Simon Clucas

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.

Solar particle enhancements of single-event effect rates at aircraft altitudes

Data from space-borne detectors and ground-level neutron monitors, together with radiation transport codes, are applied to calculate the single-event effects environments in the atmosphere due to large solar particle events. Calculations are validated against data from Concorde and show the importance of allowing for the increased penetration of particles to lower latitudes during geomagnetic storms. For the solar particle events studied, high-latitude subsonic routes are shown to experience higher neutron fluxes than Concorde routes. Upset rates for typical commercial memories are calculated for a number of situations, and implications for aircrew dose are discussed.

Assessment of neutron- and proton-induced nuclear interaction and ionization models in Geant4 for Simulating single event effects

This paper examines the performance of the Geant4 radiation transport toolkit for the simulation of energy deposition from proton- and neutron-nuclear interactions in silicon microelectronics. The results show that for large (/spl sim/300 /spl mu/m) to small (/spl sim/0.5 /spl mu/m) feature-size devices, the nucleon-nuclear and electromagnetic interaction models within the toolkit provide energy deposition spectra and single event upset rate predictions that are in good agreement with experimental data. The new Binary Cascade and Classical Cascade models, together with the nuclear pre-equilibrium model in Geant4, do not significantly differ in the results they produce. For small feature-size devices, it is shown that it is necessary to consider the effects of ionization by particles produced by nuclear interactions several micrometers above the sensitive volume.

Observation of solar particle events from CREDO and MPTB during the current solar maximum

Data obtained from the Microelectronics and Photonics Test Bed (MPTB) since July 2000 show a large number of solar particle events. Measurements from the Cosmic Radiation Environment Dosimetry (CREDO) Experiment show that the relative importance of proton and heavy ion fluxes varies greatly from event to event and even with time during an event. Single-event upsets in the analog-to-digital converter experiment are compared with calculations based on the measured fluxes; this demonstrates the importance of both contributions. Three major events show that the CREME96 worst-day model can be equaled in ions and exceeded in protons.

Measurements of the atmospheric radiation environment from CREAM and comparisons with models for quiet time and solar particle events

Flight data on neutron fluxes and dose rates obtained over a wide range of latitudes and altitudes are compared with several models of atmospheric radiation for both quiet-time and solar particle events. For quiet-time, general agreement between the models and measurements is within about 25%. Geomagnetic disturbances can increase atmospheric cosmic ray fluxes by up to 10% due to suppression of cut-off rigidities. Solar particle events are more difficult to model but for this solar maximum reasonable agreement has been obtained with a measurement during the event of 15 April 2001. On 20 January 2005 the second largest ground level event in recorded history could have raised atmospheric fluxes at flight altitudes by several hundredfold at high southern latitudes.

Observation of the solar particle events of October and November 2003 from CREDO and MPTB

Proton fluxes, linear energy transfer spectra, and single-event effects (SEEs) rates for the five solar particle events occurring during October 26 to November 6, 2003, are compared with previous events of the current solar maximum and with environment models. It is shown that this period includes the worst week for the current solar maximum and that this exceeds the October 1989 worst week in protons but not in ions. The SEE rates illustrate the importance of allowing for both proton and heavy ion contributions, the relative importance of which varies from event to event and with time during individual events.

Neutron Energy-Deposition Spectra Measurements, and Comparisons with Geant4 Predictions

Comparisons are presented between measured and predicted energy deposition spectra from neutron-nuclear events within a silicon semiconductor detector. Experiment data are from irradiations performed at the TRIUMF and TSL neutron beam facilities. Predictions are based on the Geant4 radiation transport toolkit, which is applied both to quantify the effects of interactions in the detector and scattering of the neutron source by other experiments upstream of the detector. The predictions highlight the importance of quantifying scattering effects in large experiment setups, the lower-energy neutrons in the continuum of the quasi-monoenergetic spectrum suffering greater scattering. The results are used to correct neutron-induced single event upset cross-section data