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Apparatus for effecting and examining the disintegration of the nitrogen atom.
T, brass tube.
t, t, inlet and outlet tubes for gases.
R, source of a-particles.
a, absorbing layer of silver foil, etc.
S, scintillating screen of zinc sulphide.
M, microscope.
P, pointer.
G, graduated scale for measuring thickness of absorbing air. |
When α-particles, emitted from radium C or other suitable source, pass through hydrogen and some other gases, they produce a few swiftly moving particles which have a range of about four times that of the exciting α-particles, and which can therefore be detected independently of these by the scintillations which they produce on a screen of zinc sulphide. The later publications of Rutherford and his collaborators, and others, leave no doubt that these particles are "protons," or hydrogen nuclei, each bearing unit positive charge, which are ejected from the nucleus of an atom by elastic or end-on collisions with the α-particles. In the case of hydrogen, one collision in about one thousand million produces a swift particle.
The apparatus consisted of a tube, containing the gas under examination, in which is fixed a plate bearing the radioactive preparation, and at one end opposite to this a brass plate, in the centre of which is cut a small opening which is closed by a thin plate of metal, such as silver, aluminium, or iron, whose stopping power for the α-particles lies between 4 and 6 cm. of air. The zinc sulphide screen is placed opposite the opening outside the tube and distant 1 or 2 mm. from the metal covering. A small portion of the screen, about 2 mm. in diameter, is viewed by a microscope which has a magnification of about 40. The tube is encircled by the poles of an electro-magnet, which deflects the swift β-rays or negative electrons away from the screen.
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Absorption of H-particles derived from nitrogen and hydrogen.
Curve 1. Particles from nitrogen forward.
Curve 2. Particles from nitrogen backward.
Curve 3. Particles from hydrogen. |
In this apparatus, hydrogen, and then other elements and compounds have been tested for the emission of swift particles of long range. The following description is quoted mainly from one of Rutherford's papers: When the hydrogen is replaced by dry oxygen or carbon dioxide there are still a few scintillations which were believed to be due to traces of hydrogen. The number of scintillations was increased three to four times by substituting air in the tube, and in a greater proportion by pure nitrogen. When a-particles having a uniform range of 7 cm. of air are employed, the swift particles derived from hydrogen have a range of just under 30 cm., and no scintillations can be detected after absorption by this thickness of air or an equivalent thickness of other absorbing substance. The particles from nitrogen, however, can be detected after an absorption of 40 cm. of air or its equivalent. Those between 30 and 40 cm. cannot, therefore, be ascribed to traces of hydrogen or its compounds which might be present in the nitrogen. These particles are deflected by a magnetic field like the H-particles, with which they appear to be identical in all respects except in velocity and range. Their maximum velocity is 1.6 v, where v is the velocity of the α-particle. Some are ejected in a direction contrary to that of the bombarding a-particles; the maximum backward range being 18 cm. The number emitted in all directions by a million α-particles of range 8.6 cm. is about 20. The same particles can be emitted from the nuclei of other elements (see below), and in particular phosphorus, which gives about the same number as nitrogen under the same conditions, but of a greater range, equal to 65 cm. of air. No element of higher atomic weight than phosphorus was found to give the swift H-particles.
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| Production of swift H-particles from the nitrogen atom (after Rutherford). |
It has been calculated that the chance of liberation of an H-particle from nitrogen is not more than 1/20th of the chance of liberating one from hydrogen in corresponding motion. The a-particle has to penetrate more deeply into a more complicated system of electrons in the former case. That such a penetration may also cause a backward ejection was shown by the simple sketch reproduced from Rutherford's paper. Certain conclusions as to the structure of the atomic nuclei have been drawn from these results. In the first place, it is noticed that H-particles are only liberated from elements the atomic weights of which are given by the series: A=4n+2 or 4n+3 (where "n" is a whole number). In the case of nitrogen, A =4×3+2, and in that of phosphorus, A=4×7+3.
Carbon and oxygen, with atomic weights of exactly 4n, give no particles, and helium also (in which n=1) has never been disintegrated by bombardment with α-particles. In the formation of the helium nucleus great energy is liberated and there is loss of mass, since the 4 protons coalesce to give a mass of 4 instead of 4×1.0077; but in the atoms of atomic weight, 4n+2 and 4n+3, the odd protons are probably more loosely held and may retain their mass. "If it be supposed that the nitrogen nucleus is made up of 3 helium atoms of total mass 12, and 2 hydrogen atoms, the mass of the nitrogen atom should be not 14.00 but more nearly 14.01." In this connection it should be noted that nitrogen contains no mass-spectra isotopes. The tracks of ionised particles may be made visible and photographed as lines of condensed water when the a-particles pass through a vessel of supersaturated water-vapour (method of C. T. R. Wilson). It may be expected that a more complete record of the collisions should thus be obtained. Such photographs have been taken in an automatic form of Wilson's apparatus by a camera which exposed two standard cinematograph films at right angles. Twenty-three thousand photographs were taken, and a few of these showed the expected effect in a very striking manner.
