A technique involving ^{55}Fe X-rays provides a straightforward method tomeasure the response of a detector. The detector's response can lead directlyto a calculation of the conversion gain (e^- ADU^{-1}), as well as aid detectordesign and performance studies. We calibrate the ^{55}Fe X-ray energy responseand pair production energy of HgCdTe using 8 HST WFC3 1.7 \micron flight gradedetectors. The results show that each K$\alpha$ X-ray generates 2273 \pm 137electrons, which corresponds to a pair-production energy of 2.61 \pm 0.16 eV.The uncertainties are dominated by our knowledge of the conversion gain. Infuture studies, we plan to eliminate this uncertainty by directly measuringconversion gain at very low light levels.
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