Thousands Of Electrons Research Articles

Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite

The European Space Agency (ESA) Gaia satellite has 106 CCD image sensors which will suffer from increased charge transfer inefficiency (CTI) as a result of radiation damage. To aid the mitigation at low signal levels, the CCD design includes Supplementary Buried Channels (SBCs, otherwise known as `notches') within each CCD column. We present the largest published sample of Gaia CCD SBC Full Well Capacity (FWC) laboratory measurements and simulations based on 13 devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs in the upper half of each CCD that are systematically smaller by two orders of magnitude (<50 electrons) compared to those manufactured pre-2004 (thousands of electrons). Gaia's faint star (13 < G < 20 mag) astrometric performance predictions by Prod'homme et al. and Holl et al. use pre-2004 SBC FWCs as inputs to their simulations. However, all the CCDs already integrated onto the satellite for the 2013 launch are post-2004. SBC FWC measurements are not available for one of our five post-2004 CCDs but the fact it meets Gaia's image location requirements suggests it has SBC FWCs similar to pre-2004. It is too late to measure the SBC FWCs onboard the satellite and it is not possible to theoretically predict them. Gaia's faint star astrometric performance predictions depend on knowledge of the onboard SBC FWCs but as these are currently unavailable, it is not known how representative of the whole focal plane the current predictions are. Therefore, we suggest Gaia's initial in-orbit calibrations should include measurement of the onboard SBC FWCs. We present a potential method to do this. Faint star astrometric performance predictions based on onboard SBC FWCs at the start of the mission would allow satellite operating conditions or CTI software mitigation to be further optimised to improve the scientific return of Gaia.

Expeditious Stochastic Approach for MP2 Energies in Large Electronic Systems

A fast stochastic method for calculating the second order Møller-Plesset (MP2) correction to the correlation energy of large systems of electrons is presented. The approach is based on reducing the exact summation over occupied and unoccupied states to a time-dependent trace formula amenable to stochastic sampling. We demonstrate the abilities of the method to treat systems with thousands of electrons using hydrogen passivated silicon spherical nanocrystals represented on a real space grid, much beyond the capabilities of present day MP2 implementations.

Computational challenges of large-scale, long-time, first-principles molecular dynamics

Plane wave density functional calculations have traditionally been able to use the largest available supercomputing resources. We analyze the scalability of modern projector-augmented wave implementations to identify the challenges in performing molecular dynamics calculations of large systems containing many thousands of electrons. Benchmark calculations on the Cray XT4 demonstrate that global linear-algebra operations are the primary reason for limited parallel scalability. Plane-wave related operations can be made sufficiently scalable. Improving parallel linear-algebra performance is an essential step to reaching longer timescales in future large-scale molecular dynamics calculations.

One Electron Makes Current Flow

Even the smallest components in a modern desktop computer use tens of thousands of electrons at a time to implement classical logic (the conventional 1s and 0s of binary computation). A computer that operated on single electrons, however, could in principle implement quantum logic functions (quantum bits or “qubits” that could perform computational tasks that are beyond the ability of classical computers). One proposed architecture for a single-electron computer could be realized in a two-dimensional electron gas (2DEG), a special kind of reservoir in which electrons can travel without dissipation (1). On page 1169 of this issue, Fève et al. (2) report on an important step toward this vision: a device that can emit single electrons into a 2DEG and absorb them again on nanosecond time scales. With such a controllable electron source, researchers will now be able to set up one or more electrons in well-defined quantum states, which are crucial for any future quantum computer.

Subpicosecond conduction through thin SiO2 layers triggered by heavy ions

Heavy ions impinging on thin silicon dioxide layers generate a dense plasma of electrons and holes. Under particular conditions, such as in thin oxide layer surrounding isolated conductive lines, this plasma can act as a conductive medium, able to carry current for very short times (10−14s). We studied this phenomenon by using large data set obtained on state-of-the art floating gate memory arrays. Floating gates hit by ions experience a charge loss linearly dependent on ion linear energy transfer and on the electric field across the tunnel oxide. Despite its absolute low value, reaching at most thousands of electrons, charge lost from floating gate exceeds by orders of magnitude that expected from relatively simple models, such as generation and recombination. The model we are proposing is fully consistent with a broad range of theoretical and experimental results, and has excellent fitting capabilities.

