We characterize cosmic ray interactions in blanked-off \JWST NIRSpec ``dark'' exposures. In its Sun/Earth-Moon L2 halo orbit, \JWST encounters energetic ions that penetrate NIRSpec's radiation shielding. The shielded cosmic ray hit rate decreased from approximately 4.3 to 2.3ionscm−2s−1 during the first three years of operation. A typical hit affects about 7.1~pixels necessitating mitigation during calibration and deposits around 6keV in the λco=5.4μm HgCdTe detector material (equivalent to ∼5200 charges). The corresponding linear energy transfer is about 0.86keVμm−1. As we are currently near solar maximum, galactic cosmic ray flux is expected to increase as solar activity declines, leading to an anticipated rise in the NIRSpec rate from 2.3 to 4.3ionscm−2s−1 by early 2027 and potentially reaching ∼6ionscm−2s−1 in the early 2030s. We investigate rare, large ``snowball'' hits and, less frequently, events with secondary showers that pose significant calibration challenges. We explore their possible origins as heavy ions, secondary particles from shielding, or inelastic scattering in the HgCdTe detector material. We discuss the implications of these findings for future missions including the Nancy Grace Roman Space Telescope.
@article{arxiv.2602.02134,
title = {JWST NIRSpec's Cosmic Ray Experience at L2},
author = {Bernard J. Rauscher and D. J. Fixsen},
journal= {arXiv preprint arXiv:2602.02134},
year = {2026}
}