Characterisation and modelling of the interaction between sub-Kelvin bolometric detectors and cosmic rays

Abstract : We have studied the effect of cosmic rays in detectors using a composite NTD germanium bolometer at low temperatures and an alpha particle source as a generic source of pulses. We have characterised this bolometer, finding that its pulse shape is due to a combination of its impulse response function (the sum of two double exponentials), and position-dependent effects arising from thermalisation of ballistic phonons into thermal phonons in its absorber. We have derived a scheme for describing the pulse shape in this bolometer, comparing a generic mathematical pulse shape with a second description based on thermal physics. We find that ballistic phonon thermalisation, followed by thermal diffusion, play a significant role in the pulse shape, along with electro-thermal coupling and temperature-dependent electrical effects. We have modelled the pulses, finding that their behaviour can be reproduced accounting for ballistic phonon reflection off the absorber border, with a strong thermal coupling to the bolometer’s central sensor. With these findings, we also investigate the effects of cosmic rays on the Athena X-Ray Integral Field Unit (X-IFU), producing simulated timelines and testing the average RMS temperature increase on the detector wafer, showing that the expected cosmic ray thermal flux is within the same order of magnitudeas the maximum allowed ΔTRMS, posing a threat to the instrument’s energy resolution budget.
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Samantha Lynn Stever. Characterisation and modelling of the interaction between sub-Kelvin bolometric detectors and cosmic rays. Instrumentation and Methods for Astrophysic [astro-ph.IM]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLS009⟩. ⟨tel-02091039v2⟩

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