We provide an overview of the design and capabilities of the near-infrared spectrograph (NIRSpec) onboard the James Webb Space Telescope. NIRSpec is designed to be capable of carrying out low-resolution (R = 30− 330) prism spectroscopy over the wavelength range 0.6 − 5.3µm and higher resolution (R = 500− 1340 or R = 1320− 3600) grating spectroscopy over 0.7 − 5.2µm, both in singleobject mode employing any one of five fixed slits, or a 3.1×3.2 arcsec 2 integral field unit, or in multiobject mode employing a novel programmable micro-shutter device covering a 3.6×3.4 arcmin 2 field of view. The all-reflective optical chain of NIRSpec and the performance of its different components are described, and some of the trade-offs made in designing the instrument are touched upon. The faint-end spectrophotometric sensitivity expected of NIRSpec, as well as its dependency on the energetic particle environment that its two detector arrays are likely to be subjected to in orbit are also discussed.
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 ¼ 0:6-5 μm and found that it exceeds unity for photon energies E γ > ð2:65 AE :2ÞE g , where E g is the HgCdTe bandgap energy. This corresponds to λ ≲ 2 μm for NIRSpec's 5 μm cutoff HgCdTe.
We report the direct observation of transferred-electron effect in unintentionally doped GaN epilayers grown by metalorganic chemical vapor deposition. The negative differential resistivity (NDR) was observed from the current-electric field characteristics in GaN using a metal-semiconductor-metal (M-S-M) system. The threshold field for the onset of NDR was independent of the spacing of M-S-M fingers, and was measured to be 1.91×105 V/cm for GaN with an n-type carrier concentration of 1014 cm−3. This value is very close to the value obtained from theoretical simulation. This observation is an experimental evidence of transferred-electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn-effect devices using GaN materials.
In the course of developing microcalorimeters as detectors for astronomical X-ray spectroscopy, we have undertaken an empirical characterization of ''non-ideal" effects in the doped semiconductor thermometers used with these detectors, which operate at temperatures near 50 mK. We have found three apparently independent categories of such behavior that are apparently intrinsic properties of the variable-range hopping conduction mechanism in these devices: 1/f fluctuations in the resistance, which seems to be a 2D effect; a departure from the ideal coulomb-gap temperature dependence of the resistance at temperatures below T 0 /24; and an electrical nonlinearity that has the time dependence and extra noise that are quantitatively predicted by a simple hot electron model. This work has been done largely with ion-implanted Si : P : B, but similar behaviors have been observed in transmutation doped germanium.
The transition between normal conduction and superconductivity in superconducting materials can be exploited as a highly s e n s i t i v e thermometer. Transition temperatures can be tailored through the selection of materials, their c o m p o n e n t material thicknesses, and the comparative ratios i n cases of more than one material. Two b i l a y e r configurations, AgIAI and AuIMo, are e x a m i n e d , including details of preparation, testing, and encountered difficulties. Proposed designs f o r spaceflight detector applications are discussed.
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