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Summary  

Neutron activation analysis (NAA) offers advantages for detecting impurity levels of select isotopes that have suitable neutron cross sections. Secondary ion mass spectrometry (SIMS) on the other hand detects most isotopes, but suffers various molecular interferences and covers only a small beam size volume per run. These two methods are combined here to study a large number of isotopes in titanium thin films in an electrolytic cell experiment. Nine isotopes are covered by NAA and over 50 with SIMS. An overlap in the data sets allows a normalization of SIMS data to the more accurate NAA measurements.

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Abstract  

A comparison of special low-background germanium counting systems used at the Pacific Northwest Laboratory will be presented. These vary from specially modified instruments in the laboratory to low-cosmic-exposure detectors operated deep underground. The underground detectors have copper cryostats completely electroformed from low-background copper. Electroforming is a process analogous to zone refining in its ability to remove chemical impurities. Shielding techniques and their merit are compared as to difficulty and benefit. Active cosmic veto is directly compared to passive overburden shielding. Special attention is paid to cosmic activation of the cryostat and the germanium crystal itself.

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Abstract  

This work addresses the energy spectrum correction due to increased charge carrier collection times in larger HPGe spectrometers. The energy of the radiation interaction is expected to be proportional to the total collected charge. This is increasingly not true with larger HPGe spectrometers. Some charge is lost as the total charge travels from the interaction location to the collection electrode. This path dependent loss of charge results in decreased energy resolution. In HPGe spectrometers, this process is characterized by the charge carrier lifetime constant and is given as an exponential function of the charge carrier collection time divided by this constant. Thus large detectors can experience exponential decrease in energy resolution as charge carrier collection time increases. We studied the effect of charge carrier lifetime on energy resolution for a p-type point contact HPGe spectrometers using pulse shape analysis. We present a method using the rise time to correct for the charge carrier lifetime on a pulse by pulse basis for a given HPGe spectrometer.

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Nuclear Techniques in National Security Studies on Contraband Detection

IEC-based neutron generator for security inspection system

Journal of Radioanalytical and Nuclear Chemistry
Authors: G. H. Miley, L. Wu, and H. J. Kim

Summary  

The Federal Radiological Monitoring and Assessment Center (FRMAC) is authorized by the Federal Radiological Emergency Response Plan to coordinate all off-site radiological response assistance to state and local governments, in the event of a major radiological emergency in the United States. The FRMAC is established by the U.S. Department of Energy, National Nuclear Security Administration, to coordinate all Federal assets involved in conducting a comprehensive program of radiological environmental monitoring, sampling, radioanalysis, quality assurance, and dose assessment. During an emergency response, the initial analytical data is provided by portable field instrumentation. As incident responders scale up their response based on the seriousness of the incident, local analytical assets and mobile laboratories add additional capability and capacity. During the intermediate phase of the response, data quality objectives and measurement quality objectives are more rigorous. These higher objectives will require the use of larger laboratories, with greater capacity and enhanced capabilities. These labs may be geographically distant from the incident, which will increase sample management challenges. This paper addresses emergency radioanalytical capability and capacity and its utilization during FRMAC operations.

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Abstract  

The International Monitoring System (IMS) of the Comprehensive Test Ban Treaty Organization (CTBTO) is currently under construction. The IMS is intended for monitoring of nuclear explosions. The radionuclide part of the IMS monitors the atmosphere for short-lived radioisotopes indicative of a nuclear weapon test, and includes field collection and measurement stations, as well as laboratories to provide reanalysis of the most important samples and a quality control function. The Pacific Northwest National Laboratory in Richland, Washington hosts the United States IMS laboratory, with the designation “RL16.” Since acute reactor containment failures and chronic reactor leakage may also produce similar isotopes, it is tempting to compute ratios of detected isotopes to determine the relevance of an event to the treaty or agreement in question. In this paper we will note several shortcomings of simple isotopic ratios: (1) fractionation of different chemical species, (2) difficulty in comparing isotopes within a single element, and (3) the effect of unknown decay times. While these shortcomings will be shown in the light of an aerosol sample, several of the problems extend to xenon isotopic ratios. Due to the difficulties listed above, considerable human expertise will be required to convert a simple mathematical isotope ratio into a criterion which will reliably categorize an event as ‘reactor’ or ‘weapon’.

