Alpha Radiation
A positively charged particle emitted by certain radioactive material
consisting of two neutrons and two protons, the nucleus of a helium
atom. A dangerous carcinogen when inhaled or ingested. Alpha radiation
can penetrate the body to just below the dead skin, but is blocked
by clothing or even a sheet of paper. When released inside our bodies
from material we breathe or swallow, alpha rays are able to transfer
their energy at short range to damage body cells.
Beta
Radiation
A beta particle is a single high-energy electron moving at high
speed and carrying a negative charge. They can travel about one
metre through air and can penetrate the skin, to reach internal
tissue. Can cause skin burns and, when ingested, cancer. Beta rays
are especially dangerous when emitted inside the body.
Gamma
Radiation
Gamma rays are electromagnetic waves or photons emitted from the
nucleus (center) of an atom. They have no electrical charge and
penetrate deeply into the body, or pass through it, creating ions
as they collide with atoms along their path. Gamma rays are similar
to X-Rays, but are much more powerful.
Neutron
Radiation
Neutrons are the neutral particles that are normally contained in
the nucleus of all atoms. Neutron radiation occurs when the nucleus
of a heavy element like uranium decays into a lighter element and
emits a neutron. In a nuclear reactor core or atom bomb, enough
neutrons are released to split more uranium atoms, releasing more
neutrons and creating a critical mass: a self sustaining reaction.
In a nuclear weapon, the reaction is uncontrolled and leads to a
massive explosion and burst of neutron radiation. In a nuclear reactor,
the critical mass is 'moderated' or slowed, generating tremendous
heat without actually exploding. Neutron radiation is perhaps the
most dangerous for living creatures.
The
nature of atoms
All matter is made up of atoms: tiny bubbles of force so small they
can scarcely be imagined. These entities seem to exist as pointlike
knots of condensed energy (the nucleus) surrounded by a shimmering
cloud of electrons.
The nucleus itself is
normally composed of two kinds of particle: positively charged protons
and uncharged neutrons. For an atom to be considered electrically
neutral (that is, have no charge), it must have one negatively charged
electron in the cloud to balance each positively charged proton
in the nucleus. The number of neutrons, while affecting the mass
of the atom, has no influence on electric charge.
Hydrogen, the simplest
kind of atom, normally has no neutrons at all: it is simply a single
proton coupled to one whirling electron. Carbon, a more complex
element, contains six protons, six electrons, and between six and
eight neutrons.
It's a common misconception
that matter and energy are two different things, but in the early
years of this century, Albert Einstein introduced the notion that
an atom is a tightly concentrated, meta-stable bundle of energy
arising from the universal field (a concept that might seem familiar
to ancient cosmologies). According to the theory, this energy can
be liberated under certain circumstances: when an atom is split,
for example, or when it fused with other particles to make a heavier
element.
Atomic
and Mass Numbers
Each element is commonly referred to as having an atomic number
and a mass number. The atomic number is simply the number of protons
in the nucleus, and the mass number is the combined total of protons
and neutrons.
Elements are sometimes
written in the following format: 238U or Uranium-238.
The 238 is the mass number of the element.
Knowing that uranium
has an atomic number of 92, you can calculate that it must also
have 146 neutrons by subtracting 92 from 238 - at the scale of atoms,
this makes it a monster.
Isotopes
The variability in the number of neutrons packed into the core of
the atom is what gives rise to the phenomenon of isotopes. Most
naturally occuring elements are mixtures of several isotopes, and
many of these are stable under normal conditions. Uranium, with
92 protons, has three flavours: 234U, 235U
and 238U. While all isotopes of an element are chemically
identical to each other, the isotopes have differing physical properties
- this is one of the reasons uranium is subject to exotic (and dangerous)
refinement and enrichment processes.
Half
Lives and Radioactive Decay
Uranium-238, the most prevalent isotope in uranium ore, has a half-life
of about 4.5 billion years; that is, half the atoms in any sample
will decay in that amount of time. Uranium-238 decays by alpha emission
into thorium-234, which itself decays by beta emission to protactinium-234,
which decays by beta emission to uranium-234, and so on. The various
decay products, (sometimes referred to as "progeny" or "daughters")
form a series starting at uranium-238.
After several more alpha
and beta decays, the series ends with the stable isotope lead-206.
The property of uranium important for nuclear weapons and nuclear
power is its ability to fission, or split into two lighter
fragments when bombarded with neutrons releasing energy in the process.
Of the naturally-occurring uranium isotopes, only uranium-235 can
sustain a chain reaction -- a reaction in which each fission produces
enough neutrons to trigger another, so that the fission process
is maintained without any external source of neutrons. In contrast,
uranium-238 cannot sustain a chain reaction, but it can be converted
to plutonium-239, which can.
Plutonium-239, virtually
nonexistent in nature, was used in the first atomic bomb tested
July 16, 1945 and the one dropped on Nagasaki on August 9, 1945.
