Abstract
The development of neutron imaging from a qualitative inspection tool towards a quantitative technique in materials science has increased the requirements for accuracy significantly. Quantifying the thickness or the density of polycrystalline samples with high accuracy using neutron imaging has two main problems: (i) the scattering from the sample creates artefacts on the image and (ii) there is a lack of specific reference attenuation coefficients. This work presents experimental and simulation results to explain and approach these problems. Firstly, a series of neutron radiography and tomography experiments of iron, copper and vanadium are performed and serve as a reference. These materials were selected because they attenuate neutrons mainly through coherent (Fe and Cu) and incoherent (V) scattering. Secondly, an ad hoc Monte Carlo model was developed, based on beamline, sample and detector parameters, in order to simulate experiments, understand the physics involved and interpret the experimental data. The model, developed in the McStas framework, uses a priori information about the sample geometry and crystalline structure, as well as beamline settings, such as spectrum, geometry and detector type. The validity of the simulations is then verified with experimental results for the two problems that motivated this work: (i) the scattering distribution in transmission imaging and (ii) the calculated attenuation coefficients.
Highlights
Introduction and motivationNeutron imaging is a well established technique for nondestructive two, three- and four-dimensional evaluation of samples (e.g. Anderson et al, 2009; Strobl et al, 2009; Kaestner, Mnch et al, 2011)
Where I0ðÞ is the initial intensity of the beam, IðÞ is the transmitted intensity, ÆtotalðÞ is the attenuation coefficient of the material and t is the thickness of the sample in the beam direction
In x2 we describe the underlying physics and neutron instrument parameters which constitute the basis for the Monte Carlo model
Summary
Neutron imaging is a well established technique for nondestructive two-, three- and four-dimensional evaluation of samples (e.g. Anderson et al, 2009; Strobl et al, 2009; Kaestner, Mnch et al, 2011). To be able to quantify the attenuation of a sample, we approach two unsolved questions: how to avoid scattered neutrons adding intensity to the transmission images and how to obtain adequate cross section values. There is a tradeoff between the highest possible spatial resolution and the quantification of the transmitted signal with state-of-the-art neutron instruments This is because the closer the detector is to the sample, the larger is the angle of the scattering cone covered by the detector and in particular by the sample projection (Fig. 1). The results of these experiments show the difficulties in correctly measuring polycrystalline structures, even with simple geometries. A validation of the model by comparison with radiography and tomography experiments and a discussion of the results are given
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