3.1.

Pulse Height Distribution and Signal Losses Due to Camera Noise

The statistical output of the EMCCD gain register is a function of the operating gain and the number of input electrons.¹⁴ We refer to the probability distribution of output electrons from the register as the PHD, an example of which is shown in Fig. 1 for 1, 3, and 10 electron inputs to the gain register operating at a mean gain of 1000. Single electron input to the gain register results in an exponential output distribution, while multiple electron input results in a broad gaussian-shaped profile. The profile introduces a noise factor¹⁵ (the “excess noise factor”) that reduces the SNR by a factor of $\sqrt{2}$ for signals originating prior to the gain register. One can think of this (“analog”) mode of operation effectively halving the QE of a conventional CCD operating at unity gain, albeit with much lower effective noise (which could result in higher SNR depending on the circumstance). The excess noise factor is eliminated in the photon counting limit of low signal levels when it can be assured that there will not be more than one signal electron in an image pixel in a given frame. In this limiting scenario, the “brightness” of a pulse measured from the gain register has no correlation with the actual brightness of the source due to the exponential PHD. It is the frequency of pulses measured from a given detector location provides this information.

Fig. 1

Three theoretical PHDs for an EMCCD with 604 multiplication elements operating at a gain of $1000\times$ with 1, 3, and 10 electron input. The simulation does not include shot noise in the input signal.

The exponential nature of the photon counting EMCCD PHD has the consequence that some fraction of events are hidden by read noise, and the gain must be chosen to minimize this fraction while balancing the impact of high gain on other performance parameters, such as charge transfer efficiency. The gain is chosen to be the minimum value that meets the photon counting efficiency requirements. The magnitude of the read noise therefore defines the gain that is required to detect a sufficiently large fraction of events. For Roman CGI, we require at least 90% of events to have a signal level at the output of the gain register that is greater than five times the read noise. This ensures high detection efficiency while reducing the possibility of detecting spurious read noise events to near zero. One could experiment with lower thresholds as a means of reducing the required gain as long as leakage of other noise sources above the threshold level remains negligible. The required gain is determined from the exponential PHD for single electron input to the EMCCD gain register¹⁴

Fig. 2

A measured PHD from a CCD201 with 604 multiplication elements operating at a gain of $3300\times$. The histogram shows the distribution of values measured in the image, and it can be seen that since most pixels have no signal that the gaussian read noise dominates at low signal levels. The plot includes a fit to the read noise and the extrapolated distribution of amplified events. The vertical dashed line shows the $5\sigma$ threshold above which events are unambiguously detected.

The photon counting efficiency factor ${ϵ}_{PC}$ is the probability that any pulse from the EMCCD gain register will have an amplitude greater than a specified threshold $t$. This quantity is the integral of Eq. (2) for $x>t$, or

If we expand the exponential and solve for the minimum gain required to amplify events above a threshold $t$ with efficiency ${ϵ}_{\mathrm{PC}}$ then we find

Therefore with a typical read noise of 100 electrons, the $5\sigma$ threshold would be $t=500$ electrons, which could be exceeded with 90% (${ϵ}_{PC}$) probability with a gain greater than $5000\times$.¹⁶ A reasonable rule of thumb would be to operate the EMCCD with a ratio of gain/read noise greater than 50 to maximize throughput in photon counting mode.

3.2.

Coincidence Losses

All photon counting systems will at some point lose track of incident light due to photon arrival times too close to distinguish. As discussed in the last section, the statistics of the EMCCD gain register do not allow multiple detections in a single pixel to be discriminated without added noise terms, so pixels with multiple electrons will simply be counted as one. The magnitude of this effect can be approximated by considering a $3\times 3\text{pixel}$ region of the sensor and a frame time $t$ s chosen long enough that on average a single pixel will be illuminated in each frame with flat field illumination. In this case the flux is $0.11/\mathrm{t}\text{counts-}{\text{pixel}}^{-1}\text{-}{\mathrm{s}}^{-1}$. In a period of time $2t$ s, two pixels will be illuminated in the same $3\times 3$ frame, and 11% of the time they will be the same pixel. This means that in a frame of length $t$ at this flux rate, 5.5% of detections will be lost due to coincidence. Following the efficiency factor definition of the last section, we define the coincidence loss efficiency factor ${ϵ}_{\mathrm{CO}}$. For the example described

The coincidence loss efficiency factor is useful for understanding how to set the frame length for a given target. For low flux targets, it may be the case that the addition of extraneous signals such as dark current or background light should be considered in the estimation of this effect.

