Sometime later, Rutherford showed that we have in the swift a-particle a localization of energy intense enough to effect a disruption of the nucleus. The experiments were of great im portance not only as advancing knowledge of the constitution of the nucleus, but also as the first in which a change in the chemi cal nature of an atom has been deliberately provoked. (See TRANSMUTATION OF ELEMENTS.) As a preliminary to the description of Rutherford's method it may be well to consider the impact of an a-particle on a very light nucleus. Whereas in the case of heavy nuclei (the nucleus of a gold or of a silver atom, say) we can neglect the motion of the struck nucleus, in the case of a hydrogen nucleus we must devote special attention to the motion produced by the collision. It is easy to show that the velocity u of the struck nucleus will depend upon the masses M and m of the a-particle and struck nucleus respectively, and upon the angle 0 which the path of the nucleus makes with the original path of the a-particle, whose velocity is v. If there is no loss of energy in the collision it is given by the expression: which becomes 1.6 v cos0 when the struck nucleus is a proton (M=4, in= I ). It can further be shown that the range of a proton in a given gas is nearly the same as (actually a little less than) that of an a-particle of the same initial velocity, the effect of the reduced mass and of the reduced charge nearly annulling one another in this respect. It is an established result that the range of an a-particle is proportional to the cube of its velocity, whence for a direct impact (0 =o) the range of a struck proton is about 4.1 times the range of the a-particle itself in the same gas. With an a-particle from Radium C, whose range in hydrogen is 31 cm., the range 'of the proton which it strikes cen trally should be 117 cm. in hydrogen. This result makes it possible to distinguish easily between struck protons and a-par ticles themselves, since the greater range of the former allows them to be detected much further from the source than the latter. It is only the full collisions that lead to so large a range: protons struck at glancing angles have, by equation (2), smaller velocities, and hence smaller ranges. A connection between range and angle can be calculated for equation (2), and this distribution is actually confirmed for slow a-particles. It is a significant obser vation, however, that for swift a-particles passing through hydro gen far more protons are thrown directly forward than the formula indicates. It was this consideration that first led Rutherford to assume the plate-like form of the a-particle in close collision, to which reference has been made.
The great range of the light proton struck by the heavier a-particle is the key to Rutherford's first experiments on the dis ruption of the nucleus. The apparatus used is represented in fig. 4. The brass disc D bears a deposit of radium C, which is the source of the a-particles. This is contained in a brass box AA which can be exhausted or filled with any gas. An opening S in one end is closed by a thin metal foil, which, as far as its power of stopping a-particles is concerned, is equivalent to about 5 cm. of air. The particles which pass through this foil, and any other foils which may be placed between S and F, produce scin tillations on the phosphorescent zinc sulphide screen F. These scintillations are observed with the microscope M. With this apparatus filled with dry hydrogen various results on the range of the struck protons were obtained, of which mention has already been made.
Long-range protons were also obtained from hydrogen com pounds, such as paraffin wax in thin films. The particles were identified as protons not only by their range, but also by deflec tion in an electric and magnetic field. More recently Stetter has also shown that the ratio for the particles knocked out of paraffin wax has the value pertaining to a proton, by using as a source a very thin tube of wax containing radium emanation, and employing an apparatus built on the lines of Aston's mass spectro graph. (See POSITIVE RAYS, ISOTOPES.) The mean of his deter minations agrees within 1% with the value to be expected.
When the apparatus of fig. 4 was filled with air, or pure nitrogen (for the effect was soon traced to this gas) particles were ob served whose range in air was not less than 28 cm., which is about four times that of the a-particles themselves, or just what we should expect the range of a struck hydrogen nucleus to be. The particles are not due to hydrogen contaminations, such as films of grease, for their number is proportional to the pressure of the nitrogen. Further, when oxygen or oxygen compounds, such as carbon dioxide, are substituted for the nitrogen, nothing but an occasional long-range particle is detected. A large number of control experiments were made, as a result of which it was definitely established that the passage of the a-particles through nitrogen produces particles whose long range gives very strong reason to believe that they must be protons. Rutherford proved that this really was the nature of the particles by deflecting them in a magnetic field.
The only possible source of the long range protons, whose existence was established by these experiments, is the nucleus of the nitrogen atom. We are forced to the belief that the a-particle, when it hits a nitrogen nucleus fair and square, is able to detach from it one of the protons which go to make up its structure, and hurl it in the forward direction. The extreme forward range was later found to be not 28 cm., but 4o cm., which makes the proof of the nuclear origin of the proton even more definite, energy be ing derived from the nucleus itself. More precise information as to the direction in which the expelled proton proceeds was fur nished by subsequent experiments of Rutherford and Chadwick.
Rutherford and Chadwick investigated the passage of a-parti cles through various light elements, with the object of finding out if protons are expelled from their nuclei. (With heavy nuclei this is not to be expected, as the high nuclear charge exerts so large a repulsive force on the a-particle that the close ap proach necessary to detach a proton cannot occur.) Films of solid compounds of the various metals were placed in the path of the a-particles, and a search made for long-range particles. Such particles were detected with the elements boron, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, and potassium, a s well as the original nitrogen, careful controls in each case establishing that the particles were actually protons, and that their source was the element named. The results obtained with these elements indicate the generalisa tion that the nuclei of atoms of odd atomic number are more easily broken, at any rate by a-particles, than those of even atomic number, since with the latter the protons, if present at all, are of comparatively short range.