Parallel task processing in ab initio electronic structure calculations

In order to perform precise investigation of low-symmetry effects, a direct numerical method for electronic structure modeling has been proposed. The radial orbitals are split in accordance with the site symmetry of the ion in the crystal and its electronic structure is calculated with a nonspherical ab initio potential of nearest neighbours. The introduction of parallel techniques is discussed from two points of view. The physical model is based on an atomistic approach for solids which is totally equivalent to usual ab initio quantum chemistry approximations. `Non-effective' physical treatment becomes attractive computationally due to the possibility of solving, simultaneously and up to self consistency, a common equation duplicated many times with different parameters. Thus, parallel task processing on the set of equations for atomic shells together with message passing of single electron orbitals calculated in each process allows co-ordination of thousands of processors for the thousands of electrons in the crystal.

On the Linear Extrapolation to Low Doses

Abstract While radiobiological data are conveniently fitted by a linear quadratic formula to data of limited dynamic range at doses typically exceeding 1 Gy, they are extrapolated linearly to doses below a milligray for the evaluation of low dose RBEs. However a single relativistic electron passing through a cell nucleus deposits a 'dose' there in the neighbourhood of a milligray. The validity of the linear extrapolation then rests on the demonstration that a single electron transit through a cell nucleus can cause inactivation or mutation or can lead to cancer induction. The extrapolation made is a huge one, of some three orders of magnitude, from thousands of electrons traversing a cell to a single cell traversal, and with only qualitative and rhetorical foundation (one track-two tracks), and ignoring the statistical fluctuations of track intersections with targets. Existing data obtained with beams of electrons, protons, X ray photons, incorporated tritium and 125I, demonstrate that hundreds of electrons may traverse a cell for inactivation and millions may be required for cancer induction. If linear extrapolation were valid these numbers would be reduced to one. These contradictions suggest serious reconsideration of accepted radiation protection standards.

On the Linear Extrapolation to Low Doses

While radiobiological data are conveniently fitted by a linear quadratic formula to data of limited dynamic range at doses typically exceeding 1 Gy, they are extrapolated linearly to doses below a milligray for the evaluation of low dose RBEs. However a single relativistic electron passing through a cell nucleus deposits a 'dose' there in the neighbourhood of a milligray. The validity of the linear extrapolation then rests on the demonstration that a single electron transit through a cell nucleus can cause inactivation or mutation or can lead to cancer induction. The extrapolation made is a huge one, of some three orders of magnitude, from thousands of electrons traversing a cell to a single cell traversal, and with only qualitative and rhetorical foundation (one track-two tracks), and ignoring the statistical fluctuations of track intersections with targets. Existing data obtained with beams of electrons, protons, X ray photons, incorporated tritium and 125I, demonstrate that hundreds of electrons may traverse a cell for inactivation and millions may be required for cancer induction. If linear extrapolation were valid these numbers would be reduced to one. These contradictions suggest serious reconsideration of accepted radiation protection standards.

Simple and Efficient Method to Eliminate Spike Noise from Spectra Recorded on Charge-Coupled Device Detectors

