![]() Baumbach succeeded in blowing such fine tubes very uniform in thickness. This fine tube, which was sealed on a larger capillary tube B, was sufficiently thin to allow the α particles from the emanation and its products to escape, but sufficiently strong to withstand atmospheric pressure. The equilibrium quantity of emanation from about 140 milligrams of radium was purified and compressed by means of a mercury-column into a fine glass tube A about 1.5 cms. The experimental arrangement is clearly seen in the figure. We have recently made experiments to test whether helium appears in a vessel into which the α particles have been fired, the active matter itself being enclosed in a vessel sufficiently thin to allow the α particles to escape, but impervious to the passage of helium or other radioactive products. If one atom of helium appeared for each α particle expelled, calculation and experiment might still agree, and yet the α particle itself might be an atom of hydrogen or of some other substance. For example, it might be argued that the appearance of helium in the radium emanation was a result of the expulsion of the α particle, in the same way that the appearance of radium A is a consequence of the expulsion of an α particle from the emanation. The proof of the identity of the α particle with the helium atom is incomplete until it can be shown that the α particles, accumulated quite independently of the matter from which they are expelled, consist of helium. The methods of attack on this problem have been largely indirect, involving considerations of the charge carried by the helium atom and the value of e/m of the α particle. In particular, the good agreement between the calculated rate of production of helium by radium and the rate experimentally determined by Sir James Dewar, is strong evidence in favour of the identity of the α particle with the helium atom. The values of several radioactive quantities, calculated on the assumption that the α particle is a helium atom carrying two unit charges, have been shown to be in good agreement with the experimental numbers. The number of α particles from one gram of radium have been counted, and the charge carried by each determined. ![]() In recent papers, Rutherford and Geiger have supplied still further evidence of the correctness of this point of view. The experimental evidence collected during the last few years has strongly supported the view that the α particle is a charged helium atom, but it has been found exceedingly difficult to give a decisive proof of the relation. The Nature of the α Particle from Radioactive Substances Įrnest Rutherford & Thomas Royds, Philosophical Magazine 17, 281-286 (1909) It does, however, illustrate the simple and careful experimental methodology of Rutherford. (I would say "helium nucleus," but Rutherford had not yet discovered the nucleus.) In importance to the development of understanding about the atom, this paper does not rank with those cited above. The selection reproduced below represents the final step in the identification of the α particle as a positively-charged helium atom. Rutherford characterized the α particle in work extending over several years with a variety of co-workers. During the World War, Rutherford discovered that some atoms could be induced to fall apart in a process of artificial transmutation. The experiments which found a physically measurable quantity associated with atomic number were also carried out in his laboratory. His correct interpretation of that scattering led to the realization that most of the mass of an atom is concentrated in a tiny core or nucleus thus it is to Rutherford that we owe the nuclear atom and nuclear physics. Large-angle scattering of α particles was first reported in his laboratory. But some of his most important work was still ahead. This work was sufficient to earn Rutherford a Nobel Prize in 1908. With Frederick Soddy, he realized that radioactive decay actually tranforms an atom of one element into an atom of a different element. He worked out the time-dependence of radioactive decay and introduced the term half-life. Thomson's laboratory, Rutherford distinguished between two different forms of radioactivity, alpha (α) and beta (β). No one contributed more than Ernest Rutherford (1871-1937 see portrait at National Portrait Gallery (UK)) to an understanding of radioactivity and its domain, the atomic nucleus, in the early years of the 20 th century. What is the Alpha Particle? Rutherford Elements and Atoms: Chapter 19
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