NOVEMBEE 21, 1919] 



SCIENCE 



469 



recoil atom carries a double charge after a 

 collision, it is 1o be expected that its range 

 would only be about one quarter of the corre- 

 sponding range if it carried a single charge. 

 It follows that we can not expect to detect the 

 presence of any recoil atom carrying two 

 charges beyond the range of the a-particle, 

 but we can calculate that any recoil atom, of 

 mass not greater than oxygen and carrying a 

 single charge, should be detected beyond the 

 range of the a-particle. For example, for a 

 single charge the recoil atoms of hydrogen 

 and helium should travel 4 E, lithium 2.8 E, 

 carbon 1.6 E, nitrogen 1.3 E, and oxygen 1.1 

 E, where E is the range of the incident a-par- 

 ticles. We thus see that it should be possible 

 to detect the presence of such singly charged 

 atoms, if they exist, after completely stopping 

 the d-particles by a suitable thickness of ab- 

 soi'bing material. This is a great advantage, 

 for the number of such swift recoil atoms is 

 minute in comparison with the number of a- 

 particles, and we could not hope to detect them 

 in the presence of the much more numerous 

 a-particles. 



In order to calculate the number of recoil 

 atoms scattered through any given angle from 

 the direction of flight of the oc-particles, it is 

 necessary, in addition, to make assumptions as 

 to the constitution of the atoms and as to the 

 nature and magnitude of the forces involved 

 in the collision. Consider, for example, the 

 case of a collision of an ct-^particle with an 

 atom of gold of nuclear charge 79. Assuming 

 that the nucleus of the a-particle and that of 

 the gold atom behave like point charges, re- 

 pelling according to the inverse square law, it 

 can readily be calculated that, for direct col- 

 lision, the a-particles from radium C, which is 

 turned through an angle of 180°, approaches 

 within a distance D ^ 3.6 X 10"^- cm. of the 

 center of the gold nucleus. This is the closest 

 possible distance of approach of the a-particle, 

 and the distance increases for oblique collis- 

 ions. For example, when the a-particle is 

 scattered through an angle of 150°, 90°, 30°, 

 10°, 5°, the closest distances of approach are 

 1.01, 1.2, 2.4, 6.2, 12 D respectively. 

 In the experiments of G-eiger and Marsden, 



the niunber of a-particles scattered through 

 5° was observed to be about 200.000 times 

 greater than the number through 150°. The 

 variation with angle was in close accord with 

 the theory, showing that the law of inverse 

 squares holds for distances between 3 6X10"" 

 cm. and 4-3X10"^^ cm. in the case of the 

 gold atom. The experiments of Crowther in 

 1910 on the variation of scattering of /?-rays 

 with velocity indicate that a similar law 

 holds also in that case, and for even greater 

 distances from the nucleus. 



We have seen that Marsden was able by 

 the scintillation method to detect hydrogen 

 atoms set in swift motion by a-particles up 

 to distances about four times the range of the 

 incident a-particle. In Marsden's experi- 

 ments a thin-walled glass tube filled with 

 radium emanation served as an intense source 

 of rays. Since the lack of homogeneity of 

 the a-radiation and the absorption in the 

 glass are great drawbacks in making an ac- 

 curate study of the laws controlling the pro- 

 duction of swift atoms by impact, I have 

 found it best to use for the purpose a homo- 

 geneous source of radium C by exposing a 

 disc in a strong source of emanation. Fifteen 

 minutes after removal from the emanation the 

 a-rays from the disc are practically homo- 

 geneous, with a range in air of 7 cm. By 

 special arrangements very intense sources of 

 a-radiation can be produced in this way, and 

 in the various experiments discs have been 

 used the 7-ray activity of which has varied 

 between 5 to 80 milligrams of radium. Allow- 

 ance can easily be made for the decay of the 

 radiation with time. 



In the experiments with hydrogen the 

 source was placed in a metal box about 3 cm. 

 away from an opening in the end covered by 

 a thin sheet of metal of sufficient thickness 

 to absorb the a-rays completely. A zinc sul- 

 phide screen was mounted outside about 1 

 mm. away from the opening, so as to allow for 

 the insertion of absorbing screens of alumi- 

 nium or mica. The apparatus was fQled with 

 dry hydrogen at atmospheric pressure. The 

 H atoms striking the zinc sulphide screen 

 were counted by means of a microscope in the 



