S.A. Wender

Predicting the number of fatal soft errors in Los Alamos national laboratory's ASC Q supercomputer

Early in the deployment of the Advanced Simulation and Computing (ASC) Q supercomputer, a higher-than-expected number of single-node failures was observed. The elevated rate of single-node failures was hypothesized to be caused primarily by fatal soft errors, i.e., board-level cache (B-cache) tag (BTAG) parity errors caused by cosmic-ray-induced neutrons that led to node crashes. A series of experiments was undertaken at the Los Alamos Neutron Science Center (LANSCE) to ascertain whether fatal soft errors were indeed the primary cause of the elevated rate of single-node failures. Observed failure data from Q are consistent with the results from some of these experiments. Mitigation strategies have been developed, and scientists successfully use Q for large computations in the presence of fatal soft errors and other single-node failures.

A fission ionization detector for neutron flux measurements at a spallation source

Abstract The construction of a neutron flux monitor that can measure absolute neutron intensities in the neutron energy range from below 1 MeV to over 500 MeV is described. The detector consists of an ionization chamber with several thin deposits of fissionable material. The ionization chamber is thin enough that it does not significantly affect the neutron beam and may be left in the neutron flight path during experimental measurements to continuously monitor the beam flux. The use of this monitor at the continuous-energy spallation neutron source at the WNR target area at LAMPF is described.

Single event phenomena in atmospheric neutron environments

Describes direct experimental measurements of neutron-induced single event effect (SEE) rates in commercial high-density static random access memories in a neutron environment characteristic of that at commercial airplane altitudes. The first experimental measurements testing current models for neutron-silicon burst generation rates are presented, as well as measurements of charge collection in silicon test structures as a function of neutron energy. These are the first laboratory SEE and charge collection measurements using a particle beam having a continuum energy spectrum and with a shape nearly identical to that observed during flight. Significant inaccuracies in the presently accepted models for predicting SEU rates in an atmospheric environment are noted, and an experimental basis for development of a more accurate model is provided. >

Neutron-induced single event burnout in high voltage electronics

Energetic neutrons with an atmospheric neutron spectrum, which were demonstrated to induce single event burnout in power MOSFETs, have been shown to induce burnout in high voltage (>3000 V) electronics when operated at voltages as low as 50% of rated voltage. The laboratory failure rates correlate well with field failure rates measured in Europe.

Single event upset and charge collection measurements using high energy protons and neutrons

RAMs, microcontrollers and surface barrier detectors were exposed to beams of high energy protons and neutrons to measure the induced number of upsets as well as energy deposition. The WNR facility at Los Alamos provided a neutron spectrum similar to that of the atmospheric neutrons. Its effect on devices was compared to that of protons with energies of 200, 400, 500 and 800 MeV. Measurements indicate that SEU cross sections for 400 MeV protons are similar to those induced by the atmospheric neutron spectrum. >

First observations of power MOSFET burnout with high energy neutrons

Single event burnout was seen in power MOSFETs exposed to high energy neutrons. Devices with rated voltage /spl ges/400 volts exhibited burnout at substantially less than the rated voltage. Tests with high energy protons gave similar results. Burnout was also seen in limited tests with lower energy protons and neutrons. Correlations with heavy-ion data are discussed. Accelerator proton data gave favorable comparisons with burnout rates measured on the APEX spacecraft. Implications for burnout at lower altitudes are also discussed.

The Effects of Neutron Energy and High-Z Materials on Single Event Upsets and Multiple Cell Upsets

Neutron-induced charge collection data and computer simulations presented here show that the presence of high-Z materials, like tungsten, can increase the single event upset (SEU) and multiple cell upset (MCU)cross sections of high critical charge (Qcrit) devices exposed to the terrestrial neutron environment because of interactions with high energy ( >; 100 MeV) neutrons. Time-of-flight data and computer simulations presented here demonstrate that 14 MeV neutrons do not produce highly ionizing secondary particles. Thus, 14 MeV neutrons can only simulate the SEU response of 65 nm SRAM devices in the terrestrial neutron environment for devices with a Qcrit <; 27fC, and can simulate the 2-bit MCU response to within a factor of two only for very low Qcrit devices, <; 1.2 fC.Additionally, it is shown that 14 MeV neutrons cannot adequately simulate the 3 or more bit MCU response for typical 65 nm SRAM devices.

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.

Measurements and analysis of neutron-reaction-induced charges in a silicon surface region

We directly measured neutron-reaction-induced charges in the silicon surface region using silicon-on-insulator (SOI) test structures. Because the neutron beam used has an energy spectrum similar to that of sea-level atmospheric neutrons, our charge collection data correspond to those induced by cosmic ray neutrons. Measured charge collection spectra were dependent on the SOI thickness and agreed with simulated results. An application for the neutron-induced upset rate prediction was also discussed. Furthermore, the charge collection components were separated by our charge collection simulator.

Biological Effects of High-Energy Neutrons Measured In Vivo Using a Vertebrate Model

Abstract Interaction of solar protons and galactic cosmic radiation with the atmosphere and other materials produces high-energy secondary neutrons from below 1 to 1000 MeV and higher. Although secondary neutrons may provide an appreciable component of the radiation dose equivalent received by space and high-altitude air travelers, the biological effects remain poorly defined, particularly in vivo in intact organisms. Here we describe the acute response of Japanese medaka (Oryzias latipes) embryos to a beam of high-energy spallation neutrons that mimics the energy spectrum of secondary neutrons encountered aboard spacecraft and high-altitude aircraft. To determine RBE, embryos were exposed to 0–0.5 Gy of high-energy neutron radiation or 0–15 Gy of reference γ radiation. The radiation response was measured by imaging apoptotic cells in situ in defined volumes of the embryo, an assay that provides a quantifiable, linear dose response. The slope of the dose response in the developing head, relative to reference γ radiation, indicates an RBE of 24.9 (95% CI 13.6–40.7). A higher RBE of 48.1 (95% CI 30.0–66.4) was obtained based on overall survival. A separate analysis of apoptosis in muscle showed an overall nonlinear response, with the greatest effects at doses of less than 0.3 Gy. Results of this experiment indicate that medaka are a useful model for investigating biological damage associated with high-energy neutron exposure.