Mixed Silver Research Articles

Luminescence of Silver Bromoiodide Crystals

Crystals of simple and mixed silver halides are known to show a strong fluorescence but little afterglow at low temperatures. However, the slow buildup of luminescence under weak excitation indicates the presence of effective trapping processes. These processes have been studied in more detail on silver bromoiodide crystals. The lifetimes of the trapping states were estimated by observing the dependence of buildup rates on the dark interval between two exposures. The trapping states are also revealed by the effects of infrared radiation. If applied during excitation, it reduces the stationary brightness; if it is applied in the interval between exposures, it reduces the rate of re-excitation. The pattern of behavior of these phosphors is interpreted by a model of the type generally used in the description of zinc sulfide and similar photoconducting phosphors. If the absence of afterglow is taken into account, the model requires that the efficiency of the phosphor should increase with the square root of the exciting intensity. This has been confirmed experimentally over a range of 3 powers of 10. An afterglow can be produced in the silver bromoiodide by incorporating certain divalent cations (such as ${\mathrm{Cd}}^{++}$ or ${\mathrm{Sr}}^{++}$) into the crystal. From an extension of the model to this more complex system, a number of emission and stimulation effects can be deduced which have been found in a series of qualitative observations.

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It seems probable that the fluctuations in plant growth in northern Scandinavia to which Dr. Hustich directs attention are examples of a general phenomenon which will prove to be traceable over the whole of the North Temperate region. Two further examples are shown, in the accompanying figure. The first curve represents the mean diameter increment of eight old trees of oak (Quercus sessiliflora Sals.) growing in unenclosed woodland near Brook in the New Forest, Hampshire, England. Exactly the same trend is shown by rather younger oak (about two hundred years old) in adjacent enclosures, and by pedunculate oak (Q. robur L.) in other parts of the New Forest. The second curve represents the volume increment of mixed silver fir–spruce forest in the Swiss Jura1, in silves per hectare, between successive enumerations made at intervals of approximately five years. (A ‘silve’ is a volume of one cubic metre estimated from the diameter of the trunk and the known average diameter–volume relationship.)

Estimation of the halogens in mixed silver salts

Estimation of the halogens in mixed silver salts