Abstract
A fast neutron tomography system based on the use of real-time pulse-shape discrimination in 7 organic liquid scintillation detectors is described. The system has been tested with a californium-252 source of dose rate 163μSv/h at 1m and neutron emission rate of 1.5×107 per second into 4π and a maximum acquisition time of 2h, to characterize two 100×100×100mm3 concrete samples. The first of these was a solid sample and the second has a vertical, cylindrical void. The experimental data, supported by simulations with both Monte Carlo methods and MATLAB®, indicate that the presence of the internal cylindrical void, corners and inhomogeneities in the samples can be discerned. The potential for fast neutron assay of this type with the capability to probe hydrogenous features in large low-Z samples is discussed. Neutron tomography of bulk porous samples is achieved that combines effective penetration not possible with thermal neutrons in the absence of beam hardening.
Highlights
The interaction properties of neutrons with matter complement those of X-rays due to their lack of electrostatic interaction
We reported recently [52] on the use of fast neutrons and a single liquid scintillation detector for the purposes of discerning rebar in concrete samples with a 75 MBq 252Cf source
The experimental data presented in this paper demonstrate that fast neutron tomography with real-time pulseshape discrimination (PSD) and a relatively small number of organic scintillation detectors is feasible [67]
Summary
The interaction properties of neutrons with matter complement those of X-rays due to their lack of electrostatic interaction. Their scattering properties are largely independent of the atomic number of the elements that constitute the scattering medium whilst, they interact significantly with light (i.e. low-Z) isotopes when thermalized This is especially relevant for hydrogen (often as a constituent of water) and for the other widespread elemental constituents of low-Z porous materials such as carbon, nitrogen and oxygen. The application of neutron radiation is of interest as a complementary tool to X-ray computerized tomography (CT) because X-rays highlight high-Z features very effectively (such as the iron and cobalt in reinforcing bars and calcium in the surrounding cementitious matrix) whilst neutrons are effective for low-mass, lowdensity features such as porosity, hydration and fissures that attract low-mass deposits Both neutron tomography (NT) and X-ray CT have a role in the non-destructive evaluation of the internal state and integrity of a wide variety of materials in use throughout the world
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