PROPERTIES OF THE RADIATIONS
The activity of these radio-active bodies consists in the emission of
certain radiations which may be separated into rays and studied
through the phenomena which they cause.
Ionization of Gases
One of these phenomena is the power of forming ions or carriers of
electricity by the passage of the rays through a gas, thus ionizing
the gas. The details of an experiment will serve to make the meaning
of this ionization clear.
[Illustration: FIG. 1.--IONIZATION OF GASES.]
When this apparatus is set up a minute current will be observed
without the introduction of any radio-active matter. This, as
Rutherford says, has been found due mainly to a slight natural
radio-activity of the matter composing the plates. If radio-active
matter is spread on plate _A_, which is connected with one pole of a
grounded battery, and if plate _B_ is connected with an electrometer
which is also connected with the earth, a current is caused which
increases rapidly with the difference of potential between the plates,
then more slowly until a value is reached that changes only slightly
with a larger increase in the voltage.
According to the theory of ionization, the radiation produces ions at
a constant rate. The ions carrying a positive charge are attracted to
plate _B_, while those negatively charged are attracted to plate _A_,
thus causing a current. These ions will recombine and neutralize their
charges if the opportunity is given. The number, therefore, increases
to a point at which the ions produced balance the number recombining.
When an electric field is produced between the plates, the velocity of
the ions between the plates is increased in proportion to the strength
of the electric field. In a weak field the ions travel so slowly that
most of them recombine on the way and consequently the observed
current is very small. On increasing the voltage the speed of the ions
is increased, fewer recombine, the current increases, and, when the
condition for recombination is practically removed, it will have a
maximum value. This maximum current is called the saturation current
and the value of the potential difference required to give this
maximum current is called the saturation P.D. or saturation voltage.
The picture, then, is this. The radiations separate the components of
the gas into ions, or carriers of electricity, half of which are
charged negatively and half positively. In the electric field those
negatively charged seek the positive plate and those positively
charged seek the negative plate. If time is given, these ions meet and
recombine, their charges are neutralized, and there is no current.
Experimental Confirmation
This theory of the ionization of gases has been most interestingly
confirmed by direct experiment. For instance, the ions may form nuclei
for the condensation of water, and in this way the existence of the
separate ions in the gas may be shown and the number present actually
counted.
When air saturated with water vapor is allowed to expand suddenly, the
water present forms a mist of small globules. There are always small
dust particles in air and around these as nuclei the drops are formed.
These drops will settle and thus by repeated small expansions all dust
nuclei may be removed and no mist or cloud will be formed by further
expansions.
If now the radiation from a radio-active body be introduced into the
condensation vessel, a new cloud is produced in which the water drops
are finer and more numerous according to the intensity of the rays. On
passing a strong beam of light through the condensation chamber, the
drops can readily be seen. These drops form on the ions produced by
the radiation.
Application of Electric Field
If the condensation chamber has two parallel plates for the
application of an electric field like that already described, the
ions will be carried at once to the electrodes and disappear. The
rapidity of this action depends upon the strength of the electric
field and experiment shows that the stronger the field the smaller the
number of condensation drops formed. If there is no electric field, a
cloud can be produced some time after the shutting off of the source
of radiation, showing that time is required for the recombination of
the ions.
Size and Nature of Ions
If the drops are counted (there being special methods for this) and
the total current carried accurately measured, then the charge carried
by each ion may be calculated. This has been determined. The mass of
an ion compared with the mass of the molecules of gas in which it was
produced can also be approximately estimated. In the study of these
ions the view has been held that the charged ion attracted to itself a
cluster of molecules which surrounded the charged nucleus and traveled
with it. It is roughly estimated that about thirty molecules of the
gas cluster around each charged ion.
Photographing the Track of the Ray
Utilizing the fact that these ions with their clusters of molecules
form nuclei for the condensation of water vapor, C. T. R. Wilson has
by instantaneous photography been able to photograph the track of an
ionizing ray through air. The number of the ions produced, and hence
the number of drops, is so great that the trail is shown as a
continuous line. In the copy of this photograph it will be seen that
at some distance from its source the straight trail is slightly but
abruptly bent. Near the end of its course there is another abrupt and
much sharper bend. These bends show where the ionizing ray, in this
case an alpha particle, has been deflected by more or less direct
collision with an atom. These collisions and the final disappearance
of the ray will be discussed later.
[Illustration: FIG. 2.--PHOTOGRAPH OF THE TRACK OF AN
IONIZING RAY.]
Action of Radiations on Photographic Plates
Taking up now other means of examining these radiations, it is well to
consider their action upon a photographic or sensitive plate. It will
be recalled that this was the method by which their existence was
originally detected. To illustrate the method, the following account
of how one such photograph was taken may be given.
The plate was wrapped in two thicknesses of black paper. The objects
were placed upon this and the radio-active ore, separated by a board
one inch thick, was placed above. The exposure lasted five days. The
action is much less rapid and the result not so clearly defined as in
the case of photographs taken by _X_ rays. Of course, the removal of
the board and the use of more concentrated preparations of radium
would give quicker and better results. The method, however, on
account of time consumed and lack of definition is ill adapted to
accurate work.
