Showing papers in "IEEE Transactions on Nuclear Science in 1985"

The Use of Sieves to Stabilize Images Produced with the EM Algorithm for Emission Tomography

Abstract: Images produced in emission tomography with the expectation-maximization (EM) algorithm have been observed to become more 'noisy' as the algorithm converges towards the maximum-likelihood estimate. We argue in this paper that there is an instability which is fundamental to maximum-likelihood estimation as it is usually applied and, therefore, is not a result of using the EM algorithm, which is but one numerical implementation for producing maximum-likelihood estimates. We show how Grenader's method of sieves can be used with the EM algorithm to remove the instability and thereby decrease the 'noise' artifact introduced into the images with little or no increase in computational complexity.

Inverse Monte Carlo: A Unified Reconstruction Algorithm for SPECT

Abstract: Inverse Monte Carlo (IMOC) is presented as a unified reconstruction algorithm for Emission Computed Tomography (ECT) providing simultaneous compensation for scatter, attenuation, and the variation of collimator resolution with depth. The technique of inverse Monte Carlo is used to find an inverse solution to the photon transport equation (an integral equation for photon flux from a specified source) for a parameterized source and specific boundary conditions. The system of linear equations so formed is solved to yield the source activity distribution for a set of acquired projections. For the studies presented here, the equations are solved using the EM (Maximum Likelihood) algorithm although other solution algorithms, such as Least Squares, could be employed. While the present results specifically consider the reconstruction of camera-based Single Photon Emission Computed Tomographic (SPECT) images, the technique is equally valid for Positron Emission Tomography (PET) if a Monte Carlo model of such a system is used. As a preliminary evaluation, experimentally acquired SPECT phantom studies for imaging Tc-99m (140 keV) are presented which demonstrate the quantitative compensation for scatter and attenuation for a two dimensional (single slice) reconstruction. The algorithm may be expanded in a straight forward manner to full three dimensional reconstruction including compensation for out of plane scatter.

Correlation of Radiation Effects in Transistors and Integrated Circuits

Abstract: The effects of ionizing radiation on discrete MOS n- and p-channel transistors are correlated with performance degradation of CMOS integrated circuits. The individual components of radiation induced charge, oxide-trapped charge and interface-state charge, are separated using a subthreshold current technique. Processing splits and post-irradiation biased anneals are used to vary the ratio of oxide-trapped charge to interface-state charge. It is shown that the effective channel mobility depends to first order on the interface-state charge density. Static power supply current is correlated with the n-channel leakage at zero gate voltage while output drive currents are a function of both threshold voltage and channel mobility. Changes in propagation delay of signals through integrated circuits can be understood when both mobility and threshold voltage are considered as a function of the bias dependent charge buildup. A new transistor switching time figure of merit, t/C, which measures the drain to source drive over a full logic level voltage swing at the drain node, is introduced. This index is then shown to correlate with propagation delay in an IC. Finally, performance changes in an IC are modeled using only the measured buildup of oxide-trapped and interface-state charges from transistors as a function of radiation.

Hole Removal in Thin-Gate MOSFETs by Tunneling

Abstract: Fast time-resolved measurements of the response of thin-oxide MOSFETs show that radiation-induced holes are removed from the gate oxide by a tunneling process. A tunneling rate of 0.35 nm/decade from each interface is found for SiO2 at 77 K. Fast time-resolved measurements performed at room temperature are in qualitative agreement with low-temperature annealing data. Uncertainties in the room-temperature data did not allow extraction of firm and reliable values for the tunneling parameters.

Scatter Compensation Techniques for SPECT

Abstract: Compton scattered photons included within the photopeak pulse-height window decrease the image contrasts of lesions. Furthermore, in the absence of an effective compensation procedure, the accuracy of the SPECT measurement is reduced. In the following sections the effect of scatter on SPECT is reviewed, and the influence of scattered photons on the effective linear attenuation coefficient used with multiplicative attenuation compensation methods is described. Criteria for evaluating scatter compensation procedures are proposed, and approaches to reducing the effect of scatter on SPECT imaging are described.

