An intuitive method of separating isobars in AMS is the measurement of the characteristic X-rays emitted when the fast ions pass through a suitable target foil. The method is reputed to suffer from low detection efficiency [1], but how low is the detection efficiency really? We are investigating the technique for beam energies of 150-200 MeV available from the 14UD accelerator for heavy ions of mass 180-209. Crucially, it is the characteristic L-x-rays with typical energies of ~8 keV that are detected rather than K-x-rays. Cross-sections for the former are orders of magnitude higher than for the latter.
Further, the x-ray production depends strongly on energy and target material [2]. These dependences have been explored using an experimental setup consisting of an ionization chamber preceding the target foil, and a germanium X-ray detector in close proximity to the foil. X-rays are detected in coincidence with the ionization chamber signal, resulting in very clean spectra free of room background or background from other ion species with the same m/q as the ion of interest. Cross sections for L-x-ray production by hafnium ions and its energy dependence will be presented. With the known energy dependence the optimum thickness of a foil can be derived. The optimal material has an L- or K-shell approximately matching the L-shell of the projectile, but its characteristic lines should not interfere with the L-X-rays of the projectile.
The mechanism leading to these enhanced cross-sections has been explained by the “Electron Promotion Model” [3,4], in which a quasi-molecule is formed during the collision of the heavy ions. The promoted inner-shell electrons get trapped by crossings of their molecular orbitals with those of outer shells. This will leave one (or both) of the colliding atoms with an inner-shell vacancy after the collision.
[1] see for example: J. E. McAninch, G. S. Bench, S. P. H. T. Freeman, M. L. Roberts, J. R. Southon, J. S. Vogel and I. D. Proctor. Nucl. Inst. and Meth. in Phys. Res. B 99 (1995), 541
[2] W. Meyerhof, A. Rüetschi, Ch. Stoller, M. Stöckli, and W. Wölfli. Phys. Rev A 20 (1979), 154.
[3] M. Barat and W. Lichten. Phys. Rev. A 6 (1972), 211.
[4] U. Fano and W. Lichten. Phys. Rev. Lett. 14 (1965), 627.
See more of Poster Session II
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)