Recent attention to international safeguards has stimulated interest in nondestructive analysis techniques. These NDA techniques include high- and low-resolution gamma-ray spectrometry, active and passive neutron counting, and physical measurements. Often, the NDA measurements are made abroad under field conditions, and in these cases, portability is important. In other cases, the measurements are made under laboratory conditions but no calibration materials are available. This paper describes several NDA applications in support of international safeguards projects, all involving international cooperation.
Authors:S. Pellock, A. Thompson, K. He, C. Mecklin, and Jie Yang
Naturalization is the introduction and establishment of a nonnative species with sustainable populations in a novel environment. The success of nonnative species may be influenced by their relatedness to the native flora. Darwin proposed that if a nonnative plant species is introduced into an environment without native congeners, the nonnative species will have a greater chance of becoming naturalized. To test Darwin’s naturalization hypothesis, we compiled a Kentucky plant database consisting of 821 vascular plant species and subsequently selected species traits and distribution information to determine the effect of congeneric species and traits on the probability of successful naturalization and invasion. The predictors used include reproductive traits, growth form, abundance, habitat type, native congeners, and biogeographical origin. We fit three sets of generalized linear mixed models (GLMMs) with a binomial family and a logit link. Backward selection based on minimizing the Akaike Information Criterion (AIC) was used in the analyses. Our results from these three sets of models clearly indicate that the validity of Darwin’s hypothesis is invasion stage dependent. More specific, the naturalized and invasive models (predicting the probability of being naturalized and invasive respectively) did not support Darwin’s naturalization hypothesis. The number of native congeners had no effect on the likelihood that a particular species would naturalize and become invasive. Our results suggest that Darwin’s naturalization hypothesis is more relevant during the early stage of establishment as demonstrated by the native model (predicting the probability of being native) and it becomes irrelevant during the late stages of invasion as indicated by the naturalized and invasive models. Thus, it can be generalized that biotic interactions, especially competition, is a critical determinant of initial success for nonnative species in the recipient communities. Once established, the fate of non-native species during the late stages of invasion may be more related to other factors such as biogeographic origin and habitat conditions. Furthermore, we found reproductive traits such as flowering phenology and flower type are associated with invasion success. We also recognized contrasting traits between native and nonnative species, indicating niche differentiation between these two groups of species. Niche overlapping was found as well among species regardless of the status of being native or otherwise. Our study provides a novel approach to advance the understanding of phylogenetic relatedness between nonnative species and native flora by integrating traits and niche concepts at the regional scale.
Authors:A. Schilk, K. Abel, D. Brown, R. Thompson, M. Knopf, and C. Hubbard
A novel scintillating-fiber sensor for detecting high-energy beta particles has been designed and built at the Pacific Northwest Laboratory to characterize238U and90Sr in surface soils. High-energy betas generate unique signals as they pass through multiple layers of scintillating fibers that make up the active region of the detector. Lower-energy beta particles, gamma rays, and cosmic-ray-generated particles comprise the majority of the background interferences. The resulting signals produced by these latter phenomena are effectively discriminated against due to the combination of the sensor's multi-layer configuration and its interlayer coincidence/anti-coincidence circuitry.
Authors:K. Abel, A. Schilk, D. Brown, M. Knopf, R. Thompson, and R. Perkins
A large area beta scintillation detector has been developed which is currently capable of determining Sr-90/Y-90 contamination in surficial soils. The detector system employs scintillating fiber optic arrays, with active dimensions approximately 15 cm wide by 100 cm long, both ends of which are coupled to multiple photomultiplier tubes (PMTs). Electronic processing includes coincidence requirements to optimize sensitivity and selectivity for the 2.28 MeV (maximum) beta particle from Y-90. Low energy beta particles and gamma rays are discriminated against using double ended and multi-layer coincidence requirements. The detector system is personal-computer-software controlled and data restored in a format compatible with standard database software for ease of final data reduction. Experimental calibration studies have shown a linear response for Sr-90/Y-90 soil concentrations from 12 to over 500 pCi/g and a discrimination factor of 50 to 1 versus Cs-137.
Authors:T. Bowyer, K. Abel, C. Hubbard, A. McKinnon, M. Panisko, R. Perkins, P. Reeder, R. Thompson, and R. Warner
A fully automatic radioxenon sampler/analyzer (ARSA) has been developed and demonstrated for the collection and quantitative
measurement of the four xenon radionuclides,131mXe(11.9 d),133mXe(2.2 d),133Xe(5.2 d), and135Xe(9.1 hr), in the atmosphere. These radionuclides are important signatures in monitoring for compliance to a Comprehensive
Test Ban Treaty (CTBT). Activity ratios of these radionuclides permit source attribution. Xenon, continuously and automatically
separated from the atmosphere, is automatically analyzed by electron-photon coincidence spectrometry providing a lower limit
of detection of about 100 μBq/m3. The demonstrated detection limit is about 100 times better than achievable with reported laboratory-based procedures for
the short-time collection intervals of interest.