The Measurement of f-values for Lines of Astrophysical Interest by Beam-Foil Spectroscopy

Extract

The necessity of having accurate oscillator strengths in astrophysical applications is well known. The apparent discrepancy which existed between the solar and meteoritic abundance of iron is just one example of the problems which can arise from poor f-values. An excellent critique of methods for determining both absolute and relative f-values has been given by Blackwell & Collins (1972). Their comments on life-time techniques provide a clear indication of both the advantages and difficulties associated with these techniques: “In principle, a life-time method, as exemplified by the technique of beam foil spectroscopy, described for example by Wiese (1970), has the fundamental advantage that in some restricted circumstances its application does not depend upon a temperature measurement or any assumption of themodynamic equilibrium in the source: in addition it gives an absolute result without the need of an absolute number density of atoms. The hope is sometimes expressed that the method of beam foil spectroscopy will yield oscillator strengths of the required accuracy. In practice, the technique suffers from the difficulty that although the life-time of an excited state can be measured with reasonable accuracy, it is also necessary to measure in a separate experiment the branching ratios for radiative de-excitation. As these ratios are usually measured by an arc method, the accuracy of the final oscillator strengths is limited by the deficiencies of this source. Also, some atoms in the beam may be excited to higher levels than the one being examined, and because of the nature of the initial excitation is unknown, radiative de-excitation (cascading) takes place to this lower level in a way that is wholly unpredictable. This difficulty is especially important for levels of low excitation.” In this talk techniques will be described for overcoming the cascading problem in beam foil spectroscopy and for measuring the associated branching ratios.