Positron Annihilation Spectroscopy to Characterize Irradiation Induced Vacancy Type Defects in Materials for Nuclear Fission and Fusion

Abstract

In nuclear reactors, materials are submitted to irradiations and modification of their macroscopic properties such as swelling or hardening is observed… It is of first importance to understand the origin of these evolutions and how damage at the atomic scale such as vacancy and interstitials type defects can interact with each other or with solutes and impurities to make the microstructure evolved. So the properties of defects have to be determined.Positron annihilation spectroscopy (PAS) is a well-established technique to characterize materials [1]. Due to its positive charge, positron is sensitive to the local variations of the Coulomb potential in solids. As the anti-particle of the electron, both particles can annihilate leading to the emission of gamma rays with energy depending on the momentum of the electron positron annihilated pair. The detection of these gamma rays arises original information on the defects because positron can be trapped and annihilate in localized state such as vacancies. Two different and complementary annihilation characteristics can be measured. Firstly Positron Annihilation Lifetime Spectrometer (PALS) allows to measure the positron lifetime which depends on the local electron density at the annihilation site. The second characteristic is the momentum distribution of the electron positron annihilated pairs and is obtained by measuring the energy spectrum of the gamma annihilation rays using a Doppler Broadening Spectrometer (DBS). Both characteristics give the signature of the defects and allow determining some of their properties such as their nature, concentration, chemical environment... By using slow positron accelerators which produce monokinetic positrons beams with energy varying from 0.1 to 30 keV it is possible to study the depth profile of defects in thin layers from 0.1 to about 5 µm depending on the density of materials with a resolution of the order of 0.1xdepth. 22Na source based beams [2] or user facilities taking advantages of high positrons flux available around nuclear reactor (FRMII, Garching [3]) or LINAC( as in AIST in Japan [4]) are now available to study defects in materials. PAS has numerous advantages among them, it is non-destructive and can be used for conducting as well as insulating materials crystalline or amorphous. This technique is especially useful to probe vacancy defects in metals for which it is one of the only direct characterization. The annihilation characteristics can be predicted by first principle calculations and these data when available can help to identify defects. Sensitive to the single vacancy, PAS can allow to determine the size of the vacancy clusters up to a maximum of 1 nm approximately in the concentration range from 10 15 to a few 10 18 cm -3 .In nuclear science this technique is very powerful to characterize not only damage induced by irradiation but also to determine some fundamental properties of defects that are required for modeling of the microstructural evolution of materials such as formation, migration, agglomeratio...

Studies on the role of ion mass and energy in the defect production in irradiation experiments in tungsten

“…The τ 1 values of both samples are in agreement with the theoretical value of 105 ± 3 ps, corresponding to the perfect W lattice [27]. The defect (vacancy) density of the recrystallized samples was calculated using the defect model of positrons [28] with a specific trapping rate of ∼4 × 10 14 s −1 [29]. (For the RC1 samples, the vacancy cluster density was found to be about 2.4 × 10 23 m −3 [18] whereas, for the RC2, it was about 5.03 × 10 23 m −3 ).…”

“…where μ is the specific trapping concentration (4 ± 2 ×10 14 s [29]). For the un-irradiated RC1 and RC2 samples the trapping rate was calculated from the lifetime data and was found to be 6 ×10 −6 and 8.4 ×10 −6 , respectively.…”

Studies on the near-surface trapping of deuterium in implantation experiments

“…Positron studies of the self-ion implanted tungsten have shown that the vacancy clusters also can have a different slope compared to defect-free sample in the S-W space [65]. For our experimental conditions, the simulations indicate that the contribution of vacancy clusters to the total vacancy population is negligible.…”

“…The defect lifetime was 245 ± 5.8 ps with 25% intensity which corresponds to 5-6 vacancy-clusters [63]. The vacancy cluster density is calculated by assuming a trapping model of positrons in the defects [64] with a specific trapping rate of ∼4 × 10 14 s −1 [65] and was found to be about 2.4 × 10 23 m −3 .…”