3.3.

Cosmic Ray Effects on EMCCD Throughput

The EMCCD gain register is optimized for single electron input, but some bright sources in an image may not be avoidable. One example of these are cosmic rays. These energetic particles are primarily comprised of muons at sea level, and occur at a rate of about $1{\mathrm{cm}}^{-2}\text{-}{\min }^{-1}$.¹⁸ In the Roman L2 environment, the particles are primarily protons ($\sim 74\%$) with energies greater than 100 MeV¹¹ and rates predicted to be up to 300 times higher than at sea level on Earth, or $5{\mathrm{cm}}^{-2}\text{-}{\mathrm{s}}^{-1}$.¹⁹ The particles are sufficiently energetic that they are effectively unshielded by even a heavy camera housing, and each produces hundreds of electrons spread over a handful of pixels in the thin CCD. This amount of charge is not compatible with the EMCCD gain register, because it is multiplied beyond the capacity of the gain register pixels. The effect is to overspill the pixel boundaries in the serial direction, and to fill the pixel wells to the point that the charge interacts with the surface. Traps at the surface oxide are filled in this scenario and leave a trail of charge behind each cosmic ray that can be hundreds of pixels long. The detrapped charge can itself be amplified by the remaining register pixels, forming additional discreet events on top of an exponential tail. For the purpose of estimating the impact of these cosmic ray tails, we collected a series of long integrations with a commercial CCD201 EMCCD at a range of gain from 2500 to 7700 as shown in Fig. 3. It is apparent that at the gain needed for photon counting, the rate of cosmic rays at L2 presents a motivation to keep individual photon counted frames as short as possible (counterbalanced by a preference for long frames to minimize CIC). For the commercial CCD201 at a gain of 5000, the contamination of individual frames by cosmic rays would be in the neighborhood of 30% for 120 s long individual frames.

Fig. 3

The distribution of cosmic ray tail contamination (number of pixels above the five sigma photon counting threshold) versus gain for the commercial CCD201.

We define the efficiency factor ${ϵ}_{\mathrm{CR}}$ to account for signal pollution by cosmic ray tails as follows:

The tails of the cosmic rays are reasonably fit with an exponential function that includes short and long time constants; however, as shown in Fig. 4, we have had good success simply subtracting a median filtered row from each line of data, which subtracts much of the cosmic ray tail but leaves the sharply defined photon events behind. An analysis of this data reveals that the cosmic ray tail itself can lead to additional apparent photon events by releasing electrons into the gain register that mix in with real photoelectrons. Figure 5 shows a comparison of the residual event rate in regions where tails have been subtracted to the rest of the detector. We have determined that the cosmic ray tail increases the noise significantly in the underlying pixels, and therefore it is best to eliminate the tails by other means if possible (either by shortening the frame time or optimizing the hardware).

Fig. 4

The blue traces show a row of raw photon data with a representative cosmic ray tail. The green trace shows a fit to the tail generated by a median filter. The orange trace shows the fit-subtracted data, revealing an excess of events in the underlying pixels.

Fig. 5

A comparison of noise in image row pixels with cosmic rays before the cosmic ray is detected (blue), underneath the tail (orange), and after the tail has faded (green). These reveal that the cosmic ray tails enhance the background significantly even in regions hundreds of pixels after the cosmic ray is detected.

3.4.

