W. L. Imhof

Space Charging Currents and Their Effects on Spacecraft Systems

The range and limits on the space charging electron currents available in the near-geosynchronous orbit have been identified from a large SCATHA satellite data base. The most intense current densities observed were 0.8 nA/cm2 at 1 keV and 0.5 pA/cm2 at 1 MeV. The effects of these currents on both surface and internal charging of dielectrics have been modeled. In exposed dielectrics radiation-induced effects significantly increase the conductivity within the first few ¿m of the surface, produce permanent radiation damage, and affect the final potential of the sample. The calculated electric field profiles and surface potential of a 127 ¿m Kapton® sample are found to be consistent with the voltage measured aboard the SCATHA satellite during a charging event. The calculated field strength of 2×105 V/cm is below the spontaneous breakdown level. Transient electrical pulses observed in association with the charging may therefore be due to capacitive coupling effects rather than to breakdown. The electric fields and voltage internal to both plane-parallel and coaxial geometries containing a Teflon® dielectric enclosed between two conductors have also been modeled. Electric field strengths of a few x 105 V/cm and internal potentials of several kV are calculated for these typical configurations when exposed to the direct electrons in unusually energetic events.

A Satellite-Borne High-Resolution Ge(Li) Gamma-Ray Spectrometer System Part 1: Description of the Instrument and Gamma-Ray Backgrounds in Earth Orbit

Gamma radiation from terrestrial and extraterrestrial sources has been investigated for the first time with a high-resolution lithium-drifted germanium spectrometer-cryogen system flown on board a low-altitude, spin-stabilized, polar-orbiting satellite (1972-076B) launched on October 2, 1972. Gamma-ray pulses corresponding to energies between 40 keV and ~ 2.8 MeV are analyzed by a 4096-channel pulse-height analyzer with an overall system resolution of 3.5 - 4.0 keV FWHM (1.33 MeV Co60). A description of the instrument, its performance in earth orbit and preliminary results of the gamma-ray backgrounds are presented.

A High-Resolution, Multiple Particle Spectrometer for the Measurement of Solar Particle Events

A particle spectrometer possessing high spectral and spatial/temporal resolution for the measurement of energetic protons, electrons and alpha particles over a broad energy and intensity range during solar particle events (SPE) was successfully launched aboard a polar-orbiting satellite on 17 October 1971. The spectrometer consists of a multi-element, silicon detector stack mounted within a plastic scintillator guard counter to form a particle telescope. The spectra of protons in the energy range 1.2-100 MeV, electrons from 0.88-1.6 MeV and alpha particles from 1.6-500 MeV are measured in four time-shared modes of operation with dual 256-channel pulse-height analyzers (PHA) of the successive-approximation type. Integral counting rates from each of the main elements of the telescope and the guard counter are measured every 32 milliseconds with low-power CMOS digital counters to provide high spatial/temporal resolution on the solar particle fluxes. Complete spectra on all three particle types over the above energy ranges are obtained every few seconds. A detailed description of the spectrometer, including a discussion of the mode logic which enhances the counting statistics in selected energy ranges, the dual pulse-height analyzers and the methods of obtaining the high temporal resolution is presented. Preliminary worldwide flight data obtained from two identical spectrometers oriented at different directions to the particle flux are also presented.

High Resolution Gamma-Ray Spectroscopy on the P78-1 Satellite

On February 24, 1979, two cooled germanium spectrometers operating in the range 0.05 to 2.5 MeV were successfully launched into polar orbit on the P78-1 satellite. Each independent spectrometer system consists of a single large (85 cm3) IGe detector which is shielded by a NaI, polyscin, anticoincidence scintillator and cooled by either one or both of two mechanical Stirling cycle refrigerators. The instrument design and performance during the first 6 months in orbit are presented. After approximately 70 days in orbit, radiation damage effects became evident by the gradual degradation of the resolution from 3.0 - 3.5 keV at 0.511 MeV to greater than 20 keV after about 125 days. An important aspect of these results is the apparent capability of stabilizing the radiation effects by partially restoring the performance of these detectors to resolutions of 15 - 20 keV at 511 keV by annealing at ambient (~280 K) temperatures for a few days.

