a two-dimensional finite-element model with a virtual z-axis (using the same process as described in prior work [20]). However, unlike the X-IFU case, the LiteBIRD JCAP09(2021)013 model consists only of one thermal layer due to the lack of a metallisation on the surface, i.e.…”