Showing papers in "IEEE Transactions on Nuclear Science in 1976"
TL;DR: This algorithm is shown to yield an image which is unbiased, which has the minimum variance of any estimator using the same measurements, and which will perform better than any current reconstruction technique, where the performance measures are the bias and viariance.
Abstract: The stochastic nature of the measurements used for image reconstruction from projections has largely been ignored in the past. If taken into account, the stochastic nature has been used to calculate the performance of algorithms which were developed independent of probabilistic considerations. This paper utilizes the knowledge of the probability density function of the measurements from the outset, and derives a reconstruction scheme which is optimal in the maximum likelihood sense. This algorithm is shown to yield an image which is unbiased -- that is, on the average it equals the object being reconstructed -- and which has the minimum variance of any estimator using the same measurements. As such, when operated in a stochastic environment, it will perform better than any current reconstruction technique, where the performance measures are the bias and viariance.
224 citations
TL;DR: In this article, the trivalent silicon and the interstitial oxygen donor centers were shown to be responsible for the heat and radiation generated positive space charge build-up (oxide charge) in thermally grown silicon oxide.
Abstract: The randomly located trivalent silicon atoms are shown to account for the thermally generated interface states at the SiO2-Si interface. The interface state density is greatly reduced in water containing ambients at low temperatures (450°C) by forming trivalent silicon hydroxide bonds. Interface states are regenerated when the ?Si-OH bonds are broken by ionizing radiation and the OH ions are drifted away. In the bulk of the oxide film, the trivalent silicon and the interstitial oxygen donor centers are shown to be responsible for the heat and radiation generated positive space charge build-up (oxide charge) in thermally grown silicon oxide.
170 citations
TL;DR: In this article, the authors report significant new results in defect studies and indicate the impact of these ideas on today's problems, and present a survey of defect studies in the literature.
Abstract: The purpose of this work is to report significant new results in defect studies and indicate the impact of these ideas on today's problems. 13 figs, 2 tables, 49 refs.
163 citations
TL;DR: In this paper, the hole-electron pairs are generated in the SiO/sub 2/ insulating layer by ionizing radiation and the holes remain essentially immobile at or near their point of creation.
Abstract: MOS devices, such as charge-coupled devices and MOSFET's, are being investigated for use in imaging, signal processing and detector preamplifier applications in which the detector and its immediately associated electronics are operated at low temperatures (less than or equal to 80 K). Recent experiments on MOS capacitors and devices at approximately 80 K have established that when hole-electron pairs are generated in the SiO/sub 2/ insulating layer by ionizing radiation the electrons are rapidly swept out of the oxide while the holes remain essentially immobile at or near their point of creation. As a result, the holes cause large flatband or threshold voltage shifts per unit dose in MOS structures. In contrast to the highly process-dependent ''permanent'' trapping of a small fraction of the radiation-generated holes observed at room temperature, the retention of the holes in SiO/sub 2/ at approximately 80 K is largely process-independent and is a consequence of a strongly temperature-dependent hole transport mechanism. The present paper discusses two related investigations of radiation-induced charge buildup in SiO/sub 2/ at approximately 80 K. In the first study, the retention of holes at low temperature is exploited to provide a direct measure of hole-electron pair yield per unit radiation dosemore » as a function of electric field for high-energy ionizing radiation. In the second study the response of MOS capacitors to short pulse high dose irradiation at approximately 80 K is investigated, and mechanisms which impose limits on the oxide charge buildup are examined.« less
158 citations
TL;DR: In this article, two Anger scintillation cameras have been combined into a positron imaging system with high sensitivity (200 cts/sec/?Ci), high resolution (system resolution less than 10 mm FWHM) and useful clinical count rates up to 8000 cts /sec.