Enhanced Radiation Doses to High-Altitude Spacecraft during June 1980

During the period June 9 to 14, 1980 fluxes of high energy electrons were greatly intensified at high altitudes. Since electrons with energies greater than a few MeV can penetrate nominal spacecraft shielding and damage electronic components, the increased fluxes during this period were investigated to determine the nature and extent of damage they might cause to spacecraft in equatorial orbits near an altitude of 30000 km. It is concluded that short term effects such as charging and subsequent discharge in dielectrics from the accumulation of electrons stopped in the material in a period of the order of a day are more likely than total dose damage over the few days duration of high flux levels.

The morphology of widespread electron participation at high latitudes

Abstract The morphology of widespread energetic electron participation processes at high latitudes has been studied using an extensive set of satellite Bremsstrahlung X-ray measurements. Long term averages of the X-ray (> 50 keV) intensities and energy spectra observed at 750 km altitude over a broad field of view are presented. These data provide for the first time long term averages of Bremsstrahlung X-rays produced by electrons precipitating into the atmosphere over wide local time and latitude intervals. The average X-ray fluxes were found to be ~30% larger near local noon than near local midnight. The net intensities were generally higher at larger values of the AE index but wide variations in this pattern were observed. For AE > 800 the average flux was a factor of ~100 higher than the flux when AE

High resolution measurements of space radiation effects on silicon surface-barrier detectors

Two high-resolution charged particle spectrometers employing silicon surface-barrier detectors have been operated in an 800 km altitude polar orbit for a 26-month period between October 1971 and December 1973-Based on existing trapped radiation models and the measured solar flare environment during this period the detectors were exposed to a proton fluence of 2.8 t 1013 protons/cm2 at energies > 400 keV and an electron fluence of 4.5 t 1013 electrons/cm2 at energies > 50 keV. High-resolution pulse height spectra of the alpha particles from low-intensity Americium-241 in-flight calibration sources show that the resolutions of the detectors did not degrade more than 5 percent and that the gains of the system did not vary more than 1.2 percent as a result of this exposure. Two aspects of detector operation are considered responsible for this performance. First, the incident detectors were operated with the rear, aluminum contact exposed to the radiation environment instead of the front, gold contact. The more intense lower energy radiation is therefore stopped in the rear ohmic contact region instead of in the front surface-barrier junction region and the overall damage effects are significantly reduced. Second, a detector bias technique was employed that automatically maintained a constant depletion potential across the detector independent of variations in leakage current due to radiation damage.

Effects of 33-MeV Proton Bombardent on the Performance of CdTe Gamma-Ray Detectors

A measurement program to investigate the performance of CdTe gamma-ray detectors after bombardment with 33-MeV protons has been undertaken in order to evaluate the effects of energetic protons encountered on earth-orbiting satellites. The pulse heights and energy resolutions of the 59.6 keV line from Am241 and the 122 keV line from Co57 and the leakage currents of the detectors were monitored as a function of proton fluence. Of the two types of CdTe detectors tested, far greater radiation effects were observed in those obtained from Tyco (chlorine doped) than in the sensor obtained from Hughes (indium doped). In the former detectors the relative pulse heights appeared to decrease slowly with increasing fluence up to about (5-9) × 109 protons/cm2 beyond which the gain degraded at a faster rate. The energy resolution and leakage currents were observed to improve at moderate fluence levels near (1-3) × 109 protons/cm2. In one of the two chlorine doped sensors tested, the intrinsic detector resolution at 59.6 keV improved from 4.3 keV to 2.6 keV at a fluence of 2 × 109 protons/cm2 and subsequently degraded beyond its original performance level after 5 × 109 protons/cm2.

Satellite observations of energetic electron precipitation during the 1979 solar eclipse and comparisons with rocket measurements

Abstract During the solar eclipse of 26 February 1979, the P78-1 satellite passed near Red Lake, Ontario, at an altitude of ~600 km. On two consecutive orbits spanning the time of total eclipse, energetic electrons were measured with two silicon solid state detector spectrometers having excellent energy and angular resolution. Significant fluxes of precipitating electrons were observed near the path of totality. Comparisons of flux intensities and energy spectra with those measured from a Nike Orion and two Nike Tomahawk rockets launched near Red Lake before and during total eclipse give good agreement and indicate that the electron precipitation was relatively uniform for more than an hour and over a broad geographical area.