JWST NIRSpec's Cosmic Ray Experience at L2

TL;DR

This study characterizes cosmic ray interactions with JWST NIRSpec at L2, observing a decrease in hit rate over time, analyzing the nature of hits, and discussing implications for future space telescopes amid changing cosmic ray flux.

Contribution

It provides the first detailed analysis of cosmic ray interactions in JWST NIRSpec, including hit rates, energy deposition, and rare events, informing calibration and future mission planning.

Findings

Cosmic ray hit rate decreased from 4.3 to 2.3 ions/cm²/s over three years.

Typical hits affect about 7 pixels and deposit around 6 keV in the detector.

Anticipated cosmic ray flux will increase as solar activity declines, impacting future observations.

Abstract

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 $2.3~\mathrm{ions~cm^{-2}}~s^{-1}$ during the first three years of operation. A typical hit affects about 7.1~pixels necessitating mitigation during calibration and deposits around $6~\mathrm{keV}$ in the $\lambda_\mathrm{co} = 5.4~\mu$m HgCdTe detector material (equivalent to $\sim5200$ charges). The corresponding linear energy transfer is about $0.86~\mathrm{keV~\mu m^{-1}}$. 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 $4.3~\mathrm{ions~cm^{-2}}~s^{-1}$ by early 2027 and potentially reaching $\sim6~\mathrm{ions~cm^{-2}}~s^{-1}$ 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.