A detector efficiency model based on energy extraction from neutrons is described and used to compare 4He detectors with liquid scintillators (EJ301/NE-213). Detector efficiency can be divided into three regimes: single neutron scattering, multiple neutron scattering, and a transition regime in-between. For an average fission neutron of 2MeV, the amount of 4He needed would be about 1/4 of the amount of the mass of EJ301/NE-213 in the single-scattering regime. For about 50% neutron energy extraction (1MeV out of 2MeV), the two types of detectors (4He in the transition regime, EJ301 still in the single-scattering regime) have comparable mass, but 4He detectors can be much larger depending on the number density. A six-tube 11-bar-pressure 4He detector prototype is built and tested. Individual electrical pulses from the detector are recorded using a 12-bit digitizer. Differences in pulse rise time and amplitudes, due to different energy loss of neutrons and gamma rays, are used for neutron/gamma separation. Several energy spectra are also obtained and analyzed.

We present a novel concept of the SNM imaging system based on cosmic-ray muon tracking in coincidence with neutron/gamma detection. The cosmic-ray flux at sea level is about 1 muon/sq. cm/minute. It is composed of nearly equal numbers of μ+ and μ-. In previous work, we have demonstrated that these muons can be used to image nuclear threats in relatively short times by measuring their multiple scattering through objects. Here we propose to image nuclear objects by combining tracking of the muons into a scene with measurements of the secondary particles produced when the muons stop in dense potentially fissile materials. We use multiple drift tube planes to trace incoming cosmic rays. Plastic scintillator serves as a detector of outgoing neutrons and gamma-rays. Additionally, the same plastic scintillator is used to estimate the energy of incoming cosmic-rays. We use a coincidence of n/gamma detection with the initial cosmic-ray trigger to suppress the background. The fissions produced by the stopped μ-generate fission chains that die away after several (~5) fissions. Each fission produces ~10 energetic gamma rays and ~2.5 neutrons. Although a self-shielding needs to be considered, it is likely that tens of neutrons and gamma rays will escape from the object of typical configuration. The efficiency of detecting at least one of the products within ~100 ns could be close to 100% for a detector of reasonably large solid angle (~2 ster). Ten minutes of data should produce 50 trajectories from μ-stopped in 20 kg of U. These numbers can be scaled for other size objects. Our approach has no active source, and therefore it is safe for humans and has no effect on the object under inspection. The detectors are scalable and portable. The drift tubes of the detectors are sealed and do not need the gas replenishment. Detection and localization of SNM is achieved with automatic reconstruction algorithm, which can be run at a standard computer.

The design, construction, and performance of a type of sealed 3He drift tubes for neutron detection are presented. Because the 3He pressure is in the 25–300mbar range, the detector costs are not dominated by the 3He gas. Intrinsic neutron detection efficiencies up to 5% have been observed by using high-density polyethylene moderation. Sensitive measurements of the detector lifetime are achieved by monitoring the full-energy peak of the 3He(n, p)3H reaction as a function of time. The neutron peak position shows a 24-h cycle that may be explained by the physical adsorption of gases onto the wall. The estimated lifetimes of the detectors are sufficiently long and therefore, the design and the construction are robust and practical for applications such as fissile material detection.

Helium-3 gas has long been a popular medium for neutron detection because of its enormous 5320 barn thermal cross-section. Unfortunately, helium-3 does not naturally occur in significant quantities on Earth. Increasing competition over the world's finite helium-3 reserves has recently created an urgent need to utilize our existing supplies of the gas in the most efficient manner possible. A concept called cost efficiency is introduced to quantify this. In cost efficiency, the amount of helium-3 gas used and geometric constraints are considered costs and detector efficiency is considered a return-on-investment. Two figures of merit (FOM) are introduced to compare the cost efficiency of various detector designs. A new design concept for moderating neutron detectors has been developed at Los Alamos National Laboratory (LANL). This design uses an array of detectors nestled inside a polyethylene lattice. This design is demonstrated to provide an increase in cost efficiency of a factor of 2.5 to 3.5 over previous systems. In this thesis, low-pressure helium-3 drift tubes are compared to the high-pressure tubes currently in widespread use. However, the concept can be applied to many other types of detectors as well. A prototype detector containing 72 low-pressure helium-3 tubes was constructed at LANL for benchmarking purposes as well as supporting standoff interrogation experiments. A semi-deterministic method for quantifying detector responses to environmental neutron scattering, called efficacy, is introduced as a means of customizing detector implementations for specific open-laboratory UNCLASSIFIED ADC Review by Alexander Saunders, P-25, on 3/23/2009 LA-UR-09-