Alex Hands

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.

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

Single Event Effects in Power MOSFETs and SRAMs Due to 3 MeV, 14 MeV and Fission Neutrons

Various SRAM and MOSFET devices were exposed to 3 MeV and 14 MeV neutrons at a fusion facility and to a fission neutron spectrum with a californium-252 source. Single event burnout (SEB) was observed in several of the MOSFETs in all three environments-the first time this phenomenon has been observed at neutron energies below 10 MeV. In addition to observing single event upsets (SEU) and single event latchup (SEL) in the SRAMs, two devices experienced significant multiple cell upset (MCU) effects which dominated the upset rate. The physical mechanisms underlying these phenomena and the consequences for various radiation environments are discussed.

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.

Advances in Atmospheric Radiation Measurements and Modeling Needed to Improve Air Safety

Air safety is tied to the phenomenon of ionizing radiation from space weather, primarily from galactic cosmic rays but also from solar energetic particles. A global framework for addressing radiation issues in this environment has been constructed, but more must be done at international and national levels. Health consequences from atmospheric radiation exposure are likely to exist. In addition, severe solar radiation events may cause economic consequences in the international aviation community due to exposure limits being reached by some crew members. Impacts from a radiation environment upon avionics fromhigh-energy particles and low-energy, thermalized neutrons are now recognized as an area of active interest. A broad community recognizes that there are a number of mitigation paths that can be taken relative to the human tissue and avionics exposure risks. These include developing active monitoring and measurement programs as well as improving scientific modeling capabilities that can eventually be turned into operations. A number of roadblocks to risk mitigation still exist, such as effective pilot training programs as well as monitoring, measuring, and regulatorymeasures. An active international effort toward observing theweather of atmospheric radiation must occur to make progress in mitigating radiation exposure risks. Stakeholders in this process include standard-making bodies, scientific organizations, regulatory organizations, air traffic management systems, aircraft owners and operators, pilots and crew, and even the public.

A Technique for Measuring Dose Equivalent and Neutron Fluxes in Radiation Environments Using Silicon Diodes

Radiation transport codes and multifacility empirical measurements are used to characterize a solid-state-based radiation monitor to provide measurements of dose equivalent in an aviation environment and neutron fluxes at accelerated testing facilities. A new millidosimetry technique for using millimeter-scale solid-state technology to assess dose equivalent in a mixed radiation field is described and validated in detail. Empirical data from aviation dose monitoring and test facility beam monitoring are presented.

SEU Rates in Atmospheric Environments: Variations Due to Cross-Section Fits and Environment Models

The wide range in predicted SEU rates in avionics devices is analyzed in relation to fitted device cross-section curves (including thermal neutron sensitivity) and the variation between atmospheric radiation models and current standards.

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.

Single Event Effects in Power MOSFETs Due to Atmospheric and Thermal Neutrons

Eight commercially available n-channel power MOSFETs were exposed to high energy spallation neutrons and thermal neutrons in separate experiments. Single event burnout (SEB) was observed in several of the devices in both environments. Measurements of SEB at derated drain-source voltages show very strong reductions in burnout cross-sections, but suggest that current recommendations for safe operation of devices may need updating for high voltage devices. In one device a different failure mode was observed, with subsequent investigations suggesting that single event gate rupture (SEGR) was responsible. This first observation of SEGR in accelerated neutron testing of power MOSFETs represents a new consideration for designers of high voltage control systems.