Radiation-Induced Traps

Energetic protons from flares passing through the detector will displace silicon atoms and create damage with a variety of typical forms.^20–22 Depending on the type of defect, the result will be a “trap” capable of capturing a single electron and holding it for a period of time dependent on the operating temperature. These traps reveal themselves in conventional damaged-CCD images either as elevated dark current (in the form of discreet “hot pixels”), or as “tails” behind the direction of charge transfer. The traps have an attractive charge when they are empty, and a neutral charge when full. It is typical²³ to add some charge to the image (a “flash” or “fat zero”) in order to neutralize the traps and to improve charge transfer; however, the fat zero adds shot noise, defeating its purpose for a photon counting application. Furthermore, photon counting is sensitive to the impact of long time constant traps, which collect charge during readout but hold onto it long enough that it does not reappear until the next image (or longer). In this case the charge is lost and the most significant effect on the detector is to reduce its dQE, rather than to degrade the image quality since image tails typically have a very low SNR and may not be detectable.

We have characterized this effect in radiation damaged CCD301 test sensors. A line image was projected near the top row of a detector through a neutral density filter (typically ND5 or ${10}^{-5}$ transmission). We measured the ratio of the image brightness with and without the filter (a measurement of the filter transmission) as a function of the source intensity. Measurements without the filter achieve high SNR in seconds at unity gain, while measurements with the filter may take up to 100 hrs in photon counting mode. The commercial detector measurements reveal that as the brightness decreases, so does the apparent dQE when the sensor has been exposed to proton radiation. Furthermore, we found that lengthening the individual frame time had a strong effect on the faint light images, driving CGI to baseline the longest possible frames allowed by the observed cosmic ray tail length ($\sim 120\mathrm{s}$). We interpret that in the faintest illumination conditions the detector traps are mainly empty. When the source is weak and there is no background, a significant fraction of the charge can be captured before the readout. The ratio of the measured filter transmission in faint light to the measured transmission in bright light is taken as a proxy for the charge collection efficiency factor, ${ϵ}_{\mathrm{CC}}$.

The final efficiency factor relates to so-called “hot pixels,” which are those with increased dark current as a result of radiation damage. Similar to the experience documented for Hubble Space Telescope detectors,²³ we find that a distribution of high dark current pixels develops with radiation exposure, and for CGI $\sim 5\%$ of pixels fail the dark current requirements after exposure to the tested level. Our approach is to mask the hot pixels, which can be considered a reduction in the detector area. In aggregate this impacts the dQE with a factor we refer to as ${ϵ}_{\mathrm{HP}}\sim 0.95$.

4.1.

Proton Irradiation

The expected mission dose of radiation into the silicon detectors was initially modeled for an L2 orbit and a 1 cm tantalum alloy radiation shield using a NOVICE (Experimental and Mathematical Physics Consultants, Gaithersburg, Maryland, United States) software simulation. Tantalum had been selected in the preliminary design phase but was later changed to a slightly denser tungsten alloy HD17 (17 versus $16.7\mathrm{g}\text{-}{\mathrm{cm}}^{-3}$) due to its availability. The shielding capability of the two materials is nearly identical as a result. The modeled dose is primarily protons, and includes the contribution from both solar and galactic sources. Without shielding, the galactic component would be negligible relative to the solar one. Inside the heavily shielded CGI camera, the relative contribution of GCR is enhanced to about 7% of the total because the shield does not reduce their number. The NOVICE code modeled the spatial and energy distributions through a preliminary three-dimensional mechanical model of the camera radiation shield and detector. Uncertainties in the transport model were accounted for by increasing the modeled dose by a “radiation design factor” (RDF) of two.

