A Positron Trapping Study of Molybdenum Irradiated with 2,10, and 40 MeV Electrons

Molybdenum samples of two different impurity levels (one set of a few parts per million and another set of some tens of parts per million concentration) were irradiated at about 60 degrees C with electrons of energies 2, 10, and 40 MeV. Subsequent measurements at 20 degrees C indicated that the average lifetime for positrons trapped in the created vacancies and vacancy clusters increase with irradiation energy from about 190 ps to about 250 ps. It is argued that the former lifetime is due to trapping in monovacancies, whereas the latter results from trapping in a mixture of monovacancies and small vacancy clusters of average size 3-4 vacancies. Rough estimates indicate the concentration of clusters for the 10 and 40 MeV samples to be of the order of 10-50% of the monovacancy concentration.