Vacancy defects identification in Au 4 MeV irradiated UO₂ using Positron Annihilation Spectroscopy

Marie-lyne Amany1

M Gerardint1,2, G. Carlot2, P. Desgardin1, M-F Barthe1.

1CNRS/CEMHTI, CNRS UPR 3079/CEMHTI, 45071 Orleans, France;

2CEA, IRESNE, DEC, Centre de Cadarache, 13108, Saint-Paul-lez-Durance, France.


Uranium dioxide is the most widely used fuel material. In reactors, 235U fission causes the formation of various defects and gaseous fission products that can interact with each other leading to structural damage and significant evolution of the fuel properties. In order to better understand and quantify the effect of such damage on the structural evolution of the fuel, separated effect experiments are performed based on heavy ions implantations in UO2. In the present study, we deal with the results of experiments performed on Au 4 MeV irradiated samples to simulate the damage induced by fission products in the fuel. In order to identify vacancy defects and characterize their thermal evolution they are investigated using Positron Annihilation Spectroscopy (PAS).

PAS is a non-destructive method allowing to study open volume defects in solids near the surface (from nanometer scale up to 160 µm) using two properties of the positron. First, positron as the anti-particle of the electron can annihilate with it leading to 511 keV gamma rays emission. Secondly, positrons can be trapped in defects. It is complementary to TEM, RBS and DRX measurements. In this way, two types of annihilation characteristics are investigated in this study using two positron annihilation techniques allowing us to characterize defects in materials: Doppler broadening of the annihilation radiation (DB)
and positron Lifetime spectroscopies. The first technique allows to measure the momentum
distribution of electrons which annihilate with positrons. The second gives the positron lifetime which is inversely proportional to the electron density probed by the particle.

In this work, Doppler broadening spectroscopy (DBS) is performed in the first 800 nm under the surface using a slow mono-energetic positron beam. A predominance of defects associated with one uranium vacancy of VU+nVO (n=0-2) type was detected in these irradiated samples. Experimental results suggest also the trapping of positrons in larger vacancy clusters of (2VU+4VO)2- and also in negative ions. In order to provide new elements for the identification of the nature of these vacancy defects, the DBS measurements have been carried out as a function of the sample temperature between 50 and 400 K. To complete the study, some positron lifetime measurements were performed, using fast positrons. The measured lifetimes are compared to calculated lifetimes of vacancy defects available in the literature. Lifetime results provide additional information on the defects evolution depending on the fluence or annealing of such irradiated samples. These results were simulated using various positron trapping models in order to evaluate the proportion and nature of the detected vacancy clusters and
negative ions.

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Thursday – 16th September 2021
Marie-lyne Amany