['Subsequent', 'bubble', 'enlargement', 'can', 'then', 'only', 'occur', 'either', 'through', 'increased', 'temperature', 'or', 'by', 'radiation', 'induced', 'mechanisms', 'which', 'increase', 'the', 'number', 'of', 'vacancies', 'in', 'the', 'bubble', 'and', 'reduce', 'the', 'lattice', 'strain', '.']