Using Nuclear Resonance Fluorescence for Nondestructive Isotopic Analysis

Nuclear resonance fluorescence (NRF) has been studied as one of the nondestructive analysis (NDA) techniques currently being investigated by a multi-laboratory collaboration for the determination of Pu mass in spent fuel. In NRF measurements specific isotopes are identified by their characteristic lines in recorded gamma spectra. The concentration of an isotope in a material can be determined from measured NRF signal intensities if NRF cross sections and assay geometries are known. The potential of NRF to quantify isotopic content and Pu mass in spent fuel has been studied. The addition of NRF data to MCNPX and an improved treatment of the elastic photon scattering at backward angles has enabled us to more accurately simulate NRF measurements on spent fuel assemblies. Using assembly models from the spent fuel assembly library generated at LANL, NRF measurements are simulated to find the best measurement configurations, and to determine measurement sensitivities and times, and photon source and gamma detector requirements. A first proof-of-principal measurement on a mock-up assembly with a bremsstrahlung photon source demonstrated isotopic sensitivity to approximately 1% limited by counting statistics. Data collection rates are likely a limiting factor of NRF-based measurements of fuel assemblies but new technological advances may lead to drastic improvements

Increased UV transmission by improving the manufacturing process for FS

ABSTRACT Optical designers have been designing ultraviolet (UV) systems at wavelengths in the UV region for many years. With increasing demand for deep UV applications, special considerations that are not applicable to traditional visible optics must be taken to produce the optics. Specifically as the wavelength of incident light decreases, the importance of very smooth surfaces increases. The intent of this project is to increase the performance of UV optics in a four-phase project. The first phase consists of characterizing sub-surface damage using destructive methods to enable process control, the second phase (presented here) focuses on polishing methods, the third phase will include cleaning and possible etching protocols and the fourth phase will be improving thin film coating performance. Keywords: Ultraviolet, fused silica, polishing, coating INTRODUCTION As trends in UV optical system design shift to shorter UV wavelengths, optical manufacturing has to be more conscious of the effect that subsurface damage, surface features, residual contamination from polishing and cleaning and coating have on the residual performance of the optics in their systems. For many years, researchers have tackled partial aspects of these problems. For example, Bloembergen 1 stated that cracks and pores on an optical surface will lead to laser damage (LD) when incident with a laser beam. Neauport et al. 2 spoke to two of the main damage initiators of LD, sub-surface damage (SSD) and nano-absorbing centers, focusing mainly on the latter. They used fused silica optics in high power laser applications at 351nm. Higher cerium concentration on the surfaces strongly correlated with increased damage density. Aluminum, copper and iron did not have similar correlations. Neauport et al. also tried to correlate the presence of cerium with damage morphology but the results were inconclusive. Yoshiyama et al. 3 studied the effects of polishing, etching, cleaving and water leaching on the UV damage of fused silica. The surfaces were all exposed to a Nd:YAG laser at 355nm. Micropits were found on the polished surface. Their analysis found high concentrations of Al, B, Ce and Zr. The concentrations of the Al, B and Zr all decreased rapidly to less than 10% of the maximum value at a depth of 50nm, but the Ce required ~100nm before decreasing to less than 10% of its maximum value. A second sample etched with a buffered HF solution had a lower pit density than the polished surface. The pit density decreased exponentially with the etched layer thickness indicating that the cerium is a precursor to laser damage. Micropits found on the cleaved surface indicated that cerium contamination is not the only cause of damage. It is hypothesized that damage initiated because of residual stresses and permanent mechanical damage from the cleaving process. Hydrolyzed cleaved surfaces were found to decrease the laser damage threshold. Camp et al. 4 determined that the zirconia conventionally polished surfaces have a higher laser damage threshold at 355nm compared to ceria polished surfaces. They also observed that damage typically centered around scratches or digs on the surface of the parts. Néauport et al