On Collinear Wake Field Acceleration

Abstract: In the Voss-Weiland scheme of wake field acceleration a high current, ring-shaped driving bunch is used to accelerate a low current beam following along on axis. In such a structure, the transformer ratio, i.e. the ratio of the maximum voltage that can be gained by the on-axis beam and the voltage lost by the driving beam, can be large. In contrast, it has been observed that for an arrangement in which driving and driven bunches follow the same path, and where the current distribution of both bunches is gaussian, the transformer ratio is not normally greater than two. This paper explores some of the possibilities and limitations of a collinear acceleration scheme. In addition to its application to wake field acceleration in structures, this study is also of interest for the understanding of the plasma wake field accelerator.

Relation between Field Energy and RMS Emittance in Intense Particle Beams

Abstract: An equation is presented for continuous beams with azimuthal symmetry and continuous linear focusing, which expresses a relationship between the rate of change for squared rms emittance and the rate of change for a quantity we call the nonlinear field energy. The nonlinear field energy depends on the shape of the charge distribution and corresponds to the residual field energy possessed by beams with nonuniform charge distributions. The equation can be integrated for the case of an rms matched beam to yield a formula for space-charge-induced emittance growth that we have tested numerically for a variety of initial distributions. The results provide a framework for discussing the scaling of rms emittance growth and an explanation for the well-established lower limit on output emittance.

Maximum-Likelihood Reconstruction for Single-Photon Emission Computed-Tomography

Abstract: A mathematical model is formulated for a gamma camera used to observe single-photon emissions from multiple view angles. The model accounts for the statistics of radioactive decays, nonuniform attenuation, and a depth-dependent point-spread function. The maximum-likelihood method of statistics is used with the model to derive an algorithm for estimating the distribution of radioactivity.

Techniques of Microprocessor Testing and SEU-Rate Prediction

Abstract: Several different approaches have been used in the past to assess the vulnerability of microprocessors to SEU. In this paper we discuss the advantages and disadvantages of each of these test methods, and address the question of how the microprocessor test results can be used to estimate upset rate in space. Finally, as an application of the above techniques, we present the test results and predicted upset rates in synchronous orbit for a selected group of microprocessors.

Optimizing and Controlling the Radiation Hardness of a Si-Gate CMOS Process

Abstract: A quick-turnaround technique is presented for optimizing the radiation hardness of parts produced by a Si-gate CMOS process. The technique involves (1) fabricating MOS capacitors on control wafers that can be "pulled" at different stages during the fabrication process, and (2) defining "hardness" in terms of the amount of radiation-induced charge. We demonstrate the use of this technique in monitoring radiation hardness before and after key process steps, as well as in determining the cumulative effects of many process steps on hardness. It is shown that the optimum temperature for gate oxidation in dry O2 is 1000°C for both Al- and Si-gate technologies. In addition, In-Source evaporated Al is a preferred process step over sputtered Al deposition for an Al-gate technology. By choosing process steps that optimize hardness, the hardness of parts fabricated using a Si-gate CMOS process can be increased by over an order of magnitude. Finally, we report a different electric field dependence for the buildup of interface states in Alversus Si-gate technologies.

Hole Transport and Trapping in Field Oxides

Abstract: The electric field, temperature, and oxide thickness dependencies of the transport of radiation-generated holes through thick (> 100 nm) SiO2 layers was measured using fast capacitance-voltage techniques. The transport is radically slowed with respect to that in thinner oxides under similar conditions and is well described by the continuous time random walk model. Significant hole trapping in the oxide bulk is also observed.

Timing Electronics and Fast Timing Methods with Scintillation Detectors

Abstract: Time spectroscopy involves the measurement of the time relationship between two events. This paper reviews time pick-off techniques, practival time-pickoff circuits, and timing with scintillation detectors. A detailed comparison is made between leading-edge timing and constant-fraction timing. Typical timing resolution results are given for 60Co.

