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We are developing an Immersion Grating (IG) made of CdZnTe which is designed for a high-dispersion midinfrared spectrograph (10-18 μm, R = λ/Δλ ∼ 30, 000) to be onboard the next-generation infrared space telescope GREXPLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer). The adoption of an IG will reduce the spectrometer size to 1/n in length (1/n3 in volume, n: refractive index) compared to conventional diffraction gratings. In order to determine the absorption coefficient of the high-resistivity CdZnTe, we developed a new measurement system for transmittance in 10-18 μm with cryogenic common-path double beam optics equipped with filament lamp source inside the vacuum chamber, which enables accurate determination of the transmittance at the cryogenic temperature by considering the effect of the multiple Fresnel reflection at the sample surface. By the new transmittance measurement system, the CdZnTe sample can be cooled down to ~6 K by employing cooled long wavelength band pass filter (λ > 7 μm) to attenuate the peak emission of the filament lamp (λ ~ 2 μm). In the present paper, we report the results of transmittance measurement with high precision (δτ~0.03%) by our new equipment for the high-resistivity CdZnTe, and the absorption coefficient α of high-resistivity CdZnTe. By applying the value of refractive index n at T > 5.7 K reported recently, α was estimated to be 0.00225 cm-1 and 0.00036 cm-1 at T~300 K and ~12 K, respectively at λ~10 μm in wavelength. In contrast to low-resistivity CdZnTe, the obtained values for α of high-resistivity CdZnTe have shown only slight temperature dependence, and the absorption coefficient values were smaller than the requirement: α<0.01 cm-1 for the IG material. The high-resistivity CdZnTe was likely to be a candidate material of IG for GREX-PLUS high-resolution spectrograph..
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