Showing papers in "IEEE Transactions on Nuclear Science in 1978"

A Method for Attenuation Correction in Radionuclide Computed Tomography

Abstract: The development of algorithms for Radionuclide Computed Tomography (RCT) is complicated by the presence of attenuation of gamma-rays inside the body. Some of the existing RCT reconstruction algorithms apply approximation formulas to the projection data for attenuation correction, while others take attenuation into account through some iterative procedures. The drawbacks of these algorithms are that the approximation formulas commonly used are generally inadequate and the iterative procedures are usually very time-consuming. The method for attenuation correction in RCT, which we propose, applied a simple, effective two-step procedure to the uncorrected image. In this procedure the filtered back-projection algorithm is used for its fast speed. A simple mathematical basis and description of the procedure together with some illustrative computer results are given in this paper.

Cosmic Ray Induced in MOS Memory Cells

Abstract: A model for estimating the cosmic ray induced bit error rate in dynamic MOS RAMs is developed and used to calculate the bit error rate in NMDS dynamic RAMs used in an operating satellite system. The calculated error rate agrees sufficiently well with the observed error rate to conclude that cosmic ray ionization is a likely cause of observed satellite bit errors. The susceptibility of M1S RAMs to cosmic ray induced error is a result of the small charge (sub-picocoulamb) typically stored on a MDS gate to represent a data bit. Relatively small energy deposition (a few MeV) can discharge a storage node, resulting in a bit error. The heavy ion particles present in galactic cosmic rays can provide this energy, resulting in a significant bit error rate for large nmeory systems in satellites. The dynamic RAM operational factors and design factors affecting ionization-induced bit error rates are discussed.

The Development of an MOS Dosimetry Unit for Use in Space

Abstract: A space qualified integrating radiation dosimeter, using a special extra-sensitive metal-oxide-semiconductor (named the TOT 201 transistor) as the sensing device has been put into geostationary orbit aboard a European ccmTunication satellite (OTS). This paper describes the fabrication and calibration of a specially sensitive MOS transistor (sensitivity of 3.8 10-4 Volt rad-1) and the design and calibration of × the flight circuit. This is the first known use of a special MOS device to measure the internal environment of a space vehicle. Weight size and power demand are so low that the unit could be widely used as a 'housekeeping monitor' in future orbital payloads.

A Computer Assisted Ringdector Positron Camera System for Reconstruction Tomography of the Brain

Abstract: A ring detector positron camera system for brain metabolism studies is being constructed at the Institute of Physics, University of Stockholm, in collaboration with the Departments of Neuroradiology and clinical Neurophysiology, Karolinska Sjukhuset, Stockholm, Sweden. The instrument utilizes 95 NaI(Tl) detectors and will simultaneously record coincidences from 1900 detector combinations. Each detector is coupled in coincidence with forty detectors on the opposite side of the ring. In the conventional stationary mode of operation the experimental system resolution is 10.5 mm. A new sampling technique has been developed to reduce the system resolution. Based on this technique, an experimental system resolution of 7 mm has been obtained.

Inelastic Interactions of Electrons with Polystyrene: Calculations of Mean Free Paths, Stopping Powers, and CSDA Ranges

Abstract: A theoretical description of the inelastic interactions of electrons with solid polystyrene is presented. The response of the valence electrons to energy and momentum transfers is determined by a model insulator theory; carbon K-shell ionization cross sections are derived from atomic, generalized oscillator strengths. Contributions to the inverse mean free path and stopping power due to these two excitation processes are derived and tabulated for incident electrons with energies from 10 eV to 10 keV. Electron ranges in the continuous-slowing-down approximation are calculated and tabulated for electrons with energies from 15 eV to 10 keV.

A Simple Model for Predicting Radiation Effects in MOS Devices

Abstract: A mathematical model for the electrical consequence of charge buildup in metal-oxide-semiconductor devices has been investigated and found to be useful. It can be used (a) for comparing experimental data from different sources and (b) for making predictions of the lifetime of MOS circuits exposed to radiation in space or laboratory environments. A wide range of experimental data has been collected together and is presented in the form of a normalised oxide radiation-sensitivity parameter A, representing the probability of hole capture. The results show that the simple model, based upon a thin sheet of hole traps, must be modified when the rate of interface-state creation is commensurate with the trap filling rate but that this does not destroy its usefulness for a wide range of applications.

