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

在计划中的项目中，我们希望探索钼和钨中氦团簇成核过程的潜在物理原理。该项目的主要目标是揭示晶格缺陷在He诱导团簇成核中的作用，并通过实验确定钼和钨两种模型体系中氦原子在最简单的俘获位点（即氦占比低的单空位）的结合能。众所周知，在大多数金属中，只要达到足够的氦浓度和温度，He团簇就会成核并长成气泡。这可能是由于氦原子进入金属而发生的，尽管这里需要克服高达几个电子伏特的相当高的能垒，或者是由于金属内部的α衰变。由于这种现象发生在核裂变和核聚变装置中，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已经得到了很好的理解，但迄今为止还没有一个实验证实的模型来描述原子尺度上的He团簇成核。近十年来，越来越多的计算研究得出结论，氦团簇的成核和生长需要两种机制的结合，即氦自俘获和陷阱突变。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。我们计划制备具有明确缺陷类型和浓度的样品，随后将在FRM II上世界上最亮的正电子源（NEPOMUC）上使用巧合多普勒增宽光谱仪（CDBS）和正电子湮灭寿命谱（PALS）进行表征。NEPOMUC由第一个申请者作为正电子实验专家运行。然后将这些样品以低于位移损伤阈值的能量注入氦。一旦用氦离子源对CDBS进行拟议的升级完成，原位He装载也将完成。利用离子束分析（IBA）（核反应分析和弹性反冲探测分析）研究氦在近地表区域的分布。最后，程序升温解吸（TPD）将得到俘获点氦原子的结合能。结果将与CDBS和PALS获得的正电子分析结果相关联，以便将TPD峰分配到特定的捕获位点。实验得到的结合能可以直接与计算研究中得到的文献值进行比较。IBA和TPD研究将在第二名申请者的实验室进行，他在氢和氢金属相互作用方面的经验将很有价值。

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.