Low-Temperature Photoconductivity of Additively Colored KCl

The transient photoconductivity of additively colored crystals of KCl has been investigated at temperatures down to 10°K. The spectral response reflects the structure in optical absorption in the ultraviolet reported by Lüty. Quantum yields are found to be less than 1.0 at 10°K but are much higher in the region of the $L_1$ (3.6 ev) and $L_2$ (4.2 ev) bands than in the $K$ band on the high-energy side of the $F$ band. The beginning of electron collection or saturation effects was found in the case of thin, lightly colored crystals which indicates that electron ranges were in the vicinity of ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^2$/volt at 10°K. Shallow traps effective below 26°K were found in the crystals studied so far. These traps have a large product of concentration times cross section and compete favorably with the $F$ center at 10°K. They can be partly filled following illumination at low temperature with the result that enhanced photoresponse is produced in the red and near infrared.