1915] on Radiations from Exploding Atoms 50;] 



is a definite limit to the frequency of the radiation which can l)e 

 excited in a given atom. Theory does not provide us with an answer 

 to this problem, since little is known al)out the conditions of excita- 

 tion, nor even of the nature of such high-frequency vibrations. A 

 study of the frequency of the 7 rays from radio-active substances is 

 of great importance, as it throws much light on this problem. 



As we have seen, the energy of the /3 particle escaping from the 

 nucleus of radium C is equivalent to that acquired by an electron 

 moving in an exhausted space under a potential difference of several 

 million volts. This high-speed electron passes through the electronic 

 distribution in its escape from the atom. Notwithstanding such 

 ideal conditions for the excitation of high-frequency radiations of 

 the atom, the highest frequency in the radiation emitted by radium 

 C is only about twice that obtainable from an ordinary hard X-ray 

 tube excited by 100,000 volts. It thus appears probable that there 

 is a definite limit to the frequency of the radiation obtainable from 

 a given atom, however high the speed of the disturbing electron. 

 This limiting frequency is determined not by the speed of the electron 

 but Vjy the actual structure of the atom. Since the y radiation from 

 radium C gives a line spectrum, it would appear that the highest fre- 

 quency obtainable is due to a definite system of electrons which is set 

 into characteristic vibration by the escape of a ^ particle. In order to 

 throw farther light on this point, Prof. Barnes, Mr. H. Richardson 

 and myself have recently made experiments to determine the maxi- 

 mum frequency obtainable from an X-ray tube for different constant 

 voltages. The Coolidge tube, which has recently been ptit on the 

 market, is ideal for this purpose, as it provides powerful radiation at 

 any desired voltage. The anti-kathode is of tungsten of atomic 

 weight 181, so that we are dealing in this case with the possible 

 modes of vibration of a heavy atom. The maximum frequency of 

 the radiation was deduced by measuring the absorption by aluminium 

 of the most penetrating rays emitted at different voltages. The 

 absorption of X-rays of different frequencies Ijy aluminium has been 

 examined over a very wide range, and can be expressed by simple 

 formula. It was found that for 20,000 volts the frequency of the 

 radiation was slightly lower than that to be expected if Planck's 

 relation held. With increasing voltage there is a rapid departure 

 from Planck's relation. The frequency reaches a maximum at 

 about 115,000 volts, and no increase was observable up to the 

 maximum voltage employed, viz. 175,000 volts. The experiments 

 thus show that the frequency of radiation reaches a definite maximum, 

 which is no doubt dependent on the atomic weight of the particular 

 radiator employed. It is of interest to note that the maximum 

 penetrating power of the X-rays from the Coolidge tube in aluminium 

 is about the same as the y rays from radium B, but is about three- 

 tenths of the y rays from radium C. There is evidence which suggests 

 that the very penetrating y rays from raditim C correspond to the octave 



