Single-event Rate Research Articles

Selective Cyclooxygenase-2 Inhibition and Cardiovascular Effects

The differential effects of selective cyclooxygenase-2 (COX-2) inhibitors compared with nonspecific nonsteroidal anti-inflammatory agents (NSAIDs) on platelet aggregation and prostacyclin/thromboxane balance have led to concerns that COX-2 inhibitors may increase the risk for cardiovascular thrombotic events. Empirical studies have generally been limited to analyses of secondary end points with low event rates in clinical trials and single event rates in observational studies, all of which have come to conflicting conclusions. This observational cohort study examines the cardiovascular risk of COX-2 inhibitors compared with nonspecific NSAIDs in Maryland Medicaid enrollees, a high-risk population. Medical and prescription claims were analyzed for noninstitutionalized Medicaid enrollees who received at least a 60-day supply of a COX-2 inhibitor or other prescription NSAID between June 2000 and June 2002 and who did not use these drugs for at least 6 months prior. Naproxen users were excluded. We developed a logistic model of propensity for treatment with COX-2 inhibitors and stratified patients by quintiles of their propensity score. The model adjusted for demographics, indications for COX-2 inhibitors, and cardiovascular risk factors. The study population comprised 1005 patients using COX-2 inhibitors and 5245 patients using a nonnaproxen NSAID. Of the 6250 patients, 70% were female, 50% were African American, and 30% were older than 50 years. Overall, 12% of the patients had at least 1 cardiovascular thrombotic event after treatment within the follow-up period. The propensity-adjusted odds ratio showed no significant effect of COX-2 inhibitor use on this percentage of patients (odds ratio, 1.09; 95% confidence interval, 0.90-1.33). We did not find that COX-2 inhibitors increased cardiovascular risk over nonnaproxen NSAIDs in a high-risk Medicaid population.

Design and performance of a data acquisition system for VUB-PET

A flexible and high-speed VME-based data acquisition system (DAQ) has been designed to process data from the University of Brussels (VUB) small-animal positron emission tomography (PET) system. The core tasks of the system are performed by two custom VME modules: a trigger module and a fast readout module (FRO). Following the detection of a valid coincidence event by the trigger unit, the raw data from all analog-to-digital converter channels involved are read out simultaneously via a FRO. An onboard digital signal processor (DSP) processor checks and calibrates these data. To increase the transfer throughput of the list mode data to a SUN Ultra Sparc 1 workstation, the DSP processor compresses the data format to a 6-byte data packet per event. The maximum average sustained acquisition rate of the system is 200000 counts per second (cps), while peak rates up to 2 Mcps can be handled. The DAQ also provides online measurement of the random event rate, the single event rate and event losses due to electronic dead time.

Application of a single-event kinetic model in the simulation of an industrial riser reactor for the catalytic cracking of vacuum gas oil

A kinetic model is developed for the catalytic cracking of vacuum gas oil in terms of the elementary steps of carbocation chemistry. The composition of the hydrocarbon mixture is detailed up to the C-number. Next to the individual n-paraffins, the i-paraffins, n- and i-olefins, mono- di- tri- and tetra-naphthenes, the corresponding aromatics and the naphtheno-aromatics are lumped per C-number so that a link with modern analytical techniques is possible. Within each lump all likely chemical species are considered and accounted for in the network generation. The latter is performed by means of Boolean matrices and auxiliary vectors characterizing the molecules and carbocations. The kinetic modeling leads to the so-called single-event rate parameters which are invariants of the type of elementary steps they are related with. Some 50 single-event rate parameters were determined from an extensive experimental program on the catalytic cracking of key components with the relevant structures. The kinetic model was then inserted into a reactor model for the simulation of riser catalytic cracking. The results are represented in terms of the yields of 646 lumps or of the usual oil fractions, obtained from an interface post-processing the results generated at the elementary step level.

The effect of the angle of incidence on proton induced single events in devices-a critical assessment by modeling

The effect of the angle of incidence on the proton induced single events in devices with finite dimensions is studied using a modified Monte Carlo model. The physical reasons for the existence or the absence of an anisotropy of the single event occurrence (with respect to the proton angle of incidence) are highlighted for the first time. Several different cases are found: (1) A significant anisotropy is predicted for devices with small lateral dimensions, a smaller sensitive layer thickness, and a large critical energy. The origin of this angular dependence is shown to be the contribution of the highly anisotropic medium mass fragments which is dominant for these conditions. (2) Published experimental data indicate only a small anisotropy in practical devices, which are characterized by a small critical deposited energy needed for SEU. It is shown that for these devices, most of the charge is created by the heavy mass fragments, which are isotropic. (3) In surface barrier detectors (SBDs) which have large lateral dimensions the effect of the isotropic light mass fragments takes over and the anisotropy for proton events is small even for large critical energies and a small sensitive layer thickness. The conclusion of the present work is that the single event rate in space can be evaluated by the normal incidence cross sections for all devices which have a significant sensitivity for proton induced events.

