Evolution and impact of defects in a p-channel CCD after cryogenic proton-irradiation

P-channel CCDs have been shown to display improved tolerance to radiation-induced charge transfer inefficiency (CTI) when compared to n-channel CCDs. However, the defect distribution formed during irradiation is expected to be temperature dependent due to the differences in lattice energy caused by a temperature change. This has been tested through defect analysis of two p-channel e2v CCD204 devices, one irradiated at room temperature and one at a cryogenic temperature (153K). Analysis is performed using the method of single trap pumping. The dominant charge trapping defects at these conditions have been identified as the donor level of the silicon divacancy and the carbon interstitial defect. The defect parameters are analysed both immediately post irradiation and following several subsequent room-temperature anneal phases up until a cumulative anneal time of approximately 10 months. We have also simulated charge transfer in an irradiated CCD pixel using the defect distribution from both the room-temperature and cryogenic case, to study how the changes affect imaging performance. The results demonstrate the importance of cryogenic irradiation and annealing studies, with large variations seen in the defect distribution when compared to a device irradiated at room-temperature, which is the current standard procedure for radiation-tolerance testing

Postirradiation behavior of p-channel charge-coupled devices irradiated at 153 K

The displacement damage hardness that can be achieved using p-channel charge-coupled devices (CCD) was originally demonstrated in 1997, and since then a number of other studies have demonstrated an improved tolerance to radiation-induced CTI when compared to n-channel CCDs. A number of recent studies have also shown that the temperature history of the device after the irradiation impacts the performance of the detector, linked to the mobility of defects at different temperatures. This study describes the initial results from an e2v technologies p-channel CCD204 irradiated at 153 K with a 10 MeV equivalent proton fluences of 1.24×109 and 1.24×1011 protons cm-2. The dark current, cosmetic quality and the number of defects identified using trap pumping immediately were monitored after the irradiation for a period of 150 hours with the device held at 153 K and then after different periods of time at room temperature. The device also exhibited a flatband voltage shift of around 30 mV / krad, determined by the reduction in full well capacity

The relationship between pumped traps and signal loss in buried channel CCDs

Pocket-pumping is an established technique for identifying the locations of charge trapping sites within the transport channels of CCDs. Various parameters of the pumping process can be manipulated to increase the efficiency, or allow characterisation of the trap sites effective during nominal operating modes. A CCD273 was irradiated in a triangular region by protons to a 10 MeV equivalent fluence of 1.2E9 p cm2, ensuring a suitably low trap density for ease of automated trap recognition. X-rays of 5,898 eV were incident on the CCD above the region irradiated with the triangle, such that events could be analysed having passed through an increasing length of irradiated silicon and hence number of trapping sites. Here we present the relationship between the number of traps identified by pocket pumping within the parallel transport channels of a CCD273 and the amount of signal that is deferred by the trapping process during readout

Modelling charge transfer in a radiation damaged charge coupled device for Euclid

As electrons are transferred through a radiation damaged Charge Coupled Device (CCD), they may encounter traps in the silicon in which they will be captured and subsequently released. This capture and release of electrons can lead to a 'smearing' of the image. The dynamics of the trapping process can be described through the use of Shockley-Read-Hall theory, in which exponential time constants are used to determine the probability of capture and release. If subjected to a hostile radiation environment, such as in space where the dominant charged particle is the proton, these incident protons can cause displacement damage within the CCD and lead to the formation of stable trap sites. As the trap density increases, the trapping and release of signal electrons can have a major impact on the Charge Transfer Efficiency (CTE) to the detriment of device performance. As the science goals for missions become ever more demanding, such as those for the ESA Euclid and Gaia missions, the problem of radiation damage must be overcome. In order to gain a deeper understanding of the trapping process and the impact on device performance, a Monte Carlo simulation has been developed to model the transfer of charge in a radiation damaged CCD. This study investigates the various difficulties encountered when developing such a model: the incorporation of appropriate clocking mechanisms, the use of suitable trap parameters and their degeneracy, and the development of methods to model the charge storage geometry within a pixel through the use of three-dimensional Silvaco simulations

Developing a high-resolution x-ray imager using electron-multiplying (EM) CCDs

Applications at synchrotron facilities such as macromolecular crystallography and high energy X-ray diffraction require high resolution imaging detectors with high dynamic range and large surface area. Current systems can be split into two main categories: hybrid pixel detectors and scintillator-coupled Charge-Coupled Devices (CCDs). Whilst both have limitations, CCD-based systems (coupled to fibre-optics to increase imaging area) are often used in these applications due to their small pixels and the high resolution. Electron-Multiplication CCDs (EM-CCDs) are able to suppress the readout noise associated with increased readout speed offering a low noise, high speed detector solution. A previous pilot study using a small-area (8 mm × 8 mm) scintillator-coupled EM-CCD found that through high frame-rates, low noise and novel uses of photon-counting, resolution could be improved from over 80 μm to 25 μm at 2 fps. To further improve this detector system, high speed readout electronics can be used alongside a fibre-optic taper and EM-CCD to create a “best of both worlds” solution consisting of the high resolution of a CCD, along with the low noise, high speed (high dynamic range) and large effective area of pixel detectors. This paper details the developments in the study and discusses the latest results and their implication on the system design

Neuropathologic basis of frontotemporal dementia in progressive supranuclear palsy.

