Peter Truscott

New Geant4 based simulation tools for space radiation shielding and effects analysis

We present here a set of tools for space applications based on the Geant4 simulation toolkit, developed for radiation shielding analysis as part of the European Space Agency (ESA) activities in the Geant4 collaboration. The Sector Shielding Analysis Tool (SSAT) and the Materials and Geometry Association (MGA) utility will first be described. An overview of the main features of the MUlti-LAyered Shielding SImulation Software tool (MULASSIS) will follow. The tool is specifically addressed to shielding optimization and effects analysis. A Java interface allows the use of MULASSIS by the space community over the World Wide Web, integrated in the widely used SPENVIS package. The analysis of the particle transport output provides automatically radiation fluence, ionising and NIEL dose and effects analysis. ESA is currently funding the porting of this tools to a low-cost parallel processor facility using the GRID technology under the ESA SpaceGRID initiative. Other Geant4 present and future projects will be presented related to the study of space environment effects on spacecrafts.

Neutron-Induced Single Event Effects Testing Across a Wide Range of Energies and Facilities and Implications for Standards

Neutron test data on single event effects for a wide range of SRAMs, facilities (monoenergetic and continuum) and energies (thermal to 800 MeV) are compared. Many modern devices are found to be sensitive to thermal neutrons and rates from this source can dominate in many situations. A significant number of devices suffer latchup and the cross-sections increase with operating voltage and beam energy implying that most test facilities will underestimate the problem for the natural atmospheric environment. Upset sensitivity at 3-5 MeV varies from 5 to 600 less than at high energies and will be of most significance for sources of fission neutrons. These results are related to current and developing standards

Advances in Measuring and Modeling the Atmospheric Radiation Environment

New radiation monitors based on solid-state detectors have been developed to perform wide-ranging measurements of the atmospheric environment and provide warnings of sudden increases during solar particle events. Results have been obtained during the current deep solar minimum across the full range of latitudes and from sea level to 13 km altitude. Results for ambient dose equivalent agree very closely with Tissue Equivalent Proportional Counters carried on the same flights. Values of 10 ¿Sv/hr are being reached at 12 km altitude and high latitude. Comparisons are made with the QinetiQ Atmospheric Radiation Model and the need to include cosmic-ray heavy ions is demonstrated.

Solar Particle Events in the QinetiQ Atmospheric Radiation Model

Models have been embodied within the QinetiQ atmospheric radiation model (QARM) to compute enhancements during major solar particle events and the influence of any concurrent geomagnetic storms that lower the cut-off rigidity. Predictions are compared with available in-flight measurements and extended to give worst case estimates for a number of aircraft routes. It is found that during solar particle events the gradients of particle fluxes with respect to altitude and geomagnetic latitude are so steep that doses and fluences are extremely sensitive to both geomagnetic conditions and the exact route.

Low-angle scattering of protons on the XMM-Newton optics and effects on the on-board CCD detectors

Monte Carlo simulations are presented where protons are shown to propagate through X-ray telescope optics by grazing incidence scattering processes. If these protons reach charge coupled device (CCD) detectors in sufficient numbers, they can induce a radiation background, and at certain energies, the nonionising dose can cause degradation of the charge transfer efficiency (CTE). In the course of this work, the telescope optics of the Chandra and XMM-Newton observatories were simulated.

Charge Collection in Power MOSFETs for SEB Characterisation—Evidence of Energy Effects

Charge collection is used as a non-destructive technique to analyze the statistical response of vertical power MOSFETs and their single-event burnout (SEB) rate as a function of the incident ion energy. Two effects are observed at either low or high energy. At low energy, the collected charge significantly decreases because of the limited ion range and energy straggling in the thick epitaxial layer. Because of this limited range effect, using low energy ions for SEB testing can significantly underestimate the SEB rate. At high energy, the presence of thick source bond wires, which partially cover the die area, as typically encountered in power MOSFETs, induce a large shadowing effect. When crossing the bond wires, high energy ions loose energy and can have a higher LET (but still a significant range) when they reach the active die. As a result, they can deposit more charge in the thick sensitive epitaxial layers of the transistors than the primary beam. A significant probability of high collected charge events is then observed at high energy. Contrary to the low energy (range) effect, the shadowing at high energy contributes to overestimating the SEB rate. General rules for the SEB radiation hardness assurance, related to the ion energy versus the power MOSFET voltage rating, are provided to avoid both range and shadowing effects.

