Keith A. Ryden

ESA's tools for internal charging

Electrostatic discharges, caused by bulk charging of spacecraft insulating materials, are a major cause of satellite anomalies. A quantitative knowledge of the charge build-up is essential in order to eliminate these problems in the design stage. This is a presentation of ESAs tools to assess whether a given structure is liable to experience electrostatic discharges or not. A study has been made of the physical phenomenon, and an engineering specification has been created to be used to assess a structure for potential discharge problems. The specification has been implemented in new software, DICTAT. The implementation of tests in dedicated facilities is an important part of the specification, and tests have been performed to validate the new tool.

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.

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.

Observations of Internal Charging Currents in Medium Earth Orbit

The Galileo global navigation system will employ an array of satellites in medium Earth orbit (MEO). Internal charging is one of the primary hazards for any spacecraft in MEO, and accordingly, the Galileo test satellite, Giove-A, carries a detector, SURF, to undertake the measurement of internal charging currents at three different shielding depths. The currents are due to electrons stopped in three aluminum plates: the first plate is 0.5 mm thick and located under 0.5-mm Al-equivalent shielding, the second is 0.5 mm thick and located underneath the first plate, and the third is 1.0 mm thick and located underneath the second plate. Giove-A was successfully launched on December 28, 2005 into a 23 300-km circular 56deg inclination orbit. In this paper, we provide data on the internal charging currents observed in 2006, with particular emphasis on two large charging events, one in mid-April and one in mid-December. Comparisons are made to predictions using the DICTAT internal charging tool and the FLUMIC ldquoworst caserdquo trapped electron belt model. In general, the charging currents observed are safely within the standard DICTAT 3.5/FLUMIC 3.0 predictions but are exceeded in the most shielded plate on five calendar days during December 2006. The December event was unique in 2006 in that it was triggered by the arrival of a coronal mass ejection (CME) rather than being due to the effects of a fast solar wind stream flowing from a coronal hole. The currents measured in the December event were, however, an order of magnitude lower than those predicted using the ldquoanomalously large eventrdquo supplement to FLUMIC which models the most extreme electron enhancements associated with CMEs.

Cosmic Radiation Dose Measurements from the RaD-X Flight Campaign

Abstract The NASA Radiation Dosimetry Experiment (RaD-X) stratospheric balloon flight mission obtained measurements for improving the understanding of cosmic radiation transport in the atmosphere and human exposure to this ionizing radiation field in the aircraft environment. The value of dosimetric measurements from the balloon platform is that they can be used to characterize cosmic ray primaries, the ultimate source of aviation radiation exposure. In addition, radiation detectors were flown to assess their potential application to long-term, continuous monitoring of the aircraft radiation environment. The RaD-X balloon was successfully launched from Fort Sumner, New Mexico (34.5°N, 104.2°W) on 25 September 2015. Over 18 hours of flight data were obtained from each of the four different science instruments at altitudes above 20 km. The RaD-X balloon flight was supplemented by contemporaneous aircraft measurements. Flight-averaged dosimetric quantities are reported at seven altitudes to provide benchmark measurements for improving aviation radiation models. The altitude range of the flight data extends from commercial aircraft altitudes to above the Pfotzer maximum where the dosimetric quantities are influenced by cosmic ray primaries. The RaD-X balloon flight observed an absence of the Pfotzer maximum in the measurements of dose equivalent rate.

Analyses of CCD images of nucleon-silicon interaction events

Sets of image frames were captured from a CCD device exposed to continuous beams of high energy neutrons. The tracks of short-range fragments from nuclear spallation interactions in the silicon of the devices pixels have been analyzed in respect of their frequency, intensity, directionality and other pertinent parameters. A comparison between these results and the predictions of computer code models of nucleon interactions in silicon is presented. Comparisons are also made with equivalent images of neutron events in an APS camera.

The Disappearance of the Pfotzer-Regener Maximum in Dose Equivalent Measurements in the Stratosphere

The NASA Radiation Dosimetry Experiment (RaD-X) successfully deployed four radiation detectors on a high altitude balloon for a period of approximately twenty hours. One of these detectors was the RaySure in-flight monitor, which is a solid-state instrument designed to measure ionizing dose rates to air crew and passengers. Data from RaySure on RaD-X show absorbed dose rates rising steadily as a function of altitude up to a peak at approximately 60,000 feet, known as the Pfotzer-Regener maximum. Above this altitude absorbed dose rates level off before showing a small decline as the RaD-X balloon approaches its maximum altitude of around 125,000 feet. The picture for biological dose equivalent, however, is very different. At high altitudes the fraction of dose from highly ionizing particles increases significantly. Dose from these particles causes a disproportionate amount of biological damage compared to dose from more lightly ionizing particles and this is reflected in the quality factors used to calculate the dose equivalent quantity. By calculating dose equivalent from RaySure data, using coefficients derived from previous calibrations, we show that there is no peak in the dose equivalent rate at the Pfotzer-Regener maximum. Instead the dose equivalent rate keeps increasing with altitude as the influence of dose from primary cosmic rays becomes increasingly important. This result has implications for high altitude aviation, space tourism and, due to its thinner atmosphere, the surface radiation environment on Mars

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.

Internal Charging Measurements in Medium Earth Orbit Using the SURF Sensor: 2005–2014

The SURF internal charging monitor that measures deposited currents in three shielded aluminum plates is one of the sensors within the Merlin radiation monitoring instrument launched on Giove-A in December 2005 into a 23300-km circular 56° inclination medium earth orbit. The instrument has now completed 8.5 years in orbit despite being originally intended for a two-year operational life. Here, we report on the instruments health, provide further recent data, and review the overall results in terms of their engineering significance. It is concluded that the instrument remains in good health, with no significant deterioration, and a near continuous data set from December 2005 to April 2014 is now available. The maximum plate currents to date were recorded over the period April 6-8, 2010, during a significant outer electron belt enhancement (also observed by Geostationary Operational Environmental Satellite (GOES)-12 in geostationary orbit), which persisted for several days. From April 7 to April 9, 2010, the current in the most shielded plate (1.0-mm thickness with 1.5-mm shielding) exceeded the widely used 0.1-pA cm-2 safety threshold for internal charging (the only days in the mission where this occurred), and on these days, it also exceeded the reasonable worst case current predications given by the DICTAT internal charging tool (the other plate currents remained within DICTAT predictions). The 0.02-pA cm-2 safety threshold used in European ECSS charging standards for dielectrics below 25 °C has been exceeded on a far larger number of days; for example, in the most shielded plate, it has been exceeded on 55 days (8% of the total).