A System For The Determination Of Uranium In Meteorites By Delayed Neutron Counting

The counting of delayed neutrons emitted by fission is presented as a valuable technique for the measurement of uranium concentrations in a variety of matrices. The concentrations successfully analyzed can vary from well under one part per million to the high concentrations found in uranium ores. In a sample analysis, fission is induced in the uranium in the sample by irradiation in a thermal neutron flux, then the sample is rapidly transferred to a counting assembly capable of detecting delayed neutrons. In the system described, irradiation is performed in a TRIGA reactor, and counting is done in a paraffin-moderated assembly of BF3 gas-filled detectors. All equipment needed for the analysis and the necessary procedures are discussed. The delayed fission neutron (DFN) counting technique is compared to other methods of analysis for uranium, and the experiences of other researchers using the DFN technique are summarized. When compared to many other methods, DFN counting is relatively free of interferences. The interferences which may occur, such as high energy gammas, unknown neutron-emitting nuclides or strong neutron absorbers in the sample, are discussed. Any uncertainties associated with a DFN measurement are also analyzed. DFN counting has been used in many applications, such as the measurement of uranium in geological samples, phosphate products and seawater adsorbers. It can also be used for the measurement of thorium in many samples. These applications are presented, and the results of many different analyses are listed. Experience gained at Oregon State University is examined in detail, and several improvements are suggested.

Bulk uranium items are often measured using active neutron interrogation systems to take advantage of the relatively high penetrability of neutrons, providing the ability to quickly and accurately measure uranium masses in large, dense configurations. Active techniques employ an external neutron source to induce fission in the uranium and subsequently measure emitted prompt fission or delayed neutrons. Unfortunately, the emitted neutrons from 235U [uranium-235] and 238U [uranium-238] are, for all practical purposes, indistinguishable; therefore, commonly used systems such as the Active Well Coincidence Counter, the 252Cf [californium-252] Shuffler, and other systems based on measurement of prompt or delayed fission neutrons require many representative calibration standards and/or well-known isotopic information to interpret the results (i.e., extract an isotopic mass from the effective fissionable mass), thus limiting these techniques for safeguards applications. The primary objective of this research was to develop and demonstrate a dual-energy neutron interrogation technique using a 252Cf Shuffler measurement chamber for determination of uranium enrichment, thus eliminating the need for a (traditionally separate) gamma isotopic measurement. This new technique exploits the change in fission rates as a function of interrogating neutron energy to independently determine the 235U and 238U content in the measurement item. Dual neutron interrogation energies were achieved by adding a deuterium- tritium (D-T) neutron generator into the measurement chamber of the Oak Ridge National Laboratory 252Cf Shuffler. Results from traditional 252Cf measurements and the new D-T measurements were then used to develop a relationship between uranium enrichment and the ratio of the two delayed neutron count rates. Parameter studies were performed to optimize the measurements for each source using a combination of modeling/simulation and experimental measurements. This dissertation presents the detailed development of this novel dual-energy neutron interrogation technique. The results are promising and with engineering refinements could be deployed for routine assay of certain types of materials.

This publication addresses recent developments in neutron generator (NG) technology. It presents information on compact instruments with high neutron yield to be used for neutron activation analysis (NAA) and prompt gamma neutron activation analysis in combination with high count rate spectrometers. Traditional NGs have been shown to be effective for applications including borehole logging, homeland security, nuclear medicine and the on-line analysis of aluminium, coal and cement. Pulsed fast thermal neutron analysis, as well as tagged and timed neutron analysis, are additional techniques which can be applied using NG. Furthermore, NG can effectively be used for elemental analysis and is also effective for analysis of hidden materials by neutron radiography. Useful guidelines for developing NG based research laboratories are also provided in this publication.