F center formation in KCl and KBr crystals below 4.2K

The efficiencies of the F center formation, eta sub(F), and of the intrinsic luminescence, eta sub(L), in KCl and KBr crystals have been studied under the irradiation of a pulsed electron beam at temperatures mainly below 4.2K. eta sub(F) at 1.7K was about 1/5 of that at 4.2K, while eta sub(L) was almost a constant value down to 1.7K, suggesting no change of the efficiency of the Vsub(k) type relaxation of the self trapped hole. The decrease of eta sub(F) follows an Arrhenius' equation with a thermal activation energy of --2 MeV and --1 MeV in KCl and KBr crystals, respectively. A tentative mechanism of the F center formation is discussed. (auth.)