D.E. Beutler

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. >

Solutions to heavy ion induced avalanche burnout in power devices

A review of normal breakdown and current induced avalanche (CIA) breakdown mechanisms in silicon power transistors is presented. The applicability of the CIA model to heavy ion induced burnout is shown, and solutions to CIA in silicon power semiconductors are given. It is noted that solving the problem of CIA burnout in npn bipolar and n-channel DMOS devices is, at best, difficult. Several techniques of hardening these devices to the effects of heavy ion, dose-rate induced failure, and any other condition producing CIA are discussed. The most effective techniques are those that minimize the emitter current injection by reducing the emitter injection efficiency or making the parasitic bipolar more difficult to turn on. However, it is believed that the simplest solution to the problem is to use pnp bipolar and p-channel DMOS devices whenever possible. >

A dose rate independent pMOS dosimeter for space applications

A dual-dielectric pMOS dosimeter (RADFET) has been recently designed at Sandia. The RADFET consists of a thermally grown oxide and a CVD deposited nitride. With a negatively applied bias, holes are generated in the SiO/sub 2/ transport and are trapped at the SiO/sub 2//Si/sub 3/N/sub 4/ interface producing a measurable threshold-voltage shift. Because holes are trapped away from the Si/SiO/sub 2/ interface, hole neutralization by tunneling and interface-trap buildup are minimized resulting in little fade or annealing of the RADFET output response. RADFETs were irradiated at dose rates from 0.002 to 50 rad(Si)/s with biases from -5 to -20 V. RADFETs were also annealed for times up to 10/sup 7/ s at temperatures up to 100/spl deg/C. Within experimental uncertainty, no difference in RADFET output response at a given bias was observed over the dose rate range examined and for 25/spl deg/C anneals. At an anneal temperature of 100/spl deg/C only a 20% decrease in RADFET output response was observed. These results show that Sandias RADFETs exhibit little or no fade of their output characteristics and are ideal for low dose rate space applications.

High-brightness, high-spatial-resolution, 6.151keV x-ray imaging of inertial confinement fusion capsule implosion and complex hydrodynamics experiments on Sandia’s Z accelerator (invited)

When used for the production of an x-ray imaging backlighter source on Sandia National Laboratories’ 20MA, 100ns rise-time Z accelerator [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], the terawatt-class, multikilojoule, 526.57nm Z-Beamlet laser (ZBL) [P. K. Rambo et al., Appl. Opt. 44, 2421 (2005)], in conjunction with the 6.151keV, Mn–Heα curved-crystal imager [D. B. Sinars et al., Rev. Sci. Instrum. 75, 3672 (2004)], is capable of providing a high quality x radiograph per Z shot for various high-energy-density physics experiments. Enhancements to this imaging system during 2005 have led to the capture of inertial confinement fusion capsule implosion and complex hydrodynamics images of significantly higher quality. The three main improvements, all leading effectively to enhanced image plane brightness, were bringing the source inside the Rowland circle to approximately double the collection solid angle, replacing direct exposure film with Fuji BAS-TR2025 image plate (read with a Fuji BAS-5000 sc...

Radiation field from an extended planar-anode diode on HERMES III

The bremsstrahlung field from an extended planar-anode diode with an annular cathode tip on the the 16-TW HERMES III electron accelerator is measured and compared with predictions of the MAGIC-CYLTRAN model. Measurements confirm predictions and demonstrate that the diode provides a versatile large-area source of gamma radiation. Versatility is obtained by adjustment of the anode-cathode gap, which affects electron trajectories while simultaneously maintaining constant diode impedance. The adjustment permits the generation of average dose rates from about 1.2*10/sup 12/ rad/s over 3100 cm/sup 2/ to about 5.6*10/sup 12/ rad/s over 700 cm/sup 2/ without destruction of the bremsstrahlung target. >

Comparison of experimental pulse-height distributions in germanium detectors with Integrated-Tiger-Series-code predictions

Pulse-height distributions in two different types of Ge detectors have been measured for a variety of medium-energy X-ray bremsstrahlung spectra. These measurements have been compared to predictions using the Integrated Tiger Series (ITS) Monte Carlo electron/photon transport code. In general, the authors find excellent agreement between experiments and predictions using no free parameters. These results demonstrate that the ITS codes can predict the combined bremsstrahlung production and energy deposition with good precision (within measurement uncertainties). The one region of disagreement observed occurs for low-energy ( >

Experimental verification of bremsstrahlung production and dosimetry predictions as a function of energy and angle

Measurements of energy deposition from bremsstrahlung production as a function of angle and beam energy (5-25 MeV) are shown to be in excellent agreement with Monte Carlo predictions. Dosimetry measurements are made and predicted in both equilibrated and underequilibrated radiation environments. In the latter case the quality of the agreement requires an accurate prediction of both the photon and electron spectra produced by the primary electron beam. An improved empirical equation for predicting bremsstrahlung production is also presented. This empirical relation can be used to estimate doses without resorting to expensive calculational efforts. It also gives an analytical relationship for dose as a function of energy and angle for a converter optimized for bremsstrahlung production using 15.5 MeV electrons. >

Dosimetry considerations for the high-energy photon/electron environment of HERMES III: implications for experiments and modeling

Measurements of energy deposition as a function of equilibrator thickness and position in the HERMES III radiation field are compared to ITS code predictions. These measurements demonstrate the combined photon/electron nature of the radiation field and the importance of the electron field in both measurements and calculations. >

Experimental verification of bremsstrahlung production and dosimetry predictions for 15.5 MeV electrons

The radiation produced by a 15.5-MeV monoenergetic electron beam incident on optimized and nonoptimized bremsstrahlung targets is characterized using the ITS Monte Carlo code and measurements with equilibrated and nonequilibrated TLD dosimetry. Comparisons between calculations and measurements verify the calculations and demonstrate that the code can be used to predict both bremsstrahlung production and TLD response for radiation fields that are characteristic of those produced by pulsed simulators of gamma rays. The comparisons provide independent confirmation of the validity of the TLD calibration for photon fields characteristic of gamma-ray simulators. The empirical Martin equation, which is often used to calculate radiation dose from optimized bremsstrahlung targets, is examined, and its range of validity is established. >

Characterization of flash γ-ray detectors that operate in the Trad/s range

Abstract Compton-diode detectors, scintillator-photodiode detectors, and Cherenkov-photodiode detectors, designed to measure the intense pulsed bremsstrahlung field of HERMES III, are described and characterized in the field of HERMES III. Measurements and modeling show that (1) the Compton-diode detector measures dose rate and is capable of linear operation up to 2.5×1012 rad/s, (2) the scintillator-photodiode detector measures dose rate only when the rate is less then 2×1011 rad/s, and (3) the Cherenkov photodiode detector can be used o place limits on the radiation pulse over the range ∼ 2.5×1010 to ∼ 2.5×1012 rad/s. A by-product of the measurements and modeling is the determination of the temporal behavior of the forward field of HERMES III near the beam axis.