On the Excitation of Characteristic X-Rays from Light Elements
Proceedings of the Royal Society A
I. I n t r o d u c t io n . 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 sufficient to cause the bombarded element to emit its
... c radiations with measurable intensities. These characteristic radiations were detected, and their intensities measured, by their photo-electric 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 abscissse and the measures of the photo-electric effect 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 A = 57*5 A. and A = 358 A. respectively, and for boron about A = 83'5 A. and A = 505 A. respectively. Hughes' results appear to be the only ones as yet obtained with the element boron, but with carbon other researches have been carried out, and differing values have been found for the critical absorption wave-lengths of the K and L series for this element. Kurth'sf values are A = 42-6 A. (Acc. Pot. = 283 volts) and A = 375 A.