Iterative Three-Dimensional Reconstruction from Twin-Cone Beam Projections

Abstract: The twin-cone beam geometry has been used to reconstruct a three-dimensional object from projections. The reconstruction has been performed by means of the additive algebraic reconstruction technique (ART) extended to three dimensions. Several simulation experiments have been undertaken to explore the effect of pseudo-projections, analytic projections, iteration-dependent damping and interpolation schemes on the performance of the reconstruction. The results indicate that the ART-reconstruction from twin-cone beam projections can be achieved without blurring artefacts. The measured reconstruction error is of the same order of magnitude as the error of a ART-multislice reconstruction from coaxial projections.

The Decrease in Effective Photocurrents Due to Saddle Points in Electrostatic Potentials near Differentially Charged Spacecraft

Abstract: The reported investigation had the objective to illustrate the presence of important multidimensional effects in spacecraft charging. Two-dimensional codes have been under development by Parker (1976). A description is presented of a calculation which was performed using the three-dimensional NASA Charging Analyzer Program (NASCAP). NASCAP was run to calculate the electrostatic potentials on the surface of, and in the space surrounding, a sunlit Teflon-coated sphere. Currents to the sunlit surfaces were determined on the basis of an approximate photosheath model for strong differential charging.

Enhanced Flatband Voltage Recovery in Hardened Thin MOS Capacitors

Abstract: The short-term recovery of hardened pyrogenic SiO2 MOS capacitors exposed to pulsed electron-beam irradiation was studied as a function of oxide thickness between 200 and 1000 A. Two sets of samples were studied: one set grown to a single thickness and then etched back to various thicknesses, and the other set grown for different lengths of time to varying thicknesses. In both cases, the recovery time was observed to vary approximately as the fourth power of oxide thickness. This anomalous superlinear dependence of recovery time on the SiO2 thickness agrees with the predictions of the stochastic model of hole transport based on a continuous-time random walk. Combining the highly field activated character of hole transport in SiO2 with the thickness dependence reported here demonstrates that very significant gains in the short-term recovery speed can be made by reducing the oxide thickness and/or increasing the oxide field.

Temperature- and Field-Dependent Charge Relaxation in SiO2 Gate Insulators

Abstract: Experimental results are presented for the time dependence of the relaxation of flat-band voltage shift (?VFB) induced by ionizing radiation in metaloxide-semiconductor (MOS) capacitors incorporating radiation-hard wet (pyrogenic H20) thermally-grown SiO2 gate insulators. ?VFB was observed from 0.4 ms to 800 s following short-pulse sample irradiation at temperatures from 79 to 295 K and with applied fields from -6 to + 6 MV/cm. The results illustrate the strong temperature and field dependences of hole transport in the SiO2 which is responsible for the early annealing of the radiation-induced ?VFB. Examples are given of the manner in which these data may be applied to estimate the response of radiation-hard MOS devices at various times after irradiation, particularly in the potentially troublesome low temperature (80-150 K) regime.

A High-Resolution Proportional Chamber Positron Camera and Its Applications

Abstract: A positron camera consisting of two high density proportional chambers is described. It provides a spatial resolution of 2.4 mm FWHM, a maximum data rate of 3000 c.p.s. and a sensitivity of 25 c.p.s. per ?Ci. Results of its application to angular correlation studies of condensed matter and to phantom and in vivo medical imaging are presented. A Fourier deconvolution technique is described for obtaining three-dimensional medical images. It uses a generalized matrix inversion by singular value decomposition to modify the low frequency Fourier components.

