Uranium (8 10^9 years) 238.5 -> [alpha]
-> [alpha]
/ Uranium X (35.5 days) (230.5) -> [beta]&[gamma]
-> ([beta]) /
Ionium (5 10^4 to 10^6 years) (230.5) -> [alpha]
/ Radium (2,500 years) 226.4 -> [alpha]
/ Emanation (5.57 days) (222.4) -> [alpha]
/ Radium A (4.3 minutes) (218.4) -> [alpha]
/ Radium B (38.5 minutes) (214.4) -> ([beta]) / Radium C_{1} (28.1 minutes) { (214.4) -> [alpha]
{ -> [beta]&[gamma]
{ / Radium C_{2} (1.9 minutes) { (210.4) -> [beta]&[gamma]
/ Radium D (24 years?) (210.4) -> ([beta]) / Radium E (7.25 days) (210.4) -> [beta]&[gamma]
/ Radium F (Polonium 202 days) (210.4) -> [alpha]
/ Radium G (probably lead) (206.8)
Actinium (?) / Radio-Actinium (28.1 days) -> [alpha]
-> ([beta]) / Actinium X (15 days) -> [alpha]
/ Emanation (5.6 seconds) -> [alpha]
/ Actinium A (0.0029 second) -> [alpha]
/ Actinium B (52.1 minutes) -> ([beta]) / Actinium C_{1} (3.10 mins.) { -> [alpha]
{ / Actinium C_{2} (?) { -> [alpha]
/ Actinium D (7.4 minutes) -> [beta]&[gamma]
/ Actinium E (unknown)
Thorium (4 10^{10} years?) 232.4 -> [alpha](?) / Mesothorium_{1} (7.9 years) / Mesothorium_{2} (8.9 hours) -> [beta]&[gamma]
/ Radiothorium (2.91 years?) -> [alpha]
/ Thorium X (5.35 days) -> [alpha]
/ Emanation (76 seconds) -> [alpha]
/ Thorium A (0.203 second) -> [alpha]
/ Thorium B (15.3 hours) -> ([beta]) / Thorium C_{1} (79 minutes) { -> [alpha]
{ / Thorium C_{2} (?) { -> [alpha]
/ Thorium D (4.5 minutes) -> [beta]&[gamma]
/ Thorium E (unknown)
FIG. 6.--DISINTEGRATION SERIES FOR URANIUM, ACTINIUM, AND THORIUM, AS GIVEN BY SODDY.]
CHAPTER IV
NATURE OF THE ALPHA PARTICLE
Disintegration of the Elements
The remarkable disintegrations related in the last chapter, in which the heaviest known elementary atom--that of uranium (at. wt. 238)--is by successive stages changed into others of lower atomic weight, afford a clue to the nature of the atom and to that goal of the chemist, the final const.i.tution of matter. The composite nature of the atom and some sort of interrelation of the elements had previously been made apparent from a study of the Periodic System and data gathered still earlier, but all attempts at working out a so-called genesis of the elements had proved vague and unsatisfactory.
Identification of the Rays
To get an understanding of the disintegration occurring in radio-active substances, the nature of the rays produced must be known. These rays are the cause of the activity and their emission accompanies the changes or disintegration. They have for the sake of convenience been called the alpha, beta, and gamma rays. The gamma rays have been identified with the _X_ rays discovered by Rontgen and are a form of energy a.n.a.logous to light. The beta rays are particles of negative electricity or electrons. With these, then, we have some degree of familiarity. But what are the alpha rays? An answer to this question should make clearer the character of the changes taking place, and should give some insight into the composition and mechanism of the atom.
The Alpha Rays
It has already been stated that these alpha rays are similar or a.n.a.logous to the ca.n.a.l rays, but this advances the matter very little, as the nature of these ca.n.a.l rays has not been fully determined. The full ident.i.ty with them, if proved, should have an important theoretical bearing.
Alpha Rays Consist of Solid Particles
In the first place, these alpha rays have been found to be made up of solid particles, that is, of what we are accustomed to call matter.
Since it has become more and more difficult to draw a clear distinction between matter and energy, it would perhaps be better to say that these particles appear to have some of the properties. .h.i.therto attributed solely to matter. The best evidence that these particles are of atomic ma.s.s is furnished by their deflection in electric and magnetic fields.
