Supplement to Nature 



No. 2808 



AUGUST 25, 1-923 



The Life History of an a-Particle.^ 



By Sir Ernest Rutherford, F.R.S. 



IN this lecture I propose to*' discuss some of the 

 properties of the high-speed a-particle which is 

 spontaneously ejected from radioactive substances. 

 This flying atomic nucleus is not only the most ener- 

 getic projectile known to us, but it is also an agent of 

 great power in probing the structure of atoms, so that 

 an account of the effects produced by it is of wide 

 scientific interest. 



It is now well established that the a-particle expelled 

 from radioactive bodies is in all cases a helium atom, 

 or, to be more precise, the nucleus of a helium atom of 

 mass 4 carrying two positive charges of electricity. 

 It is only when the expelled nucleus is stopped by its 

 passage through matter that it captures the two 

 negative electrons required to convert it into the 

 neutral helium atom. It is natural to suppose that 

 the helium nucleus, which is shot out at great speed 

 from the heavy nucleus of a radioactive atom, formed 

 part of its structure. For some reason, which is not 

 as yet understood, occasionally one of the radioactive 

 nuclei breaks up with explosive violence, ejecting the 

 component helium nucleus with high velocity. It is 

 probable that the a-particle in escaping from the 

 radioactive nucleus acquires part of its great energy 

 of motion in passing through the repulsive electric 

 field surrounding the latter, but at present we do 

 not know the nature of the forces which hold the 

 complex nucleus together, or whether the a-particle 

 is at rest or in orbital motion in the nuclear structure 

 before instability sets in. We know, however, that 

 there is a very wide range of stability exhibited by 

 different radioactive elements. In a substance like 

 radium A, the average life of the radioactive atom 

 before ejection of an a-particle is about 4-3 minutes, 

 for radium itself 2250 years, while in the case of a 

 very slowly changing element like uranium the average 

 life is of the order of 7000 million years. 



It is known that the a-particles from a given element 

 are all shot out with the same speed but that this speed 

 varies from element to clement. There is apparently 

 a close connexion between the velocity of ejection 

 of the a-particle and the average life of the parent 

 clement. The shorter the average life of the element, 

 the swifter is the speed of expulsion. This interesting 



' Discourse delivered at the Royal Institution on Fri<lay, Juno 15. 



relation between the violence of the explosion and the 

 average life of the element holds in the majority of 

 cases, but it is difficult at present to be at all clear of 

 its underlying meaning. Sir William Bragg long ago 

 showed that the a-particle travels through matter 

 nearly in a straight line, and has a definite range of 

 travel in a substance. This is well illustrated by the 

 tracks of a-particles obtained by Wilson's expansion 

 method. The majority of the tracks are seen to be 

 quite straight, apart from an occasional deflexion near 

 the end of the path. At the end of the range the 

 photographic and ionising effects of the a-particle 

 apparently cease with great suddenness. On account 

 of its great energy of motion, the individual a-particle 

 can be detected by the scintillation it produces in 

 crystalline zinc sulphide, by the effect on a photo- 

 graphic plate, and by special electrical methods, while 

 the beautiful expansion method of Wilson shows the 

 trail of each individual u-particle through the gas. 



We are enabled, particularly by the scintillation 

 method, to count the individual particles, and thus we 

 have at our command a method of great delicacy for 

 studying the effects produced by the passage of a-par- 

 ticles through matter. In travelling through a gas 

 the a-particle passes through the outer electronic 

 structure of a large number of atoms and liberates 

 electrons, thus giving rise to an intense ionisation 

 along the track. The ionisation increases to a maxi- 

 mum near the end of the path of the a-particle and 

 then falls rapidly to zero. 



A careful study has been made of the law of decrease 

 of velocity of the a-particle in passing through matter 

 by studying the deflexion in a magnetic field of a pencil 

 of a-particles before and after its passage through a 

 known thickness of matter. In most of these ex- 

 periments we employ the a-particles of radium C, 

 which have a range of about 7 cm. in air under ordinary 

 conditions. The initial velocity Vq of these particles 

 is known to be 19,200 kilometres per second, and the 

 reduction of velocity can readily be followed down to 

 about 0'4 Vfl. At this stage the emergent range of 

 the a-particlcs is less than one centimetre, and measure- 

 ments are difficult, owing to the fact that a beam of 

 a-particles becomes heterogeneous and contains par- 

 ticles moving with different velocities. 



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