Phase Tomography in Neutron Interferometry

Perfect crystal neutron interferometry permits the simultaneous investigation of different interactions, like absorption, small angle scattering, and forward scattering. The coherent forward scattering of neutrons in materials or magnetic fields generates phase shifts, which can be detected with high sensitivity. The tomographic reconstruction of phase projections is similar to that of intensity projections in transmission tomography, but due to the larger fluctuation of count numbers and phases, an optimized maximum likelihood algorithm has to be engaged. We present first experimental results, the analysis of isotope mixtures, and the investigation of a metal alloy; both materials are nearly transparent to thermal neutrons. The neutron phase tomography proves its strength in extreme applications where other methods fail, e.g., the complete 3D analysis of non or weak absorbing substances and isotope distributions, the sensitive detection of liquids, residues and corrosion in metals, and the investigation of magnetic domains in bulk materials. Introduction: The PCT (Phase Contrast Tomography) imaging technique was first invented in x-ray tomography [1,2], and has then successfully been transferred to neutron interferometry [3]. The perfect crystal interferometer is a very sensitive device for the detection of scattering effects in the sample, and it allows distinguishing between different interactions, like absorption, small angle scattering (SAS), and coherent forward scattering. The coherent scattering in the sample yields detectable phase differences between the two interfering beams, the object beam through the sample, and the reference beam [4]. While x-ray PCT is sensitive to the electron density and the atomic number Z, the neutron (nPCT) technique depends solely on nuclear and magnetic interaction. Therefore x-ray and neutron PCT are complementary techniques, always sensing different material features. The sensitivity of the new nPCT method has to be compared with attenuation tomography. It is found that the nPCT sensitivity is three orders of magnitude higher than that of conventional CT for most isotope mixtures. Interferometric imaging is experimentally more demanding because it requires a strong neutron source, temperature stabilization and vibration shielding. But nPCT could be of particular interest in extreme applications, like the investigation of substances with very weak absorption, the visualization of isotopic distributions with high sensitivity, and the analysis of magnetic domains in bulk materials. Theory: The principle of interferometric imaging is sketched in Fig. 1. The entrance beam, monochromatic neutrons of wavelength λ ≅ 0.192 nm, is coherently split into the reference beam and the object beam at the first crystal lamella. When the object beam passes the sample it experiences phase shifts which then generate detectable phase differences between the object and reference rays. The accumulated phase shifts Φ through the sample create a phase-sensitive intensity pattern at the interferometer output. The interference pattern is modulated by an auxiliary phase shifter, which generates a series of controlled phase differences ∆j between object and reference beams. This measurement procedure allows the determination of Φxz for every individual pencil beam of cross section dxdz in the corresponding detector pixel at position (x,z). The phase-modulated count numbers in the detector pixels (x,z) follow a harmonic oscillation: ( ) ( cos V 1 2 ) t 1 ( I I xz j xz xz 0 , xz j , xz Φ + ∆ + + = )