Albert E. Evans

Gamma-Ray Response of a 38-mm Bismuth Germanate Scintillator

The response of a 38-mm-diam by 38-mm-long bismuth germanate (BGO) scintillator to gamma radiation from 0.12 to 8.28 MeV has been measured. The full-energypeak efficiency of the detector, which varies from 80% at 0.12 MeV to 10.8% at 8.29 MeV, is 5.6 times as great at 2.75 MeV as the full-energy efficiency of a similar NaI (Tl) detector. Although the energy resolution of the detector is inferior to that which may be obtained with NaI (Tl), its high peak-to-Compton ratio and high full-energy-to-escape-peak ratio is advantageous. This detector shows promise of application in many fields.

Unfolding bismuth-germanate pulse-height distributions to determine gamma-ray flux spectra and dose rates

Abstract We report a new procedure for unfolding gamma-ray pulse-height distributions acquired with bismuth-germanate detectors. The equipment used for acquiring the distributions consists of a LeCroy 3500 data acquisition and analysis system and eight bismuth-germanate scintillation detectors 7.62 cm in diameter and 7.62 cm long. The system was calibrated and characterized from 0.12 to 8.28 MeV by using gamma-ray spectra from a variety of radioactive sources and from the 14 N(p,γ) 15 O reaction. By fitting these pulse-height distributions with a function containing 17 parameters, we determined theoretical response functions and used them to obtain the gamma-ray flux spectra at multiple space points from a variety of radioactive objects of interest to nuclear safeguards. We used two flux-spectrum-to-dose-rate conversion curves to obtain dose rates. For a composite source, consisting of several sources with accurately known strengths, the result of our procedure agreed with the expected value to within less than 10%. Direct use of measured spectra and the flux-spectrum-to-dose-rate curves to obtain dose rates avoids the errors that can arise because of spectrum dependence in simple gamma-ray dosimeter instruments.

High Rfsolution Fast Neutron Spectrometry without Time-of-Flight

Performance tests of a spectrometer tube of the type developed by Cuttler and Shalev show that the measurement of fast neutron spectra with this device can be made with an energy resolution previously obtainable only in large time-of-flight facilities. In preliminary tests, resolutions of 16.4 keV for thermal neutrons and 30.9 keV for 1-MeV neutrons were obtained. A broad-window pulse-shape discrimination (PSD) system is used to remove fram pulse-height distributions most of the continua due to 3He-recoil events, noise, and wall effect. Use of PSD improved the energy resolution to 12.9 keV for thermal neutrons and 29.2 keV for l-MeV neutrons. The detector is a viable tool for neutron research at nominally equipped accelerator laboratories.

Radiation damage to 3He proportional counter tubes

Abstract Neutron damage to 3 He proportional-counter tubes is inhibited by coating the cathodes of the tubes with activated charcoal. This coating also improves pulse-height resolution.

Influence of Materials and Counting-Rate Effects on 3He Neutron Spectrometry

The high energy resolution of the Cuttler-Shalev 3He neutron spectrometer causes spectral measurements with this instrument to be strongly susceptible to artifacts caused by the presence of scattering or absorbing materials in or near the detector or the source, and to false peaks generated by pileup coincidences of the rather long-risetime pulses from the detector. These effects are particularly important when pulse-height distributions vary over several orders of magnitude in count rate versus channel. A commercial pile-up elimination circuit greatly improves but does not eliminate the pileup problem. Previously reported spurious peaks in the pulse-height distributions from monoenergetic neutron sources have been determined to be due to the influence of the iron in the detector wall.

Energy dependence of the response of a 3He long counter

Abstract A polyethylene-moderated 3 He neutron detector that had previously been found to have an energy-independent efficiency for broad-spectrum neutron sources is apparently resonant to 450 keV neutrons.

Nondestructive Assay of Fissile Material Samples in Support of Nuclear Safeguards

Samples of fissile material can be assayed by bombarding with 300-to 600-keV neutrons and counting delayed neutrons from fission. Interrogating neutron energy selection is based upon considerations of sample penetrability and insensitivity of response to nonfissile isotopes. Significant cost savings in nuclear safeguards and quality control are possible.

Evaluation of LMFBR Fuel-Motion Diagnostics Instrumentation with PARKA

To aid in the design of LMFBR safety test experiments and safety test faciities (STF), a program of evaluation of concepts for fuel-motion diagnostics instrumentation has been undertaken. A part of this evaluation is being done at PARKA, a Rover project critical assembly which has been modified to study the self nuclear image from driven FTR-type fuel assemblies. Feasibility of obtaining fast-neutron images of single-pin voids in assemblies of up to 127 fuel pins has been demonstrated, albeit marginally for the larger fuel bundles. The feasibility of using in-core detectors as fuel-motion monitors has also been studied. Use of PARKA in a pulsed mode to study STF transient phenomena is discussed.

Hodoscope Performance Tests on a 91-Pin Fuel Bundle at PARKA

Joint LASL/ANL studies of the fuel motion performance capability of a fast-neutron hodoscope have been performed on a 91-pin fuel bundle. Optimization of the neutron detectors used in the hodoscope was also investigated. The neutron detectors tested were a Hornyak button and a stilbene detector with pulse shape discrimination. These studies show that a hodoscope can provide useful fuel motion detection capability for large bundles and that the amount of information provided is greatly increased by the use of neutron detectors with high neutron thresholds and high detection efficiency. These measurements indicate that the TREAT hodoscope can be upgraded to meet most of the fuel motion diagnostic requirements of new safety facilities.

The Expanding Role of the Small Van De Graaff in Nuclear Nondestructive Analysis

In support of Nuclear Safeguards, a 3-MeV Van de Graaff accelerator was installed at LASL to aid research in techniques for nondestructive assay of fissionable materials. Monoenergetic neutrons make possible fissile assay techniques less influenced by self-absorption than thermal interrogation and more discriminating of fissile over fertile material than use of partially moderated high-energy neutrons. Delayed fission-neutron counting, a valuable assay technique, is facilitated by pulsed neutron on-to-off ratios in excess of 109, achieved with special beam-handling techniques. Where high neutron backgrounds, e.g. from samples containing 240Pu, preclude delayed-neutron counting, fission prompt-neutron counting with an energy-biasable detector is applied. The increasing facility workload, which includes detector development and calibration, research in trace analysis by proton-induced x-ray fluorescence and research in light-element isotopic assay by means of charged-particle induced reactions, indicates that technological application may more than supplant dwindling nuclear research as a market for accelerators of this class.