E.G. Stassinopoulos

Space, atmospheric, and terrestrial radiation environments

The progress on developing models of the radiation environment since the 1960s is reviewed with emphasis on models that can be applied to predicting the performance of microelectronics used in spacecraft and instruments. Space, atmospheric, and ground environments are included. It is shown that models must be adapted continually to account for increased understanding of the dynamics of the radiation environment and the changes in microelectronics technology. The IEEE Nuclear and Space Radiation Effects Conference is a vital forum to report model progress to the radiation effects research community.

Probability model for cumulative solar proton event fluences

A new model of cumulative solar proton event fluences is presented. It allows the expected total fluence to be calculated for a given confidence level and for time periods corresponding to space missions. The new model is in reasonable agreement with the JPL91 model over their common proton energy range of >1 to >60 MeV. The current model extends this energy range to >300 MeV. It also incorporates more recent data which tends to make predicted fluences slightly higher than JPL91. For the first time, an analytic solution is obtained for this problem of accumulated fluence over a mission. Several techniques are used, including Maximum Entropy, to show the solution is well represented as a lognormal probability distribution of the total fluence. The advantages are that it is relatively easy to work with and. To update as more solar proton event data become available.

Characterizing solar proton energy spectra for radiation effects applications

The Weibull distribution for smallest values is shown to be a useful description for solar proton event energy spectra. One advantage is its compact analytic expression, which allows easy conversion between differential and integral spectra. Another is its versatility, which is necessary for describing the highly variable spectra of concern. Furthermore, the Weibull distribution appears to be appropriate for use over broad energy ranges extending out to GeV. Examples are shown and comparisons to previously used distributions are made. An especially useful consequence of this approach for radiation effects applications is that it allows both predictive model spectra and observed spectra to be described by the same distribution. This allows spectra to be systematically ranked by severity of radiation damage caused in microelectronics. It further allows observed spectra to be related to predictive model parameters such as confidence levels. These points are demonstrated by evaluating the ionization dose deposited by various spectra in silicon behind aluminum shielding appropriate for spacecraft.

Probability model for worst case solar proton event fluences

A predictive model of worst case solar proton event fluences is presented. It allows the expected worst case event fluence to be calculated for a given confidence level and for periods of time corresponding to space missions. The proton energy range is from >1 to >300 MeV, so that the model is useful for a variety of radiation effects applications. For each proton energy threshold, the maximum entropy principle is used to select the initial distribution of solar proton event fluences. This turns out to be a truncated power law, i.e., a power law for smaller event fluences that smoothly approaches zero at a maximum fluence. The strong agreement of the distribution with satellite data for the last three solar cycles indicates this description captures the essential features of a solar proton event fluence distribution. Extreme value theory is then applied to the initial distribution of events to obtain the model of worst case fluences.

A simple algorithm for predicting proton SEU rates in space compared to the rates measured on the CRRES satellite

A new simulation code, the Clemson Omnidirectional Spallation Model for Interaction in Circuits (COSMIC), is described and its predictions agree with SEU data from four devices flown as part of the microelectronics package experiment on the CRRES satellite. The code uses CUPID for determining the energy depositions in the sensitive volumes. It allows proton exposures with arbitrary angles of incidence including random omnidirectional exposure; and the user specifies the thickness of shielding on six sides of the sensitive volume. COSMIC is used as part of an algorithm developed to predict the rate proton induced single event upsets occur in the space radiation environment given by AP-8. In testing the algorithm, the position coordinates are taken from the satellites ephemeris data, but calculations based on position coordinates from orbital codes were also in agreement with the measured values. >

Proton-induced transients in optocouplers: in-flight anomalies, ground irradiation test, mitigation and implications

We present data on recent optocoupler in-flight anomalies and the subsequent ground test irradiation performed. Discussions of the single event mechanisms involved, transient filtering analysis, and design implications are included. Proton-induced transients were observed on higher speed optocouplers with a unique dependence on the incidence particle angle. The results indicate that both direct ionization and nuclear reaction-related mechanisms are responsible for the single events observed.

SEDS MIL-STD-1773 fiber optic data bus: Proton irradiation test results and spaceflight SEU data

Proton test and space-flight single event effect data for NASAs first fiber optic data bus are presented. Bit error rate predictions based on a proton direct ionization model agree well with flight data for proton-belt and solar-flare effects. Specifically, the authors discuss the SEUs (single event upsets) seen in space during the first months of the SAMPEX (Solar Anomalous Magnetospheric Particle Explorer) mission, including a solar flare from October 30 through November 6, 1992, their impact, and comparison to predicted SEU rates. The fact that the interpretation of test results leads to a flight model which assumes direct ionization effects suggests that the occurrence of retransmissions could be significantly reduced with design changes to the receiver. A comparison of the measured and predicted rates for the present implementation with those expected for a III-IV based detector showed that improvement would follow from both a smaller physical cross-section, and greatly reduced particle pathlengths through a thinner direct band-gap detector. >

Model for solar proton risk assessment

A statistical model for cumulative solar proton event fluences during space missions is presented that covers both the solar minimum and solar maximum phases of the solar cycle. It is based on data from the Interplanetary Monitoring Platform and Geostationary Operational Environmental Satellites series of satellites, which are integrated together to allow the best features of each data set to be used to the best advantage. This allows the fluence-energy spectra to be extended out to energies of 327 MeV.

The Damage Equivalence of Electrons, Protons, and Gamma Rays in MOS Devices

In most cases of device testing or characterization in the laboratory, Cobalt-60 sources are commonly used, since they are an inexpensive and convenient source of ionizing radiation. However, some scientists and engineers had suspected for some time that the damage effects from gamma rays in MOS devices are not equivalent to those produced by electrons and protons. The issue of the validity of device test data obtained with Cobalt-60 sources arises when the devices are intended for use on satellites, because the effective space radiation environment consists predominantly of charged particles. This paper reports on an experiment which was designed to establish the relation among the damage effects obtained with Cobalt-60, electron, and proton radiation in order to derive relative damage factors. These factors would then greatly increase the validity of assessments of the vulnerability of MOS LSI devices in space.

Charge generation by heavy ions in power MOSFETs, burnout space predictions and dynamic SEB sensitivity

The transport, energy loss, and charge production of heavy ions in the sensitive regions of IRF 150 power MOSFETs are described. The dependence and variation of transport parameters with ion type and energy relative to the requirements for single event burnout in this part type are discussed. Test data taken with this power MOSFET are used together with analyses by means of a computer code of the ion energy loss and charge production in the device to establish criteria for burnout and parameters for space predictions. These parameters are then used in an application to predict burnout rates in a geostationary orbit for power converters operating in a dynamic mode. Comparisons of rates for different geometries in simulating SEU (single event upset) sensitive volumes are presented. >