Abstract: Two Anger scintillation cameras have been combined into a positron imaging system. Count rate capability has been extended by using absorbers to reduce scattered radiation from the object, by using 1" thick crystals to improve detection efficiency for annihilation radiation, and by implementing pulse shortening techniques and low dead time D. C. coupled electronics. The main characteristics of the device in its present form are high sensitivity (200 cts/sec/?Ci), high resolution (system resolution less than 10 mm FWHM) and useful clinical count rates up to 8000 cts/sec.
153 citations
TL;DR: In this paper, a model that assumes that the space charge region is produced by injected electrons being trapped by deep acceptors explains most of the effects observed, and it is demonstrated that MIS structures are also a practical means to suppress the polarization.
Abstract: Polarization has been studied in semi-insulating cadmium telluride compensated with chlorine. Several parameters have been considered including surface preparation, nature of the contact, temperature, as well as type and strength of the source. A model that assumes that the space charge region is produced by injected electrons being trapped by deep acceptors explains most of the effects observed. The possible nature of the center is discussed. Finally it is demonstrated that MIS structures are also a practical means to suppress the polarization.
149 citations
135 citations
TL;DR: In this paper, the transient response of a number of pedigreed SiO2 gate insulator MOS capacitors following exposure to a pulsed 13-MeV electron beam was studied as a function of time, temperature, and applied bias.
Abstract: The transient response, or flat-band voltage recovery, in a number of pedigreed SiO2 gate insulator MOS capacitors following exposure to a pulsed 13-MeV electron beam was studied as a function of time, temperature, and applied bias A quantitative comparison of the response characteristics of the different oxides is made It is found generally that the response consists of two stages The first (early time) stage in most cases encompasses most of the recovery and is dominated by hole transport through the insulator film to the interface This hole transport recovery stage in all the oxides studied is well described by the stochastic model of hopping transport based on a continuous time random walk used previously in the analysis of hole transport in SiO2 It is further shown that all the main features of the hole transport are consistent with a model of the holes moving via polaron hopping between localized sites in the SiO2 The second or long-term stage of recovery commences after cessation of the hole transport in the oxide and is found to vary significantly in its importance and time of onset among the various oxides This stage involves a radiation-induced buildup of interface states and possibly, in some cases, an annealing of a trapped-hole distribution near the SiO2/Si interface
129 citations
TL;DR: In this article, the authors derived the form of the maximum likelihood (ML) estimate of the scintillation point given the photomultiplier counts in an Anger Scintillation camera.
Abstract: We derive the form of the maximum likelihood (ML) estimate of the location of the scintillation point given the photomultiplier counts in an Anger Scintillation camera. This estimate is also Maximum a Posteriori (MAP) if the prior probability density on the scintillation point is uniform in the object plane. The form of the estimate suggests the possibility of an optical filtering implementation. We note that the ML estimate implies a solution that is remarkably similar to the "optimum position arithmetic" derived by Tanaka, et al.
111 citations
TL;DR: In this paper, a comprehensive analysis of theoretical and practical aspects of position sensing by charge division from resistive electrodes is presented, and properties of transformer decoupling of the resistive electrode from detection bias voltage are analyzed and compared to the usual capacitive decoupled methods.
Abstract: A summary of a comprehensive analysis of theoretical and practical aspects of position sensing by charge division from resistive electrodes is presented. Properties of transformer decoupling of the resistive electrode from detection bias voltage are analyzed and compared to the usual capacitive decoupling methods. Optimization and limitation of signal shaping is discussed as a function of diffusion time constant, signal rise times, and noise.
106 citations
TL;DR: In this article, a positron camera system was designed to record the coincidences from 2 × 736 (or 2 × 1116) detector combinations in a circular array with a single layer of 64 (or 72) NaI(T1) crystals and PM tubes.
