(modifi ed from Eisler 1994, 2007
and Wikipedia 2011
Actinides. Elements of atomic numbers 89 to 103: actinium, thorium,
protactinium, uranium, neptunium, americium, curium, berkelium,
californium, einsteinium, fermium, mendelevium, nobelium,
lawrencium (Ac, Th, Pa, U, Np, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lw).
Activity. The activity of a radioactive material is the number of
nuclear disintegrations per unit time. Up to 1977, the accepted unit
of activity was the curie (Ci), equivalent to 37 billion disintegrations/
second (d/s)—a number that approximated the activity of 1 gram of
radium-226. The present unit of activity is the becquerel (Bq) equivalent
to 1 d/s.
Alpha particles An alpha particle is composed of 2 protons and 2
neutrons and a charge of +2; essentially it is a helium nucleus without
orbital electrons. Alpha particles usually originate from the decay of
radionuclides of atomic number >82 and are detected in samples that
contain uranium, thorium, or radium. Alpha particles react strongly
with matter and consequently produce large numbers of ions per
unit length of their paths. As a result, they are not very penetrating
and traverse only a few cm of air. Alpha particles are unable to
penetrate clothing or the outer layer of skin; however, when internally
deposited alpha particles are often more damaging than most other
types of radiations because comparatively large amounts of energy
are transferred in a small volume of tissue. Alpha particle absorption
116 The Fukushima 2011 Disaster
involves ionization and orbital electron excitation, Ionization occurs
whenever the particle is suffi ciently close to an electron to pull it from
its orbit. The alpha also loses kinetic energy by exciting orbital electrons
with interactions that are insuffi cient to cause ionization.
Atom. The smallest part of an element that has all the properties of that
element. An atom consists of one or more protons and neutrons in the
nucleus and of one or more electrons.
Atomic number. The number of electrons outside the nucleus of a
neutral (nonionized) atom plus the number of protons in the nucleus.
Becquerel (Bq). The present accepted unit of activity is the becquerel
equivalent to one disintegration/second (1 dps). About 0.37 Bq=1
Beta particles. Beta particles are electrons that are spontaneously ejected
from the nuclei of radioactive atoms during the decay process. They
may be either positively or negatively charged. A positively charged
beta, called a positron, is less frequently encountered than its negative
counterpart, the negatron. The neutrino, a small particle, accompanies
beta emission. The neutrino has little mass and is electrically neutral.
But neutrinos conduct a variable part of the energy of transformation
and account for the variability in kinetic energies of beta particles
that are emitted from a given radionuclide. Positrons are emitted
by many of the naturally and artifi cially produced radionuclides;
they are considerably more penetrating than alpha particles but less
penetrating than X-rays and gamma rays. Beta particles interact with
other electrons and with nuclei in the travel medium. The ultimate fate
of a beta particle depends on its charge. Negatrons, after their kinetic
energy is spent, combine with a positively charged ion or become free
electrons. Positrons also dissipate kinetic energy through ionization and
excitation; the collision of positrons and electrons causes annihilation
and release of energy that is equal to the sums of their particle masses.
Breeder reactor. A nuclear chain reactor in which transmutation
produces a greater number of fi ssionable atoms than the number of
consumed parent atoms. Breeders were at fi st considered superior to
light water reactors because of their superior fuel economy. Interest
waned after the 1960s as more uranium reserves were found and new
methods of uranium enrichment reduced fuel costs.
Cosmic rays. Highly penetrating radiations that originate in outer

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