The effect of protons on the performance of swept-charge devices

This is the pre-print version of the final paper published in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2009 Elsevier B.V.The e2v technologies CCD54, or swept-charge device (SCD) has been extensively radiation tested for use in the Chandrayaan-1 X-ray Spectrometer (C1XS) instrument, to be launched as a part of the Indian Space Research Organisation (ISRO) Chandrayaan-1 payload in 2008. The principle use of the SCD is in X-ray fluorescence (XRF) applications, the device providing a relatively large collecting area of 1.1 cm2, and achieving near Fano-limited spectroscopy at −15 °C, a temperature that is easily obtained using a thermoelectric cooler (TEC). This paper describes the structure and operation of the SCD and details the methodology and results obtained from two proton irradiation studies carried out in 2006 and 2008, respectively to quantify the effects of proton irradiation on the operational characteristics of the device. The analysis concentrates on the degradation of the measured FWHM of various elemental lines and quantifies the effects of proton fluence on the observed X-ray fluorescence spectra from mineralogical target samples

Proton damage comparison of an e2v technologies n-channel and p-channel CCD204

Comparisons have been made of the relative degradation of charge transfer efficiency in n-channel and p-channel CCDs subjected to proton irradiation. The comparison described in this paper was made using e2v technologies plc. CCD204 devices fabricated using the same mask set. The device performance was compared over a range of temperatures using the same experimental arrangement and technique to provide a like-for-like comparison. The parallel transfer using the p-channel CCD was then optimized using a trap pumping technique to identify the optimal operating conditions at 153 K

New Levels of Language Processing Complexity and Organization Revealed by Granger Causation

Granger causation analysis of high spatiotemporal resolution reconstructions of brain activation offers a new window on the dynamic interactions between brain areas that support language processing. Premised on the observation that causes both precede and uniquely predict their effects, this approach provides an intuitive, model-free means of identifying directed causal interactions in the brain. It requires the analysis of all non-redundant potentially interacting signals, and has shown that even “early” processes such as speech perception involve interactions of many areas in a strikingly large network that extends well beyond traditional left hemisphere perisylvian cortex that play out over hundreds of milliseconds. In this paper we describe this technique and review several general findings that reframe the way we think about language processing and brain function in general. These include the extent and complexity of language processing networks, the central role of interactive processing dynamics, the role of processing hubs where the input from many distinct brain regions are integrated, and the degree to which task requirements and stimulus properties influence processing dynamics and inform our understanding of “language-specific” localized processes

Radiation damage analysis of the swept charge device for the C1XS instrument

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis is concerned with ensuring high energy resolution from the swept charge device (SCD) CCD54, essentially a non-pixellated version of the charge coupled device (CCD), for use in the Chandrayaan-1 X-ray Spectrometer (C1XS). Of particular interest is the effect on performance due to the radiation damage, caused by protons, the CCD54s used in C1XS will receive during the transfer to the Moon and during the two years in lunar orbit. Chapter 2 reviews the atomic structure, the formation and detection of X-rays, and the operation of a CCD. Chapter 3 discusses the space radiation environment and the damaging effects it has on CCDs, for example increasing dark current and charge transfer inefficiency. Chapter 4 presents the basic laboratory equipment and procedure used during the experimental work, and details the initial optimisation and characterisation, the pre-flight characterisation of devices available for use in C1XS, the measurement of the depletion depth, and quantum efficiency of the CCD54. Chapter 5 details the results of the initial proton irradiation study, intended to demonstrate the ability of the CCD54 to provide excellent scientific data over the two years at the Moon. Chapter 6 describes a second irradiation study covering a more detailed investigation of the damage effects, investigating dark current, trap energy levels, and charge transfer inefficiency. Chapter 7 describes work conducted to assist the C1XS science team in the development of an X-ray fluorescence model, to be used with X-ray spectra provided by the X-ray solar monitor and the spectra detected by C1XS, to provide elemental abundance information of the lunar surface. It also presents the initial C1XS results from the Moon, and a brief comparison of the CCD54 with other semiconductor X-ray fluorescence detectors. Chapter 8 describes the final conclusions and recommendations for further work, including a study of the radiation damage effects during the two years at the Moon and the future development of SCD detectors for use in space