Abstract: The transverse axial 3-dimensional image reconstruction concepts developed in transmission tomography brought the nuclear medicine and diagnostic radiology into the realm of quantitative analysis of the 3-dimensional image hitherto superimposed in two dimensions Two factors make it appropriate to design a positron camera system at this time, (1) analogy between the emission of photons following a positron annihilation and the transmission of photons, and (2) the already demonstrated success of positron camera multidetector systems The camera consists of one layer of 64 (or 72) NaI(T1) crystals and PM tubes that simultaneously record coincidences from 2 × 736 (or 2 × 1116) detector combinations in a circular array
TL;DR: In this paper, a whole body positron emission transaxial tomograph (PETT III) has been designed, constructed and is presently under clinical evaluation and the design of PETT III spatial resolution and sensitivity of annihilation coincidence detection (ACD), sampling considerations and importance of proper shielding are evaluated.
Abstract: A whole body positron emission transaxial tomograph (PETT III) has been designed, constructed and is presently under clinical evaluation. The design of PETT III spatial resolution and sensitivity of annihilation coincidence detection (ACD), sampling considerations and importance of proper shielding are presented to evaluate some design considerations for an ACD tomograph. Tomographic studies of the brain and heart with human subjects are presented to demonstrate the image quality of PETT III under clinical conditions. (auth)
TL;DR: In this article, a statistical model is described and employed to calculate inverse mean free paths and stopping powers for electrons of energies from a few eV to 10 keV above the Fermi level in Al, Si, Ni, Cu, Ag and Au.
Abstract: A statistical model is described and employed to calculate inverse mean free paths and stopping powers for electrons of energies from a few eV to 10 keV above the Fermi level in Al, Si, Ni, Cu, Ag, and Au. Brief tables of mean free paths and stopping powers for these solids are presented. In some cases graphical displays of inverse mean free paths and stopping powers are also included. The calculations based on this model are discussed and compared with previous work.
TL;DR: In this paper, the authors describe three ion sources working in a laboratory that deliver multiply charged ion beams, which are characterized by the fact that the electrons are emitted by the plasma itself and are accelerated to the adequate energy through electron cyclotron resonance (E.C.R.).
Abstract: We describe three ion sources working in our laboratory that deliver multiply charged ion beams. All of them are E.C.R. ion sources and are characterized by the fact that the electrons are emitted by the plasma itself and are accelerated to the adequate energy through electron cyclotron resonance (E.C.R.). They can work without interruption during several months in a quasi-continuous regime. (Duty cycle: <.5). Their charge state distributions (C.S.D.) depend on the size and the characteristics of the plasma. For the smallest source the performances are comparable to those of P.I.G. sources. For the biggest source, the C.S.D. tend towards those obtained by EBIS-type sources. Experimental results and guiding ideas concerning these three ion sources are reviewed.
TL;DR: A review of the electron beam sources of highly-stripped ions, developed at the Joint Institute for Nuclear Research during 1967-75, is given in this paper, where the effective experimental ionization cross sections of positive C, N. Ar, and Xe ions by electron impact are presented.
Abstract: A review of the electron beam sources of highly-stripped ions, developed at the Joint Institute for Nuclear Research during 1967-75, is given. The effective experimental ionization cross sections of positive C, N. Ar, and Xe ions by electron impact are presented. Possible fields of application of electron beam ion sources are indicated.
TL;DR: In this article, the authors review the charging phenomenon, its hazards and the response of the scientific and engineering community during the past few years, and discuss the similarities and differences between spacecraft charging and SGEMP.
Abstract: A potentially catastrophic environmental phenomenon was recently discovered; large discharges and arcs on spacecraft. This phenomenon, induced by magnetospheric substorm events, was probably responsible for numerous spacecraft malfunctions and at least one spacecraft failure. More often, however, the electromagnetic interference resulting from the arc is induced into various spacecraft circuits which leads to triggering of integrated circuit switches, multivibrators and one-shots. In addition, the arcing phenomenon has given rise to degradation of thermal blankets (by vaporizing the vacuum deposited aluminum [VDA] present on the blanket), contamination of surfaces by vaporization products, triggering of light sensitive devices, and anomalous functioning of scientific sensors. The general program, undertaken by the engineering and scientific community, in response to this phenomenon, includes the following major elements: 1) the development of a standard environmental specification to cover the geomagnetic substorm environment; 2) the development of general design requirements and criteria-guidelines for design of spacecraft to be resistant to the effects of geomagnetic substorms; 3) the development of analytic tools to determine and predict stress levels induced by the substorm charging environment; 4) the development of test programs to verify the immunity of spacecraft to environmental substorm charging and arcing; and 5) the requirement for housekeeping data, environmental data, and laboratory test data. The purpose of this paper is to review the charging phenomenon, its hazards and the response of the scientific and engineering community during the past few years. The similarities and differences between spacecraft charging and SGEMP will also be discussed.
