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and ions give rise to biological effects. The damage to the cells caused the
burns of the earlier workers in x-rays, and the sterility which follows
damage to the reproductive cells. It is also the basis of the ray therapy of
malignant disease as by introducing the rays on to the structure to be
destroyed by different paths, the cells of the structure itself might be killed
while no harm is done to the normal tissues through which the rays pass.
The source of these harmful rays is usually outside the body ; but it may
be inside, having been deposited there by inhalation, ingestion or by
penetration of the skin through wounds.
Radioactivity. An atom is accepted as comprising a positively
charged heavy nucleus surrounded by a cloud of neutrons just sufficient
to balance the nuclear charge. Except in the case of hydrogen where the
nucleus is a single proton, the nuclei of all types of element are built up
of protons and neutrons, particles of roughly equal mass and differing
only in that the proton carries a positive charge. The number of protons
in any nucleus determines the chemical nature of the element, and the
neutrons supply additional mass. Heavier elements tend to have more
neutrons than protons. Most nuclei are stable in ordinary circumstances;
but a few of the heavier elements are inherently unstable. Sooner or
later a violent re-arrangement occurs in the nucleus, a charged particle is
ejected, leaving the nucleus differently charged and therefore being a
different element. This is the phenomenon of radio-activity. The atom
of uranium is unstable; sooner or later it disintegrates, emitting a
helium nucleus. The uranium then becomes thorium. This is unstable
and breaks down to radium, which in turn becomes radon, passing
through polonium and other unstable intermediaries to reach stable lead.
Some of these changes are very slow, some almost instanstaneous, each
type of nucleus having its own characteristic half-life which is the period
over which one half of the atoms disappear.
In addition to these radiations are the alpha and beta particle emissions.
The beta emission is the commoner, being released from most unstable
nuclei with the result that neutron is converted into a negative proton,
resulting in a daughter nucleus with an atomic number one greater than
the parent. Beta emission from a radio-active source is usually associated
with gamma rays. Alpha and beta rays present a special problem from
the aspect of internal harm right inside the body.
Nature of Isotopes. The nuclei of some elements contain more
neutrons than is usual for that element. As neutrons determine the mass,
these nuclei are heavier ; but as the proton which determines the nature
of the element is the same, chemically these elements are the same as those
carrying the usual number of neutrons. These unusual nuclei are known
as isotopes. They are usually unstable, and are radio-active.
Sources of Radiation. We are exposed to radiation from many
sources. There is that from outer space, from the surface of the earth
from the food, water and air we take in, and from the radio-active elements
in our own bodies. This is the background radiation about which nothing
can be done. On top of that we have for some years now been subjected
to man-made additions.