C.S. Dyer

MULASSIS: a Geant4-based multilayered shielding simulation tool

Radiation shielding analysis is a crucial process in the spacecraft and space instrument development cycle. Simple tools such as SHIELDOSE-2 are used traditionally. With advances in computer and network technology and in energetic particle transport simulation codes, detailed Monte Carlo (M-C) simulation is no longer an approach reserved for research scientists but is widely becoming an important engineering tool. In this paper, we report on the main features of the multilayered shielding simulation software tool (MULASSIS) developed as part of the European Space Agency (ESA) activities in the Geant4 collaboration. It is based on the Geant4 M-C simulation toolkit developed by a large international collaboration lead by CERN. The software will be used as a general tool within ESA for radiation fluence, dose, and effects analysis. It has been integrated into the ESA SPENVIS system, thus, making it accessible to the wider space community over the World Wide Web.

An atmospheric radiation model based on response matrices generated by detailed Monte Carlo Simulations of cosmic ray interactions

A comprehensive model of the energetic radiation in the atmosphere has been developed. It is based on the use of a dynamic cosmic ray radiation model, an efficient rigidity cutoff calculation tool, and the use of detailed atmospheric response matrices to monochromatic cosmic ray particle incidences. The response matrices were produced by detailed Monte Carlo (MC) simulations of cosmic ray interactions with the atmosphere. This model can predict the secondary particle (proton, neutron, electron, gamma, charged pion, or charged muon) spectrum at any given location and time in the atmosphere.

Improvements to and Validations of the QinetiQ Atmospheric Radiation Model (QARM)

The QinetiQ atmospheric radiation model (QARM) is a comprehensive model of the energetic radiation in the atmosphere. In this paper we report on the improvement and validation activities for this model. The improvements include the implementation of two additional cosmic ray models, new response matrix, dose rate and flight dose calculation facilities. Tests/validations of the model have been carried out at individual component level as well as at system level. We will also report on the web interface developed to allow easy access to the model.

Calculations and observations of solar particle enhancements to the radiation environment at aircraft altitudes

Solar particle events can give greatly enhanced radiation at aircraft altitudes, but are both difficult to predict and to calculate retrospectively. This enhanced radiation can give significant dose to aircrew and greatly increase the rate of single event effects in avionics. Validation of calculations is required but only very few events have been measured in flight. The CREAM detector on Concorde detected the event of 29 September 1989 and also four periods of enhancement during the events of 19-24 October 1989. Instantaneous rates were enhanced by up to a factor ten compared with quiet-time cosmic rays, while flight-averages were enhanced by up to a factor six. Calculations are described for increases in radiation at aircraft altitudes using solar particle spectra in conjunction with Monte Carlo radiation transport codes. In order to obtain solar particle spectra with sufficient accuracy over the required energy range it is necessary to combine space data with measurements from a wide range of geomagnetically dispersed, ground-level neutron monitors. Such spectra have been obtained for 29 September 1989 and 24 October 1989 and these are used to calculate enhancements that are compared with the data from CREAM on Concorde. The effect of cut-off rigidity suppression by geomagnetic activity is shown to be significant. For the largest event on record on 23 February 1956, there are no space data but there are data from a number of ground-level cosmic-ray detectors. Predictions for all events show very steep dependencies on both latitude and altitude. At high latitude and altitude (17 km) calculated increases with respect to cosmic rays are a factor 70 and 500 respectively for 29 September 1989 and 23 February 1956. The levels of radiation for high latitude, subsonic routes are calculated, using London to Los Angeles as an example, and can exceed 1 mSv, which is significantly higher than for Concorde routes from Europe to New York. The sensitivity of the calculations to spectral fitting, geomagnetic activity and other assumptions demonstrates the requirement for widespread carriage of radiation monitors on aircraft.

An experimental study of single-event effects induced in commercial SRAMs by neutrons and protons from thermal energies to 500 MeV

Irradiations have been performed, with neutrons and protons over a wide particle energy range, on six different commercial-grade 4-Mbit SRAM parts from the manufacturers Hitachi, Toshiba, Mitsubishi, and Samsung. At energies above 20 MeV, the single-event upset (SEU) cross sections are similar for protons and neutrons. While the proton cross sections fall off rapidly below this energy due to package shielding and Coulomb barrier effects, the neutron cross sections at 14.5 MeV are close to the high energy plateau values. Four of the devices show significant thermal neutron SEU cross sections and in two cases these exceed the high energy plateau values. The most modern of the parts tested is susceptible to latchup from both protons and neutrons but the response is shifted to higher energies compared with SEU.

Monte Carlo calculations of the influence on aircraft radiation environments of structures and solar particle events

Radiation transport codes are applied to calculate the influence of aircraft structure and loads on the internal environment from galactic cosmic rays. Significant shielding effects are predicted, while thermalization of the neutron flux can lead to enhanced single-event upset rates in devices containing boron. The enhanced environments due to large solar particle events are calculated and shown to be very important for both single-event effects in electronics and dose to crew and passengers.

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.

Observations of single-event upsets in non-hardened high-density SRAMs in Sun-synchronous orbit

Observations of single-event upset (SEU) activity in nonhardened static and dynamic RAMs of both low (16-kb) and high (256-kb, 1-Mb), density are presented for a family of small spacecraft in low-earth, near-polar, Sun-synchronous orbits. The observation of single-event multiple-bit upset (MBU) in these devices is discussed, and the implications of such events for error-protection coding schemes are examined. Contrary to expectations, the 1-Mb static RAMs (SRAMs) are more resilient to SEU than the 256-kb SRAMs, and one type of commercial 1-Mb SRAM shows a particularly low error rate. >

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

Comparison between SRAM SEE cross-sections from ion beam testing with those obtained using a new picosecond pulsed laser facility

A new pulsed laser facility has been developed to extend laser testing techniques to generate upset cross-section curves. The objective has been to establish an economical laser-based bulk screening capability for SEE susceptibility.