Nucleation of Helium clusters in metals studied by positron annihilation combined with ion beam analysis and temperature programmed desorption

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Within the planned project we want to explore the underlying physics of the helium cluster nucleation process in molybdenum and tungsten. The main goal of the project is to reveal the role of lattice defects for He induced cluster nucleation, and to determine experimentally the binding energies of helium atoms at the simplest trapping site, i.e. a mono-vacancy with a low helium occupancy level, for the two model systems molybdenum and tungsten.The nucleation of He clusters and their growth to bubbles is known to occur in most metals as soon as a sufficient helium concentration and temperature is reached. This can either happen due to helium atoms entering the metal, although here a fairly high energy barrier of up to several eV needs to be overcome, or as consequence of alpha decay inside the metal. Since this occurs in nuclear fission and fusion devices, the phenomenon has been studied extensively in the context of nuclear engineering. Therefore, the implications of helium cluster growth for the macroscopic properties of many reactor relevant materials are well understood, but an experimentally confirmed model describing the He cluster nucleation on an atomic scale is not available so far. In the last decade an increasing number of computational studies have come to the conclusion that a combination of two mechanisms, namely helium self-trapping and trap-mutation is required to explain He cluster nucleation and growth. However, this hypothesis has not been tested experimentally. Within the scope of this project we aim to deliver this experimental test. We plan to prepare samples with a well-defined defect type and concentration, which will be subsequently characterized using the coincidence Doppler broadening spectrometer (CDBS) and positron annihilation lifetime spectroscopy (PALS) at the worlds brightest positron source (NEPOMUC) at FRM II. NEPOMUC is run by the first applicant as expert for positron experiments. These samples will then be implanted ex situ with helium at energies below the displacement damage threshold. Once the proposed upgrade of the CDBS with a He ion source is completed, in situ He loading will be done as well. Ion beam analysis (IBA) (nuclear reaction analysis and elastic recoil detection analysis) will be performed to study the distribution of helium in the near-surface region. Finally temperature programmed desorption (TPD) will give the binding energies of helium atoms at the trapping sites. The results will be correlated with the positron analytics results obtained by CDBS and PALS in order to assign TPD peaks to specific trapping sites. The experimentally obtained binding energies can then be directly compared to the literature values derived in computational studies. The IBA and TPD studies will be carried out in labs run by the second applicant, whose experience in hydrogen-and hydrogen metal interaction will be of great value.