Accounting for Dose-Enhancement Effects with CMOS Transistors

Abstract: Dose-enhancement effects are monitored with standard CMOS transistors by measuring thresholdvoltage shifts due to oxide-trapped charge and interface states. These results, in conjunction with studies of the effects of trapped-hole annealing and electron-hole recombination, are used to correlate the responses of transistors irradiated with Co-60 gamma rays or 10 keV X rays.

Transient Radiation Effects in SOI Memories

Abstract: This paper presents the first measurements of transient radiation effects on SOI discrete devices and an LSI memory. A commercially processed LSI SOI memory, a 4K × 1 SRAM on SIMOX, was tested for SEU, and transient ionizing radiation effects as a function of bias conditions and dose rate. The SEU error rate was found to be between 1.5 and 2.5 × 10-8 errors/bit-day for the 10% worst-case orbit model. The output voltage logic upset level was greater than 1.6 × 1010 rad(Si)/sec for Vcc supply voltage variations of -10% and +20% with Vsub at -10 V. For the discrete devices and memory, the measured transient photocurrents were larger than the calculated volumetric photocurrent generated in the active device region. This increased transient response is postulated to be due to the gain of the parasitic phototransistor of the dielectrically isolated MOS device.

Ion Track Shunt Effects in Multi-Junction Structures

Abstract: Charge collection processes are discussed for heavy ion hits across multiple p-n junctions in bipolar transistor or CMOS structures. The concept of a resistive-like ion track shunt bridging two like conductivity regions is introduced and a first-order-model developed for the charge transported along the ion track shunt. This model is shown to be consistent with charge collection measurements on multi-junction CMOS-like structures. It is found that the charge collection at a given p-n junction is influenced and can even be changed in sign by voltages present at a second p-n junction when the ion track penetrates both junctions. This has important consequences for the design of radiation hard integrated circuits and such ion track shunt effects become more important as device dimensions are scaled to smaller values.

Measurements of Stability Limits for a Space-Charge-Dominated Ion Beam in a Long a. G. Transport Channel

Abstract: The Single Beam Transport Experiment at LBL consists of 82 electrostatic quadrupole lenses arranged in a FODO lattice. Five further lenses provide a matched beam from a high-current high-brightness cesium source for injection into the FODO channel. We call the transport conditions stable if both the emittance and current remain unchanged between the beginning and end of the channel, and unstable if either the emittance grows or the current decreases because of collective effects. We have explored the range of single-particle betatron phase advance per period from ?0 = 45° to 150° to determine the stability limits for the space-charge depressed phase advance, ?. No lower limit for ? (down to 7°) has been found at ?0= 60°, whereas limits have clearly been identified and mapped in the region of ?0 above 90°.

The Effect of Operating Frequency in the Radiation Induced Buildup of Trapped Holes and Interface States in MOS Devices

Abstract: Both n-channel and p-channel MOS transistors, fabricated with a radiation hardened 3-4 um process, were irradiated at various operating frequencies and biases The n-channel devices showed a larger rebound effect when alternately biased 'off' and 'on' during irradiation than for the cases of being biased either 'off' or 'on' during the irradiation The p-channel devices showed a smaller threshold voltage shift when alternately biased 'off' and 'on' during irradiation compared to devices which were biased either 'off' or 'on' during the irradiation The contributions of trapped holes and interface states on the observed threshold voltage shifts were determined through subthreshold current measurements This research showed the source of this alternating bias effect on the radiation response of MOS devices to be reduced hole trapping and increased interface state buildup in n-channel MOS devices In p-channel devices, reduced hole trapping is the primary source of this effect

The Metal Vapor Vacuum ARC (MEVVA) High Current Ion Source

Abstract: An ion source is described in which a metal vapor vacuum arc is used to create the plasma from which the ions are extracted. Beams of a variety of ions have been produced, ranging from lithium up to uranium. At an extraction voltage of 25 kV we've measured an ion beam current of over 1 Ampere, with over 550 ema of the beam in an emittance of 0.07 ? cm. mradians (normalized). The ion charge state distribution varies with cathode material and with arc power; for uranium a typical distribution is peaked at U5+, with up to 40% of the beam current in this charge state.

