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

在计划的项目中，我们希望探索钼和钨中氦团簇成核过程的基本物理。该项目的主要目标是揭示晶格缺陷对He诱导团簇成核的作用，并通过实验确定氦原子在最简单的捕获位置（即具有低氦占有水平的单空位）的结合能，对于钼和钨这两个模型系统，He团簇的成核和长大成气泡是已知的，在大多数金属中，只要有足够的He，达到浓度和温度。这可能是由于氦原子进入金属，尽管这里需要克服高达几个eV的相当高的能量势垒，或者由于金属内部的α衰变。由于这种现象发生在核裂变和聚变装置中，因此在核工程的背景下对这种现象进行了广泛的研究。因此，氦团簇生长对许多反应堆相关材料的宏观性质的影响是很好理解的，但到目前为止，在原子尺度上描述氦团簇成核的实验证实的模型还没有。在过去的十年中，越来越多的计算研究得出的结论是，两种机制，即氦自捕获和陷阱突变的组合需要解释氦团簇的成核和生长。然而，这一假设尚未得到实验验证。在这个项目的范围内，我们的目标是提供这个实验测试。我们计划准备一个定义明确的缺陷类型和浓度，随后将其特征在于使用符合多普勒展宽光谱仪（CDBS）和正电子湮没寿命谱（帕尔斯）在世界上最亮的正电子源（NEPOMUC）在FRM II的样品。NEPOMUC由第一个申请人作为正电子实验专家管理。然后，这些样品将在低于位移损伤阈值的能量下用氦进行非原位注入。一旦完成了用He离子源对CDBS的拟议升级，也将进行原位He装载。将进行离子束分析（IBA）（核反应分析和弹性反冲检测分析），以研究近地表区域的氦分布。最后，程序升温脱附（TPD）将给出氦原子在俘获位置的结合能。将结果与通过CDBS和帕尔斯获得的正电子分析结果相关联，以便将TPD峰分配给特定的捕获位点。实验获得的结合能，然后可以直接比较计算研究中得出的文献值。IBA和TPD研究将在第二个申请人的实验室中进行，第二个申请人在氢和氢金属相互作用方面的经验将具有重要价值。

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.