When evaluating radiation damage in the detector, both the total ionizing dose (TID) and displacement damage dose (DDD) are computed. The ionizing dose can result in trapped charge in the detector oxides, causing offsets in the required operating voltages. The displacement dose causes damage to the silicon lattice itself and creates traps for signal electrons during charge transfer. Sometimes these are caused by different types of particles with different energy spectra, but in the case of the CGI at L2, the model assumes the same protons cause both effects. The TID impact on the EMCCD was evaluated previously by CEI²⁶ and found to be small, $0.14\mathrm{V}\text{-}{\mathrm{krad}}^{-1}$. The results of the NOVICE simulation provided the target dosages for the test campaign shown in Table 4. Because the protons result mainly from solar flares, and since a large fraction of the total dose may come from a small number of flare events, it is important to recognize that a significant amount of granularity is possible in the time to reach a given dose level. For test purposes, the results represent the dose that will not be exceeded with 95% confidence during a goal 5.25-year mission lifetime.

Table 4

Target radiation dosage at L2 through a 1-cm tantalum shield including an RDF of 2.

Input particles were assumed to be isotropically distributed and have an energy spectrum based on measured solar flare characteristics. To determine the optimal proton energy for the test campaign, we weighted the energy distribution of protons arriving at the detector by the “non-ionizing energy loss” factor,²⁷ which represents the relative amount of energy absorbed by the silicon as a function of incident proton energy. The weighted mean of this function provided the “damage weighted energy” of $\sim 85\mathrm{MeV}$ that we used in our test. Note that protons lose a significant amount of energy traversing the shield, so the source particles outside the shield would be closer to 150 MeV and on the high energy tail of the solar flare.

We irradiated CCD301 and CCD302 test sensors at James M. Slater MD Proton Treatment and Research Center, Loma Linda University Medical Center (Loma Linda, California, United States). The sensors were each installed on a support bracket as shown in Fig. 7. Each sensor was shielded by a 15-mm-thick stainless steel shield over the control half of the detector, while the bare portion (which included the serial register) was exposed to the proton beam. The sensors were irradiated in an unbiased, room temperature condition. The “5 year” dose of 85 MeV protons applied to the unshielded half of the detector was $7.4\times {10}^9\text{protons}\text{-}{\mathrm{cm}}^{-2}$ (corresponding to a DDD of $2.4\times {10}^7\mathrm{MeV}/\mathrm{g}$ and a TID of 790 Rads). The reported displacement damage dose scales to $2.6\times {10}^9\text{protons}\text{-}{\mathrm{cm}}^{-2}$ “10 MeV equivalent.”²⁸ As the shield design has matured, predictions for the 5.25 year, 85 MeV lifetime dose have fallen significantly and therefore we expect the current results are conservative, particularly with consideration of the required 21-month instrument lifetime.

Fig. 7

(a) The proton irradiation test configuration at Loma Linda Medical Center. (b) A close-up of the EMCCD and test shield. (c) A test package behind the shield (without silicon) revealing the unshielded half of the package.

4.2.

Test Equipment

The majority of the undamaged sensor characterization was performed at the Centre for Electronic Imaging of the Open University under contract to JPL. CEI utilized an XCam (Northampton, UK) XCU-A 1 MHz programmable CCD controller with a custom vacuum cryostat interfaced to a Polycold (Petaluma, CA) Cryotiger cooling system shown in Fig. 8. An x-ray fluorescence tube with a Mn target was housed inside the vacuum to provide charge transfer calibrations similarly to an ${}^{55}\mathrm{Fe}$ technique. Measurements were performed at $-105°\mathrm{C}$. The CEI tests emphasized performance characteristics, such as full well, dark current, and the CCD302 custom amplifier glow, which were not expected to be significantly impacted by the difference in readout speed from the flight 10 MHz implementation.

Fig. 8

The CEI test camera utilizes a 1 MHz XCam XCU-A programmable controller coupled to a custom vacuum cryostat. The headboard and CCD, cooled by a Polycold Cryotiger to $-105°\mathrm{C}$, is shown in the inset. An x-ray source is incorporated inside the vacuum for use in charge transfer calibration.

