Hyperfine Interaction Investigations of Helium Trapping in Metals
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HYPERFINE INTERACTION INVESTIGATIONS OF HELIUM TRAPPING IN METALS
H. DE WAARD Laboratorium voor Algemene Natuurkunde,
University of Groningen,
The Netherlands
ABSTRACT Helium decorated vacancies may be trapped to a high degree and with a high activation enthalpy at impurities in metals. This effect is studied by means of the hyperfine of 1 the radio-active impurities ttt 1 1 interaction 1 19 9 In Cd, ' Sb gSn and 12 9 mTe , 1291, implanted into a number of f.c.c. metals and into iron and cobalt. Large changes of the magnetic hyperfine field are caused at 11gSn impurities in Ni, Co and Fe and hiqh values of the electric quadrupole interaction strength 111 are found at Cd impurities in Al, Cu, Ag, Ni and Pt when helium decorated vacancies are trapped at the impurities. These effects provide a new method for studying the trapping and desorption mechanism of helium in metals. INTRODUCTION The presence of helium in a metal has a strong influence on its mechanical properties. Studies of heliumxembrittlement are important in connection with the development of suitable materials for fusion reactors. In order to understand the observed effects in a more than phenomenological manner, we must try to obtain a microscopic insight in the defect structures produced in a metal by the penetration of helium and in the trapping and detrapping of helium by such defects. Also it is important to study the influence of the presence of impurities in the metal, because these may sensitively affect the helium trapping and detrapping stages. In general, helium may be trapped by isolated vacancies, vacancy clusters, vacancies associated with impurities or at grain boundaries, If none of these defects were present, the helium would rapidly evaporate from most metals through interstitial diffusion already below room temperature. One useful technique that has been employed to study the behaviour of helium in metals is by measuring its desorption from the metal as a function of temperature. As discussed, for instance, by Kornelson and Edwards [i] and by Reed [2],this desorption often occurs at a number of discrete, sharply defined temperatures, each corresponding to a particular vacancy-helium cluster. Systematic studies have allowed the identification of several of such clusters. We have recently started to explore another method for studying the behaviour of helium in metals, namely by measuring the hyperfine interaction of impurities with associated vacancies into which helium may be trapped. To this end we have implanted radioactive impurities, suitable for time differential perturbed angular correlation spectroscopy (DPAC) or for Mtssbauer spectroscopy (MS) into various metals and we have looked for changes of their hyperfine interaction patterns after helium doping and subsequent annealing. Striking effects were first found in the DPAC spectra of sources of "'In implanted in Ni and Cu which were post-implanted with helium [3]. When the helium dose is high enough, helium decorated vacancy clusters are formed with close to 100% effid
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