Radiation environment measurements from CREAM and CREDO during the approach to solar maximum

Results from the Cosmic Radiation Environment Monitors (CREAM and CREDO) have been reported from a range of platforms during the declining phase of solar cycle 22 and a number of implications drawn for radiation environment and shielding models. Since these reports, the CREAM monitor has flown on a number of Shuttle visits to MIR during 1997-1998 with an extended deployment on MIR during January to May 1998. In all cases an active monitor measured charge-deposition spectra at various locations, while passive packages comprising neutron activation foils, neutron bubble detectors and thermoluminescent dosimeters obtained integrated data at these and other sites. Movement of the South Atlantic Anomaly is observed and cannot be fitted by simply updating the geomagnetic field model. The data from MIR are compared with those from previous Shuttle missions and show comparable secondary neutron fluencies and dose rates. Meanwhile a CREDO-3 particle telescope has been included in the Microelectronics and Photonics Test Bed in highly eccentric, high inclination orbit and has been returning data since November 1997. This experiment measures proton fluxes greater than 38 MeV and linear energy transfer spectra of cosmic rays and solar particle events in the range 100-20000 MeV/(g cm-2). The data have been extended to July 2000 and are used both to correlate with device behavior and to compare with models of trapped radiation, cosmic rays and solar particles. A number of solar particle events have been observed as cycle 23 builds up. Following a relatively quiet year in 1999, the recent event of July 14, 2000 is observed to compete with the October 1989 events in terms of proton fluency but has a somewhat lower heavy ion fraction.

The low Earth orbit radiation environment and its evolution from measurements using the CREAM and CREDO experiments

The new regime of trapped protons centered around L=2.5 and observed by CRRES (Combined Release and Radiation Effects Satellite) between March and October 1991 has been observed by CREAM (Cosmic Radiation Environment and Activation Monitor) on Shuttle missions at an altitude of 569 km in September 1991 (STS-48) and at altitudes of 385 km and 326 km in December 1992 on STS-53. CREDO (Cosmic Radiation Environment and Dosimetry instrument) on UOSAT-3 at 800 km observed the decay of the belt from March 1991 to March 1992. At high latitudes cosmic-ray fluxes have increases by a factor of two between June 1991 and March 1993. The effect of Shuttle shielding on cosmic rays is to increase the fluxes of low LET (linear energy transfer) secondaries and neutrons. Trapped protons and their dose contribution are attenuated, but again significant fluxes of secondary neutrons are produced. >

Radiation environment measurements on Shuttle missions using the CREAM experiment

The Cosmic Radiation Environment and Activation Monitor (CREAM) was successfully deployed in the middeck area on Shuttle missions STS-48 and STS-44 during September and November 1991 with the aim of monitoring those aspects of the primary and secondary radiation environment responsible for single event upsets in microelectronics and background noise in sensors. Results are compared with the outputs of standard radiation environment models. For the accurate location of trapped protons the choice of geomagnetic field model is shown to be critical, while results at high latitudes show the low-altitude manifestation of the new trapped proton belt observed to follow the March 1991 solar flare event. From deployment at a number of locations there is clear evidence for a significant build-up with shielding of secondary charged particles and neutrons. >

Particle transport simulation for spaceborne, NaI gamma-ray spectrometers

Radioactivity induced in detectors by protons and secondary neutrons limits the sensitivity of spaceborne gamma-ray spectrometers. Three-dimensional Monte Carlo transport codes were used to simulate particle transport of cosmic rays and innerbelt protons in various representations of the Gamma-Ray Observatory Spacecraft and the Oriented Scintillation Spectrometer Experiment. Results are used to quantify accurately the contributions to the radioactive background, assess shielding options, and examine the effects of detector and spacecraft orientation in anisotropic trapped proton fluxes. >