New and Better Detectors for the JWST Near-Infrared Spectrograph

Abstract

ESA and NASA recently selected two 5 μm cutoff Teledyne H2RG sensor chip assemblies (SCA) for flight on the James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec). These HgCdTe SCAs incorporate Teledyne’s “improved barrier layer” design that eliminates the degradation that affected earlier JWST H2RGs. The better indium barrier, together with other design changes that Teledyne phased in from other programs over the years, has improved the performance and reliability of JWST’s SCAs. In this article, we describe the measured performance characteristics that most directly affect scientific observations including read noise, total noise, dark current, quantum efficiency (QE), and image persistence. As part of measuring QE, we inferred the quantum yield over the full NIRSpec pass band of λ = 0.6–5 μm and found that it exceeds unity for photon energies Eγ > (2.65 ± .2)Eg, where Eg is the HgCdTe bandgap energy. This corresponds to λ ≲ 2 μm for NIRSpec’s 5 μm cutoff HgCdTe.