Section III, 1922 [125] Trans. R.S.C. 



Primary and Secondary 0-Rays 



By J. A. Gray, F.R.S.C. 



(Read May Meeting, 1922) 



In the previous paper in these Transactions, Miss A. V. Douglas 

 has given an account of measurements of the absorption and effective 

 range of the /3-rays of radium E in various substances. Using paper 

 as absorbing material, the writer^ carried out similar experiments in 

 1912, including some on the secondary /3-rays excited in lead by 

 X-rays^ which had been formed by the |3-rays of radium E (the primary 

 /3-rays) in another piece of lead. In this paper an account will be 

 given of these experiments on secondary jS-rays. This term secondary 

 /3-rays is here confined to ;ô-rays excited by X-rays. 



As a source of X-rays the following arrangement was used. A 

 strong preparation of radium {D-\-E) was placed between two plates 

 of lead 0. 1 mm. thick and the lead was covered above and below by 

 plates of graphite 3 mm. thick. The latter precaution was necessary 

 because the lead plates were not thick enough to absorb all the primary 

 /3-rays. Under these circumstances, about 80 per cent, of the rays 

 coming through the graphite have been formed by primary /3-rays in 

 the lead. The remaining rays are 7-rays from radium D. Previous 

 experiments^ had shown that the combined rays have a mass ab- 

 sorption coefficient in lead of 3.88 (for small thicknesses only) and 

 0.074 in carbon. These rays, therefore, excite a very much larger 

 number of secondary j8-rays in a thin sheet of lead than in a corre- 

 sponding sheet of paper. This being the case we can examine the 

 secondary /3-rays from lead in the manner described below. 



To measure their intensity an iron electroscope of 15 cm. cube 

 was used. The bottom of it was cut out and replaced by very thin 

 aluminium leaf, appropriately supported by wires. The source of 

 X-rays was placed below the electroscope, and a sheet of lead foil 

 0.0173 gramme/cm.- placed above it. The foil was first placed just 

 beneath the electroscope, then 9 mm. away and finally 1.7 cm. In 

 this position of the foil sheets of paper, each of mass 0.00877 gramme/ 

 cm.^, were placed above the foil. The reading of the electroscope was 



iGray, Roy. Soc. Proc, Series A, vol. 87, p. 487, 1912. 



*It has been thought preferable to use the term X-rays instead of 7 or secondary 

 7-rays which the writer has used in previous papers. The term 7-rays is here con- 

 fined to 7-rays from radioactive substances. 



'Gray, Roy. Soc. Proc, Series A, vol. 87, p. 489, 1912. 