High-Brightness Photoemitier Injector for Electron Accelerators

Abstract: A free-electron laser (FEL) oscillator, driven by an rf linac, requires a train of electron bunches delivered to an undulator. The electron-beam brightness requirement exceeds that available from a conventional buncher. The demonstrated high peak brightness of laser-illuminated photoemitters indicates that the conventional buncher system might be eliminated entirely without the usual large loss in beam brightness that occurs in bunchers. A photoemitter with a current density of about 200 A/cm2 is located on an end wall of an rf cavity to accelerate a 60-ps bunch of electrons to 1 MeV as rapidly as possible. Preliminary experimental work and simulation calculations are presented.

Current Induced Avalanche in Epitaxial Structures

Abstract: A correlation is made between observed photoionization induced avalanche breakdown in epitaxial structures and the analysis of high-current effects in these devices using Poisson's equation. The analysis shows that a photocurrent-stimulated conductivity modulation mechanism can lead to avalanche at the epitaxial-substrate junction at bias levels far below the usual breakdown voltages for the structures. Experimental data are presented for both VDMOS power-FET devices and bipolar npn epitaxial transistors which show junction avalanche at low bias levels.

Control of Cavities with High Beam Loading

Abstract: Beam loading of RF cavities is a subject of great concern for the design and operation of high current circular accelerators and storage rings. The steady state and transient perturbations of the accelerating voltage by the beam current lead to undesirable beam behaviour, like for instance the onset of instabilities or particle loss by lack of longitudinal acceptance. To some extent, it is possible to alleviate or even completely suppress these effects by a proper control of the RF power amplifier - cavity combination, using feedback or feedforward techniques. Several new developments in this field have taken place during the past years. For instance, the implementation of fast feedback technology, the use of cavity compensation schemes or digital filtering of signals in long delay feedback systems have resulted in considerably improved machine performance. Such techniiques will be reviewed in this paper and their performance and limitations will be presented.

Radiation Effects of Protons on Samarium-Cobalt Permanent Magnets

Abstract: At TRIUMF the use of samarium-cobalt permanent magnet quadrupoles as the first element in a secondary channel has been studied as a means of increasing the solid angle acceptance of the channel. The high remanent induction Br and high coercive force Hc of rareearth cobalt (REC) can be utilized to produce a high-gradient quadrupole field in an extremely compact magnet. Although many properties of REC material have been measured, little is known about the effect of charged particle radiation on the magnetic behaviour. As the TRIUMF application requires the magnets to operate in a high radiation environment it was considered essential to study this effect. This paper describes the results of exposing samples of samarium-cobalt and other permanent magnet materials to a beam of protons.

Digital Position Estimation for the Modular Scintillation Camera

Abstract: Digital implementation of position estimation for the modular scintillation camera is presented. The modular camera system, introduced at the 1983 IEEE Nuclear Science Symposium, comprises an array of small, mechanically and electronically independent, all-digital scintillation cameras. This paper addresses how to best determine the event position given the PMT signals. Two Bayesian estimators are investigated, namely maximum-likelihood (ML) and minimum-meansquare-error (MS) estimators. Both estimators performed similarly. The more important aspect of the estimation problem was found to be the shape of the detector responses as a function of position.

Defect Production in SiO2 by X-Ray and Co-60 Radiations

Abstract: Hole trapping, interface-state production and annealing were studied in NMOS transistors irradiated with x-rays and Co-60. Hole trapping showed a photon energy dependence that can be explained by existing models. However, interface-state generation showed an energy dependence that cannot be explained using previously available models. The results given here suggest that hole production and trapping in the oxide may involve different mechanisms than the generation of interface states. In addition, annealing of hole trapping observed in what is called "super recovery" or "rebound" was found to be reversible.