Approaches to proton single-event rate calculations

This article discusses the fundamentals of proton-induced single-event upsets and of the various methods that have been developed to calculate upset rates. Two types of approaches are used based on nuclear-reaction analysis. Several aspects can be analyzed using analytic methods, but a complete description is not available. The paper presents an analytic description for the component due to elastic-scattering recoils. There have been a number of studies made using Monte Carlo methods. These can completely describe the reaction processes, including the effect of nuclear reactions occurring outside the device-sensitive volume. They have not included the elastic-scattering processes. The article describes the semiempirical approaches that are most widely used. The quality of previous upset predictions relative to space observations is discussed and leads to comments about the desired quality of future predictions. Brief sections treat the possible testing limitation due to total ionizing dose effects, the relationship of proton and heavy-ion upsets, upsets due to direct proton ionization, and relative proton and cosmic-ray upset rates.

Single event kinetics of catalytic cracking

A fundamental kinetic model of catalytic cracking on zeolite catalysts has been developed. The complete network of elementary steps of the carbenium ion reaction intermediates is generated by means of an algorithm based on Boolean relation matrices. Elementary steps on Lewis and Broensted sites are taken into consideration. Single-event rate parameters are estimated for the cracking of paraffins, accounting for the thermodynamic constraints. Because of their fundamental chemical nature, these rate parameters are independent of the feedstock. It is shown how the model can be adapted to the availability of a lumped analysis of the feedstock. Lumped Hougen-Watson rate equations, incorporating a maximum of information about the complex reaction networks, are derived. The lumped rate parameters are reconstructed from the fundamental kinetics by means of multiplication with lumping coefficients.

Model for CMOS/SOI single-event vulnerability

A lumped-parameter model derived from transistor characterization data has been used in SPICE analyses to study and predict the single-event-upset thresholds for SIMOX SOI (separation by implantation of oxygen, silicon-on-insulator) SRAMs (static random-access memories) with a variety of cell designs. The modeling of CMOS/SOI transistors with fully bottomed sources and drains includes direct representation of the parasitic lateral bipolar structure. Results indicate that, in the SOI devices investigated, single events simulate a localized bipolar response, even in devices with bodies electrically tied to active nodes. The bipolar response enhances the destabilizing effect of an ion event. The total current impulse contributing to upset can be significantly greater than that produced by direct ionization within the hit transistor, i.e., devices can be upset by ions that deposit less than the total charge required to initiate logic state reversal. In light of this, advanced CMOS/SOI-SOS logic with short channel lengths (and therefore significant parasitic bipolar gain) may exhibit critical LETs (linear energy transfers) lower than expected from simple scaling rules, and thinning of the active regions may not significantly reduce single-event rates in such CMOS/SOI digital circuits. >

Suggested Single Event Upset Figure of Merit

This paper examines a number of concepts that are connected, directly or indirectly, with the problem of assigning a single event upset figure of merit to a specific oevice. Single event rates depend both on device and circuitry, through the critical charge requisite for upset, and upon the device geometry and technology, which determine the target size and charge collection capability. Each of these factors must be taken into account when determining device susceptibility. Upset rates in space additionally depend on the environment. Device response in trapped proton belts and in the cosmic ray environment is sufficiently oifferent that a single susceptibility measure is inadequate. We conclude that devices should be characterizea by a proton susceptibility, and by an upset rate in a reference cosmic ray environment. We present a simple expression, based on laboratory measurement, that approximates the cosmic ray upset rate ano propose it as a figure of merit. Calculated and measured values for upset rates are sensitive to several factors. The field funneling effect is known to increase both the magnitude of collected charge and the effective sensitive circuit volume during single events relative to the static parameters. Thus, experimental sensitive area measurements obtained from single event data exceed values predicted from inspection of static circuit layout configurations. Also, more charge is collected from any specific event than is predicted by using depletion region extent to determine charge collection volumes. This effect must be included when critical upset charge values are determined from experimental upset thresholds.