Characterization of a Portable Neutron Coincidence Counter
Neutron coincidence counting is a technique widely used for qualitative and quantitative analysis of nuclear material. Because different isotopes possess different coincident neutron characteristics, the coincident neutron signature can be used to identify and quantify a given material. In an effort to identify unknown nuclear samples in field inspections, a new portable neutron coincidence counter has been developed. An indepth analysis has been performed to establish whether the nuclear material in an unknown sample could be quantified with some confidence. The analysis was performed by comparing the true measurements of the system to the calculated output produced using MCNPX and the neutron coincidence point model. Based on the analysis, it is evident that this new portable system can play a useful role in identifying nuclear material for verification purposes. Introduction Neutron coincidence counting is an important passive, nondestructive-assay method that can be used to characterize nuclear material in bulk samples, namely plutonium. Because the number of neutrons emitted simultaneously during the fission process is particular to each isotope, the coincidence neutrons create a useful signature that can be used to describe the isotopes in a sample. Coincidence measurements can be done regardless of background or (α, n) neutrons making them less sensitive to external factors. For plutonium, Pu, Pu, and Pu all have dominant spontaneous fission yields producing coincidence neutrons. The measurement of these spontaneous fission neutrons, however, is complicated by the presence of induced fission neutrons. Induced fission is dependent on the fissile isotopes, primarily Pu, and is caused by the absorption of neutrons in the sample. Both spontaneous fission and induced fission produce coincidence neutrons. Because these neutrons are emitted practically simultaneously, they create a useful signature that can be used to quantify a particular nuclear material. The Portable Neutron Coincidence Counter was developed by Los Alamos National Laboratory to aid inspectors in field measurements of bulk samples. In order to benchmark and characterize the detector, a computer model was created using the Monte Carlo N-Particle eXtended (MCNPX) code and its embedded advanced multiplicity capabilities. The results determined using MCNPX were then compared to the results calculated using the traditional Neutron Coincidence Point Model. This paper discusses both these results and their implications. Background The PNCC consists of four individual high-density polyethylene slabs. The slabs are approximately 7-in. (length) x 10-in. (height) x 3-in. (width) and weigh approximately 8
Feasibility Study of a Portable Coupled 3He Detector with LaBr3 Gamma Scintillator for Field Identification and Quantification of Nuclear Material
Feasibility Study of a Portable Coupled He Detector with LaBr3 Gamma Scintillator for Field Identification and Quantification of Nuclear Material. (May 2010) Daniel Canady Strohmeyer, B.S., Texas A&M University; Chair of Advisory Committee: Dr. William Charlton In recent years, there have been several research endeavors to increase the ability to identify and quantify special nuclear material in field measurements. These have included both gamma spectroscopy and neutron coincidence systems that are portable and work in a variety of environments. In this work, a Monte Carlo Neutral Practicle X (MCNPX) model was used to design an instrument that includes four gamma detection slabs placed within four neutron detection slabs. The combination of gamma spectroscopy and neutron coincidence counting in a single instrument allows for direct measurement of plutonium (Pu) mass without need for assumptions or operator declarations. A combined neutron-gamma instrument was designed for use in characterizing and quantifying Pu in field samples. This detector consists of a plastic scintillator containing LaBr3 nanoparticles and a polyethylene slab containing four He tube detectors. The system was tested via simulation with MCNPX for four Pu samples of known quality and quantity. These samples had masses ranging from 100-300 g of Pu. It was found that the designed detector system could be used to determine Pu-effective
Monte Carlo simulations of magnetovolume instabilities in anti-Invar systems
We perform constant pressure Monte Carlo simulations of a spin-analogous model which describes coupled spatial and magnetic degrees of freedom on an fcc lattice. Our calculations qualitatively reproduce magnetovolume effects observed in some rare earth manganese compounds, especially in the anti-Invar material YMn2. These are a sudden collapse of the magnetic moment which is connected with a huge volume change, and a largely enhanced thermal expansion coefficient.