Steady-State and Transient Radiation Effects in Precision Quartz Oscillators

Abstract: Radiation effects on 5 MHz 5th overtone AT cut Premium-Q quartz resonators are evaluated, comparing sweeping processes and resonator fabrication procedures. The resonators were irradiated at operating temperatures in oven controlled oscillator test beds with pulsed 10 MeV electrons from 200 rads up to 1 Mrad as well as with continuous 60Co gamma rays. Steady-state (permanent) and transient radiation-induced frequency changes were measured, the persistence of radiative preconditioning was investigated and the effect on drift rate was evaluated. From saturation characteristics of the steady-state frequency offset as function of total electron dose, the formation rate cross sections of several radiation-induced crystal defects have been evaluated. Formation of at least one defect is sensitive to differences in the sweeping process. In some resonators, frequency recovery after irradiation is dominated by an exponential decay mechanism with a time constant of 3.7 days. Under continuous gamma ray exposure with dose rates between 10 and 90 rad/hour, the frequency response is characterized by a steep initial rise followed by different patterns of saturation and recovery.

Toward the Energy Resolution Limit of Mercuric Iodide in Room Temperature Low Energy X-Ray Spectrometry

Abstract: Development of mercuric iodide nuclear detectors fabricated from vapor grown single crystals continues with emphasis in our work on their capabilities as room temperature operable low-energy X-ray spectrometers. The influence of trapping in the detector crystal, electronic noise and statistical fluctuations on energy resolution of the detectors are discussed. A room temperature energy resolution value of 490 eV (FWHM) has been measured for Fe-55 (with a corresponding 420 eV value of electronic noise) using a 2mm2 HgI2 detector. The K? and Ks lines of Fe-55 at 5.9 keV and 6.5 keV have been separated, to our knowledge, for the first time at room temperature. A new Fano-factor value of 0.46 has been established for mercuric iodide. In the room temperature HgI2 detector - FET preamplifier combination the latter is the largest contributor to the width of the spectral line in the low energy region. Our efforts are therefore directed toward reducing room temperature preamplifier noise. A room temperature preamplifier noise level of 360 eV (HgI2) has been attained. We are currently investigating use of low noise, low capacitance FET devices in a more optimized preamplifier configuration which should further reduce the noise value (H.E. Kern, IEEE Trans. Nucl. Sci. NS-17). Also under investigation is the effect of Peltier cooling of the input stage of the FET preamplifier.

A Picosecond Resolution Time Digitizer for Laser Ranging

Abstract: The Time Digitizer capable of covering a range of 0.34 sec in 9.76 psec increments is described. The time interval between a pair of start-stop pulses is digitized coarsely in 20 nsec periods by a very accurate 50 MHz reference clock. The residual fractions of a clock period at the start and the stop end of the measured interval are stretched in two interpolators and digitized in 9.76 psec increments. An equivalent digitizing frequency of 102.4 GHz is thus achieved. The digitizer is built in a CAMAC minicrate and communicates via a standard crate controller. It is intended for use in the laser ranging between ground stations and the Laser Geodetic Satellite (LAGEOS). It is shown that the distribution in any two adjacent 9.76 psec channels of a small number of identical test time intervals is essentially binomial. The performance of the digitizer and test results are given.

Semi-Autonomous Controller for Data Acquisition the Brilliant ADC

Abstract: A set of high speed 16-bit computers and ADC's has been designed and built for the data collection, compression, and correction system of the SLAC/LBL Mark II Magnetic Detector. The "Brilliant ADC" controls the analog multiplexing of a CAMAC crate of data acquisition modules, digitizes the analog data, and executes microprogrammed algorithms for data handling and correction.

Hardness Assurance Considerations for Long-Term Ionizing Radiation Effects on Bipolar Structures

Abstract: The purpose of this work was to assess the theoretical understanding of long-term ionization effects in semiconductor bipolar devices in support of developing hardness assurance techniques. The principal effort was directed at studying transistor gain degradation mechanisms by use of models relating semiconductor physical and electrical parameters to surface properties. Ionizing radiation effects on surface properties were used to identify critical physical parameters for use in hardness assurance procedures. Model implications and predictions were then compared with existing data to evaluate their accuracy and usefulness as a hardness assurance tool.

Suppression of Ring Artefacts in CT Fan-Beam Scanners

Abstract: A software algorithm is presented which is able to suppress ring-like artefacts which occur in CT fan-beam scanners when the measurements are slightly erroneous in their offset or gain.