TL;DR: In this article, it was shown that at room temperature for high applied fields, nearly all electrons and holes generated in the oxide by a pulse of ionizing radiation drift to the interfaces, whereas at low temperatures only electrons contribute to observed transport for relatively low fields.
Abstract: Studies of the temporal, temperature, and electricfield dependences of radiation-induced charge transport have been performed for radiation-hardened SiO2 films. At room temperature for high applied fields, nearly all electrons and holes generated in the oxide by a pulse of ionizing radiation (5-keV electrons) drift to the interfaces, whereas at low temperatures only electrons contribute to observed transport for relatively low fields. Below ~130°K at high fields, field-induced emission of trapped holes occurs, giving rise to collection within seconds of a significant fraction of the total number of holes generated. The present hole transport data are accounted for quite well in terms of a multiple-trapping model with a spread in trap levels ranging from ~0.3 to ~0.5 eV from the valence band. Comparison with the stochastic hopping transport model is made and that model is found to be less satisfactory in explaining these data. Charge buildup was examined in a Co60 environment and it is demonstrated that oxides exhibiting radiation tolerance at room temperature display severe radiation-induced changes at 77°K. It is also demonstrated that low-temperature charge buildup problems can be alleviated either by employing an ion-implanted oxide or by applying a relatively high field to the oxide during irradiation.
TL;DR: In this article, the continuity and Poisson's simultaneous equations were solved to obtain the space charge profile and the field strength profile by solving the continuity equation and the Poisson equation.
Abstract: Space charge profile and consequent field strength profile were obtained by solving the continuity and Poisson's simultaneous equations. Experimental electron deposition curve and radiation-induced conductivity curve were used for calculation. The calculated results explained well the observed replacement current and electrical breakdown phenomena. Dose rate, dose, and temperature effects on charge accumulation were also investigated.
TL;DR: In this article, two aspects of the determination of the optical quality of ion beams are considered: namely, the formation of an ion beam by extraction from a plasma source and the space charge neutralization of ion beam.
Abstract: Two aspects of the determination of the optical quality of ion beams are considered: namely, the formation of an ion beam by extraction from a plasma source and the space charge neutralization of ion beams. It is noted that there is a relation between these problems since they both depend on the perveance of the beam and become more important as the perveance increases. (JFP)
TL;DR: In this article, a new nuclear Betavotaic battety is described, which uses a Schottky barrier in place of the more standard p-n junction diode, along with 147Pm metal film rather than Pm2O3 oxide as in the commercially available Betacel.
Abstract: A new nuclear betavotaic battety is described. It uses a Schottky barrier in place of the more standard p-n junction diode, along with 147Pm metal film rather than Pm2O3 oxide as in the commercially available Betacel. Design details of the battery including measurement of absorption, conversion efficiency, thickness etc. as functions of & resistivity and other cell parameters are described. A prototype design is discussed and its performance assessed.
TL;DR: In this paper, the quality of the ion beam emitted from the source has been investigated in terms of emittance, brightness, and energy distribution, and the range of beams obtained has been extended with beams of Li- (1?A) and BO- (2.5?A).
Abstract: The recently described ANIS source of the cesiumdoped, sputtering type has been the subject of further investigation. The range of beams obtained has been extended with beams of Li- (1 ?A) and BO- (2.5 ?A). The quality of the ion beam emitted from the source has been investigated in terms of emittance, brightness, and energy distribution.
