In situ trap properties in CCDs: the donor level of the silicon divacancy

The silicon divacancy is one of the main defects of concern in radiation damage studies of Charge-Coupled Devices (CCDs) and, being immobile at room temperature, the defect is accessible to a variety of characterisation techniques. As such, there is a large amount of (often conflicting) information in the literature regarding this defect. Here we study the donor level of the divacancy, one of three energy levels which lie between the silicon valence and conduction bands. The donor level of the divacancy acts as a trap for holes in silicon and therefore can be studied through the use of a p-channel CCD. The method of trap-pumping, linked closely to the process of pocket-pumping, has been demonstrated in the literature over the last two years to allow for in-situ analysis of defects in the silicon of CCDs. However, most work so far has been a demonstartion [sic] of the techinique [sic]. We begin here to use the technique for detailed studies of a specific defect centre in silicon, the donor level of the divacancy. The trap density post-irradiation can be found, and each instance of the trap identified independently of all others. Through the study of the trap response at different clocking frequencies one can measure directly the defect emission time constant, and through tracking this at different temperatures, it is possible to use Shockley-Read-Hall theory to calculate the trap energy level and cross-section. A large population of traps, all with parameters consistent with the donor level of the divacancy, has been studied, leading to a measure of the distribution of properties. The emission time constant, energy level and cross-section are found to have relatively large spreads, significantly beyond the small uncertainty in the measurement technique. This spread has major implications on the correction of charge transfer inefficiency effects in space applications in which high precision is required

Development of EM-CCD-based X-ray detector for synchrotron applications

A high speed, low noise camera system for crystallography and X-ray imaging applications is developed and successfully demonstrated. By coupling an electron-multiplying (EM)-CCD to a 3:1 fibre-optic taper and a CsI(Tl) scintillator, it was possible to detect hard X-rays. This novel approach to hard X-ray imaging takes advantage of sub-electron equivalent readout noise performance at high pixel readout frequencies of EM-CCD detectors with the increase in the imaging area that is offered through the use of a fibre-optic taper. Compared with the industry state of the art, based on CCD camera systems, a high frame rate for a full-frame readout (50 ms) and a lower readout noise (<1 electron root mean square) across a range of X-ray energies (6–18 keV) were achieved

Exactly solvable model of dissipative vortex tunneling

I consider the problem of vortex tunneling in a two-dimensional superconductor. The vortex dynamics is governed by the Magnus force and the Ohmic friction force. Under-barrier motion in the vicinity of the saddle point of the pinning potential leads to a model with quadratic Hamiltonian which can be analytically diagonalized. I find the dependence of the tunneling probability on the normal state quasiparticle relaxation time $\tau$ with a minimum at $\omega_0\tau\sim 1$, where $\omega_0$ is the level spacing of the quasiparticle bound states inside the vortex core. The results agree qualitatively with the available experimental data.Comment: RevTeX, 6 pages, 2 figures. Published versio

Introduction to the issue on novel and specialty fibers

The fiber optical communication revolution has been fueled by well publicized and relentless improvements of transmission fiber. Since the demonstration of the first low-loss optical fiber in 1972, there has been a continual stream of technology improvements designed to reduce impairments due to propagation loss and pulse dispersion. This steam of fiber technology has led the industry from multimode fiber operated at 800 nm, to standard single-mode fiber used at 1310 nm, then on to transmission fibers that now have attributes tuned for particular applications such as terrestrial or submarine transmission. There is every reason to believe that advances in technology will continue at the accelerating pace we have seen in the past decade, adding to the family of available transmission fibers. The special issue is dedicated to the increasing family of specialty fibers, and includes exciting papers on fibers for gratings and a unique amplification fiber. Fibers for specialized transmission spanning a broad range of applications are also described in three important articles. As is appreciated by all optical scientists, progress can be made only as quickly as one can improve measurement capabilities, so the issue includes two excellent papers dealing with the important measurement of chromatic dispersion.We hope that you enjoy the papers of this issue as much as we the editors have enjoyed reading and reviewing them

