Grant J. Lockwood

Simultaneous Integral Measurement of Electron Energy and Charge Albedos

Electron energy and charge albedos were obtained for Be, Al, Ti, Mo, Ta, and U targets over the range of incident energies from 0.1 to 1.0 MeV. In the case of Al, Ta, and U the measurements were extended down to approximately 25 keV. Measurements were made for incident angles from 0° (normal) to 60°, and in some cases 75°, in 15° increments. The results agree well with existing experimental data and with Monte Carlo predictions, except at low energies where the discrepancies are only partially understood.

Optimization of Bremsstrahlung Energy Deposition

Thick-target bremsstrahlung energy deposition in a thin, gold calorimeter has been measured as a function of converter foil thickness for Mo, Sn, La, Ce, Pr, Nd, and Ta converters at source electron energies of 0.5, 0.75, and 1.0 MeV. The thickness-maximized deposition is not a monotonically increasing function of converter atomic number, but instead exhibits a broad maximum near the type 4f rare-earth region. This maximum is substantially greater than the deposition from the Ta converter--Ta being a material routinely employed in flash x-ray sources. Theoretical models also predict higher deposition from La converters than from Ta converters and are used to show that this enhancement is a consequence of higher characteristic x-ray production in La. Nevertheless, there remain significant discrepancies between theoretical predictions and measured data, even when improved numerical cross sections which have just become available are employed in the calculations.

Improved calorimetric method for energy deposition measurement

An improved technique has been developed for the measurement of electron energy deposition versus depth profiles using thin foil calorimeters. A square-wave modulated beam is employed, and the time derivative of the thermocouple signal is measured at a specified time after beam switch. The method and apparatus are described, and results are presented for Al and Ta for 0.3, 0.5, and 1.0 MeV normally incident electrons. Data are also presented for the same energies and 60° (relative to normal) incidence in Al. Deposition data in a layered Ta and Al system at 1.0 and 0.5 MeV energy (normal incidence) are also presented.

Electron Energy Deposition in Multilayer Geometries

A high precision calorimetric method has been employed to measure electron energy deposition profiles in multilayer configurations for normally incident source electrons at an energy of 1.0 MeV. The results are compared with the predictions of a coupled electron/ photon Monte Carlo transport model. Excellent agreement between theory and experiment is obtained for an Al/Au/Al configuration, while significant discrepancies are observed in the case of C/Au/C and Be/Au/Be configurations. Several potential problems are discussed but no satisfactory explanation for the discrepancies has been found.

Absolute Measurement of Low Energy Electron Deposition Profiles in Semi-Infinite Geometries

Two different techniques have been used to measure absolute electron energy deposition profiles in semi-infinite geometries. The first, employing an ionization chamber, was used to measure energy deposition by normally incident electrons at incident energies of 1.0, 0.5, and 0.3 MeV in aluminum, and 1.0 MeV in tantalum. The second method is unique in that it represents the first calorimetric measurement of energy deposition from a monoenergetic D. C. source. Results are given for normally incident electrons at an incident energy of 1.0 MeV in aluminum, and at incident energies of 1.0 and 0.5 MeV in tantalum. The latter method circumvents some of the difficulties inherent in the ion chamber technique and provides more precise results.

Photonic methods of high-speed analog data recording

We have developed a photonic high‐speed analog data recording system that, when compared with conventional oscilloscope‐based systems, offers greater economy, higher bandwidth, electrical isolation, and compactness. The basis of this approach is to record analog photonic signals rather than analog electrical signals. A key component is the prototype high‐speed multi‐ channel data recorder. Initial demonstrations of this photonic system in pulsed power experiments have already shown its versatility and usefulness. We describe the system and its integration with newly developed photonic sensors and discuss its overall impact in diagnostic technology.

Theoretical and Experimental Studies of Multiple Foil Bremsstrahlung Sources

The bremsstrahlung output obtained from the interaction of a single relativistic electron beam with multiple converter foils in a wire-initiated plasma channel has been studied experimentally and theoretically. Agreement of model predictions with the measured spatial variation in thermoluminescent dosimetry is satisfactory in view of the limited source diagnostics. The utility of the model for investigating parameter dependences in this complex scheme is demonstrated. Preliminary experimental results are analyzed in order to compare the multiple-foil approach with a more conventional single-foil scheme employing Bz transport and compression. The maximum dose-area product obtained from multiple-foil experiments was approximately 50% higher than that obtained from the standard single-foil configuration.

Mine detection using backscattered x-ray imaging of antitank and antipersonnel mines

The use of backscattered x-rays to image buried land miens and distinguish between surface and buried features has been well documented. Laboratory imaging experiments, being conducted at Sandia National Laboratories/New Mexico, have been used to develop preliminary data acquisition hardware and software for an upcoming advanced technology demonstration (ATD). In addition image processing techniques, developed by the Department of Nuclear Engineering at the University of Florida, are utilized. Previous buried land mine imaging studies focused on antitank mines buried in screened sand and have included well defined surface features such as a board or a small diameter rock. In the present study we have examined imaging under a variety of practical environmental conditions. We have successfully imaged antitank miens (ATM) buried in sand and rocky New Mexico soil. Images have been obtained for bare surfaces as well as four surfaces covered with limestone road coarse base, snow, water, and native grass. In addition, we have imaged buried ATM and surface antipersonnel mines covered with debris consisting of various size rocks, a log, and leaves such that no mine was visible to the eye. Contour plots of the images obtained for the various environmental conditions are presented.

Land mine detection using backscattered x-ray radiography

The implementation of a backscattered x-ray landmine detection system has been demonstrated in laboratories at both Sandia National Laboratories (SNL) and the University of Florida (UF). The next step was to evaluate the modality by assembling a system for fieldwork and to evaluate the systems performance with real laboratories. To assess the systems response to a variety of objects, buried simulated plastic and metal antitank landmines, surface simulated plastic antipersonnel landmines, and surface metal fragments were used as targets for the field test. The location of the test site was an unprepared field at SNL. The tests conducted using real landmines were held at UF using various burial depths. The field tests yielded the same levels of discrimination between soil and landmines that had been detected in laboratory experiments. The tests on the real landmines showed that the simulated landmines were a good approximation. The real landmines also contained internal features that would allow not only the detection of the landmines, but also the identification of them.

Electron Transport in Reactor Materials

The transport method used in the analysis of pulsed electron beam experiments to determine the equations of state of Liquid-Metal-Fast-Breeder-Reactor materials has been verified experimentally. A calorimetric technique has been employed in conjunction with a steady-state monoenergetic (¿ 1.0 MeV) accelerator to measure electron albedos and high-resolution energy deposition profiles in reactor materials. Energy deposition profiles have been obtained in a multilayer carbon-uranium-carbon configuration and in semi-infinite uranium. Systematic measurements of the saturated number and energy albedos for UO2 have been obtained as a function of source energy and incident angle. All experimental results have been compared with predictions of the Monte Carlo code employed in the analysis of the pulsed-beam experiments. The good agreement substantially enhances our confidence in the resulting equation-of-state data, as well as in the more general predictive capability of the transport technique.