Direct evidence for several impurity-interstitial configurations in molybdenum

Abstract

Abstract 3 MeV electron irradiations on Mo57Co Mössbauer samples were performed with various fluences at 100 K irradiation temperature. Using the Mossbauer technique the trapping of interstitials, mobile at irradiation temperature, in different configurations around the 57Co impurities, could be directly observed from the appearance of additional absorption lines in the Mössbauer spectra. Analysing the irradiation dose dependent population of these defect lines, four different stable sites for trapped single interstitials and two di-interstitial sites were found. They are characterized by a nearly unique isomer shift of +0.46 mm/s relative to a substitutional lattice site and large quadrupole splittings ranging from 1.82 to 0.81 mm/s. At higher irradiation doses interstitial clustering at the 57Co impurities was found, resulting in a defect line with 0.39 mm/s quadrupole splitting and a broadened line width. The general trapping behaviour of the Mo57Co system could be described by the unsaturable trap model with a trapping radius of rco/rv = 0.23. The microscopic picture for the population of the observed trapping sites for interstitials is discussed within this model.