BackgroundProgressive supranuclear palsy (PSP) is a neurodegenerative disorder characterized by neuronal loss in the extrapyramidal system with pathologic accumulation of tau in neurons and glia. The most common clinical presentation of PSP, referred to as Richardson syndrome, is that of atypical parkinsonism with vertical gaze palsy, axial rigidity, and frequent falls. Although cognitive deficits in PSP are often ascribed to subcortical dysfunction, a subset of patients has dementia with behavioral features similar to the behavioral variant of frontotemporal dementia. In this study we aimed to identify the clinical and pathological characteristics of PSP presenting with frontotemporal dementia.MethodsIn this study, we compared clinical and pathologic characteristics of 31 patients with PSP with Richardson syndrome with 15 patients with PSP with frontotemporal dementia. For pathological analysis, we used semiquantitative methods to assess neuronal and glial lesions with tau immunohistochemistry, as well image analysis of tau burden using digital microscopic methods.ResultsWe found greater frontal and temporal neocortical neuronal tau pathology in PSP with frontotemporal dementia compared with PSP with Richardson syndrome. White matter tau pathology was also greater in PSP with frontotemporal dementia than PSP with Richardson syndrome. Genetic and demographic factors were not associated with atypical distribution of tau pathology in PSP with frontotemporal dementia.ConclusionsThe results confirm the subset of cognitive-predominant PSP mimicking frontotemporal dementia in PSP. PSP with frontotemporal dementia has distinct clinical features that differ from PSP with Richardson syndrome, as well as differences in distribution and density of tau pathology. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society

Determination of <i>in situ</i> trap properties in CCDs using a "single-trap pumping" technique

The science goals of space missions from the Hubble Space Telescope through to Gaia and Euclid require ultraprecise positional, photometric, and shape measurement information. However, in the radiation environment of the space telescopes, damage to the focal plane detectors through high-energy protons leads to the creation of traps, a loss of charge transfer efficiency, and a consequent deterioration in measurement accuracy. An understanding of the traps produced and their properties in the CCD during operation is essential to allow optimization of the devices and suitable modeling to correct the effect of the damage through the postprocessing of images. The technique of “pumping single traps” has allowed the study of individual traps in high detail that cannot be achieved with other techniques, such as deep level transient spectroscopy, whilst also locating each trap to the subpixel level in the device. Outlining the principles used, we have demonstrated the technique for the A-center, the most influential trap in serial readout, giving results consistent with the more general theoretical values, but here showing new results indicating the spread in the emission times achieved and the variation in capture probability of individual traps with increasing signal levels. This technique can now be applied to other time and temperature regimes in the CCD to characterize individual traps in situ under standard operating conditions such that dramatic improvements can be made to optimization processes and modeling techniques

The noise performance of electron-multiplying charge-coupled devices at X-ray energies

Electron-multiplying charge-coupled devices (EMCCDs) are used in low-light-level (L3) applications for detecting optical, ultraviolet, and near-infrared photons (10–1100 nm). The on-chip gain process is able to increase the detectability of any signal collected by the device through the multiplication of the signal before the output node. Thus, the effective readout noise can be reduced to subelectron levels, allowing the detection of single photons. However, this gain process introduces an additional noise component due to the stochastic nature of the multiplication. In optical applications, this additional noise has been characterized. The broadening of the detected peak is described by the excess noise factor. This factor tends to a value of √2 at high gain (>100x). In X-ray applications, the situation is improved by the effect that Fano factor f has on the shot noise associated with X-ray photon detection (f ≈ 0.12 at X-ray energies). In this paper, the effect of the detection of X-ray photons in silicon is assessed both analytically and through a Monte Carlo model of the gain amplification process. The excess noise on the signal is predicted (termed the modified Fano factor) for photon detection in an EM-CCD at X-ray energies. A hypothesis is made that the modified Fano factor should tend to 1.115 at high levels of gain (>10x). In order to validate the predictions made, measurements were taken using an 55 Fe source with Mn k-alpha X-ray energy of 5898 eV. These measurements allowed the hypothesis to be verified

Rod Electroretinograms Elicited by Silent Substitution Stimuli from the Light-Adapted Human Eye

YesPurpose: To demonstrate that silent substitution stimuli can be used to generate electroretinograms (ERGs) that effectively isolate rod photoreceptor function in humans without the need for dark adaptation, and that this approach constitutes a viable alternative to current clinical standard testing protocols. Methods: Rod-isolating and non-isolating sinusoidal flicker stimuli were generated on a 4 primary light-emitting diode (LED) Ganzfeld stimulator to elicit ERGs from participants with normal and compromised rod function who had not undergone dark-adaptation. Responses were subjected to Fourier analysis, and the amplitude and phase of the fundamental were used to examine temporal frequency and retinal illuminance response characteristics. Results: Electroretinograms elicited by rod-isolating silent substitution stimuli exhibit low-pass temporal frequency response characteristics with an upper response limit of 30 Hz. Responses are optimal between 5 and 8 Hz and between 10 and 100 photopic trolands (Td). There is a significant correlation between the response amplitudes obtained with the silent substitution method and current standard clinical protocols. Analysis of signal-to-noise ratios reveals significant differences between subjects with normal and compromised rod function. Conclusions: Silent substitution provides an effective method for the isolation of human rod photoreceptor function in subjects with normal as well as compromised rod function when stimuli are used within appropriate parameter ranges. Translational Relevance: This method of generating rod-mediated ERGs can be achieved without time-consuming periods of dark adaptation, provides improved isolation of rod- from cone-based activity, and will lead to the development of faster clinical electrophysiologic testing protocols with improved selectivity