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Mössbauer Spectroscopy at Cork

In 1957, Rudolf Mössbauer, in the course of his graduate work at Heidelberg , discovered the effect that is now named after him. He was subsequently (1963) awarded a Nobel prize for this discovery. What was special about his work that warranted this award? Essentially what he developed was a very high resolution spectroscopy ( better than 1 in 1013 resolution!) involving the recoilless, or zero phonon emission of gamma-rays. When there are no lattice vibrations generated during the emission of a gamma ray, the linewidth is determined by the nuclear levels alone. These are typically about 10-8 eV hence the very high resolution. The effect was first observed using an isotope of iridium and as such would perhaps have remained a curiosity. Within a short time, however , the effect was observed using Fe57 and Sn119 and hence the whole area of application increased hugely. The number of papers on the Mössbauer effect published annually in scientific journals now runs into many thousands.

With such high resolution it was possible to measure very small shifts in spectral lines and these experiments were among the first to be carried out. The experiment of Pound and Rebka to verify the gravitational red shift is perhaps the most famous of these.

The principal source of applications arose, however, when it was realised that the resolution available was such that the observation of phenomena associated with the physical and chemical structure of the lattice containing what we will call the Mössbauer isotopes, was now possible. For example, changes in the chemical environment of a gamma-emitting nucleus will result in a very small change in the gamma-ray energy. Again the crystal surroundings of the nucleus can cause electric quadrupole and magnetic dipole splittings of the nuclear levels and hence result in the multiple emission of closely spaced gamma-rays. The magnetic effect is, of course, just the nuclear Zeeman effect. The characteristic energies in all cases are about 10-8 eV and hence, fortuitously, lie just in the region appropriate to the application of Mössbauer spectroscopy. It was this ability to determine structural, magnetic and chemical properties of solids that gave the technique its wide-reaching application and added it to the armoury of the solid state physicist and chemist.

Mössbauer spectroscopy has been conducted in this Department since 1967. Work has been carried out using Eu 151 and Sn119 but the emphasis has been mainly on Fe57 which has the advantage of being present in a huge range of materials, physical, chemical and biochemical. Because of its versatility, research involving the Mössbauer effect has been carried out in conjunction with the Chemistry, Geology and Archaeology departments as well as in the Physics department itself. For example we have examined dust from the Moon, meteorites and pieces of pottery. Currently there are several collaborations going on with the Chemistry Department here as well as with the Cork Institute of Technology.

A particular interest in the Department has been in backscatter Mössbauer spectroscopy involving internal conversion electrons and x-rays. By this means surface effects (a few nm to a few mm) can be observed. We have, using this technique, studied the stresses induced in iron and in steel when machined in different ways. We are currently engaged in studying the time-dependence of the surface magnetism in newly electrodeposited layers of iron on copper.

The technique can be readily used in industrial areas. We have recently carried out an analysis of oxidation products in the boilers at ESB Moneypoint. Here we showed that one could clearly distinguish between Fe2O3 and Fe3O4 as they occurred in the boiler under different conditions. We hope to carry out similar work in identifying the valence state of iron occurring as pollutant in atmospheric dust.