The impact of radiation damage on photon counting with an EMCCD for the WFIRST-AFTA coronagraph

Abstract

WFIRST-AFTA is a 2.4m class NASA observatory designed to address a wide range of science objectives using two complementary scientific payloads. The Wide Field Instrument (WFI) offers Hubble quality imaging over a 0.28 square degree field of view, and will gather NIR statistical data on exoplanets through gravitational microlensing. The second instrument is a high contrast coronagraph that will carry out the direct imaging and spectroscopic analysis of exoplanets, providing a means to probe the structure and composition of planetary systems.The coronagraph instrument is expected to operate in low photon flux for long integration times, meaning all noise sources must be kept to a minimum. In order to satisfy the low noise requirements, the Electron Multiplication (EM)-CCD has been baselined for both the imaging and spectrograph cameras. The EMCCD was selected in comparison with other candidates because of its low effective electronic read noise at sub-electron 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 characterised and mitigated. In addition, operation within a space environment will subject the device to radiation damage that will degrade the Charge Transfer Efficiency (CTE) of the device throughout the mission lifetime.Here we present our latest results from pre-and post-irradiation testing of the e2v CCD201-20 BI EMCCD sensor, baselined for the WFIRST-AFTA coronagraph instrument. A description of the detector technology is presented, alongside considerations for operation within a space environment. The results from a room temperature irradiation are discussed in context with the nominal operating requirements of AFTA-C and future work which entails a cryogenic irradiation of the CCD201-20 is presented.

Cryogenic irradiation of an EMCCD for the WFIRST coronagraph: preliminary performance analysis

“…The exact multiplication gain necessary to meet this requirement is dependant upon the baseline read-noise of the camera electronics, which itself depends upon the chosen pixel rate. The study performed by Harding et al (2015) 3 has shown that the baseline read noise remains below 90e…”

“…10 Comparison to the results of room temperature irradiation described in Bush et al (2015) highlight a higher factor increase for an equivalent proton fluence when compared to the room temperature case. 23 Following a one week room temperature anneal period dark current was observed to return to what may be expected for a room temperature equivalent dose, measured as (1.14±0.10)×10…”

“…An additional complication is present for EM devices whereby if a cosmic ray deposits a large amount of signal within one of more pixels it will be multiplied within the EM register and can bloom across the row. A discussion concerning one of the physical mechanisms behind this effect can be found in Bush et al (2014), and a study concerning ground-based measurements of cosmic ray fluence for the CCD201-20 can be found in Harding et al (2015).…”

“…These operating parameters were consistent with those used in a previous room temperature irradiation investigation, described in detail within Harding et al (2015) 3 and Bush et al (2015). 23 The use of consistent operating conditions and test methods allowed as fair a comparison as possible between the room temperature and cryogenic irradiation case.…”

Mapping radiation-induced defects in CCDs through space and time

“…Different irradiation conditions can be studied in intense detail and the impact and importance of cryogenic irradiations has been strongly demonstrated [21][22][23]. The technique is being used as a tool to understand the impact of radiation damage on CCDs for future mission studies, such as in the low-signal regime for WFIRST [24][25][26][27].…”