The post-irradiation characterization campaign was conducted at JPL. The system, which is shown in Fig. 9, consisted of a NuVu Cameras (Montreal, California, United States) liquid nitrogen cooled EMN2 camera driven by a CCD Controller for Counting Photons (CCCP)¹⁶ modified to adapt the entrance aperture to the unmasked design of the test sensors. The typical CCD operating temperature was $-105°\mathrm{C}$. The JPL tests emphasized measurements of quantities that were expected to be significantly impacted by the flight-design 10-MHz pixel rate. These included the low flux, photon counting mode performance as well as measurements of CIC and read noise. The low flux tests provided a measurement of the relative performance of the CCD301 image pixel designs, where the inclusion of the commercial pixel provided for an overall normalization of results. JPL testing included cross-checks of some of the CEI-measured parameters, such as photon transfer to ensure consistency between the two sets of results.

The CEI and JPL test equipment made use of a smartphone-based projection system²⁹ that enables RGB illumination with a spatial resolution similar to the capability of the EMCCD. Because the phone has an organic light-emitting diode (OLED) screen, it provides very low leakage into parts of the field that are turned off. A filter wheel adds neutral density filters that enable a wide range of flux conditions suitable for simulating the expected illumination conditions.

Fig. 9

(a) A view of the smartphone projector system used for EMCCD characterization at JPL. Inset of panel A shows an example image from the projector. (b) The JPL test camera utilizes a 10-MHz NuVu CCCP programmable controller coupled to a custom vacuum dewar and is cooled by liquid nitrogen to $-105°\mathrm{C}$. Inset of panel B shows a test EMCCD in the commercial dewar.

4.3.

Preirradiation Characterizations

4.3.1.

Photon transfer

The full well characteristics of the four image pixel designs and the serial register of the CCD301 were evaluated with a standard photon transfer technique³⁰ using flat-field illumination. The transfer curve of each design region of the sensor was evaluated separately and the result is shown in Fig. 10. As expected, the standard commercial pixel has the highest full well since the features in the other designs reduce the width of the signal channel.

Fig. 10

(a) Photon transfer curves from CEI in the 1 MHz camera for each of the four image pixel designs contained in the CCD301 test sensor. (b) Photon transfer curves for the CCD301 register, which is common to all of the image pixel designs, at three clock voltages.

We also evaluated the full well of the serial register (identical to the commercial CCD201) by binning charge from multiple image rows at each line transfer. The image full well results are presented in Table 5. It can be seen that the notch and commercial image pixel designs meet the 50,000 electron image and 90,000 electron serial full well requirements with margin.

Table 5

CCD301 image full well versus clock voltage.

Voltage	3 micron	4 micron	Notch	Commercial
(V)	(Electrons)	(Electrons)	(Electrons)	(Electrons)
10	32,000	35,000	58,000	79,000
11	34,000	40,000	62,000	84,000
12	36,000	42,000	62,000	89,000

For comparison, a photon transfer curve of a CCD302 (with the “notch” image pixel design) that was taken with the test camera at JPL at 10 MHz is shown in Fig. 11. The CCD302 devices included an extra transistor in their output amplifiers intended to improve the read noise performance, and were successful in this regard, achieving 35 electrons at 10 MHz.

Fig. 11

A photon transfer curve for the CCD302 in the JPL 10 MHz camera.

While the CCD302 exhibited excellent read noise, it was found that the amplifier glowed significantly due to the additional transistor. An example of this effect is shown in Fig. 12. The 10-s image was taken at a gain of 1500 in dark conditions and at an operating temperature of $-105°\mathrm{C}$. The amplifier glow is seen to originate in the corner closest to the output amplifier, and to extend across the entire image area. In the image shown the glow is $\sim 100$ times higher than the dark current. We found that the glow could be mitigated by reducing the output amplifier drain voltages; however, this also caused the read noise to increase and it was deemed too risky to proceed to flight with the CCD302 amplifier design given the importance of low dark current to CGI.

Fig. 12

A 10-s CCD302 image with gain set to 1500 shows glow from the corner of the imager close to the amplifier extending across the entire photosensitive area.

4.3.2.

