152 BELL SYSTEM TECHNICAL JOURNAL 



that all of them have the same mass per unit of surface-area. The 

 densities of air at sea-level, of water and of lead stand to each other in 

 about the ratios .0013 : 1 : 11.4; a metre of water is therefore "equiva- 

 lent" in this respect to about nine centimetres of lead or three- 

 quarters of a kilometre of air at sea-level. After measuring ju in a 

 material of density p, one may reduce it to a value approximately 

 equal to that which would be found in a standard material (lead, for 

 instance) of density po, by multiplying it with the factor (po/p). A 

 better approximation yet, for the harder gamma-rays, is the statement 

 that the slabs absorb the same fraction of the gamma-ray beam if the 

 atoms under each unit of surface-area have the same number of bound 

 electrons altogether. Denoting the number of bound electrons per 

 unit volume of the material in question by E, the corresponding 

 number for lead (say) by £o, we find (Eo/E) for the value of the factor 

 by which an observed value of [x is to be multiplied, in order to convert 

 it into one approximately valid for lead.^ To give a definite example, 

 I will mention simply that ten centimetres of lead intercept all but a 

 few per cent of the hardest gamma-ray corpuscles which come from 

 radioactive bodies; and that a couple of kilometres above the earth, one 

 need not worry about the influence of the radioactivity of the ground. - 



So, to avoid ionization of air by the rays of the known radioactive 

 elements at the surface of the earth, one surrounds the air which one is 

 observing by matter more than thick enough to stop all of the alpha 

 and all of the beta-particles, and thick enough to cut ofT all but a few 

 per cent of the gamma-corpuscles. Often this is done by using plates 

 of lead several cm. thick for the walls of the air-chamber. Sometimes 

 it is done by sinking the chamber into a lake, or digging a hole into the 

 ice of a glacier. A much greater thickness of water frozen or liquid is 

 required to intercept the unwanted rays, than of lead; this thickness 

 must surround the air-chamber on all sides, towards the bottom of the 

 lake, towards its edges, even (if there be overhanging mountains, or 

 radioactivity in the air) towards its surface; but water is inexpensive, 

 and does not have to be carried about. Or by going up in a balloon, 

 or sending up the air-chamber in a balloon without oneself attending it, 

 one may put enough air beneath the apparatus to screen away the 

 rays from the ground. 



But are there not radioactive atoms in the very materials used to 



1 For an element of atomic weight A and atomic number Z and density p, the 

 value of E is Zp/Amn, the symbol w// standing for the mass of the hydrogen atom. 

 For lead Z = S2, A = 206, po = 11.4. For a compound of known constitution the 

 formula can easily be worked out; if the constitution is unknown but certainly involves 

 atoms of high atomic number only, one may put Z/A = } o as a rough approximation. 



2 Miliikan in a specific case says that 4.8 cm. of lead intercept 90 per cent of the 

 gamma-rays from igneous rocks {Phys. Rev. (2), 28, p. 862; 1926). 