IBM1
TL;DR: Avalanche injection and internal photo-emission techniques have been used to study the capture of electrons by positive charges introduced into the oxide layers of MOS capacitors as discussed by the authors, and the electron capture crosssection of this charge has been found to depend on the composition of the interface.
Abstract: Avalanche injection and internal photo-emission techniques have been used to study the capture of electrons by positive charges introduced into the oxide layers of MOS capacitors. These two techniques have been used to study the positive charge in Al gate and poly-Si gate capacitors. The electron capture crosssection of this charge has been found to depend on the composition of the interface; positive charge at the Si interface tends to have a coulombic electron capture cross-section (~ 4×10-13cm2), while that at the alumninum interface has a non-coulombic electron capture crosssection (? 10-14cm2). The location of the positive charge induced by radiation under positive or negative bias has been determined in a completely non-destructive manner by photocurrent-voltage experiments. It has been found that under a positive irradiation bias, positive space charge accumulates within ~ 50 A of the Si-SiO2 interface, while under a negative bias, the space charge is within ~ 50 A of the Al-SiO2 interface. In both cases there is evidence for some charge at the other interface introduced by the irradiation.
TL;DR: In this article, the best resolution obtained is 548 e V for 5.9 keV x-rays, and the 2.01 keV phosphorous x-ray line has been resolved with a detector with an area of 14 mm2.
Abstract: Measurement results obtained with HgI2 detectors at room temperature in the low energy x-ray region down to 2 keV are reported. The best resolution obtained is 548 e V for 5.9 keV x-rays, and the 2.01 keV phosphorous x-ray line has been resolved with a detector with an area of 14 mm2. The different noise sources of the detector-amplifier system are analysed and polarization effects in the detectors are discussed. The mercuric iodide crystals were grown from the vapour phase by gradient reversal techniques.
TL;DR: In this article, a model is presented which relates MOS processing steps and design parameters to the ionizing radiation sensitivity of hardened MOS devices, based on combining the viscous properties of SiO/sub 2/ at typical processing temperatures with stresses arising in the Si system during oxidation and subsequent high-temperature cycles.
Abstract: A model is presented which relates MOS processing steps and design parameters to the ionizing radiation sensitivity of hardened MOS devices. The model is based on combining the viscous properties of SiO/sub 2/ at typical processing temperatures with stresses arising in the SiO/sub 2/ on Si system during oxidation and subsequent high-temperature cycles. The model explains the recently observed dependence of radiation-induced flatband voltage shifts upon SiO/sub 2/ thickness cubed. Quantitative comparison with experiment is obtained for the relations of flatband voltage shift on post-oxidation annealing temperature and on oxide growth temperature. Several processing changes to further harden both dry and wet oxides are suggested by the model.
TL;DR: In this paper, the authors considered charge buildup in MOS structures due to hole trapping in the oxide and the creation of sheet charge at the silicon interface and found that the contribution of hole trapping causes the flatband voltage to increase with thickness in a manner in which square and cube dependences are limiting cases.
Abstract: The model considers charge buildup in MOS structures due to hole trapping in the oxide and the creation of sheet charge at the silicon interface. The contribution of hole trapping causes the flatband voltage to increase with thickness in a manner in which square and cube dependences are limiting cases. Experimental measurements on samples covering a 200 - 1000 A range of oxide thickness are consistent with the model, using independently obtained values of hole-trapping parameters. An important finding of our experimental results is that a negative interface charge contribution due to surface states created during irradiation compensates most of the positive charge in the oxide at flatband. The tendency of the surface states to "track" the positive charge buildup in the oxide, for all thicknesses, applies both in creation during irradiation and in annihilation during annealing. An explanation is proposed based on the common defect origin of hole traps and potential surface states.
TL;DR: In this paper, a review of charge exchange cross sections of heavy ions at energies primarily in the range below 100 keV is presented, with particular emphasis on the cross sections for capture of one or more electrons in single encounters of multiply charged ions with atoms.
