**1. Introduction**

The experimental simulation of the harsh radiation environments of future nuclear fission and fusion reactors requires compromises to be made on the selection of bombarding particles and parameters such as flux, energy spectrum, or the production of transmutation elements and the change in the chemical composition of the target material. Additionally, for the characterization techniques employed, it is inevitable to consider the shape of the damage profile, particularly when using low to intermediate energy (tens of keV to hundreds of keV) of charged particles.

Among the numerous analytical techniques used in material irradiation studies [1–3], positron annihilation spectroscopy (PAS) is well known for its spectacular sensitivity to atomic-scale vacancy-type defects, and it has been widely used in the past [4]. This unique sensitivity originates from the fact that positrons are attracted to regions of the lattice with an open volume. The positron, unlike any other particle, acts as a self-seeking probe for vacancy-type defects in condensed matter. Due to its unique features and nondestructive nature, PAS has been recognized as a convenient complementary tool for the microstructural characterization of lattice defects.

**Citation:** Krsjak, V.; Degmova, J.; Noga, P.; Petriska, M.; Sojak, S.; Saro, M.; Neuhold, I.; Slugen, V. Application of Positron Annihilation Spectroscopy in Accelerator-Based Irradiation Experiments. *Materials* **2021**, *14*, 6238. https://doi.org/ 10.3390/ma14216238

Academic Editor: Wen-Tong Geng

Received: 23 August 2021 Accepted: 12 October 2021 Published: 20 October 2021

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The published data refer primarily to irradiation studies of reactor pressure vessel (RPV) steels [5,6]. Despite being a valuable contribution to understanding radiationinduced degradation processes in these materials, the lessons learned cannot be simply extrapolated to the new challenging radiation conditions of future nuclear fission and fusion reactors [7]. The use of fast neutron irradiation experiments to simulate the harsh radiation environments of future reactor systems faces two significant hurdles. While it provides reasonable displacement damage rates in producing collision cascades, it does not provide an adequate simulation of gaseous transmutation products, such as hydrogen or helium, severely impairing the defect recombination processes in the displaced matrix. The second, obvious complication arises from the induced activity of the neutron-irradiated samples; the handling of which requires dedicated nuclear facilities and radioisotope laboratories. Both issues can be conveniently solved by using ion implanters in either single-beam or dual-/triple-beam configurations. Various PAS studies of materials modified by ion bombardment, in a wide range of ion energies and fluences, have been published in the last decade. In addition to the structural materials addressed in this paper, plasma-facing materials [8–11] and nuclear fuel [12–14] have been investigated using this approach.

Comparably to ion irradiation experiments, slow positron beam experiments enable the region of interest within the studied sample to be precisely selected. This makes positron beam techniques natural approaches to studying accelerated radiation damage in materials. Most of the studies use slow positron beams with positron energy of up to a few tens of keV, corresponding to a mean implantation depth in the micrometer range in most nuclear-relevant materials. These studies have generally been aimed at ion implantation experiments using accelerating energies of up to a few MeV. There have been very few irradiation studies published on nuclear structural materials utilizing charged particle irradiation of the target at energies of 10–100 MeV [15–17]. An example of such an implantation (multi-energy He+ implantation of structural steels) is discussed in the paper by Noga et al. in this Special Issue. Moving up with the particle accelerator energy, the irradiation experiments necessarily cause activation of the material; for instance, by spallation reactions [18]. This radiological complication is, however, outweighed by extremely interesting irradiation data, involving realistic and fusion-relevant displacement damage rates, helium and hydrogen production rates, as well as quasi-homogeneous damage distribution over a bulk sample [19]. Concerning PAS, the last feature mentioned enables the convenient use of radioisotope positron sources with a continuous energy spectrum to probe the irradiated material. The comparison between conventional (radioisotope-based) PAS techniques and slow positron beam techniques is discussed in the next chapter.

While the individual PAS techniques provide excellent reproducibility of the results in various studies of semiconductors and non-metallic solids, the results obtained from the investigations of realistic alloys exposed to dissimilar irradiation conditions can rarely be found to correlate between different research studies. Here, let us omit the fundamental distinction between the near-surface studies, utilizing slow positron beams, and the "bulk" studies, based on unmoderated radioisotope positron sources. Quite substantial discrepancies between the acquired results can arise from the same kind of post-irradiation PAS examination. There are several potential reasons for this, which can be divided into two categories. The first group of issues relates to the apparatus, signal processing, and data processing. In this regard, particular attention must be paid to suppressing the noise signal originating either from transmutation elements or Compton-scattered positron annihilation gamma. While three-detector positron lifetime spectrometers [20] and a coincidence setup of Doppler broadening spectrometers [21] can potentially solve this problem, the deviations in the geometry and activity of the used positron sources, as well as in the isotopic composition of the measured samples, affect the effectivity of this solution. Another source of discrepancy between different PAS characterizations, for instance, ion-implanted samples, comes from the lack of consensus in the data evaluation. While some studies report the peak values of displacement damage and ion concentration data, others report an integral value over the whole implantation profile, or an integral value over the probing

particle stopping profile. It is important to note that the stopping profile of a monoenergetic positron is very broad at high-incident energies (tens of keV), and while the mean stopping depth of the positron can be accounted for the peak region of the ion-modified layer, a substantial amount of signal can come from either the substrate or the thermal spike region.

The first and foremost benefit of using PAS techniques in nuclear materials irradiation experiments is that it allows the detection of the slightest changes in the microstructure, associated either to defect production or defect recombination processes. Positron annihilation characteristics enable a more comprehensive interpretation of conventional TEM analyses and the results obtained from mechanical testing. To produce relevant and reliable complementary information to conventional imaging and destructive methods, numerous aspects need to be considered. This paper addresses the application of PAS techniques in the characterization of materials exposed to different types of irradiation experiments. It reviews recent studies in the field, and provides some empirical support for exploiting the full potential of positron annihilation spectroscopy in nuclear material research.
