The long period afterglow of pure and activated alkali halides excited with cathode rays has been studied at room and liquid oxygen temperatures. It has been found that the decay follows a power law I ∝ t-n between 10 and 100 seconds. The value of the power constant n lies in general between 0.7 and 1.1 and is dependent on the exciting intensity. The decay rates are different for different parts of the spectrum. An attempt has been made to explain the results by an extension of the existing theory developed for photoluminescence.