DETECTION OF INDUCED BETA-RAY EMISSION FEOM 
SUBSTANCES EXPOSED TO EONTaEN-RAYS BY 
A PHOTOGRAPHIC METHOD.=^ 
By Lewis Simons, B.Sc.Lond., 
Senior Lecturer in Physics in the University of Cape Town. 
(With Plate XIV.) 
Desiring to overcome the difficulty of having to take very many electro- 
scope readings in the study of the speeds of /^-particles emerging from 
atoms exposed to Rontgen-rays, I have attempted to record the particles 
photographically. The present paper deals with the preliminary results. 
C. T. R. Wilson was the first to photograph the images of the tracks of 
these /^-particles by means of his cloud experiments ; but in the present 
work, as in the case of the photographic study of the emission of /:^-particles 
from radioactive substances,! the particles themselves fall upon the photo- 
graphic plate and produce an impression. 
In some work that I hope to publish shortly an account is given of the 
manner in which the speeds of emergence of /5-particles from ten different 
heavy atoms differ when exposed consecutively to two types of Rontgen-rays 
of wave-lengths 0-56 x 10 ~^ cm. and 0*38 x 10~^ cm. The ultimate object 
of this work is to determine these differences photographically. It was 
found that a copious /3-radiation was emitted by a film of red-lead lightly 
dusted on to a slab of paraffin wax, and that when it was exposed to the 
Rontgen-rays of the shorter wave-length, the most rapid and normally 
directed particles moved over a maximum distance in atmospheric air of 
about 1"0 cm. This maximum range is given by the two expressions 
^mv^ = hv and = ad, where m is the mass of an electron (8 8 x 10""^^ 
h is Planck's constant (6*56 x 10~27 erg. sec), v the frequency of the 
incident Rontgen-rays giving rise to a maximum speed of emergence v of 
the particle from the atom, d the range in normal air, and a a constant for 
* The expenses of this research are partly covered by a grant from the Union 
Government through the Research Committee of the Advisory Board of Industries 
and Science of the Union of South Africa. — L. S. 
t e. g. see the reproduction of Hahn's photographs in ' Rutherford's Radioactive 
Substances and their Radiations' (1913), figs. 70a and b. 
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