Page:Radio-activity.djvu/186

 A simple method of determining the absorption in gases is shown in Fig. 36. The maximum current is measured between two parallel plates A and B kept at a fixed distance of 2 cms. apart, and then moved by means of a screw to different distances from the radio-*active surface. The radiation from this active surface passed through a circular opening in the plate A, covered with thin aluminium foil, and was stopped by the upper plate. For observations on other gases besides air, and for examining the effect at different pressures, the apparatus is enclosed in an air-*tight cylinder.

Fig. 36.

If the radius of the active surface is large compared with the distance of the plate A from it, the intensity of the radiation is approximately uniform over the opening in the plate A, and falls off with the distance x traversed according to an exponential law. Thus

I/I_{0} = e^{-[Greek: lambda]x},

where [Greek: lambda] is the "absorption constant" of the radiation for the gas under consideration. Let

x = distance of lower plate from active material, l = distance between the two fixed plates.

The energy of the radiation at the lower plate is then I_{0}e^{-[Greek: lambda]x}, and at the upper plate I_{0}e^{-[Greek: lambda](l + x)}. The total number of ions produced between the parallel plates A and B is therefore proportional to

e^{-[Greek: lambda]x} - e^{-[Greek: lambda](l + x)} = e^{-[Greek: lambda]x}(1 - e^{-[Greek: lambda]l}).

Since the factor 1 - e^{-[Greek: lambda]l} is a constant, the saturation currentparticles coming from all points of the large radio-active layer, [Greek: lambda] is not the same as the coefficient of absorption of the rays from a point source. It will however be proportional to it. For this reason [Greek: lambda] is called the "absorption constant."]