Channel Electron Multipliers: Properties, Development and Applications

Abstract: The object of this paper is to acquaint high energy physicists with the properties and numerous applications of microchannel plates as well as with the resulting improvements of particle detectors. Properties of channel electron multipliers and of microchannel plates are described. We review and discuss the major applications of these devices, with emphasis on high performance photomultipliers and on electronic imaging at high speeds and low light levels.

Radiation Testing of PIN Photodiodes

Abstract: PIN photodiodes representative of commercially available device types were radiation tested in total dose, neutron fluence, and ionization pulse environments to examine the vulnerability mechanisms that occur. Also included in the testing was a special epitaxial device that was designed to have a small ionization-sensitive volume. It was found that surface shunt paths were produced in all the devices, with the effects in the p-on-n devices with guard rings being particularly bad. These shunt paths produced a decreased optical response and an increased dark current. In devices biased to less than full depletion, lifetime degradation was observed. This caused significant optical response degradation, especially at longer wavelengths, and gave a significant contribution to the increased dark current. The epitaxial device had the predicted small ionizationsensitive volume, and no unusual vulnerability mechanism occurred in the device. In fact, it was generally at least as radiation-toleranct as any of the other devices, and thus is a good choice for use in applications requiring radiation hardness.

Radiation response of fiber optic waveguides in the 0.4 to 1.7 μ region

Abstract: The response of fiber optic waveguides to ionizing radiation has been studied. Measurements of the growth and decay of the radiation-induced loss at 0.82μ have revealed that fibers with low OH are more susceptible to damage than those with high OH. The addition of P to Ge-doped silica core fibers has been found to suppress an intense transient absorption. Spectral measurements of the radiation-induced absorption between 0.4 - 1.7μ have shown an increase in the OH overtone and combination band intensity with irradiation so that the induced loss at 1.3μ is actually less in the low OH content silica core fibers than in the high OH content fibers. Real time spectral measurements of the damage following a pulsed irradiation have lead to an identification of the absorption bands and damage mechanisms responsible for the radiation-induced absorption.

The Characteristics of Computed Tomographic Reconstruction Noise and Their Effect on Detectability

Abstract: The EMI 5005 scanner produces images with noise characteristics similar to those in simulated CT reconstructions. A detectability phantom is described which will provide a means of investigation of the effect on human detection capability of the peculiar correlations present in the noise present in CT scanner images.

The Fiber Optic Dosimeter on the Navigational Technology Satellite 2

Abstract: A fiber-optic radiation dosimeter has been developed that utilizes the darkening induced in silicate glasses by ionizing radiation. A small, light weight, low-power consuming dosimeter package was deployed on the Navigational Technology Satellite 2. Observed real-time accumulated absorbed dose behind three shield thicknesses for the period July 1977 - June 1978 are compared with both the AE4 and the AEI7 particle fluence models for outer zone trapped electrons. The observations indicate little shielding improvement above three g/cm2 due to the presence of bremsstrahlung generated within the shielding and give evidence of a harder energy spectrum than predicted by these models. Occasional large dose rate fluctuations may correlate with sunspot activity during late 1977 and early 1978.

High Accuracy Measurements in Proportional Chambers and Clarification of the Avalanche Mechanisms Around Wires

Abstract: A two-dimensional radiograph from the avalanches obtained on a single wire illustrates this property. Letters of 1.5 mm height, made by cutting through a 50 pm thick copper sheet, are illuminated by a parallel beam of 1.4 keV X-rays, the length of the sheet being parallel and centred on one anode wire, in a chamber with 2 mm wire spacing. A two-dimensional image appears, whereas the conventional methods so far used would give a single straight line (Fig. 4).

