Mr Kleeman, The nature of the ionisation, etc, 169 



The nature of the ionisation produced in a gas by 7 rays. 

 By R D. Kleeman, B.A., D.Sc, Emmanuel College, Cambridge. 

 (Communicated by Professor Sir J. J. Thomson, F.R.S.) 



[Bead 8 March 1909.] 



The ionisation in a chamber placed in the path of y rays may 

 consist of three parts. One part may consist of ions ejected by 

 the 7 rays from the gas molecules with a velocity which is so 

 small that they are unable to produce any further ions themselves. 

 One of the other parts must consist of ions made by the cathode 

 i-ays from the walls of the chamber, and the third part of ions 

 made by the cathode rays of high velocity ejected from the gas 

 molecules. The first of these parts is proportional to the mass 

 of the gas and therefore proportional to the pressure, and this is 

 also true for the second part since the ionisation of a gas by /8 

 rays has been shown to be proportional to the pressure. Since 

 the number of electrons ejected from the gas is proportional to 

 the pressure, and the number of ions each produces proportional 

 to the pressure, the third part is proportional to the square of the 

 pressure. The ionisation in the chamber may therefore be expressed 

 in the form {(A -\- B)p + Cp^}, where p denotes the pressure. 



Laby and Kaye* have shown that the ionisation in air and 

 carbon dioxide is proportional to the pressure over a wide range 

 of pressures. The term Cp^ is therefore small within this range. 

 Experiments on the variation of the ionisation with pressure will 

 not however give any information as to the relative values of 

 A and B, since both the ionisation produced by the secondary 

 cathode radiation from the walls of the chamber and that due 

 to the ejection of slow-moving ions from the molecules of the gas 

 by the 7 rays vary as the pressure. Attempts have been made 

 to obtain an estimate of the amount of ionisation in a chamber 

 which is not due to the radiation from the walls. All these 

 estimates depend on some calculations based on certain assump- 

 tions, generally involving the absorption, ionisation, etc., of the 

 cathode rays produced by the 7 rays. Now the writer has shown 

 in a paper in the course of publication f that the 7 rays give rise 

 to secondary cathode rays of very different velocities, some of 

 these rays have a range in air at atmospheric pressure of the 

 order of 2 cm. while some are as penetrating as the penetrating 

 /3 rays of radium. It cannot be assumed therefore that the 



* Phil. Mag. p. 879, Dec. 1908. 

 t Read before the Royal Society, 



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