Bijan Nemati is a senior engineer at JPL's advanced optics section. He received his Ph.D. in experimental particle physics from the University of Washington. His post-doctoral research was in the area of characterizing the decay properties of particles with the charm quark. Since 1997 he has been working on advanced optical instruments, first at Lockheed Martin and later at JPL. He developed technology for picometer-class optical truss metrology to be used for astrometric space instruments. He also co-developed the technique Synthetic Tracking for detecting very small, fast asteroids from the ground. He is currently the integrated modeling lead for the WFIRST AFTA coronagraph, scheduled to be launched in 2024.
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The Coronagraph Instrument (CGI) will be required to operate with low signal flux for long integration times, demanding all noise sources are kept to a minimum. The Electron Multiplication (EM)-CCD has been baselined for both the imaging and spectrograph cameras due its ability to operate with sub-electron effective read noise values with appropriate multiplication gain setting. The presence of other noise sources, however, such as thermal dark signal and Clock Induced Charge (CIC), need to be characterized and mitigated. In addition, operation within a space environment will subject the device to radiation damage that will degrade the Charge Transfer Effciency (CTE) of the device throughout the mission lifetime. Irradiation at the nominal instrument operating temperature has the potential to provide the best estimate of performance degradation that will be experienced in-flight, since the final population of silicon defects has been shown to be dependent upon the temperature at which the sensor is irradiated.
Here we present initial findings from pre- and post- cryogenic irradiation testing of the e2v CCD201-20 BI EMCCD sensor, baselined for the WFIRST coronagraph instrument. The motivation for irradiation at cryogenic temperatures is discussed with reference to previous investigations of a similar nature. The results are presented in context with those from a previous room temperature irradiation investigation that was performed on a CCD201-20 operated under the same conditions. A key conclusion is that the measured performance degradation for a given proton fluence is seen to measurably differ for the cryogenic case compared to the room temperature equivalent for the conditions of this study.
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