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”Ēs
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
¦¤TRMS, posing a threat to the instrument”Ēs energy resolution budget. We are now
working to expand upon this work for the next-generation CMB space mission LiteBIRD.