Mössbauer imaging

Abstract

Recoilless γ-ray resonance, or the Mössbauer effect, is a well-established spectroscopic tool in materials science¹. Mössbauer spectroscopy shares some of the basic characteristics of NMR (nuclear magnetic resonance) spectroscopy because both rely on nuclear resonance phenomena. A significant recent advance in the latter field is NMR imaging in biomedicine. The possibility of imaging in NMR arises because the frequency of the nuclear resonance is proportional to the local magnetic-field intensity, and, by imposing a magnetic-field gradient on the spin system, different frequency components in the spectroscopic signal are known to originate at different spatial locations within the sample. In this way, frequency information is translated into spatial information and a map of spin density may be recovered by using well-known image reconstruction algorithms. Here we propose the concept of Mössbauer imaging. In Mössbauer spectroscopy the relative velocity between the γ-ray source and the absorbing object is analogous to the NMR magnetic field. This suggests that an analogous approach to Mössbauer imaging is possible by suitably imposing a velocity gradient on the absorbing object. This can be achieved simply by rotating the absorbing object relative to the source, generating lines of constant Doppler shift (or equivalently, of constant γ-ray energy) across the object. From such measurements, a spatial map of the γ-ray absorption coefficient can be tomographically reconstructed. Spatial resolution is related to the rate of rotation of the absorber, but ultimately is signal-to-noise limited. Although there are no foreseeable applications of Mössbauer imaging in medicine, a variety of applications in materials science are expected. An experimental demonstration of the technique has been performed².