Noise and dark performance for FIREBall-2 EMCCD delta-doped CCD detector
Abstract
The Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall-2) is an experiment designed to observe low density emission from HI, CIV, and OVI in the circum-galactic medium around low-redshift galaxies. To detect this di↵use emission, we use a high-eciency photon-counting EMCCD as part of FIREBall-2’s detector. The flight camera system includes a custom printed circuit board, a mechanical cryo-cooler, zeolite and charcoal getters, and a N¨uv¨u controller, for fast read-out speeds and waveform shaping. Here we report on overall detector system performance, including pressure and temperature stability. We describe dark current and CIC measurements at several temperatures and substrate voltages, with the flight set-up.
1. Introduction
FIREBall-2 is a balloon-born ultraviolet (UV) multi-object spectrograph designed to observe faint line emission from the circumgalactic media (CGM) of low redshift galaxies (z ⇠ 0.7). A previous version of the FIREBall mission has flown twice, in 2006 and 2009.1, 2 Since that time, we have upgraded the spectrograph significantly to increase the field of view, throughput, and number of targets per observation.3–5 These improvements will yield a factor of 30 increase in overall sensitivity and should provide multiple detections of the CGM in emission for the first time at UV wavelengths. FIREBall-2 will also act as a test-bed for new technologies, including the use of an electron multiplying charge coupled device (EMCCD) processed at JPL to provide UV sensitivity. The delta-doping procedure,6–8 yields a device with 100% internal quantum eciency, limited only by surface reflections. Anti-reflection coatings minimize this reflectance and bring the quantum eciency of the FIREBall-2 device up to > 65% from 200-210 nm.
FIREBall-2 will use a e2v⇤ CCD201-20 in photon counting mode.
1.1 EMCCDs and noise considerations
EMCCDs9–11 consist of a normal CCD with an extra serial register added after the normal register. This addition to the serial register of an EMCCD contains additional serial register pixels which replace R2 with a DC level pixel and a high voltage (HV) clock. As photo-electrons move through the multiplication register, they undergo impact ionization when passing through the high voltage clock, generating additional electrons. The exact multiplication gain achieved is a stochastic process, which is controlled by the maximum voltage of the HV clock. Each individual transfer has a relatively low probability of multiplication (<2%), but when an electron passes through all 604 multiplication pixels (in the case of an e2v CCD201-20), the final number of electrons generated can be quite large. For our testing, we operate at an electron multiplication (EM) gain of around 1000 e/e.
The advantage of the EM gain process is that it increases the signal from a single photo-electron to a value much larger than the on-chip amplifier read-noise. This process means single events can be detected by a threshold process described more thoroughly in recent works.
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