Suppression of charge scattering in Moessbauer experiments using synchrotron radiation

The extremely small ratio of linewidth to energy combined with the character of recoil free absorption and reemission makes the low lying nuclear resonances a unique spectroscopic tool. Further, considering the simplicity of performing Moessbauer experiments, they have become very useful in a wide range of disciplines. [sup 57]Fe has remained the most widely used isotope despite its low natural abundance (2%) because iron is ubiquitous in nature and shows a strong effect. The energy width of this resonance corresponds to a lifetime of 140 nsec of the excited state. Therefore in principle time differential spectroscopy is possible. Using the time differential measurement one is no longer restricted to a nuclear source, in fact a short pulse excitation is favored in such experiments. The concept of using a pulsed source and time gating to isolate the resonant scattering was already suggested in 1962. In 1974 it was noted that synchrotron radiation (SR) was ideally suited to this task. Although the time gating technique is conceptually simple the bandwidth of SR poses significant technical difficulties. In this paper we will discuss the extraction of the nuclear signal from the overwhelming background, which is the central problem in SR-based Moessbauer experiments. Wemore » will only, consider the [sup 57]Fe-isotope, although other isotopes might be of interest in a SR-based experiment. Although SR sources have the disadvantages of creating this high background and being limited in beam-time and location, new and unique opportunities make them very attractive even with these difficulties. For this reason research and development groups at every major SR facility worldwide are increasingly working on Moessbauer experiments, stimulated by the first successful observation of nuclear Bragg scattering (NBS) using SR in 1984.« less