Coulomb excitation and recoil

Second, the ability of source experiments to use Mossbauer parent atoms permits many more elements to be used than in absorption experiments by employing the excited Mossbauer ievei. The nature of an experiment will determine how Mossbauer 7-emitters are introduced into a sample. Using an energetic Rl beam, it is possible to implant Mossbauer probes to depths of several hundred micrometers and with straggling widths as wide as several tenths of a micrometer. The measurement duration can be controlled by using a probe nuclide with an appropriate half-life for example, for Mn, the Mossbauer effect can be observed for a few minutes after implantation, whereas an in-beam experiment using Coulomb excitation and recoil implantation has a duration of only several hundred nanoseconds after implantation. 

The in-beam Mossbauer technique combining Coulomb-excitation and recoil-implantation, which was described in the Sect. 6.3.1, provides a unique feature for studying the anomalously fast diffusion, i.e., one-by-one measurement Every y-ray emission from Fe follows the implantation process. As a consequence, in the lattice the Fe probe always remains fully isolated from other Fe atoms implanted before, and therefore, the spectrum obtained with this method is completely free from overlapping cascades as well as from clustering of Fe atoms. Both of them would change completely the diffusion properties of Fe atoms. This method, therefore, guarantees an experimental condition under which we can follow a few jumps of Fe atoms immediately after the implantation into anomalously fast diffusion systems, such as a-Zr, Sc, and Pb. 

Wender and Hershkowitz [237] used the sensitivity of the recoil-free fraction in tungsten Mossbauer spectroscopy to deduce the effect of irradiation of tungsten compounds by Coulomb excitation of the resonance levels (2 states of I82,i84,i8 y with 6 MeV a-particles. While no effect of irradiation on the/-factors could be observed for tungsten metal in agreement with [233], a decrease of/was measured for WC, W2B, W2B5, and WO3 after irradiation. 

Similar situations arise, for example, in Coulomb excitation reactions. In the Ge case, the low Debye temperature of the Ge metal produces a very low recoil-free fraction. As mentioned in more detail later (p. 109), it is possible to displace the excited atoms completely out of the target material and implant them into a new matrix with a high Debye temperature, thereby obtaining a considerable improvement in the quality of the spectra. 

Very similar results were obtained using Coulomb excitation of a nickel foil target by 25-MeV oxygen ions [3], and the Coulomb-recoil implantation technique has also been demonstrated [4]. 

Coulombic excitation of Hf and °Hf by 6-MeV a-particles in targets of HfC and HfN at 78 K has shown considerable line broadening which is not a feature of these materials used as absorbers [34]. It may be presumed that the effect is a result of radiation damage associated with the recoil of the excited nuclei. The latter come to rest at lattice sites with one or more lattice vacancies nearby which generate an electric field gradient at the nucleus. 

For the measurement of lifetimes of excited states, many efficient methods have been elaborated. Some of the direct methods are electronic timing, recoil distance method, Doppler shift and blocking techniques, yX-ray coincidences and the indirect methods Coulomb excitation, (e,e ) reactions, resonance fluorescence, particle resonance spectroscopy, etc. The methods were reviewed in Berlovich et al. (1972), Nolan and Sharpey-Schafer (1979), FInyes (1986), and others. Using these techniques, it is possible to measure lifetimes in a very wide (>10 s) domain. 

This chapter describes some past and current topics in in-beam Mossbauer spectroscopy. There have been a wide variety of application-related studies of in-beam Mossbauer spectroscopy in conjunction with nuclear reactions. Coulomb excitation, recoil implantation, and short-lived Rl beams. New in-beam measurement methods and the introduction of... 

New experiments on Fe in Si with the in-beam Mossbauer spectroscopy technique were performed at the Hahn-Meitner institute in Berlin. The technique is outlined in Sect. 6.3. The results of these Coulomb excitation recoil implantation studies [26] confirmed this picture and led to an unambiguous identification of... 

Now, in a solid matrix we have a Mossbauer probe of Fe with a lifetime of 140 ns, as is shown in Fig. 6.18. The 14.4 keV first excited state of the Fe nucleus can be fed through different processes, such as electron capture from Co, jS-decay from Mn, Mossbauer absorption of 14.4 keV y-ray, and Coulomb excitation. Subsequently the 14.4 keV y-ray will be emitted resonantly without recoil (Mossbauer effect), while in Fig. 6.18, the Fe atom is jumping between... 

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