JWST NIRSpec's Cosmic Ray Experience at L2

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 $λ_\mathrm{co} = 5.4~μ$m HgCdTe detector material (equivalent to $\sim5200$ charges). The corresponding linear energy transfer is about $0.86~\mathrm{keV~μm^{-1}}$. 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.

💡 Research Summary

This paper presents a comprehensive analysis of cosmic‑ray interactions recorded in the “dark” exposures of the James Webb Space Telescope’s Near‑Infrared Spectrograph (NIRSpec) while operating in its Sun‑Earth‑Moon L2 halo orbit. Using the full‑frame dark monitoring programs from JWST Cycles 1‑3 (Program IDs 1484, 4455, 6633), the authors processed over 200 non‑destructive up‑the‑ramp reads per exposure (14.58889 s frame cadence, ~49 min total integration) with the standard JWST pipeline cosmic‑ray finder.

Key findings include a clear temporal decline in the shielded ion hit rate from ~4.3 ions cm⁻² s⁻¹ in early 2022 to ~2.3 ions cm⁻² s⁻¹ by early 2025, a reduction of roughly 46 % that correlates with the Sun’s approach to activity maximum. The decrease is interpreted as enhanced solar‑wind pressure and magnetic field modulation suppressing the galactic cosmic‑ray (GCR) flux that penetrates the instrument’s radiation shielding.

A typical hit affects an average of 7.1 pixels (approximately a 3 × 3 pixel cluster) and deposits about 6 keV of energy in the HgCdTe detector material, corresponding to ~5,200 electrons. The measured linear energy transfer (LET) of ~0.86 keV µm⁻¹ matches expectations for minimum‑ionizing protons (~2 GeV) traversing the ~5 µm thick detector layer. The authors also quantify the contribution of heavy ions and secondary particles generated within the shielding, noting that the shielding (≈20 mm SiC on the front side, ≈12 mm Mo on the back) blocks essentially all particles below ~100 MeV, while higher‑energy GCRs penetrate relatively unimpeded.

Rare, large‑scale events—dubbed “snowball” hits—are observed sporadically. These consist of extensive charge clouds affecting dozens of pixels and are sometimes accompanied by secondary “shower” patterns extending a few pixels beyond the primary track. The paper explores possible origins: (1) heavy ions (e.g., Fe, Ni) from the GCR spectrum, (2) secondary particles produced by nuclear interactions within the Mo and SiC shielding, and (3) inelastic scattering within the HgCdTe lattice itself. Because such events are not well captured by the standard pipeline detection algorithms, they pose a significant calibration challenge, potentially leading to residual artifacts in scientific data if not properly flagged and corrected.

The authors model the expected future evolution of the hit rate based on the solar cycle. With the Sun now near maximum, the GCR flux is at a minimum; as the cycle declines, the hit rate is projected to rise back to ~4.3 ions cm⁻² s⁻¹ by early 2027 and could reach ~6 ions cm⁻² s⁻¹ in the early 2030s when solar activity is near its minimum. This forecast has direct implications for observation planning, integration time budgeting, and the design of on‑board or ground‑based mitigation strategies.

A comparative discussion places the JWST environment alongside that of the ACE spacecraft at L1 and GOES satellites in geostationary orbit. ACE, also in deep space, should experience a GCR flux similar to JWST, whereas GOES benefits from the Earth’s magnetosphere, resulting in a modestly reduced flux.

Finally, the paper extrapolates its findings to the upcoming Nancy Grace Roman Space Telescope, which will also operate at L2 but with different detector technologies and shielding configurations. The authors argue that the NIRSpec measurements provide a valuable empirical baseline for Roman’s shielding design, cosmic‑ray mitigation pipeline development, and real‑time monitoring architecture. They recommend implementing dynamic calibration updates that respond to measured hit‑rate fluctuations, and suggest that pre‑flight simulations incorporate the observed “snowball” event statistics to avoid under‑estimating calibration risk.

In summary, the study delivers the first in‑flight, instrument‑specific quantification of cosmic‑ray impacts at L2 for a near‑infrared spectrograph, documents the temporal modulation of the hit rate with solar activity, characterizes the energy deposition and spatial extent of typical and rare events, and outlines actionable guidance for future L2 missions to preserve scientific data quality in the face of an ever‑changing high‑energy particle environment.