Paul L. Reeder

Glass-fiber-based neutron detectors for high- and low-flux environments

Pacific Northwest Laboratory (PNL) has fabricated cerium-activated lithium silicate scintillating fibers via a hot-downdraw process. These fibers typically have a operational transmission length (e-1 length) of greater than 2 meters. This permits the fabrication of devices which were not possible to consider. Scintillating fibers permit conformable devices, large-area devices, and extremely small devices; in addition, as the thermal-neutron sensitive elements in a fast neutron detection system, scintillating fibers can be dispersed within moderator, improving neutron economy, over that possible with commercially available 3He or BF3 proportional counters. These fibers can be used for national-security applications, in medical applications, in the nuclear-power industry, and for personnel protection at experimental facilities. Data are presented for devices based on single fibers and devices made up of ribbons containing many fibers under high-and low-flux conditions.

Calculation of Minimum-Detectable-Concentration Levels of Radioxenon Isotopes Using the PNNL ARSA System

Measurement of xenon fission product isotopes is a key element in the global network being established to monitor the Comprehensive Nuclear-Test-Ban Treaty. The automated Radio-xenon Analyzer/Sampler (ARSA), built by Pacific Northwest National Laboratory, can detect 131mXe, 133mXe, 133Xe, and 135Xe via a beta-gamma counting system. Due to the variable background and sources of these four radio-xenon isotopes, it is important to have as sensitive a detection system as possible and to quantify the Minimum-Detectable-Concentrations (MDC) that such a system will be able to detect to preclude false negative and false positive results. From data obtained from IAR in Germany MDC values for 133Xe were well below the 1 mBq/SCMA as required by the PTS for the Comprehensive Test BAn Treaty [WGB TL-11,1999].

Separation of alkaline earth from alkali metal fission products in the solar on-line mass spectrometer☆

Abstract Fission product nuclides of Sr and Ba have been separated from their Rb and Cs isobars in an on-line mass spectrometer by the use of molecular fluoride ions of Ba and Sr formed on a hot Re surface.

Advanced Large Area Plastic Scintillator Project (ALPS): Final Report

The advanced Large-Area Plastic Scintillator (ALPS) Project at Pacific Northwest National Laboratory investigated possible technological avenues for substantially advancing the state-of-the-art in gamma-ray detection via large-area plastic scintillators. The three predominant themes of these investigations comprised the following: * Maximizing light collection efficiency from a single large-area sheet of plastic scintillator, and optimizing hardware event trigger definition to retain detection efficiency while exploiting the power of coincidence to suppress single-PMT dark current background; * Utilizing anti-Compton vetoing and supplementary spectral information from a co-located secondary, or Back detector, to both (1) minimize Compton background in the low-energy portion of the Front scintillators pulse-height spectrum, and (2) sharpen the statistical accuracy of the front detectors low-energy response prediction as impelmented in suitable energy-windowing algorithms; and * Investigating alternative materials to enhance the intrinsic gamma-ray detection efficiency of plastic-based sensors.

Progress Report for the Advanced Large-Area Plastic Scintillator (ALPS) Project: FY 2003 Final

The DOE tasked PNNL to investigate possible technological avenues for substantially advancing the state-of-the-art in gamma detection via large-area plastic scintillators. This report describes progress on this project as of the conclusion of FY 2003. The primary focus of the report is on experimental tests conducted with a single large-area plastic scintillator outfitted with a variety of photomultiplier tube configurations. Measurements performed to date incude scintillator response under broad-area exposure to various point-like gamma sources, and light-output uniformity mappings obtained by varying the position of a collimated beta-source over the surface of the scintillator. Development of a Monte Carlo program for modeling the response of a large-area scintillator sensor to ionizing radiation, explicitly including resolution-broadening effects of scintillation light generation, propagation, and collection is also described.

Methods and Instruments for Fast Neutron Detection

Pacific Northwest National Laboratory evaluated the performance of a large-area (~0.7 m2) plastic scintillator time-of-flight (TOF) sensor for direct detection of fast neutrons. This type of sensor is a readily area-scalable technology that provides broad-area geometrical coverage at a reasonably low cost. It can yield intrinsic detection efficiencies that compare favorably with moderator-based detection methods. The timing resolution achievable should permit substantially more precise time windowing of return neutron flux than would otherwise be possible with moderated detectors. The energy-deposition threshold imposed on each scintillator contributing to the event-definition trigger in a TOF system can be set to blind the sensor to direct emission from the neutron generator. The primary technical challenge addressed in the project was to understand the capabilities of a neutron TOF sensor in the limit of large scintillator area and small scintillator separation, a size regime in which the neutral particle’s flight path between the two scintillators is not tightly constrained.

Progress Report on the Advanced Large-Area Plastic Scintillators (ALPS) Project

The U.S. Department of Energy tasked Pacific Northwest National Laboratory to investigate possible technological avenues for substantially advancing the state-of-the-art in gamma detection via large-area plastic scintillators. This letter report describes progress to date on this project. Early phases of the project, which commenced in January 2003, have focused on (1) quantifying the light-collection efficiency in plastic scintillator sheets as a function of photomultiplier tube positioning and edge-area coverage, (2) developing a conceptual design for a demonstration sensor, and (3) conducting initial laboratory setup and preliminary experiments using relatively small plastic scintillators for readout technique development and benchmarking of the modeling studies.

Generation of Radixenon Isotopes

Pacific Northwest National Laboratory has developed an automated system for separating Xe from air and can detect the following radioxenon isotopes, 131mXe, 133mXe, 133Xe, and 135Xe. This report details the techniques used to generate the various radioxenon isotopes that are used for the calibration of the detector as well as other isotopes that have the potential to interfere with the fission produced radioxenon isotopes. Fission production is covered first using highly enriched uranium followed by a description and results from an experiment to produce radioxenon isotopes from neutron activation of ambient xenon.

Independent isomer yield ratio of 90Rb

Abstract The independent isomer yield ratio for 90Rb from thermal neutron fission of 235U has been measured by use of a new technique involving a pulsed reactor and an on-line mass spectrometer facility. The observed isomer yield ratio of 8.7 ± 1.0 is one of the largest ratios measured for a low energy fission process. However, a statistical model analysis shows that the average angular momentum (J = 4.5) deduced from this isomer yield ratio is consistent with average angular momentum for other products from low energy fission.