Characteristics of Directional Coupler Beam Position Monitors

Abstract: The purpose of this paper is to review some of the properties of directional coupler (stripline) beam position monitors. These devices are used extensively as non-intercepting beam position monitors (BPM's) around accelerators, and offer certain advantages over the more conventional electrostatic (split plate) type pickups. The general approach will be from the engineering rather than the physics point of view. Although starting from Maxwell's equations would be more precise, it is also more pedantic, and obscures some of the more important engineering features. Due to lack of space, detailed derivations are not included, but references are given where relevant. The discussion here will be limited to cylindrical geometry couplers, primarily because the expressions are usually simpler than rectangular geometry couplers. In many cases, rectangular geometry would be preferred over cylindrical, but the basic engineering features remain the same.

Charging Dynamics of Dielectrics Irradiated by Low Energy Electrons

Abstract: A recent study by Gross et al. showed the possibility of positive charging of dielectric materials by low energy electron irradiation. In this paper we model the charging process allowing us to predict the voltage builtup or decay and the corresponding charging currents under various initial potential conditions of the dielectric. The model incorporates the secondary electron emission (SEE) yield curve and the energy distribution of the secondaries. The energy distribution is essential to obtain the correct charging behavior for beam energies smaller than the second crossover energy of the SEE yield curve. The results agree, in general, with experimental data Further it is shown theoretically that knowledge of the charging current and the corresponding voltage behavior of an irradiated sample can be utilized to determine the secondary electron yield curve for dielectrics and conductors.

A Novel RF Cavity Tuning Feedback Scheme for Heavy Beam Loading

Abstract: The beam-cavity instability threshold as given by the Robinson1 criterion for the open loop case is often improved by phase, amplitude and tuning loops. The improvement obtained is, however, limited by cross coupling as the transfer functions vary with the relative beam loading. A more powerful scheme is fast additive feedback around the final amplifier stage. With this scheme beam loading several orders of magnitude above the Robinson threshold have been achieved in the CERN ISR and AA in the absence of fast tuning loops. A novel scheme to include a tuning loop insensitive to relative beam loading is presented, and first test results from the application of this scheme to the CERN PS booster second harmonic cavity are given.

The SIS Heavy Ion Synchrotron Project

Abstract: The status of the SIS 18 synchrotron project is described along with recent modifications and supplements: the modifications of the Unilac and the experimental storage ring.

Investigation of Large Format Microchannel Plate Z Configurations

Abstract: The performance of triplet (Z) stacks of microchannel plates (MCP's) has been studied as a part of the instrument development for the Extreme Ultraviolet Explorer [1] (EUVE) satellite mission. Relatively large MCP's with a 60 mm diameter and having an 80:1 channel length to diameter (L/D) ratio were used in several configurations. These have been used in the EUVE prototype imaging detector to provide > 512 × 512 pixels with low image distortion ( 2 × 107) and tight output pulse height distributions (< 30% FWHM) may be achieved. Simple distribution techniques have also allowed low intrinsic background event rates (< .15 events cm-2 sec-1) to be obtained. Variation of the quantum efficiency of MCP's over the wavelength range 160 A to 1216 A has been investigated for a range of angles of incidence. The effect of temperature variations on MCP operating characteristics has also been evaluated.

A Thick Anger Camera for Gamma-Ray Astronomy

Abstract: The NaI(Tl) Anger camera is a natural candidate for a position sensitive detector in imaging of astrophysical ?-ray sources. Here we present laboratory measurements of the response of a relatively thick (5.1 cm) NaI(Tl) Anger camera designed for coded aperture imaging in the 50 keV to 2 MeV energy range. We obtained a position resolution of 10.5 mm FWHM at 122 keV and 6.3 mm FWHM at 662 keV. The energy resolution was 7% FWHM at 662 keV. We discuss the ability of the detector to resolve the depth of the ?-ray interaction and the use of this depth resolution to reduce back-incident and internal background.