Effects of neutrino recoil on the X-ray emission of bound electron capture atoms

Abstract

Due to neutrino emission during the capture process, the nucleus experiences a recoiling motion. If one assumes that those valence charge fractions which are localised, but also bound to ligands, cannot follow the nuclear and core motion immediately, a certain electron cloud deformation takes place besides the 'normal' static chemical polarisation. This gives rise to additional X-ray transitions from the valence band to 1s for K-electron capture. For intensity reasons in connection with radioactive sources the energy dispersive version of X-ray spectrometry is mostly applied and, therefore, a Kbeta 5-type emission cannot be resolved, but it increases the relative (to Kalpha ) Kbeta intensity and enhances the chemical sensitivity of the Kbeta /Kalpha ratio. Numerical estimates for the 51Cr decay give up to 5% effects for the limiting case of absolutely fixed valence charge fractions. 'Kinetic polarisation' is proposed to explain some experimentally observable and analytically applicable aspects of the Kbeta /Kalpha ratio of electron capture (EC) nuclides.