Gain register overspill

We evaluated the CCD302 gain register overspill feature by comparing the responses of the CCD301 and CCD302 gain registers in photon counting mode to single bright pixel events using a technique developed by CEI. Simulated cosmic rays were created by collecting a flat field image and erasing all of the pixels but one into the serial register dump gate and associated conventional amplifier sense node. A high gain image was then read out through the photon counting (LS) amplifier in such a way that 500 empty rows were clocked before and after the single bright pixel. An example of an image with a simulated cosmic ray constructed in this manner is shown in the inset of Fig. 13. Since the CCD302 has a lower noise amplifier than the CCD301, the gain and threshold that would be selected for photon counting are both lower than for the CCD301, which has a register and output amplifier that are identical to the commercial CCD201. Figure 13 compares the number of photon events detected 6000 pixels after the bright pulse with the CCD301 at a gain of $4000\times$ (with a read noise of 80 electrons and a counting threshold of 400 electrons) to the same quantity with a CCD302 at a gain of $1500\times$ (with a read noise of 30 electrons and a counting threshold of 150 electrons). We also measured the CCD302 pulse-response performance at the same gain and threshold as the CCD301 to explore the possibility of a systematic. Our results show that the overspill reduces the quantity of spurious detected events after a bright pixel pulse through the gain register by at least a factor of two. We also found the same result with the CCD302 in the low and high gain configurations, confirming the improvement in performance with this feature.

Fig. 13

A comparison of the CCD301 and CCD302 register response to a single bright input pixel in photon counting mode. The figure shows the number of pixels after the bright pulse that would be counted as photons above the counting threshold. The inset shows an example “simulated cosmic ray,” which is the result of a single bright pixel pulse to the gain register operating at high gain. Trailing pixels result from trapped charge at the register surface oxide due to the amplified signal.

4.3.3.

LoCam lifetime

While the ExCam is designed to look at the faintest objects that are detectable, the LoCam is designed to look at some of the brightest, with fluxes high enough to fill the pixel in a 1-ms frame interval. It is well known that the EMCCD gain register is susceptible to an “aging” effect in which operation with both high fluxes and gain can degrade the response of the register and in extreme cases cause it to fail. The effect is thought to be the result of charge being trapped in the oxide of the gain register, causing a flat band shift of the voltage (personal communication, P. Jerram). To study this effect, we framed a CCD301 (without an overspill) continuously with flat field illumination of $10000\text{electrons}\text{-}{\text{pixel}}^{-1}\text{-}{\text{frame}}^{-1}$ at a gain of 25 ($\sim 37\mathrm{V}$ HV) as shown in Figure 14. This configuration in the 1 MHz camera achieved a charge input to the gain register of $2\times {10}^{14}$ electrons after 24 hours, and resulted in a linear decrease of 2% in the gain, which was restored with an increase of 50 mV to the HV clock without otherwise impacting performance. The camera controller has a capability of up to 50 V, although according to product literature³¹ the charge transfer capability is expected to degrade after 4 to 5 volts of compensation. Since the total signal input to the gain register is estimated to be $1.9\times {10}^{15}$ electrons during the 21-month technology demonstration, the LoCam is expected to meet the requirements of CGI with margin.

Fig. 14

Measured electron multiplying gain as a function of signal input to the gain register at a gain of $25\times$.

4.4.

Postirradiation Characterizations

4.4.1.

Low flux results

A 100 h low flux exposure selected from the lower image area (closest to the serial register) of an irradiated CCD301 is shown in Fig. 15. The image is comprised of 100-s frames of data, each collected at a gain of $3300\times$ and $-105°\mathrm{C}$. The 100-s frame time was selected to optimize performance following initial trials that revealed poor low flux performance with short (1 to 10 s) frames. We attribute improvements at 100 s to additional dark current ($10-30\text{electrons}\text{-}{\text{column}}^{-1}$) in each frame that would be available to mitigate trap damage without increasing coincidence losses, while not reducing the cosmic ray efficiency factor ${ϵ}_{\mathrm{CR}}$ unreasonably. At the measured gain, $\sim 80\%$ of events are brighter than the $5\sigma$ threshold (in this case, 693 electrons). The lefthand side of the imager as well as the serial and gain registers were irradiated, while the right hand side was protected by a shield as shown in Fig. 7. Our dataset is comprised of three image sets at different fluxes in the range of 0.001 to $0.1\text{counts}\text{-}{\text{pixel}}^{-1}\text{-}{\mathrm{s}}^{-1}$, comparable to the stated flux requirements.

