Leonard J. Lorence

Comparison of enhanced device response and predicted X-ray dose enhancement effects on MOS oxides

The response of MOS capacitors to low- and medium-energy X-ray irradiation is investigated as a function of gate material (TaSi or Al), oxide thickness, and electric field. The measured device response is compared to the predicted response. In comparisons of 10-keV X-ray and Co-60 irradiations of Al-gate MOS capacitors at an oxide electric field of 1 MV/cm, predictions and experiments agree to within better than 20% for oxide thicknesses ranging from 35 to 1060 nm. For capacitors with TaSi/Al gates, they agree to within better than 30% at 1 MV/cm, with the largest differences occurring for 35-nm gate oxides. At other electric fields, the disagreement between experiment and prediction increases significantly for both Al- and TaSi/Al-gate capacitors. For medium-energy ( approximately 100-keV average photon energy) X-irradiations, the enhanced device response exhibits a much stronger dependence on endpoint bremsstrahlung energy than expected from TIGERP or CEPXS/ONETRAN simulations. Implications for hardness assurance testing are discussed. >

CEPXS/ONELD version 2.0: a discrete ordinates code package for general one-dimensional coupled electron-photon transport

The CEPXS/ONELD version 2.0 code package offers an easy-to-use tool for calculating the high-energy electron-photon cascade in one-dimensional geometries. Since defaults exist for the finite-differencing approximation the user is not required to be an expert in discrete ordinates techniques. Version 2.0 of CEPXS/ONELD also allows users to easily calculate response functions in adjoint or inverse transport mode. Kerma predictions and calculations in curvilinear geometries can also be obtained.<<ETX>>

The Response of MOS Devices to Dose-Enhanced Low-Energy Radiation

A series of experiments has been performed to investigate the response of MOS structures to low-energy radiation. A 10-keV x-ray source was used to irradiate MOS capacitors through a gold/aluminum bilaminate. As expected, the device response when the gold side of the bilaminate faces the device is enhanced strongly with respect to its response when the aluminum side of the bilaminate faces the device. The degree to which the enhanced radiation affects the device response, as determined by the buildup of oxidetrapped charge, depends on the thickness of the gate insulator and the thickness of the capacitor gate in ways similar to those expected from previous work. In addition, striking dependences of the enhanced device response on the energy of the incident radiation, and on the magnitude and direction of the electric field across the gate insulator during the irradiation were observed. These results may provide new insights into the details of the interaction of low-energy radiation with MOS devices.

Coupled Electron-Photon Transport Calculations Using the Method of Discrete Ordinates

A new discrete-ordinates method for coupled electron-photon transport is presented and applied to space-shielding calculations. It is found that the discrete-ordinates method is as accurate as the standard Monte Carlo method but is much more efficient.

A hybrid multigroup/continuous-energy Monte Carlo method for solving the Boltzmann-Fokker-Planck equation

A hybrid multigroup/continuous-energy Monte Carlo algorithm is developed for solving the Boltzmann-Fokker-Planck equation. This algorithm differs significantly from previous charged-particle Monte Carlo algorithms. Most importantly, it can be used to perform both forward and adjoint transport calculations, using the same basic multigroup cross-section data. The new algorithm is fully described, computationally tested, and compared with a standard condensed history algorithm for coupled electron-photon transport calculations.

An S/sub 2/ synthetic acceleration scheme for the one-dimensional S/sub eta/ equations with linear discontinuous spatial differencing

In this paper an S/sub 2/ synthetic acceleration scheme is developed for the one-dimensional S/sub n/ equations (slab and sphere) with linear discontinuous spatial differencing. A Fourier analysis shows that the scheme is unconditionally stable for a model problem. Computational results demonstrate the effectiveness of the technique for varying cell thickness and scattering anisotropy.

A differential absorption spectrometer for determining flash X-ray spectra from 10 to 2000 keV

A differential absorption spectrometer was developed and used on various bremsstrahlung sources (Pelletron, Saturn, and SPEED), with endpoint energies from 150 to 2000 keV. Spectral data were unfolded using response functions generated by TIGER and CEPXS/ONETRAN. Unfolded spectra are qualitatively in agreement with predicted spectra for steady-state bremsstrahlung sources from 150 DeV and 900 keV. Applications and limitations of the diagnostic are discussed, and modifications and simplifications are suggested. >

Experimental Verification of Non-Equilibrated Bremsstrahlung Dosimetry Predictions for 0.75 MeV Electrons

Absolute Monte Carlo predictions of the combined x-ray production and energy deposition in low-Z (Al) and high-Z (Ta) encapsulated thermoluminescent dosimeters for a 0.75-MeV electron/bremsstrahlung source have been experimentally verified over a large range of encapsulation thicknesses and conditions. The effect of the Al is primarily manifested by an alteration in the x-ray flux reaching the active CaF2 material in the dosimeter. For Ta encapsulation, however, there is clear evidence of high-Z/low-Z interface phenomena, where both fluorescence and photoelectrons from the Ta make significant contributions to enhanced energy deposition in the CaF2. During the experiment, this interface enhancement-factor for the encapsulation relative to Al encapsulation was measured directly. The good agreement between the absolute predictions and measurements reported here represents an important verification of the Monte Carlo model for predicting integral interface phenomena.

Electron photoemission predictions with CEPXS/ONETRAN

The author has written a cross-section-generating code called CEPXS that produces multigroup cross sections for use in ONETRAN. The combined CEPXS/ONETRAN code is a general-purpose fully coupled electron-photon-positron transport code. The accuracy and efficiency of electron photoemission predictions with CEPXS/ONETRAN are assessed by comparisons to both TIGERP and experiment. >

Adjoint electron-photon transport Monte Carlo calculations with ITS

A general adjoint coupled electron-photon Monte Carlo code for solving the Boltzmann-Fokker-Planck equation has recently been created. It is a modified version of ITS 3.0, a coupled electron-photon Monte Carlo code that has worldwide distribution. The applicability of the new code to radiation-interaction problems of the type found in space environments is demonstrated.