IT IS NOT within the scope of this blog to consider the
physical nature of radium and radon and their relations to each other in greater
detail than is essential to a clear understanding of their therapeutic
application in dermatology.
Radioactivity—Radium, Radon. Radioactivity
is the outward evidence of spontaneous disintegration of a radioactive
substance, or transmutation of the element, atom by atom, into other
substances. An element is therefore radioactive when it undergoes spontaneous
disintegration and transmutation.
Radium in the
elemental state is a solid grayish-white metal belonging in the alkaline earth
family. Metallic radium has been rarely prepared because of its avidity to
react to form stable salts. Preparations of the salts are spoken of in terms of
metallic (element) radium content, owing to the fact that it is this which
determines the radioactive strength regardless of the chemical combination of
the radium. The sulfate is the salt usually employed in therapeutic applicators.
Radium with
an atomic weight of 226 emits an alpha ray or, more specifically, a particle of
matter carrying two positive charges and traveling at high speed. This alpha
particle is the nucleus of a helium atom; after being slowed down and picking
up two negative electrons it becomes an atom of ordinary helium. The residue, a
gas, is a new element called radon(1), having an atomic weight of 222. Radon
undergoes a similar change, giving off an alpha particle, atomic weight 4,
which leaves as a residue a metal, RaA, atomic weight 218. Radium A likewise
disintegrates, emitting an alpha particle and producing another new metallic
element RaB, atomic weight 214. Radium B transforms by the emission of beta
rays into a new element called RaC, but without reduction of the atomic weight.
These beta rays, in their rapid escape from the radium B atom, produce
vibrations of other electrons causing the projection of high-frequency
electro-magnetic waves called gamma rays, which are analogous to x-rays,
ultraviolet, and visible light. They are generally of much higher frequency (300
billion billion cycles ), of a much shorter wavelength ( 1.4-0.01 A.) and
consequently more penetrating. RaC transforms into RaD, emitting alpha, beta
and gamma rays in the process. For therapeutic purposes the beta and gamma rays
are used, and these are derived from RaB and RaC.
The alpha,
beta, and gamma rays differ widely in their characteristics. The alpha
particles are positively charged. Although active ionizers, they are of little
therapeutic value because of their extremely slight penetrating power. The beta
rays are negatively charged electrons of various velocities, to which their powers
of penetration are proportionate. They are about one hundred times more
penetrating than the alpha rays, some of them probably reaching a depth of 1
cm. in tissue. The swiftest beta particles approximate the velocity of light,
and are known as the hard beta rays. The slower beta rays, which have far less
penetrative power, are called "soft" beta rays. Beta radiation is of
greatest value in the treatment of very superficial skin diseases, which
usually are of the benign or precancerous type. The gamma rays are unlike the
alpha and beta rays inasmuch as they are electromagnetic vibrations of
different short wavelengths similar to light. The gamma rays possess marked
powers of penetration. Like other electromagnetic waves, they are electrically
neither positive nor negative, but are electrically neutral. They are about one
hundred times as penetrating as the beta rays, and carry the energy from radium
into the deeper tissues as far as 6 to 8 inches. Lead 10 or more cm. thick is
required to absorb most of the gamma rays, whereas the beta rays are screened
off by 1.1 mm. of lead. According to their power of penetration, there are
"soft" and "hard" beta and gamma rays.
The energy
output of radium is composed of 4.8 per cent gamma radiation, 3.2 per cent beta
radiation, and 92 per cent alpha radiation, the latter not being utilized.
(1) Radon is
the name suggested in 1923 by the International Committee on Chemical Elements
to be used in place of the term "radium emanation."
The unit of measurement of radium is the milligram. This
refers to the actual radium content. The millicurie is the practical unit of
radon measurement. It has the same gamma ray activity as 1 mg. of radium in
equilibrium with its disintegration products.
Radium
provides a practically constant beta and gamma radiation for years since its “half-life
period" is 1590 years, which means a diminution of about 1 per cent in twenty-five
years, whereas radon has a “half-life period" of 3.85 days and is
completely disintegrated in about forty days. In the case of radon, 1
millicurie, if applied for its life, delivers a total of 133 millicurie hours,
50 per cent of which is delivered in the first 3.85 days.
With radioactive
cobalt, one can obtain gamma radiation. It is therapeutically as effective as
radium, only cheaper. However the half-life period is only 5.3 years, so that
when one uses cobalt 60 correction factors for the decay must be applied from
the time of its standardization to the time of its use.