Electrical Charge
It is not of first importance to discuss this or other proofs of the material nature of these particles. That they carry a charge of positive electricity is, however, a fact of very great import. The value of this charge has been carefully determined by a number of investigators working with different sources of the alpha particles and has been found to be 9.3 10^{-10} electrostatic units (.000,000,000,93 e.s.). From the consideration of the charge upon an electron previously obtained by J. J. Thomson and others, it was concluded that the alpha particle carried two unit positive charges; the fundamental unit charge, therefore, is half this value, or 4.65 10^{-10} e.s.
Helium Formed from Alpha Particles
To determine the nature of the alpha particle a crucial experiment was carried out by Rutherford and Royds, which was described as follows:
[Ill.u.s.tration: FIG. 7.--APPARATUS USED IN EXPERIMENT BY RUTHERFORD AND ROYDS.]
A large quant.i.ty of radium emanation was compressed into a fine gla.s.s tube _A_, about 1.5 cm. long. This tube, which was sealed to a larger capillary tube _B_, was sufficiently thin to allow the alpha particles from the emanation and its products to pa.s.s through, but sufficiently thick to withstand atmospheric pressure. The thickness of the gla.s.s wall was in most cases less than .01 mm. On introducing the emanation into the tube, the escape of the alpha particles from the emanation was clearly seen by the scintillations produced at some distance on a zinc sulphide screen. After this test the gla.s.s tube _A_ was surrounded by a gla.s.s tube _T_ and a small spectrum tube _V_ attached to it. The tube _T_ was exhausted to a charcoal vacuum. By means of the mercury column _H_, the gases in the tube _T_ could at any time be compressed into the spectrum tube _V_ and the nature of the gases which had been produced determined spectroscopically. It was found that two days after the introduction of the emanation into _A_ the spectrum showed the yellow line of helium, and after six days the whole helium spectrum was observed. In order to be certain that the helium, coming possibly from some other source, had not diffused through the thin walls of the tube _A_, the emanation was pumped out and helium subst.i.tuted. No trace of helium could be observed in the vacuum tube after several days, showing that the helium observed in the first experiment must have originated from the alpha particles which had been propelled through the thin gla.s.s tube into the outer tube.
Most of the alpha particles are propelled with such force that they penetrate some distance into the walls of the outer tube and some of these gradually diffuse out into the exhausted s.p.a.ce. The presence of helium in the spectrum tube can be detected after a shorter interval if a thin cylinder of lead is placed over the emanation tube, since the particles fired into the lead diffuse out more rapidly than from gla.s.s.
A still more definite proof of the ident.i.ty of the alpha particle with the helium atom was obtained by removing the outer gla.s.s tube _T_ and placing a cylinder of lead over the emanation tube in the open air.
Helium was always detected in the lead after it had remained several hours over the thin tube containing a large quant.i.ty of the emanation.
In order to test for the presence of helium in the lead, the gases present were released by melting the lead in a closed vessel. There can thus be no doubt that the alpha particle becomes a helium atom when its positive charge is neutralized.
Thus the chemist was afforded the experience of the building up of at least one element under his observation, and both the a.n.a.lysis and synthesis of matter have been revealed through the discoveries of radio-activity.
Discovery of Helium
It is of interest at this point to learn something of the history of helium and its occurrence. In 1868 there was discovered by Janssen and Lockyer a bright yellow line in the spectrum of the sun"s chromosphere. Because of its origin the name helium was given to the supposed new element causing it. Later it was found in the spectra of many of the stars, and because of its predominance in some of these they were called helium stars. Its existence on our planet was not detected for nearly thirty years.
In 1895, in connection with the discovery of argon in the atmosphere, a search was made to see if the latter element could be obtained from mineral sources. In a.n.a.lyzing certain uranium minerals Hillebrand had found considerable quant.i.ties of a gas which he took to be a peculiar form of nitrogen. Ramsay made a further examination of the gas coming from these minerals and the spectroscope revealed the yellow line of helium, thus at last proving the presence of this element on the earth. It is known now to be present in thorium minerals, in the waters of radio-active wells, and in minute amounts in the atmosphere.
Its occurrence in every case, in the light of the experiment described above, would seem to be due to the presence of radio-active changes.