Charge-coupled device (CCD) detectors have several characteristics which are suitable for spectroscopic measurements at very low light levels. The quantum efficiency of a CCD is high over a wide wavelength range, and the dark count rate is greatly reduced by cooling the detector. Further reduction of the dark count can be made by setting the pixel binning frame to the detector area that is actually illuminated. If the detector is sufficiently cooled and the signal readout is performed after appropriate binning, the only significant noise source, aside from inevitable photon shot noise, is the noise produced in the signal readout process. Since the readout noise is independent of the duration of exposure, longer exposure provides a better signal-to-noise (S/N) ratio, in principle. However, the practical limit to exposure time is set by spike noise, which is generated by cosmic rays as well as possible γ- and α-rays emitted from materials around the detector chip. Each quantum of such high-energy rays striking the CCD chip usually generates thousands of electrons in a pixel, while a single photon absorbed by the detector produces less than one electron on the average. Thus the spike noise sometimes obscures the very low light level spectrum. Distinction of spike noise from the true photon signals and reliable elimination of the spikes are required in order to further elongate the exposure time and to achieve a better S/N ratio. Occasionally, data processing with median filters is employed to remove the spikes from spectra recorded on CCD detectors. However, this filtering method is a kind of smoothing and has a possibility of distorting the spectrum, particularly when spectral linewidths are comparable to or less than those of spikes. We have developed a simple and efficient method to eliminate the spike noise without losing the original spectral information. Here, we describe the method and demonstrate its effectiveness in Raman and atomic emission spectroscopy.

Simulations of second-order Fermi acceleration of electrons: solving the injection problem

The boosting of electrons from a Maxwellian distribution into a suprathermal power-law tail is studied, using a test-particle simulation code, following the full equations of motion of tens of thousands of electrons chosen from a thermal population as they move through general time-dependent magnetic fields. This distinguishes between bulk heating of the electron population or boosting of selected electrons. Which of these occurs is found to depend on the size of the swarm thermal speed compared with the electron thermal speed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the possible source of the ionization in the nighttime Martian ionosphere: 1. Phobos 2 Harp Electron Spectrometer measurements

The measurements of electron spectra in the Martian magnetosphere by the HARP instrument on board the Phobos 2 orbiter are presented. The energy of the electrons (a few tens of electron volts) is sufficient for the impact ionization of the planetary neutral gas, and the characteristic flux of electrons (∼108 cm−2 s−1) could produce the nightside ionospheric layer with a peak density of a few thousands of electrons per cubic centimeter, which corresponds to densities observed earlier during radio occultations of the Mars 4 and 5 and Viking 1 and 2 spacecraft. The possibility of magnetospheric electron precipitation into the nightside atmosphere of Mars is in agreement with the mainly induced nature of the magnetic field in the planetary magnetotail (as at Venus), while the variability of the Martian nightside ionosphere may be explained by the partial screening of the atmosphere by a weak intrinsic magnetic field of the planet.

Reflection properties of the lower polar ionosphere, and features of the excitation and propagation of superlong waves (SLW) at high latitudes (review)

