V. A. J. van Lint

Scaling Laws for Ionization Effects in Insulators

A new formulation is presented for radiation-induced conductivity in insulators which takes cognizance of the fact that the ionization is formed along particle tracks rather than being uniformly distributed throughout the material. This leads to a linear dependence of conductivity on dose rate with, however, a possibility of an additional dependence on dose for high doses. Data are presented demonstrating this linear dependence.

Energy Dependence of Displacement Effects in Semiconductors

In a semiconductor, the formation of a space-charge region is a logical consequence of the creation of a defect cluster. The dependence of cluster size and defect density on neutron energy can be derived directly from neutron-scattering data and recoil range-energy relationships. The effect of clusters on macroscopic semiconductor properties (e.g., recombination rate, carrier removal) is more uncertain, but the shape of the dependence on cluster size is not strongly dependent on the constants used in the calculation. The relative effectiveness of 14- and l-MeV neutrons before annealing is predicted to be significantly larger for recombination rate (factor of 2.7) than for carrier removal (factor of 1.7). The annealing process can decrease this difference. Moreover, it is possible that the time scale on which annealing proceeds can be different for the larger clusters produced by 14-MeV collisions. In establishing relative damage factors between different energy neutrons, it is important to note that the relative damage curve can be a function of the property measured. In comparing the damage produced by reactor neutrons and 14-MeV neutrons, it is important to take into account the very large energy recoils (>300 keV) produced by inelastic interactions of 14-MeV neutrons. These are responsible for intense ionization as well as displacement production. If the contribution from these recoils can be shown to be negligible or to be equivalent to a number of lower-energy recoils not producing as much ionization, the effects of 14-MeV and reactor neutrons can be correlated.

Electric Breakdown of Earth in Coaxial Geometry

The resistance of earth under high electric stress was determined in 90° sector and 360° coaxial geometry by measuring the voltage at various radii. Streamers were observed to propagate outward from the center conductor after delays ranging up to 1 ms. Stable streamer configurations without arc-over between electrodes apparently exhibited relatively few (¿ 6) longer streamers in 360° × 1 m long geometry.

Electrical Conductivity and Discharge in Spacecraft Thermal Control Dielectrics

Engineering data on the radiation-induced and delayed conductivity in several common spacecraft dielectrics under conditions that approximate space exposure is presented. Direct measurements of discharge propagation velocity on Kapton, Teflon and Mylar is reported. Glass, and occasionally Mylar, exhibited discharges which propagated faster than 2 × 108 cm/s, but persist at ¿10-7 s.

Theory of Transient Electrical Effects in Irradiated Insulators

A theoretical analysis of the transient response of insulators to various types of irradiating particles is given by considering the different nature of the paths of ionized electrons that are formed by the primary radiation. Theory shows that for the class of insulators with a low dielectric constant, the transient response from a densely ionizing particle will be much less than that of a lightly ionizing particle for equal amounts of ionization energy deposition because of direct electron-ion recombination. For insulators with a high dielectric constant, the difference in response to the two types of particles is greatly reduced. In addition, a method for predicting the onset of nonlinear response as a function of radiation dose is given. It is seen that competition between direct electron-ion recombination and change in occupancy of the traps can occur. An interesting result of considering the non-uniform generation of electrons is that the response to irradiation by densely ionizing particles should depend on the orientation of the beam with respect to the applied electric field.

The Effect of Gaseous Ambient on the Initiation of Breakdown in Soil

Measurements have been made of the dependence of the breakdown behavior of soils on the gaseous ambient. Except in completely dry soils essentially no dependence on the substitution of SF6 for air in the soil was seen. This observation is in contradiction to previous results and indicates that in some configurations the breakdown behavior is not controlled by the gaseous ambient. Suggestions are made as to possible reasons for the differences between the present results and those reported previously.

SGEMP Response Investigations with Exploding-Wire Photons

The exploding-wire photon experiments produced space-charge-limited boundary layers of thickness as small as a few centimeters. Comparison between the experimental results and state-of-the-art SGEMP calculations revealed the following features. Peak electric fields, surface currents and currents coupled into simple geometries were calculated within a factor of 1.5, with no normalization other than photoemission diode measurements of the electron current and average energy. Absolute agreement at low fluxes indicate that the electron-emission coefficients are measured by the diodes to an accuracy of better than 30%. The larger scatter in data at high fluxes on the disk experiment may be due to a combination of photon spectrum and rise time variations (especially in a soft component) between pulses and could possibly be affected by differences in minor contaminants on the test object and diode surfaces. The ratio of currents on the back and front of the disk is calculated to be a factor of 1.5 greater than measured. This discrepancy cannot depend on electron emission, and may indicate a need for finer gridding in calculating the edge of a disk. The leakage of electrons into a shadowed aperture is calculated within a factor of 1.4, but appears to persist somewhat longer than the calculations indicate.

Radiation-Induced Currents in Coaxial Cables

The effects of gamma and neutron radiation on coaxial cables should be described as a current due to kinetically moved charge and a conductivity. Simple models, using known cross sections for gamma and neutron interactions can be used to calculate the amount of charge displacement. The radiation-induced conductance of a cable can also be estimated from data on the dielectric material. Experimental data frequently show larger initial response than the estimates. The observed current is proportional to instantaneous dose rate or flux, but the proportionality factor is a generally decreasing function of dose or fluence, even changing signs in some cases. The anomalies suggest relaxation of space charges created in the dielectric by the manufacturing process or by physical motion.

Silicon Solar Cell Damage from Electrical Overstress

A model for the prediction of electrical overstress failure in silicon solar cells based on bulk conduction has been developed. The model has been used to predict the threshold failure current versus pulse width for three types of concentrator cells and one flatplate cell. Threshold failure currents have been measured in each of the cell types using a high voltage pulser that was transformer coupled to the cell impedance. Threshold failure currents for a 10 ¿s exponential pulse of 3-15 kiloamperes were measured for the concentrator cells, in good agreement with model predictions. However, the measured reverse failure currents in the flatplate cell were 4-12 amperes for pulse widths of 10-100 ¿s, compared to predicted values of 200-300 amperes. The failure mechanism for the flatplate cell was related to surface or edge currents and hence would require a different model. This study was directed toward pulse widths of interest for the lightning environment but may be extended to the EMP or SGEMP environments with further analysis.

Electron Emission Spectra Produced by Exploding-Wire Photon Sources

Total yeilds and average energies were measured for the low-energy electron spectra emitted from gold, silver, aluminum and carbon by exploding-wire photon irradiation. A simple linear times exponential energy distribution function was used to fit the retarding potential curves from vacuum photoemission diodes. The yield was in the vicinity of 1014 electron/cal; the average-electron energy in the range of 1 keV. A variation of total yield was found depending on the surface treatment prior to irradiation. As little as 100 Å of carbon deposited on gold produced a factor of 3 drop in yield.