by cc Particles from Radium Emanation, 915 



for the fraction of the a particles scattered, the angle (f> 

 obtained is 150°. Thus, if the original assumptions are 

 correct, the particle would have to be turned back nearly on 

 its own track in order to produce the intensity of the lead 

 radiation observed. The probability 1 in 6000 is a maximum 

 estimate, so that it may be assumed that the collision is 

 practically of the head-on type. The law of force near the 

 nucleus has not been experimentally determined for such 

 collisions with heavy atoms, and perhaps the inverse square 

 law breaks down under such conditions. Rutherford * has 

 shown that in the case of the light atoms these very close 

 collisions happen but rarely, but when they occur the 

 nucleus of an atom like nitrogen is probably broken up 

 under the intense disturbing forces brought into action. 

 Although this does not prove that hard 7 rays are emitted 

 when the a particles collide directly with lead nuclei, it is 

 of great interest to find that with such rough experimental 

 data the order of the intensity expected from theoretical 

 considerations is that actually obtained. If it is granted 

 that such head-on collisions generate hard 7 radiation, the 

 difficulty of explaining the mechanism is still unsolved. 

 The most that can be said of the origin of this radiation is 

 that it is probably emitted when the head-on collision con- 

 dition is fulfilled. It may be due to the vibration of the 

 electrons contained in the structure of the nucleus or to 

 the vibration of the elemental portions of the nucleus.. 

 Rutherford has suggested that the origin of the very hard 

 7 rays of some radio-elements is probably in the nucleus 

 itself, since the frequencies observed are far higher than 

 correspond to the K series. It is somewhat extraordinary 

 that there is practically no hard radiation emitted when the 

 emanation atom explodes with the emission of an a. particle, 

 though it is conceivable that the shock to an unstable system 

 like the emanation nucleus on disintegration is not so violent 

 as that incurred by the stable lead nucleus on collision with 

 an a particle. There is little evidence of any hard radiation 

 being emitted from the emanation itself (see curves in 

 fig. 2). It is difficult to believe that there is no contamina- 

 tion, and this would account for the initial activity observed 

 when paper is the target bombarded. Furthermore, there is 

 nothing to show that the light atoms in the paper are not 

 emitting radiation when bombarded by the a particles, 

 though it is extremely unlikely under the experimental 

 arrangement used. 



* Rutherford, Phil. Mag. xxxvii. p. 537 (1919), 



