Benefits of the Use of Monte Carlo Simulations in Cryogenic Detector Design
The X-IFU total unrejected background spectra (i.e., the NXB) obtained with the 2017 mass model (left), including the contributions from GCR Protons, Alpha particles and electrons. The identified lines and escape peaks in the photons component of the unrejected background (right).
By Simone Lotti
Monte Carlo simulations represent a powerful tool to enhance the design and performance of cryogenic detectors, a critical component for advanced X-ray observatories such as NewAthena.
By modeling the complex interactions of Galactic Cosmic Rays expected in L1 on detector and surrounding structures, the study provides insights into the background rejection capabilities of the X-IFU Cryogenic AntiCoincidence detector (CryoAC), and their dependence on some of the recent modifications of the detector and cryogenic assembly.
Starting from the results obtained under the standard configuration foreseen in 2017, we simulated the impact of the expected maximum flux in L1 for GCR protons, α particles and electrons on the residual particle background, and explored the impact of several changes in geometry on the detector’s performance.
We conducted tests to evaluate the redefined NewAthena X-IFU concept (based on the 2023 information) effect on the residual background. As Athena is transitioning from ambient to 50 K from an active cooling approach to a passive one, the mass surrounding the instrument was significantly altered. Consequently, we explored the potential impact of removing the X-IFU external shieldings on the residual background.
We also investigated the possibility of using the filter wheel closed position to gather Non-X-ray background (NXB) data for background calibration, simulating such a configuration and comparing it to the standard open configuration expected for astrophysical observations.
Results from this activity include:
Confirmation of a uniform background distribution and resilience against reasonable misalignments between the main array and the CryoAC. Testing new FPA configurations revealed that, while a more realistic design improves reliability, it limits background mitigation options.
NXB levels remained stable despite minor absorbers' thickness and size changes.
Tests without the external cryostat layers showed a slight increase in primary flux and a reduced NXB level, due to a lower amount of mass available for the secondary particles creation.
Simulating the closed filter wheel position exhibited no significant effect on the residual background, supporting its use for background characterization and cross-calibration.
The same approach was used to reproduce the results of laboratory measurements for the DM#127 CryoAC prototype, and of the LEM focal plane detectors.
With NewAthena aiming to deliver unprecedented sensitivity and resolution in X-ray astronomy, the role of particle background is of primary importance to the observational capabilities. In this regard, the insights provided by Monte Carlo simulations are directly relevant for understanding its level and origin, and testing possible solutions to reduce its level and optimize detector performance.