Fig. 15

A 100-hr photon counted CCD301 image comprised of 100-s frames of data. A line image from the projector illuminates row 960 near the top of the array. The pixel designs are mirror-imaged on the left and right sides of the image with the shielded right hand side providing a control. The 95% dark level measured after irradiation is overlaid on each design on the left side of the figure. The yellow rectangle shows an example extraction area for the irradiated notch-design data.

Each 100-s frame of data was bias subtracted, and photon events larger than the threshold were added to the resulting 100-hr image. The image is annotated to show the locations of the different image pixel designs, mirrored on the right and left of the sensor following Fig. 6(b). The right side annotation shows the image pixel design while the mirrored left side shows the measured dark current for 95% of the damaged pixels. Some damage (in the form of hot pixels) is also visible on the right side of the image, decreasing with distance from the center and therefore interpreted as the result of shadowing of the protons under the mask, which sat $\sim 3\mathrm{mm}$ above the sensor during the irradiation. Detailed histograms of the measured dark in each region are shown in Fig. 16.

Fig. 16

Dark current histograms extracted from the photon counted image shown in Fig. 15. Each histogram is extracted from a different design region as labeled. The shielded half of the array provides the “0 year” beginning of life data while the irradiated half represents the 5-year lifetime goal at L2.

Near the top of Fig. 15 and annotated as “Row 960” there is a horizontal line, which is an image projected across the top of the array and used to evaluate the impact of traps on charge transfer. Close inspection reveals that the line is not perfectly uniform but rather has pixel-level structure that depends on the exact distribution of traps in a particular column. It is visually evident that the density of hot pixels is highest in the irradiated commercial-design pixels. It is also evident that the shielded 3 micron notch design on the far right has poor charge transfer. While we do not have a good explanation for this, it was observed on a number of the 3 micron test sensors and as a result that design was rejected pending further study. The 4 micron narrow channel design did not exhibit the charge transfer defect, but it also did not meet the full well requirement for CGI.

The resulting images were corrected for CIC (measured in a row overscan region above the image area), and the mean of the middle 68% of pixels for each row was computed within each design band (100 columns) for a 30 row section of the image centered on the projected line. We found that this “clipped mean” technique effectively removed the significant influence of hot pixels on the measurement. At the lowest fluxes, we estimate the hot pixels contributed an error of $\sim 10\%$ to the flux measurements, which we determined by comparing line profiles measured in different regions of each design band. An example fit is shown in Fig. 17.

Fig. 17

The measured “clipped mean” of 100 pixels of irradiated notch-design data in each row across the projected line image shown in the yellow rectangle in Fig. 15. The dashed line represents a Gaussian fit to the measurement. The legend shows integrated signal from the fit, the full-width at half maximum (FWHM) of the Gaussian profile, and the residual dark current background.

This analysis was completed for bright (conventional) images of the scene with no neutral density filter, and faint (photon counted) images with the filter. An example bright light image exhibiting good uniformity ($\pm 6\%$) without the ND5 filter is shown in Fig. 18. To minimize systematics, a single ND5 filter was used for all of the faint light measurements while using the projector to adjust the brightness. The results were corrected for photon counting gain ${ϵ}_{\mathrm{PC}}$ and coincidence ${ϵ}_{\mathrm{CO}}$. In Fig. 19, the ratio of the faint to bright signal is plotted against the bright light measurement scaled by the nominal $1\times {10}^{-5}$ filter transmission. The ratio of the faint light to bright light measurement reveals little change over the measured flux range for the notch and 4 micron channel designs, while the commercial design shows a large decrease in performance as the flux decreases (reciprocity failure).

