F Center Formation in KCl and KBr Crystals below 4.2 K

The efficiencies of the F center formation, η F , and of the intrinsic luminescence, η L , in KCl and KBr crystals have been studied under the irradiation of a pulsed electron beam at temperatures mainly below 4.2 K. η F at 1.7 K was about 1/5 of that at 4.2 K, while η L was almost a constant value down to 1.7 K, suggesting no change of the efficiency of the V k type relaxation of the self trapped hole. The decrease of η 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.