Beam-foil techniques

Metastable muonium atoms in the 2s state have been produced with a beam foil technique at LAMPF and at the Tri University Meson Physics Facility (TRI-UMF) at Vancouver, Canada. Only moderate numbers of atoms could be obtained. The velocity resonance nature of the electron transfer reaction results in a muonium beam at keV energies. Very difficult and challenging experiments using electromagnetic transitions in excited states, particularly the 2 Si/2 2 Pi/2 classical Lamb shift and 2 Si/2-2 P3/2 splitting could be induced with microwaves. However, the achieved experimental accuracy at the 1.5 % level [18,19,20], does not represent a severe test of theory yet. 

H.J. Andra, Fine structure, hyperfine structure tmd Lamb-shift measurements by the beam foil technique. Phys. Scr. 9,257 (1974)... 

Beam-Foil Techniques. The beam-foil method has been discussed in Sect. 6.1. It is a very general method for measuring lifetimes of atoms and ions. However, the non-selective excitation, leading to cascading decays, places heavy demands on the data analysis and sometimes a detailed study of the different cascade channels is necessary for reliable lifetime evaluations. While the nonselective excitation frequently constitutes a problem, it is also an advantage of the method since a multitude of excited states are populated. For measurements of multiply charged ions in particular, the technique provides unique measurement possibilities where other techniques are not applicable. 

These two isoelectronic sequences clearly illustrate the usefulness of the lifetime measurements made by the beam-foil technique. Since the uncertainty in the beam velocity is often less than 2 per cent, the accuracy of the results for those levels which are free of cascade is usually limited by the photon counting statistics. However, for levels which do show cascade contributions uncertainties arise, especially if the decay rates of the different exponential components are of similar magnitude. It appears that these difficulties were not always realized in some of the early beam-foil experiments. 

This view is supported by the inconsistency which has been discovered between the abundances of elements determined from photospheric absorption lines and those obtained from the intensity of coronal emission lines (Pottasch (1963, 1964)). As shown in Table 10.3,- discrepancies of at least a factor of ten exist for all of the elements in the iron group. In the case of iron this difficulty has now been resolved. Recent measurements of f-values of iron by the beam-foil technique have shown conclusively that previous f-values for iron obtained by the emission method contained serious systematic errors and there has been a consequent revision of the abundance of iron in the Sun toivards the coronal value. 

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