The55Fe x-ray energy response of mercury cadmium telluride near-infrared detector arrays

Abstract

ABSTRACT The accurate determination of a detectors fundamental parameters, including read noise, dark current, andQE, relies on a proper measurement of a detectors conversion gain ( e Š ADU Š 1 ). Charge coupling eects,such as interpixel capacitance, attenuate photon shot noise and result in an overestimation of conversion gainwhen implementing the photon transfer technique. An approach involving 55 Fe X-rays provides a potentiallystraightforward measurement of conversion gain by comparing the observed instrumental counts (ADU) to theknown charge ( e Š ) liberated by the X-ray. This technique is already preferred within the CCD community,as the pair production energy for silicon is well established. In contrast, to date the pair production energyis unknown for HgCdTe, a material commonly used for near-infrared detectors. In this paper, we derive apreliminary calibration of the 55 Fe X-ray energy response of HgCdTe using 8 HST WFC3 1.7 µ m ”ight gradedetectors. Our conversion of the X-ray intensities from counts into electrons implements a technique that restoresthe trueŽ gain via classical propagation of errors. For these detectors, our analysis yields preliminary results ofgood statistical precision: each K event generates 1849 ± 46 electrons, which corresponds to a pair productionenergy of 3.21 ± 0.08 eV. We are continuing to assess potential systematic eects to further re“ne the accuracyof this result.Keywords: Astronomical Instrumentation, Infrared Detectors, Conversion Gain, Interpixel Capacitance