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Abstract  

An implementation of the Electron Gamma Shower 4 code (EGS4) has been developed to allow convenient simulation of typical gamma ray measurement systems. Coincidence gamma rays, beta spectra, and angular correlations have been added to adequately simulate a complete nuclear decay and provide corrections to experimentally determined detector efficiencies. This code has been used to strip certain low-background spectra for the purpose of extremely low-level assay. Monte Carlo calculations of this sort can be extremely successful since low background detectors are usually free of significant contributions from poorly localized radiation sources, such as cosmic muons, secondary cosmic neutrons, and radioactive construction or shielding materials. Previously, validation of this code has been obtained from a series of comparisons between measurements and blind calculations. An example of the application of this code to an exceedingly low background spectrum stripping will be presented.

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Abstract  

A fast digital oscilloscope based pulse shape discrimination (PSD) system has been tested with intrinsic germanium detectors large enough to allow ionizing events which generate localized electron-hole pairs at a single site to be segregated from those depositing energy at several different sites in the crystal. Drift velocities of the electrons and holes result in pulses several hundred nanoseconds long. Since the electric field varies by almost a factor of 10 between the outer and inner surfaces, collection of electrons and holes can frequently be dinstinguished, and pulses due to multi-site events can be distinguished from single site events.

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Abstract  

Three years ago, state-of-the-art low-background germanium spectrometry was discussed, and speculations were advanced as to the origin of the remaining background. Some of those speculations have been shown to the incorrect. Contemporary lead shielding contains 100 Bq/kg of210Pb. Our 450-year-old lead was shown to contain <100 mBq/kg A high purity electroformed copper Marinelli shield was placed around the detector with no efffect on the background, which implied that the source is other than the 450-year-old shield. A new limit on the210Pb in this old lead shield is <9 mBq/kg. Electroformed copper components were found to contain226Ra and228Th contaminations at levels of 3500 and 110 Bq/kg, respectively. High purity H2SO4, recrystallized CuSO4, and a BaSO4 scavenge in the electroforming bath have reduced these contaminations to <25 and 9 Bq/kg, respectively. In copper, cosmic ray induced nuclear reactions are now the dominant source of raioactivity. For example,58Co can be readily measured after only a 24-hour exposure at sea level. A new germanium spectrometer containing 2150 grams of 87.44% enriched76Ge has been fabricated to mitigate the effect of cosmogenic68Ge in the background. Current background spectra are presented, and potential sources identified.

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Journal of Radioanalytical and Nuclear Chemistry
Authors: W. Hensley, A. McKinnon, H. Miley, M. Panisko, and R. Savard

Abstract  

A computer code has been written at the Pacific Northwest Laboratory (PNL) to synthesize the results of typical gamma-ray spectroscopy experiments. The code, dubbed SYNTH1, allows users to specify physical characteristics of a gamma-ray source, the quantity of the nuclides producing the radiation, the source-to-detector distance, the type and thickness of absorbers, the size and composition of the detector (Ge or NaI), and the electronic set up used to gather the data. In the process of specifying the parameters needed to synthesize a spectrum, several interesting intermediate results are produced, including a photopeak transmission function vs. energy, a detector efficiency curve, and a weighted list of gamma and x rays produced from a set of nuclides. All of these intermediate results are available for graphical inspection and for printing. SYNTH runs on personal computers, is menu driven and can be customized to user specifications. SYNTH contains robust support for coaxial germanium detectors and some support for sodium iodide detectors. SYNTH is not a finished product. A number of additional developments are planned. However, the existing code has been carefully compared to spectra obtained from National Institute for Standards and Technology (NIST) certified standards with very favorable results.

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Abstract  

A germanium diode gamma-ray spectrometer has been constructed that exhibits background levels three orders of magnitude lower than conventional low-background laboratory spectrometers in the energy region around 100 keV and five orders of magnitude lower in the energy region above 3 MeV. The steps necessary to achieve this reduction are described, and the application of this technology to construction of ultralow background laboratory based germanium diode gamma-ray spectrometers is discussed.

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