Type IIa diamonds were irradiated below 250K with electrons of 1.50, 0.60 or 0.35 MeV. The radiation-induced thermal resistivity is shown to correlate with the concentration of neutral vacancies as determined by the strength of the GR 1 optical absorption band. The production of vacancies was found to saturate at high doses and to be as large as previously observed after irradiation at about 500K.