Evolution of proton-induced defects in a cryogenically irradiated p-channel CCD

P-channel CCDs have been shown to display improved tolerance to radiation-induced charge transfer inefficiency (CTI) when compared to n-channel CCDs. This is attributed to the properties of the dominant charge-trapping defect species in p-channel silicon relative to the operating conditions of the CCD. However, precise knowledge of defect parameters is required in order to correct for any induced CTI. The method of single trap-pumping allows us to analyse the defect parameters to a degree of accuracy that cannot be achieved with other common defect analysis techniques such as deep-level transient spectroscopy (DLTS). We have analysed using this method the defect distribution in an e2v p-channel CCD204 irradiated with protons at cryogenic temperature (153K). 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. The evolution of the defect distribution over time and through each anneal phase provides insight into defect interactions and mobility post-irradiation. 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 testing

Clinical Remission in Severe Asthma : How to Move From Theory to Practice

ACKNOWLEDGMENTS Medical writing support was provided by Dan Jackson, PhD, CMPP, of CiTRUS Health Group, which was in accordance with Good Publication Practice (GPP4) guidelines. AstraZeneca (Cambridge, UK) funded this support. AMG and DBP both conceived and refined the topic for this manuscript. Both authors contributed to the discussion and revision of the content and approved the final version.Peer reviewedPostprin

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

Rules from Words: A Dynamic Neural Basis for a Lawful Linguistic Process

Listeners show a reliable bias towards interpreting speech sounds in a way that conforms to linguistic restrictions (phonotactic constraints) on the permissible patterning of speech sounds in a language. This perceptual bias may enforce and strengthen the systematicity that is the hallmark of phonological representation. Using Granger causality analysis of magnetic resonance imaging (MRI)- constrained magnetoencephalography (MEG) and electroencephalography (EEG) data, we tested the differential predictions of rule-based, frequency–based, and top-down lexical influence-driven explanations of processes that produce phonotactic biases in phoneme categorization. Consistent with the top-down lexical influence account, brain regions associated with the representation of words had a stronger influence on acoustic-phonetic regions in trials that led to the identification of phonotactically legal (versus illegal) word-initial consonant clusters. Regions associated with the application of linguistic rules had no such effect. Similarly, high frequency phoneme clusters failed to produce stronger feedforward influences by acoustic-phonetic regions on areas associated with higher linguistic representation. These results suggest that top-down lexical influences contribute to the systematicity of phonological representation

A tale of two lexica: Investigating computational pressures on word representation with neural networks

IntroductionThe notion of a single localized store of word representations has become increasingly less plausible as evidence has accumulated for the widely distributed neural representation of wordform grounded in motor, perceptual, and conceptual processes. Here, we attempt to combine machine learning methods and neurobiological frameworks to propose a computational model of brain systems potentially responsible for wordform representation. We tested the hypothesis that the functional specialization of word representation in the brain is driven partly by computational optimization. This hypothesis directly addresses the unique problem of mapping sound and articulation vs. mapping sound and meaning.ResultsWe found that artificial neural networks trained on the mapping between sound and articulation performed poorly in recognizing the mapping between sound and meaning and vice versa. Moreover, a network trained on both tasks simultaneously could not discover the features required for efficient mapping between sound and higher-level cognitive states compared to the other two models. Furthermore, these networks developed internal representations reflecting specialized task-optimized functions without explicit training.DiscussionTogether, these findings demonstrate that different task-directed representations lead to more focused responses and better performance of a machine or algorithm and, hypothetically, the brain. Thus, we imply that the functional specialization of word representation mirrors a computational optimization strategy given the nature of the tasks that the human brain faces

Importance of charge capture in interphase regions during readout of charge-coupled devices

The current understanding of charge transfer dynamics in charge-coupled devices (CCDs) is that charge is moved so quickly from one phase to the next in a clocking sequence and with a density so low that trapping of charge in the interphase regions is negligible. However, simulation capabilities developed at the Centre for Electronic Imaging, which includes direct input of electron density simulations, have made it possible to investigate this assumption further. As part of the radiation testing campaign of the Euclid CCD273 devices, data have been obtained using the trap pumping method, a method that can be used to identify and characterize single defects within CCDs. Combining these data with simulations, we find that trapping during the transfer of charge among phases is indeed necessary to explain the results of the data analysis. This result could influence not only trap pumping theory and how trap pumping should be performed but also how a radiation-damaged CCD is readout in the most optimal way