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 constitution 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 Roentgen and
are a form of energy analogous 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
analogous to the canal rays, but this advances the matter very little,
as the nature of these canal rays has not been fully determined. The
full identity 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
hitherto attributed solely to matter. The best evidence that these
particles are of atomic mass 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 x 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 x 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:
[Illustration: FIG. 7.--APPARATUS USED IN EXPERIMENT BY
RUTHERFORD AND ROYDS.]
A large quantity of radium emanation was compressed into a fine glass
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 pass through, but sufficiently
thick to withstand atmospheric pressure. The thickness of the glass
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 glass tube _A_ was
surrounded by a glass 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 substituted. 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 glass 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 space. 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
glass.
A still more definite proof of the identity of the alpha particle with
the helium atom was obtained by removing the outer glass 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 quantity 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 analysis 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 analyzing certain uranium minerals Hillebrand had
found considerable quantities 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.
Characteristics of Helium
Helium, on account of its chemical inactivity and physical properties,
is classed along with argon, neon, krypton, and xenon in the zero
group of the Periodic System, and forms with them the monatomic, inert
gases. In this class are now placed also the three radio-active gases,
emanating respectively from radium, thorium, and actinium. These are
generally known as radium emanation, thorium emanation, and actinium
emanation. The first mentioned was once called niton. Emanium was the
name originally proposed by Giesel for the body now known as actinium.
The calculated rate of production of helium in the series in
equilibrium with one gram of radium is 158 cubic millimeters per year.
This corresponds quite well with the experimental results.
Table of Constants
Some of the more important atomic and radio-active constants are given
in the following table. They are recorded here to show how helpful the
study of radio-activity has been in working out the composition of
matter, and to give some idea of the magnitude of the numbers and the
minuteness of the quantities dealt with.
Electric charge carried by each H atom in
electrolysis 4.65 x 10^{-10} e.s.[1]
Electric charge carried by each [alpha]
particle 9.3 x 10^{-10} e.s.
Number of atoms in 1 gram of H 6.2 x 10^{23}
Mass of 1 atom of H 1.6 x 10^{-24} gram
Number of molecules per cc. of any gas at
standard pressure and temperature 2.72 x 10^{19}
Number of [alpha] particles expelled per
second per gram of radium itself 3.6 x 10^{10}
Number of [alpha] particles expelled per
second per gram of radium in
equilibrium with its products 14.3 x 10^{10}
[1] The expression 10^{-10} means multiplying by .000,000,000,1;
10^{10} means multiplying by 10,000,000,000.