470 Scientific Proceedings, Royal Dublin Society. 



Secondary Radiation. 



The secondary radiation from various substances was compared by the 

 usual method of using a compound screen of two or more materials thick 

 enough to stop all the primary j3 rays, and observing how the ionisation 

 current varied with the material which was next the window. It is of 

 course necessary that the individual screens should be thick enough to 

 emit their full secondary radiation. In this way the combined effect due 

 to both Y rays and secondary j3 rays from the screen is directly measured. 

 No appreciable difference could be detected between copper, zinc, brass, 

 iron, tin, or silver. Taking the ionisation current due to y rays and 

 secondary |3 rays from any of these screens as 100, that with an aluminium 

 screen was 105, with leather 3 mm. thick 110, and with lead 112. 

 These results are in accord with those of previous investigators, who have 

 found that the " emergence " secondary radiation is a minimum for an 

 atomic weight of about 80.^ They show that it is best to use some such 

 metal as brass or iron, except where very light screening is required, 

 where aluminium is the most convenient. In this case the presence of 

 copious primary ^S radiation renders the secondary /3 radiation of small 

 importance. Experiments were also made with a brass screen about 3 mm. 

 thick, covered on top with different thicknesses of material of low atomic 

 weight. For small thicknesses visiting-cards (0'040 gram per sq. cm.) were 

 found the most suitable, six of them being almost exactly equivalent to 

 the same mass of leather. For greater thicknesses sheets of leather were 

 employed. It was found that adding successive cards increased the 

 ionisation until the total thickness of card or leather amounted to about 

 0-3 gram per cm. [equivalent to about 3 mm. of tissue]. Further increase 

 caused a slight fall, due to absorption of 7 rays. This apparently suggests 

 that the practice of placing a material such as chamois between the metal 

 screen and the skin is unwise ; but, when allowance is made for the effect 

 of distance, it will be shown later that this is not so. Practical experience 

 confirms this latter view, as it is found that the presence of a sheet of 

 chamois between the screen and the skin reduces the burning of the latter. 



Stopimig Poimr of various Siibstances. 



The results of tests on various materials are shown by the curves in fig. 2, 

 where the ordinates lepresent activity, and the abscissae mass per unit area 

 of screen. 



'E.g. Hackett, Scient. Trans. B.D.S., ix, Series 11. 9, 1909, 