Modelling charge storage in Euclid CCD structures

The primary aim of ESA's proposed Euclid mission is to observe the distribution of galaxies and galaxy clusters, enabling the mapping of the dark architecture of the universe [1]. This requires a high performance detector, designed to endure a harsh radiation environment. The e2v CCD204 image sensor was redesigned for use on the Euclid mission [2]. The resulting e2v CCD273 has a narrower serial register electrode and transfer channel compared to its predecessor, causing a reduction in the size of charge packets stored, thus reducing the number of traps encountered by the signal electrons during charge transfer and improving the serial Charge Transfer Efficiency (CTE) under irradiation [3]. The proposed Euclid CCD has been modelled using the Silvaco TCAD software [4], to test preliminary calculations for the Full Well Capacity (FWC) and the channel potential of the device and provide indications of the volume occupied by varying signals. These results are essential for the realisation of the mission objectives and for radiation damage studies, with the aim of producing empirically derived formulae to approximate signal-volume characteristics in the devices. These formulae will be used in the radiation damage (charge trapping) models. The Silvaco simulations have been tested against real devices to compare the experimental measurements to those predicted in the models. Using these results, the implications of this study on the Euclid mission can be investigated in more detail

Kernel density classification and boosting: an L2 sub analysis

Kernel density estimation is a commonly used approach to classification. However, most of the theoretical results for kernel methods apply to estimation per se and not necessarily to classification. In this paper we show that when estimating the difference between two densities, the optimal smoothing parameters are increasing functions of the sample size of the complementary group, and we provide a small simluation study which examines the relative performance of kernel density methods when the final goal is classification. A relative newcomer to the classification portfolio is “boosting”, and this paper proposes an algorithm for boosting kernel density classifiers. We note that boosting is closely linked to a previously proposed method of bias reduction in kernel density estimation and indicate how it will enjoy similar properties for classification. We show that boosting kernel classifiers reduces the bias whilst only slightly increasing the variance, with an overall reduction in error. Numerical examples and simulations are used to illustrate the findings, and we also suggest further areas of research

Charge transfer efficiency in a p-channel CCD irradiated cryogenically and the impact of room temperature annealing

It is important to understand the impact of the space radiation environment on detector performance, thereby ensuring that the optimal operating conditions are selected for use in flight. The best way to achieve this is by irradiating the device using appropriate mission operating conditions, i.e. holding the device at mission operating temperature with the device powered and clocking. This paper describes the Charge Transfer Efficiency (CTE) measurements made using an e2v technologies p-channel CCD204 irradiated using protons to the 10 MeV equivalent fluence of 1.24×109 protons.cm-2 at 153 K. The device was held at 153 K for a period of 7 days after the irradiation before being allowed up to room temperature where it was held at rest, i.e. unbiased, for twenty six hours to anneal before being cooled back to 153 K for further testing, this was followed by a further one week and three weeks of room temperature annealing each separated by further testing. A comparison to results from a previous room temperature irradiation of an n-channel CCD204 is made using assumptions of a factor of two worse CTE when irradiated under cryogenic conditions which indicate that p-channel CCDs offer improved tolerance to radiation damage when irradiated under cryogenic conditions

Influence of the Environment Fluctuations on Incoherent Neutron Scattering Functions

In extending the conventional dynamic models, we consider a simple model to account for the environment fluctuations of particle atoms in a protein system and derive the elastic incoherent structure factor (EISF) and the incoherent scattering correlation function C(Q,t) for both the jump dynamics between sites with fluctuating site interspacing and for the diffusion inside a fluctuating sphere. We find that the EISF of the system (or the normalized elastic intensity) is equal to that in the absence of fluctuations averaged over the distribution of site interspacing or sphere radius a. The scattering correlation function is $C(Q,t)=\sum_{n} \psi(t)$, where the average is taken over the Q-dependent effective distribution of relaxation rates \lambda_n(a) and \psi(t) is the correlation function of the length a. When \psi(t)=1, the relaxation of C(Q,t) is exponential for the jump dynamics between sites (since \lambda_n(a) is independent of a) while it is nonexponential for diffusion inside a sphere.Comment: 7 pages, 7 eps figure