Norbert Ensslin

Measurement variances in thermal neutron coincidence counting

Abstract Methods are presented for reliably estimating the uncertainties in quantities of plutonium as measured using neutron coincidence counting for nuclear safeguards purposes. For the conventional two-parameter assay technique, semi-empirical equations are obtained that satisfactorily describe the observed measurement variances in the total and coincidence counts for multiplying plutonium oxide and metal. For the three-parameter assay method using neutron multiplicity analysis, covariance matrix methods are developed that describe, to an accuracy of approximately 10%, the uncertainty in the calculated plutonium mass, multiplication, and alpha-n reaction rate, for samples with small multiplication.

Assay variance as a figure of merit for neutron multiplicity counters

Abstract This paper describes a procedure for estimating assay variance for the neutron multiplicity counters that are being developed to assay special nuclear materials. The procedure estimates the moments of the expected neutron multiplicity distribution and calculates the expected variance without requiring a sample measurement. Results of the procedure are compared with actual measurements made with existing prototype counters. Assay variance is used as a figure of merit to compare the counters and to identify the most important design parameters. In the future, this procedure can be used to optimize the design of new counters.

Design of a fast neutron coincidence counter

Abstract Monte Carlo simulations were carried out to simulate the response of a boron-loaded plastic scintillator array. These detectors offer potential advantages compared to moderated 3He systems because of their fast response and high efficiency. Tallies were made of neutron capture probability as a function of time and the results were coupled to assay variance estimates to evaluate detector performance. Orders-of-magnitude reductions in count time relative to a 50% efficient thermal counter are possible, in principle, for high (α,n) samples.

Neutron detection and applications using a BC454/BGO array

Neutron detection and multiplicity counting has been investigated using a boron-loaded plastic scintillator (BC454)/bismuth germanate (BGO) phoswich detector array. Boron-loaded plastic combines neutron moderation (H) and detection ({sup 10}B) at the molecular level, thereby physically coupling increasing detection efficiency and decreasing die-away time with detector volume. Separation of the phoswich response into its plastic scintillator and bismuth germanate components was accomplished on an event-by-event basis using custom integrator and timing circuits, enabling a prompt coincidence requirement between the BC454 and BGO to be used to identify neutron captures. In addition, a custom time-tag module was used to provide a time for each detector event. Time-correlation analysis was subsequently performed on the filtered event stream to obtain shift-register-type singles and doubles count rates.

A digital random pulser for testing nuclear instrumentation

Abstract A digital random pulser circuit based on a linear feedback shift register is described. The circuit provides a wide range of discrete output rates and exhibits very little deadtime. The testing of this circuit and its use with a neutron coincidence counter electronics circuit is discussed.

Evaluation of a fast neutron coincidence counter for the measurements of uranium samples

Abstract A fast neutron coincidence counter using BC454/BGO phoswich detectors has been evaluated for the purpose of rapid verification measurements of uranium items. This counter uses custom electronics to identify and count coincidence neutrons in the presence of background radiation. Measurements of uranium standards were performed to evaluate the counter. This counter is successful in measuring uranium items but has a low efficiency that results in minimal improvement over current technology. An optimized counter can be built with better performance capabilities, but it is recommended that newer technologies be used instead.

Applications of Boron Loaded Scintillating Fibers as NDA Tools for Nuclear Safeguards

Nuclear safeguards and nonproliferation rely on nondestructive analytical tools for prompt and noninvasive detection, verification, and quantitative analysis of nuclear materials in demanding environments. A new tool based on the detection of correlated neutrons in narrow time windows is being investigated to fill the niche created by the current limitations of the existing methods based on polyethylene moderated {sup 3}He gas proportional tubes. Commercially produced Boron-loaded ({sup 10}B) plastic scintillating fibers are one such technology under consideration. The fibers can be configured in a system to have high efficiency, short neutron die-away, pulse height sensitivity, and mechanical flexibility. Various configurations of the fibers with high density polyethylene have been considered which calculationally result in high efficiency detectors with short die-away times. A discussion of the design considerations and calculations of the detector efficiency, die-away time, and simulated pulse height spectra along with preliminary test results are presented.