REFLECTION PROPERTIES OF THE LOWER POLAR IONOSPHERE, AND FEATURES OF THE EXCITATION AND PROPAGATION OF SUPERLONG WAVES (SLW) AT HIGH LATITUDES (REVIEW) G. F. Remenets and M. I. Beloglazov UDC 550.38+621.37 At the present time, SLW radio waves are used to solve a wide range of problems: inves- tigation of the lower ionosphere and magnetosphere, use in global systems of communications, navigation, and in the transmission of standard frequencies. In this regard, there is special interest currently in the study of the properties of SLW propagation in high latitude regions, where a whole complex of specific ionospheric phenomena exist: these phenomena affect significantly the structure of the SLW field in the waveguide which is formed by the Earth and the ionosphere. Since the latest reviews of high latitude propagation of SLW were published [I, 2], a number of new and rather interesting data have appeared. These new results refer in part to improvements in analysis techniques, enhancements in the precision and resolving power of the methods of measurement, and the accumulation of new quantitative results which merit special consideration. It is well known that SLW propagation processes in the near-earth waveguide are extremely sensitive to the conditions in the lower ionosphere. In the present case, by the term "lower ionosphere" we mean the D-layer and the lower part of the E-layer (i.e., heights of 50-90 km, where electron densities are N e = 10-103 cm-3). We will therefore discuss first of all briefly those papers of the last 5-10 years where new or more precise results have been obtained concerning the free electron content in the lower ionosphere at high latitudes. I. SOURCES OF IONIZATION AND~METHODS OF INVESTIGATING THE ELECTRON CONTENT IN THE LOWER POLAR IONOSPHERE It is well know~ that the regular sources 0f ioniZation in the lower ionosphere which are independent of latitude are the radiation from the sun (photons) and cosmic rays of galactic origin. As has been shown in [3], the principal role in quiet daytime conditions is played by ultraviolet radiation from the sun. The major difference between the high latitude ionosphere and the ionosphere at mid- latitudes is its extraordinary variability, due to the constant incursion of corpuscular fluxes: the latter are extremely irregular in character. I.l. Brief Characterization of the Sources of Anomalous Ionization in the Lower Ionosphere at High Latitudes I.l.l. Intrusion of solar cosmic rays (mainly protons with energies of i-i00 MeV) leads to the formation of significant ionization at anomalously low heights and to the occurrence of a whole complex of effects known by the overall name of PCA (polar cap ab- sorption). The electron density may rise to values of hundreds and thousands of electrons per cubic centimeter at heights of 40-50 km. Each PCA event may last for up to 10-15 days. The region of anomalous ionization which develops occupies the region near the poles: on the average, it is bounded by the geomagnetic latitudes ~' = 65-70 ~ . The average duration of proton anomalies amounts to roughly 30-40 days per year [4]. During years when the sun is quiet, this figure falls to 15-30 days, while in years when the sun is active, it increases to 40-60 days. I.I.2. During magnetic substorms, fluxes of electrons with energies of tens of kilo- electron-volts precipitate into the earthfs atmosphere. These lead to the appearance of auroral ionization, which makes its presence felt in the form of enhanced absorption of cosmic radio noise. The statistically averaged region of auroral fonization (the auroral Leningrad State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 28, No. 12, pp, 1491-1504, December, 1985. Original article submitted February 5, 1985. 0033-8443/85/2812-1029509.50 9 1986 Plenum Publishing Corporation 1029

Skin dose from a cobalt 60 teletherapy unit.

Because of its physical characteristics, high-energy radiation offers several advantages over radiation of lower energy in the treatment of deep-seated tumors. These are: (a) an increase in percentage depth dose, (b) a decrease in side-scatter, resulting in flatter isodose curves with a smaller penumbra, and (c) the establishment of electronic equilibrium at a greater depth, resulting in a smaller skin dose. With the Co60 unit under test at this laboratory, it was found that little could be done to affect (a) or (b), with the exception of a moderate dependence of penumbra on cone design. For short cone-to-skin distances, the third advantage is greatly affected by electrons scattered from the walls of the collimating system. This report is a study of the electron build-up region and the optimum cone design necessary to preserve the third advantage. When gamma rays enter tissue, they lose energy through interactions, and accelerated electrons are produced. These electrons in turn lose their energy as they penetrate tissue by radiation processes or by removing other electrons from their atoms (ionization). If the initial electron acquires a high energy from the gamma ray, thousands of electrons and positive ions are produced by this one electron along its path, since only a small amount of energy (about 32.5 ev) is lost in producing each ion pair. The accelerated electrons penetrate the tissue to an average depth corresponding to the energies given the electrons by the gamma rays. If the beam of gamma rays is completely free of electrons before entering the tissue and if there are no back-scattered gamma rays or electrons, the ionization at the surface will be zero. When the gamma rays enter the tissue, accelerated electrons are produced, and at Co60 energies the acceleration is predominantly in a forward direction. As the tissue is penetrated, additional electrons are accelerated with each increment of depth. Thus the number of electrons, and consequently the ionization, builds up with depth. This build-up progresses until a depth corresponding to the average range of the electrons is reached. At this depth, equilibrium has been established between the primary and secondary radiation, and the maximum number of electrons that can contribute to the dosage has been achieved. Beyond this depth, the number of gamma rays per unit area decreases due to the divergence of the beam and to interactions in the preceding layers of tissue. Hence, with further increase in depth, the ionization decreases relatively slowly. There are two other sources of radiation usually present which add to the ionization in the electron build-up region. The first is back-scattered gamma rays which produce additional ionization at all depths up to and including the surface. The second is electrons (from the collimating system or from air) that contaminate the gamma-ray beam. Several workers have studied the phenomenon of electron build-up.