Neutron Degradation of Ion-Implanted and Uniformly Doped Enhancement Mode GaAs JFET's

Abstract: The degradation of the saturation transconductance of ion-implanted GaAs enhancement mode JFETs by exposure to fast neutron fluences is calculated using simple analytical expressions published by bulk GaAs. It is shown that the saturation transconductance degrades linearly with neutron fluence at about the same rate for devices with widely varying implant profile and junction location, but equal gate geometry and similar threshold voltage. Thus the degradation rate of the relative transconductance (ratio of degraded to initial transconductances) with neutron fluence is decreased by a large initial transconductance. At a fixed gate voltage, and for a given threshold voltage and gate geometry, a large initial transconductance requires high peak dopant concentration and small mean deviation of the implant profile, and a gate junction located near the position of the peak dopant concentration. Experimental data for the degradation of the transconductance by neutron fluence for two ion-implanted transistors have been matched by theory assuming the presence of an undegraded parasitic source resistance. The degradation rate of an uniformly doped epitaxial transistor is found to be 79% of that of ionimplanted devices with equal gate geometry and similar threshold voltage.

High-Energy Proton Radiation Damage of High-Purity Germanium Detectors

Abstract: Motivated by their applicability to gamma-ray spectroscopy experiments in space, quantitative studies of radiation damage effects in high-purity germanium detectors due to high-energy charged particles have been initiated with the irradiation by 6 GeV/c protons of two 1.0 cm thick planar detectors maintained at 88°K. The threshold for resolution degradation and the annealing characteristics differ markedly from those previously observed for detectors irradiated by fast neutrons. Under proton bombardment, degradation in the energy resolution was found to begin below 7 × 107 protons/cm2, and increased proportionately in both detectors until the experiment was terminated at a total flux of 5.7 × 108 protons/cm2, equivalent to about a six year exposure to cosmic-ray protons in space. At the end of the irradiation, the FWHM resolution measured at 1332 keV stood at 8.5 and 13.6 keV, with both detectors of only marginal utility as a spectrometer due to the severe tailing caused by charge trapping. The two detectors displayed a significant difference in proton damage sensitivity, which is consistent with fast neutron damage effects. To ensure that detector variability did not influence the comparision of proton-and neutron-induced damage effects, one of the detectors had been used previously in a neutron damage experiment. The threshold for high-energy proton damage was found to be markedly lower, roughly 5 × 107 protons/cm2, compared to 3 × 109 neutrons/cm2 for fast neutrons. Annealing these detectors after proton damage was found to be much easier than after neutron damage.

Response of MNOS Capacitors to Ionizing Radiation at 80°k

Abstract: MNOS capacitors with oxide thicknesses 85A-600A and silicon nitride thicknesses 200-2000A have been irradiated with 2 MeV electrons at 80°K. Measured flatband shifts are found to depend on both polarity and magnitude of the applied field, oxide thickness, nitride thickness, and variations in device processing. For negative gate bias and effective applied fields 1-2?106 V/Cm, ?VFB is independent of device processing and magnitude of the applied field. For these bias conditions, it is shown that flatband shifts in all MNOS samples may be explained by considering only generation and trapping of holes in the oxide. The holes travel a mean free path of 125± 25A in the oxide before being trapped. For positive gate bias, electrons generated in the oxide are trapped at the oxide-nitride interface and/or in the bulk of the nitride, compensating the effect of the positively charged trapped holes in the oxide, and producing a relatively smaller ?VFB for positive bias. The electron trapping process is considerably processing dependent. For high effective applied fields exceeding ± 2×106 V/cm, a strongly field-dependent mechanism of charge generation in the gate insulator is observed.

Thin Window Si(Li) Detectors for the ISEE-C Telescope

Abstract: Special detectors have been developed for the NASA ISEE-C Cosmic Ray Telescope. These are Lidrifted silicon detectors 5 mm thick and 1500 mm2 area with the criteria that thickness variations on the whole area be less than ±10 ?m and that the Li-diffused contact dead-layer not exceed 15 ?m. Techniques used to fabricate and test these detectors are presented.

Fast Track-Finding Trigger Processor for the SLAC/LBL Mark II Detector

Abstract: The SLAC/LBL Mark II Magnetic Detector consists of various particle detectors arranged in cylindrical symmetry located in and around an axial magnetic field. A versatile, programmable secondary trigger processor has been designed and built to find curved tracks in the detector. The system operates at a 10 MHz clock rate with a total processing time of 34?sec, and is used to "trigger" the data processing computer, thereby rejecting background and greatly improving the data acquisition aspects of the detector-computer combination.