[Illustration: FIG. 3--PHOTOGRAPH OF VARIOUS OBJECTS TAKEN
BY MEANS OF PITCHBLENDE]
Discharge of Electrified Bodies
The radiations from radio-active bodies can discharge both positively
and negatively electrified bodies by making the air surrounding them a
conductor of electricity. To demonstrate this, use is made of an
electroscope. If the hinged leaf of such an instrument be electrically
charged and a radio-active body be brought into its neighborhood, the
electricity will be discharged and the leaf return to its original
position. The rapidity of this discharge is used to measure the degree
of activity of the body giving off the radiation.
[Illustration: FIG. 4.--GOLD-LEAF ELECTROSCOPE.
The gold-leaf _L_ is attached to a flat rod _R_ and is
insulated inside the vessel by a piece of amber _S_ supported
from the rod _P_. The system is charged by a bent rod _CC'_
passing through an ebonite stopper. After charging, it is
removed from contact with the gold-leaf system. The rods _P_
and _C_ and the cylinder are then connected with the earth.]
Scintillations on Phosphorescent Bodies
It was found by Crookes that a screen covered with phosphorescent zinc
sulphide was brightly lighted up when exposed to the radiations. This
is due to the bombardment of the zinc sulphide by a type of ray called
the alpha ray. Under a magnifying glass this light is seen to be made
up of a number of scintillating points of light and is not continuous,
each scintillation being of very short duration. By proper subdivision
of the field under the lens, the number of scintillations can be
counted with close accuracy.
A simple form of apparatus called the spinthariscope has been devised
to show these scintillations. A zinc sulphide screen is fixed in one
end of a small tube and a plate carrying a trace of radium is placed
very close to it. The scintillations can be observed through an
adjustable lens at the other end of the tube. Outer light should be
cut off, as in a dark room. The screen then appears to be covered with
brilliant flashes of light. Other phosphorescent substances, such as
barium platino-cyanide, may be substituted for the zinc sulphide, but
they do not answer so well.
Penetrating Power
By penetrating power is meant the power exhibited by the rays of
passing through solids of different thicknesses and gases of various
depths. This power varies with different radiations and with the
nature of the solid or gas. For instance, a sheet of metallic foil may
be used and the effect of aluminum will differ from that of gold and
the different rays vary in penetrating power. In the case of gases
air will differ from hydrogen, and it is noticed that certain rays
disappear after penetrating a short distance, while others can
penetrate further before being lost.
Magnetic Deflection
If the radiations are subjected to the action of a strong magnetic
field, it is found that part of them are much deflected in the
magnetic field and describe circular orbits, part are only slightly
deflected and in the opposite direction from the first, and the
remaining rays are entirely unaffected.
[Illustration: FIG. 5.--SHOWING MAGNETIC DEFLECTION OF
[alpha], [beta], AND [gamma] RAYS.]
Three Types of Rays
By the use of these methods of investigation it is learned that the
radiations consist of three types of rays. These have been named the
alpha, beta, and gamma rays, respectively. Some radio-active bodies
emit all three types, some two, and some only one. The distinguishing
characteristic of these types of rays may be summed up as follows:
Alpha Rays
The alpha rays have a positive electrical charge and a comparatively
low penetrating power. They are slightly deflected in strong magnetic
and electric fields. They have a great ionizing power and a velocity
about one-fifteenth that of light.
Beta Rays
The beta rays are negatively charged and have a greater penetrating
power than the alpha rays. They show a strong deflection in magnetic
and electric fields, have less ionizing power than the alpha rays, and
a velocity of the same order as light.
Gamma Rays
The gamma rays are very penetrating and are not deflected in the
magnetic or electric fields. They have the least ionizing power and a
very great velocity.
The penetrating power of each type is complex and varies with the
source, so the statements given are but generalizations. The alpha
rays are projected particles which lose energy in penetrating matter.
As to the power of ionizing gases, if that for the [alpha] rays is
taken as 10,000, then the [beta] rays would be approximately 100 and
the [gamma] rays 1.
Measurement of Radiations
The rays are examined and measured in several ways:
1. By their action on the sensitive photographic plates. The use
of this method is laborious, consumes time, and for comparative
measurements of intensity is uncertain as to effect.
2. By electrical methods, using electroscopes, quadrant
electrometers, etc. These are the methods most used.
3. By exposure to magnetic and electric fields, noting extent and
direction of deflection.
4. By their relative absorption by solids and gases.
5. By the scintillations on a zinc sulphide screen.
Identification of the Rays
The alpha rays have been identified as similar to the so-called canal
rays. These were first observed in the study of the _X_ rays. When an
electrical discharge is passed through a vacuum tube with a cathode
having holes in it, luminous streams pass through the holes toward the
side away from the anode and the general direction of the stream. They
travel in straight lines and render certain substances phosphorescent.
These rays are slightly deflected by a magnetic field and in an
opposite direction from that taken by the cathode rays in their
deflection. The rays seem to be positive ions with masses never less
than that of the hydrogen atom. Their source is uncertain, but they
may be derived from the electrodes.
The beta rays are identical in type with the cathode rays and are
negative electrons.
The gamma rays are analogous to the _X_ rays and are of the order of
light. They are in general considerably more penetrating than _X_
rays. For example, the gamma rays sent out by 30 milligrams of radium
can be detected by an electroscope after passing through 30
centimeters of iron, a much greater thickness than can be penetrated
by the ordinary _X_ rays.