IRRADIATION DAMAGE CASCADES AND (n, γ) RECOIL EFFECTS IN LOW-TEMPERATURE IRRADIATED α-TIN MONITORED BY MÖSSBAUER SPECTROSCOPY

Résumé
Des échantillons d'étain α pur ont été irradiés a 4,6 K dans un réacteur nucléaire et des noyaux Sn^119m ont été produits par la réaction Sn¹¹⁸(n, γ) Sn^119m. L'effet Mössbauer a été mesuré aussi bien sans réchauffer l'échantillon qu'après un recuit graduel. Les spectres Mössbauer de l'échantillon utilisé comme absorbant indiquent une transmutation en étain métallique dans la région des cascades de dégâts d'irradiation produits par les neutrons rapides. Les spectres Mössbauer de l'échantillon utilisé comme source montrent que dans la région du processus des reculs (n, γ) il y a d'importants dégâts qui résultent, soit d'un processus de transmutation dans cette région également, soit d'atomes Mössbauer en sites interstitiels. Les dégâts disparaissent lorsqu'on chauffe l'échantillon entre 90 et 160 K.

Abstract
With Mössbauer spectroscopy as a tool for investigating lattice defects it is possible to study macroscopic changes by fission neutrons in absorber experiments and low-energy recoil events from (n, γ)-reactions in source experiments. Tin with the nuclide Sn^119m appears particularly suitable for (n, γ)-recoil investigations because of its long half life of 245 days and because the Sn¹¹⁸(n, γ) Sn^119m reaction does not produce a transmutation to another chemical element. A further advantage is the possibility to get source spectra and absorber spectra with the same sample. Finally, there is some hope to monitor the defect cascades from fast neutrons with the phase transition α-tin to tin metal. In a former experiment Goland [1] has tried to use this phase transition in the same manner, but he has irradiated at 0 °C and found no radiation damage effect. The pure α-tin samples were irradiated at 4.6 K in the Munich research reactor [2]. The Mössbauer effect was measured without warming the samples up as well as after a gradual annealing treatment. After the irradiation at 4.6 K there appears in the source and in the absorber spectra a line broadening and shifting which is interpreted as due to the appearance of an additional quadrupole-split line. The isomer shift and the quadrupole splitting of the new line (2.7 ± 0.1) mm/s and (0.4 ± 0.2) mm/s are close to tin metal (2.56 mm/s, 0.2 mm/s). Figure 1 shows the ratio R of the area of the irradiation induced lines with respect to the total area in the source and absorber experiments as a function of the irradiation dose. A striking feature is the difference in the ratio R between the source and absorber experiments, which definitely shows that damage processes locally correlated to the Mössbauer source atoms (and therefore not dose-dependent) have appeared. They must be caused by the (n, γ) recoil The damage effects disappear completely in an annealing stage at 90-160 K in the source spectra as well as in the absorber spectra.