RADIATION PROTECTION - ILEA

Historical Prologue 5
science of radioactive substances and the radiations they emit. Thus, by
1900, scientists had begun to discover and experiment with high-energy
radiations of the kind that had dominated the universe during its early
history.
At about the same time, the work of Planck in 1900 and Einstein in
1905 showed that many kinds of radiation, including heat radiation, visible
light, ultraviolet light, and radiowaves, which had previously appeared
to be transmitted as continuous waves of energy, were actually emitted as
discrete bundles of energy called photons, and that differences among
these types of radiation could be characterized in terms of the different energies
of the photons. In time, it was learned that gamma radiation, emitted
from the nuclei of atoms, and x rays, produced by the acceleration of
electrons (outside the nucleus), were also made up of photons of electromagnetic
radiation, but of much higher energies.
The discovery of the new particles and rays led to intense experimentation
on their properties and their interactions with matter. The energetic
alpha particles (actually helium nuclei) emitted by radioactive materials
were directed by Rutherford against thin gold foils. Through analysis of
the scattering pattern, he deduced in 1911 that the atom was composed of
a tiny central core, or nucleus, containing all the positive charge and almost
all the mass of the atom, and a nearly empty surrounding region containing
the light, negatively charged electrons, in sufficient number to balance
out the inner positive charge. The nucleus of the hydrogen atom,
consisting of a single particle with a charge equal in magnitude and opposite
in sign to that of the electron, was recognized as a fundamental building
block of the nuclei of all complex atoms. It was named the proton
(from the Greek protos, which means “first”). With the development of the
theory of the atomic nucleus composed of protons and other elementary
particles, it was possible to visualize how certain types of nuclei could disintegrate
and emit particles.
The emitted particles had very high energies, and the source of the energies
was a puzzle until the formulation by Einstein in 1905 of the massenergy
equation. This equation expressed in quantitative terms his conclusion
that matter could be converted into energy according to the relationship
E = mc 2 , where E was the energy, m the mass, and c the velocity of
light. If m was expressed in kilograms and c was expressed in meters per
second, the equivalent energy E was given in a unit of energy known as the
joule. When it later became possible to determine the masses of individual
particles in an instrument known as the mass spectrograph, the relationship
between mass and energy was verified experimentally. Whenever a
particle was emitted from a nucleus with high energy, it was found that the
mass of the nucleus decreased not only by the rest mass of the particles