Fig. 18

An unfiltered bright light image exhibiting $\pm 6\%$ uniformity when comparing measured brightness in the damaged and undamaged regions. The step discontinuities are due to a small misalignment of the OLED projector to the EMCCD.

Fig. 19

A comparison of the measured projected line brightness in photon counting mode (with ND5 filter) to the brightness measured without the filter at unity gain for the 4 micron, notch, and commercial designs. The estimated flux is measured from the bright-light images (minimally affected by irradiation) scaled by ${10}^{-5}$. The data show that the commercial design has significantly reduced faint-light sensitivity compared to the notch and 4 micron designs after it is damaged by radiation.

As a result of these pre- and postirradiation measurements, we are able to extract the relative performance of the CCD201, CCD301, and CCD302 sensors after the tested dose of radiation, and the results are provided in Table 6. The QE at 90% is taken as a nominal peak value for 600 nm from the datasheet. The coincidence, cosmic ray losses and hot pixel losses follow from Sec. 3, while the charge collection efficiency is taken from Fig. 19 for the faintest measurement. It can be seen that while improvements to the serial register that mitigate cosmic ray tails provide incremental benefits, the notch channel in the image section of the CCD301 has a large impact on performance in faint light conditions.

Table 6

End-of-life comparison of commercial and test EMCCD dQE performance.

	CCD201	CCD301	CCD302a
QE (%)	90	90	90
${ϵ}_{\mathrm{PC}}$	0.90	0.90	0.90
${ϵ}_{\mathrm{CO}}$	0.95	0.95	0.95
${ϵ}_{\mathrm{CR}}$	0.70	0.70	0.85
${ϵ}_{\mathrm{CC}}$	0.44	1.00	1.00
${ϵ}_{\mathrm{HP}}$	0.95	0.95	0.95
dQE (%)	22.5	51.2	62.1

^aCCD302 amplifier glow prevented the use of long integrations with gain. CCD302 performance in this table is based on CCD301 notch channel imaging combined with CCD302 serial channel cosmic ray measurements.

4.5.

Flight Sensor Design

The CCD311 flight EMCCD has been fabricated by Teledyne-e2v through a contribution by ESA to NASA. Features from both the CCD301 and CCD302 are incorporated into the flight design for maximum improvement. Flight sensors have been delivered and are being tested at JPL. The key design elements are listed in Table 7. To simplify implementation, the CCD311 uses the commercial aluminum nitride carrier and is pin-compatible with the CCD201. To provide a ruggedized thermal-mechanical interface, the aluminum nitride carrier is bonded into a nickel-plated invar package, as shown in Fig. 20.

Table 7

Design features of the Roman CGI flight CCD311.

Fig. 20

The flight CCD311 EMCCD incorporates the image pixel notch feature from the CCD301 and the gain register overspill from the CCD302. It utilizes a commercial aluminum nitride carrier bonded into a nickel-plated invar package. The CCD311 is pin compatible with the commercial CCD201.

Characterization and environmental testing of the CCD311 is underway. Early results indicate the gain register overspill is very effective at eliminating tails from cosmic rays as shown in the comparison in Fig. 21. The improvement in this key performance metric should enable the flight camera to be operated with proportionately longer frames before cosmic rays significantly obscure the image, further mitigating low signal charge transfer traps in addition to the benefit gained from the notch channel.

Fig. 21

A comparison of cosmic ray tails observed in the lab with the camera operating at high gain ($>1000\times$). (a) A $256\times 256$ image section from the CCD201 and (b) the CCD311 under similar conditions. The operating temperature was $-105°\mathrm{C}$. The improvement is quite significant with tails in the commercial sensor example of over 100 pixels and only 40 pixels in the flight design as a result of the overspill.