4i6 



NATURE 



[July 24, 1919 



the H atom carries a single charge, Darwin showed 

 that its range should be about four times the range of 

 the a-particie. This has been confirmed by experi- 

 ment. Generally, it can be shown that the range of 

 a charged atom carrying a single charge is wm^R, 

 where m. is the atomic weight, and u the ratio of the 

 velocity of the recoil atom to that of the a-particle, 

 and R the range of the a-particle before collision. 

 In comparison of theory with experiment, the 

 results agree better if the index is taken as 2-9 

 instead of 3. If, however, the recoil atom carries 

 a double charge after a collision, it is to be expected 

 that its range would only be about one-quarter of 

 the corresponding range if it carried a single charge. 

 It follows that we cannot expect to detect the presence 

 of any recoil atom carrying two charges beyond the 

 range of the o-particle, but we can calculate that anv 

 recoil atom, of mass not greater than oxygen and 

 carrying a single charge, should be detected beyond 

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

 a single charge the recoil atoms of hydrogen 

 and helium should travel 4 R, lithium 2-8 R, carbon 

 1-6 R, nitrogen 1-3 R, and oxygen i-i R, where R is 

 the range of the incident a-particles. We thus see 

 that it should be possible to detect the presence of 

 such singly charged atoms, if they exist, after com- 

 pletely stopping the o-particles by a suitable thickness 

 of absorbing 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 a-particJes, 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 o-particle with an atom of 

 gold of nuclear charge 79. Assuming that the nucleus 

 of the o-particle and that of the gold atom behave like 

 point charges, repelling according to the inverse 

 square law, it can readily be calculated that, for direct 

 collision, the a-particle from radium C, which is 

 turned through an angle of 180°, approaches within 

 a distance D = 3-6x 10-'" cm. of the centre of the gold 

 nucleus. This is the closest possible distance of 

 approach of the a-particle, and the distance increases 

 for oblique collisions. For example, when the 

 a-particle is scattered through an angle of 150°, 90°, 

 30°, 10°, 5°, the closest distances of approach are 

 i-oi, 1-2, 24, 6-2, 12 D respectively. 



In the experiments of Geiger and Marsden, the 

 number of a-particles scattered through 1;° 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 tlie law 

 of inverse squares holds for distances between 

 3-6x10-'- cm. and 43x10-" cm. in the case of the 

 gold atom. The experiments of Crovi'ther in 19 10 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 bv the scin- 

 tillation method to detect hydrogen atoms set in swift 

 motion by o-particles up to distances about four times 

 the ran^e of the incident o-particle. In Marsden'g 

 experiments a thin-walled glass tube filled with radium 

 emanation served as an intense source of rays. Since 

 the lack of homogeneity of the o-radiation and the 

 absorption in the glass are great drawbacks in makint^ 

 nn accurate study of the laws controlling the produc- 

 tion of swift atoms by impact, I have found it best 

 to use for the' purpose a homogeneous source of 



NO. 2595, VOL. 103] 



radium C by exposing a disc in a strong source of 

 emanation. Fifteen minutes after removal from the 

 emanation the o-rays from the disc are practically 

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

 special arrangements very intense sources of o-radia- 

 tion can be produced in this way, and in the various 

 experiments discs have been used the 7-ray activitv of 

 which has varied between 5 to 80 milligrams" of 

 radium. Allowance can easily be made for the decav 

 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 sulphide screen was mounted outside about 

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

 the insertion of absorbing screens of aluminium or 

 mica. The apparatus was filled with dry hydrogen at 

 atmospheric pressure. The H atoms striking the 

 zinc sulphide screen were counted by means of a 

 microscope in the usual way. The strong luminositv 

 due to the /3-rays from radium C was largely reduced 

 by placing the apparatus in a powerful magnetic field 

 which bent them away from the screen. 



If we suppose, for the distances involved in a col- 

 lision, that the o-particle and hydrogen nucleus mav 

 be regarded as point charges, it is easy to see that 

 oblique impacts should occur much oftener than 

 head-on collisions, and consequently that the stream 

 of H atoms set in motion by collisions should con- 

 tain atoms the velocities of which vary from zero to 

 the maximum produced in a direct collision. The 

 slow-velocity atoms should greatly preponderate, and 

 the number of scintillations observed should fall off 

 rapidly when absorbing screens are placed in the path 

 of the rays close to the zinc sulphide screen. 



.\ surprising effect was, however, observed. L'sing 

 a-rays of range 7 cm., the number of H atoms re- 

 mained unchanged when the absorption in their path 

 was increased from 9 cm. to 19 cm. of air equivalent. 

 After 19 cm. the number fell off steadily, and no 

 scintillations could be observed beyond 28 cm. air 

 absorption. In fact, the stream of H atoms resembled 

 closelv a homogeneous beam of o-rays of range 

 28 cm., for it is well known that, owing to scattering, 

 the number of o-particles from a homogeneous source 

 begin to fall off some distance from the end of their 

 range. The results showed that the H atoms are pro- 

 jected forward mainly in the direction of the o-particles 

 and over a narrow range of velocity, and that few, 

 if anv, lower velocitv atoms are present in the stream.. 



If we reduce the velocity of the o-particle by placing 

 a metal screen over the source, it is found that the 

 distribution of H atoms with velocity changes, and 

 that the ravs are no longer nearly homogeneous. 

 When the range of the o-rays is reduced to yz, cm., 

 the absorption of the H atoms is in close accord with 

 the value to be expected from the theory of point 

 charges. It is clear, therefore, that the distribution 

 of velocitv among the H atoms varies markedlv with 

 the speed of the incident a-particles, and this indicates 

 that a marked change takes place in the distribution 

 and magnitude of the forces involved in the collision 

 when the nuclei approach closer than a certain 

 distance. 



In addition to these peculiarities, the number of 

 H atoms is greatlv in excess of the number to be 

 exoected on the simple theory. For example, for the 

 swiftest o-ravs the number which is able to travel 

 a distance equivalent to iq cm. of air is more than 

 thirtv times greater than the calculated value. The 

 variation in number of H atoms with velocitv of the 

 incident a-particle is also entirelv different from that 

 to be expected on the theory of point charges. The 



