Infrared Stimulation and Quenching of Photoconductivity in Luminescent Powders

Abstract

Ac measurements of impedance changes in (ZnCd)S fluorescent powders upon addition of infrared radiation (a) simultaneously with the exciting ultraviolet and (b) various periods after excitation by ultraviolet or gamma radiation are described. A decrease (quenching) of conductivity is always observed in the steady state (a), but in case (b) a strong increase (stimulation) is often observed. The simultaneous quenching shows two maxima, one in the 7500 A region, the other about 12 500 A, but the stimulation decreases monotonically with increasing wavelengths. These two effects (quenching and stimulation) are superimposed; quenching can most easily be observed at relatively high conduction electron density, while stimulation is most easily observed long after excitation has ceased. The conductivity stimulation can be observed with most (ZnCd)S phosphors many days after cessation of excitation; thus these materials act as storage phosphors in which, however, the storage properties are observed by conductivity stimulation. Their light stimulability is often small. A specially activated Fonda-type storage phosphor with a very high light stimulability, however, shows only small conductivity stimulation. An explanation of this behavior and the wavelength dependence of the stimulation and quenching in terms of the action of infrared irradiation on trapped and valence-band electrons is given.