ABSTRACTS 317 



On the Excitation of Characteristic X-rays from Light Elements 



By Professor J. C. McLennan, F.R.S., and 

 Miss M. L. Clark, B.A., 

 of the University of Toronto 



(Read at May Meeting, 1922) 



A hstract 

 I. Introduction 



In an attempt to fill up the gap between the shortest ultra- 

 violet light waves hitherto produced and the longest X-ray waves 

 known, Hughes^^ recently made a study of the characteristic X-rays 

 emitted by carbon and by boron when bombarded by electrons. 

 In this investigation the energy of the bombarding electrons was 

 increased by steps, and the critical values were determined that were 

 necessary and just sufificient to cause the bombarded element to 

 emit its characteristic radiations with measurable intensities. These 

 characteristic radiations' were detected and their intensities measured 

 by their photoelectric action on an insulated electrode of nickel or of 

 silver. 



The method followed by Hughes in recording his results was 

 to plot curves with the values of the accelerating potentials of the 

 electrons as abscissae and the measures of the photoelectric efifect 

 divided by the corresponding electronic currents as ordinates. At 

 certain critical accelerating voltages it was found that these curves 

 showed marked and abrupt kinks or changes of curvature and these 

 changes were taken to connote the beginning of the emission by the 

 bombarded element of its characteristic radiations. 



By following this method he found two definite breaks in his 

 curves for both carbon and boron, and these were taken by him to 

 correspond to the critical K and L absorption wave-lengths for the 

 two elements. For carbon the breaks occurred at 215 volts and 34.5 

 volts and for boron at 148 volts and 24.5 volts. 



This would mean that the critical absorption wave-lengths of 

 the K and L X-ray series for carbon were about X 57.5^ and X 358^1 

 respectively, and for boron about X83.5yl and X505^ respectively. 



Hughes' results appear to be the only ones as yet obtained with 

 the element boron, but with carbon other researches have been 



iHughes, Phil. Mag., Jan., 1922, p. 145. 



