The production of the original flight detectors for WFC3-IR ended in Summer 2003 with the selection of two devices, FPA#64 as prime, and FPA59 as backup . FPA#64 has been packaged in its flight assembly (see Fig. 2) and underwent extensive thermal vacuum testing. In parallel with the integration and system level tests done at Ball and at the GSFC, the WFC3 team has carried out further test activities to characterize the performance of similar devices in the space environment. In particular, the effects of cosmic ray radiation have been evaluated in two runs at UC Davis, exposing the detectors to proton beams of different energy and flux while they were operated in standard data acquisition mode. A surprising result of these tests  was the appearance of an extra diffuse background between the pixels directly hit by the protons. Systematic investigation of the phenomenon vs. input proton energy demonstrates that the source is the deposition of proton energy in the thick 0.8 mm) ZnCdTe substrate, while the morphological similarity of the background to the shape of short-wavelength flat fields suggests that the physical mechanism is likely luminescence near the short-wavelength cutoff of the ZnCdTe (near 800 nm).
Scaling of the laboratory results to the on-orbit high-energy particle fluxes suggests that this mechanism could add ~ 0.25 e-/s/pixel to the diffuse background rate in the space environment, though in the absence of a detailed understanding of the physical mechanism, this estimate is highly uncertain. This level of background could lead to a significant degradation of the sensitivity of the instrument.
RSC suggested that removing the entire ZnCdTe substrate should eliminate the anomaly. Substrate removal also offers the advantage of increasing the quantum efficiency in the J-band and pushes the blue cut-off of the detector down to 0.4 ^m.
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