Abstract: A review is presented on charge exchange cross sections of heavy ions at energies primarily in the range below 100 keV. Particular emphasis is given to the cross sections for capture of one or more electrons in single encounters of multiply charged ions with atoms. The system of the noble gases as projectile ions and target atoms is studied in detail. Systematic trends are examined. Attention is given to the influence of metastable ions on the cross sections.
TL;DR: In this article, a method for three dimensional reconstruction of axial tomographic images which removes the blurred off-plane activity from a number of transverse planes simultaneously is presented, which can be implemented on a small computer having a disc system.
Abstract: A number of existing cameras for Nuclear Medicine imaging of radio-isotope distributions give depth information about the distribution. These devices have in common that they provide tomographic images of the object, that is, that images of a given object plane have that plane in focus and all other object planes contribute an out-of-focus background superimposed on the in-focus image. We present here a method for three dimensional reconstruction of these axial tomographic images which removes the blurred off-plane activity from a number of transverse planes simultaneously. The method is applicable to a number of tomographic cameras, such as the multiple single-pinhole camera, the rotating slanted-hole collimator, the Anger focussing tomographic scanner, and the positron camera. The method can be implemented on a small computer having a disc system.
TL;DR: In this article, a model for the calculation of radiation-induced signals in multiconductor electrical cables is presented, which is applicable to cables of arbitrary cross-sectional geometry and material configuration, permitting consistent treatment of such effects as gaps, residual air, spatially-dependent transient conductivity arising from dose enhancement, saturation, and polarization effects.
Abstract: A model is presented for the calculation of radiation-induced signals in multiconductor electrical cables. The model employs a Monte Carlo method for the calculation of the radiation-driven charge displacement within the cable and a Green's function to translate charge displacement into induced currents on conductors. The model, which is applicable to cables of arbitrary cross-sectional geometry and material configuration, permits consistent treatment of such effects as gaps, residual air, spatially-dependent transient conductivity arising from dose enhancement, saturation, and polarization-effects. These effects have, by improper treatment or neglect, limited the applicability and accuracy of previous models. Results are presented demonstrating the effects of gaps, residual air, dose enhancement, and dielectric liners and illustrating the distribution of signals on many-conductor cables.
TL;DR: In this article, the minimum prompt photoconductivity induced by pulses of x rays, gamma rays, and energetic electrons in various amorphous and disordered insulating organic and inorganic materials is predicted on the basis of data for the scattering of hot electrons in solids and the band gap for insulators.
Abstract: The minimum prompt photoconductivity induced by pulses of x rays, gamma rays, and energetic electrons in various amorphous and disordered insulating organic and inorganic materials is predicted on the basis of data for the scattering of hot electrons in solids and the band gap for insulators. For total doses of 3 × 104 to 30 × 104 rad or greater, the minimum prompt photoconductivity is predicted to be linear with dose rate, ?, and is given by ?(?-1cm-1)=5×10-19?0?/Eg2, where ?0 is the density (g/cm3) and Eg is the optical band gap (eV). This formula agrees well with data for a variety of plastics, mica, and borosilicate glass under widely different irradiation conditions. The formula considerably underestimates absolute values of prompt conductivities observed for Al2O3, MgO, and certain plastics, because the model does not hold for ordered materials.
TL;DR: In this paper, a numerical method for accurate and efficient calculations of charged particle flux-at-a-point in com& lex geometries is developed using the condensed history Monte Carlo technique, which is obtained by first sampling particle energy and direction at the point of interest and then tracking the particle backwards (particles speed up) to the source.
Abstract: A numerical method has been developed for accurate and efficient calculations of charged particle flux-at-a-point in com& lex geometries. Accurac is obtained using the condensed history Monte Carlo technique. Efficiency is obtained by first sampling particle energy and direction at the poinlt of interest and then tracking the particle backwards (particles speed up) to the source.