M.A. Xapsos

Nonionizing energy loss (NIEL) for heavy ions

The concept of nonionizing energy loss (NIEL) has been found useful for characterizing displacement damage effects in materials and devices. Published tabulations, however, are limited with respect to target materials, particle types and energies. In this paper we show how the NIEL database can be significantly expanded to include heavy ions in the coulombic limit by using the Monte Carlo code SRIM. The methodology used to extract NIEL from SRIM is described. This greatly adds to the number of materials and incident particles for which the NIEL concept can be applied. To show that values so derived are consistent with previous calculations, we compare alpha particle NIEL for GaAs derived from SRIM with a direct analytical calculation. The SRIM code is limited in that only coulombic interactions are considered. General rules of thumb are also described which permit prediction of NIEL for any target material over a large energy range. Tabulated values of NIEL for alpha particles incident on Si, GaAs and InP are presented.

Displacement damage in GaAs structures

High-energy knock-on atoms produced by incident protons are much more important in determining the total nonionizing energy deposited in GaAs than in Si, due to the relative size of the Lindhard correction for partitioning the recoil energy. High-energy recoils are mainly produced by inelastic nuclear interactions between the incident protons and the target atoms. A review of previous calculations indicates that both the fast cascade and the evaporation phases of the elastic interaction contribute to the average energy of the recoiling ion. New calculations are presented for the energy dependence of the nonionizing energy deposited in GaAs as a result of inelastic interaction with protons over the energy range 10-1000 MeV. These calculations are combined with the previously determined contribution from elastic interactions to obtain the energy dependence of the total nonionizing energy deposited in GaAs by protons. The calculation is compared with both new and earlier experimental data for ion-implanted GaAs resistors irradiated with protons over the energy range 40-188 MeV, in order to form a basis whereby proton displacement effects in GaAs structures can be predicted. It is shown that results obtained for 10 MeV protons, for example, can be used to predict results to be expected at much higher energies. >

Probability model for cumulative solar proton event fluences

A new model of cumulative solar proton event fluences is presented. It allows the expected total fluence to be calculated for a given confidence level and for time periods corresponding to space missions. The new model is in reasonable agreement with the JPL91 model over their common proton energy range of >1 to >60 MeV. The current model extends this energy range to >300 MeV. It also incorporates more recent data which tends to make predicted fluences slightly higher than JPL91. For the first time, an analytic solution is obtained for this problem of accumulated fluence over a mission. Several techniques are used, including Maximum Entropy, to show the solution is well represented as a lognormal probability distribution of the total fluence. The advantages are that it is relatively easy to work with and. To update as more solar proton event data become available.

Displacement damage analogs to ionizing radiation effects

Abstract We show that concepts, such as effective equivalent dose and the quality factor, which have long been found useful in comparing the effects of different kinds of ionizing radiation, are also applicable in correlating displacement damage effects in semiconductors. In the case of displacement damage, the energy deposition process is determined by the nonionizing energy loss (NIEL), instead of linear energy transfer (LET), as an ionization.

Applicability of LET to single events in microelectronic structures

Linear energy transfer (LET) is often used as a single parameter to determine the energy deposited in a microelectronic structure by a single event. The accuracy of this assumption is examined for ranges of ion energies and volumes of silicon appropriate for modern microelectronics. It is shown to be accurate only under very restricted conditions. Significant differences arise because (1) LET is related to energy lost by the ion, not energy deposited in the volume; and (2) LET is an average value and does not account for statistical variations in energy deposition. Criteria are suggested for determining when factors other than LET should be considered, and new analytical approaches are presented to account for them. One implication of these results is that improvements can be made in space upset rate predictions by incorporating the new methods into currently used codes such as CREME and CRUP. >

Characterizing solar proton energy spectra for radiation effects applications

The Weibull distribution for smallest values is shown to be a useful description for solar proton event energy spectra. One advantage is its compact analytic expression, which allows easy conversion between differential and integral spectra. Another is its versatility, which is necessary for describing the highly variable spectra of concern. Furthermore, the Weibull distribution appears to be appropriate for use over broad energy ranges extending out to GeV. Examples are shown and comparisons to previously used distributions are made. An especially useful consequence of this approach for radiation effects applications is that it allows both predictive model spectra and observed spectra to be described by the same distribution. This allows spectra to be systematically ranked by severity of radiation damage caused in microelectronics. It further allows observed spectra to be related to predictive model parameters such as confidence levels. These points are demonstrated by evaluating the ionization dose deposited by various spectra in silicon behind aluminum shielding appropriate for spacecraft.

Probability model for worst case solar proton event fluences

A predictive model of worst case solar proton event fluences is presented. It allows the expected worst case event fluence to be calculated for a given confidence level and for periods of time corresponding to space missions. The proton energy range is from >1 to >300 MeV, so that the model is useful for a variety of radiation effects applications. For each proton energy threshold, the maximum entropy principle is used to select the initial distribution of solar proton event fluences. This turns out to be a truncated power law, i.e., a power law for smaller event fluences that smoothly approaches zero at a maximum fluence. The strong agreement of the distribution with satellite data for the last three solar cycles indicates this description captures the essential features of a solar proton event fluence distribution. Extreme value theory is then applied to the initial distribution of events to obtain the model of worst case fluences.

Energy Dependence of Proton-Induced Displacement Damage in Gallium Arsenide

Nonionizing energy deposition in gallium arsenide has been calculated for protons with energies ranging from 1 to 1000 MeV. The calculations are compared with new experimental results for ion implanted gallium arsenide resistors and Hall samples irradiated with protons in the energy range 1 to 60 MeV. Results are also compared with recent studies of proton induced displacement damage in silicon.

Proton displacement damage and ionizing dose for shielded devices in space

The sensitivity of displacement damage and ionizing dose calculations to both the incident proton energy spectrum and that transmitted through shields is calculated down to 100 eV for a solar proton event. The method is also applicable to trapped proton environments.

Validation of a comprehensive space radiation transport code

The HZETRN code has been developed over the past decade to evaluate the local radiation fields within sensitive materials on spacecraft in the space environment. Most of the more important nuclear and atomic processes are now modeled and evaluation within a complex spacecraft geometry with differing material components, including transition effects across boundaries of dissimilar materials, are included. The atomic/nuclear database and transport procedures have received limited validation in laboratory testing with high energy ion beams. The codes have been applied in design of the SAGE-III instrument resulting in material changes to control injurious neutron production, in the study of the Space Shuttle single event upsets, and in validation with space measurements (particle telescopes, tissue equivalent proportional counters, CR-39) on Shuttle and Mir. The present paper reviews the code development and presents recent results in laboratory and space flight validation.