Computer-optimized design of polyethylene-moderated {sup 3}He counters for fast neutrons
Because polyethylene-moderated {sup 3}He counters are rugged and reliable, they are generally the instruments of choice for field detection of fast neutrons in gamma-ray backgrounds. Their main drawback is the bulky, massive moderator needed to reduce the incident neutron energies to the sensitive range of the {sup 3}He+n capture reaction. This report discusses an optimization approach that provides a detector with uniform angular response and the maximum detection efficiency per unit mass. The key assumption is that each parameter has a geometrical interpretation and its effect on the response can be evaluated independently from that of the others. Specifically, the detection efficiency can be written as a product of separate functions for the moderator mass, gas pressure, tube position, etc., and the uniformity of the angular response is determined by the symmetry of the moderator dimensions. This analytical model was tested by compiling a comprehensive database of detector efficiencies as functions of the different parameters, including one- versus two-tube detectors, moderator masses from 1 to 6 kg, gas pressures from 1 to 20 atm, etc. In general, the model reproduced both the magnitude and angular dependence of the efficiency to within about 10%. To a high degree, the most important parameters are polyethylene mass and the quantity of {sup 3}He gas; because of neutron diffusion out of the moderator, the optimum tube positions are near the center of the detector. The highest value of the efficiency per unit mass occurs near 3 kg, a result that requires the most compact detectors to use more than a single {sup 3}He tube. In this case, the optimum detector has two tubes and a total mass of 3.0 kg. Although they could use 4-atm tubes with 2.54-cm diameters, increasing the gas volume could easily provide a 20% increase in efficiency with no changes in other parameters.
- Research Organization:
- Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-36
- OSTI ID:
- 756871
- Report Number(s):
- LA-13695-MS; TRN: US0003860
- Resource Relation:
- Other Information: PBD: 1 May 2000
- Country of Publication:
- United States
- Language:
- English
Similar Records
Monte Carlo calculations of the effective area and directional response of a polyethylene-moderated neutron counter
The Ephithermal Neutron Multiplicity Counter Design and Performance Manual: More Rapid Plutonium and Uranium Inventory Verifications by Factors of 5-20