Abstract
The present work provides an innovative approach to the near-surface slow-positron-beam (SPB) study of structural materials exposed to ion-beam irradiation. This approach enables the use of variable-energy positron annihilation lifetime spectroscopy (PALS) to characterise a wide range of microstructural damage along the ion implantation profile. In a typical application of the SPB PALS technique, positron lifetime is used to provide qualitative information on the size of vacancy clusters as a function of the positron energy, i.e., the probing depth of the spectrometer. This approach is limited to a certain defect concentration above which the positron lifetime gets saturated. In our experiments, we investigated the back-diffusion of positrons and their annihilation at the surface. The probability of such an event is characterised by the positron diffusion length, and it depends on the density of lattice defects, even in the saturation range of the positron lifetime. Until now, the back-diffusion experiments were reported only in connection with Doppler broadening spectroscopy (DBS) of positron-annihilation radiation. To verify the validity of the used approach, we compared the obtained results on helium-implanted Fe9Cr alloy and its oxide dispersion strengthened variant with the transmission electron microscopy and “conventional” slow positron DBS analysis.
Highlights
Irradiation of bulk specimens in neutron radiation environments of various nuclear reactors inevitably leads to an induced activity of the tested samples
The obtained transmission electron microscopy (TEM) data provided the initial guess values of the layer boundaries as well as the qualitative characterisation of helium bubbles in the range of irradiation conditions, where the helium production is in the same order of magnitude as the concentrations expected in the spallation neutron target
TEM-resolvable (>1 nm) He bubbles were observed in the region of >300 nm below the surface, which corresponds to displacement damage of >2.5 dpa and helium concentration of >1000 appm
Summary
Irradiation of bulk specimens in neutron radiation environments of various nuclear reactors inevitably leads to an induced activity of the tested samples. To minimise the handling of “hot” radioactive materials, research studies are either aimed at small-scale (miniaturised) samples[1] or relatively low neutron uencies.[2,3] To experimentally simulate the neutron environment without the induced activity, neutrons can be effectively replaced in irradiation experiments by charged particles such as protons,[4] alphas[5] or self-ions,[6] respectively. Numerous studies have been published in the last two decades on SPB studies of ion-implanted samples.[9,10,11,12] The experiments were focusing either on positron annihilation lifetime spectroscopy (PALS)[9] or on measurements of the momentum distribution of electrons in a material[10,11,12] using Doppler broadening spectroscopy (DBS) of the annihilation line. There are, certain limitations of the applicability of PAS, the positron lifetime technique used in the studies of
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