Alpha particles are a
type of ionizing radiation ejected by the nuclei of
some unstable atoms. They are large subatomic
fragments consisting of 2 protons and 2 neutrons.
What are the properties of an alpha particle?
An alpha particle is
identical to a helium nucleus having two protons and
two neutrons. It is a relatively heavy, high-energy
particle, with a positive charge of +2 from its two
protons. Alpha particles have a velocity in air of
approximately one-twentieth the speed of light,
depending upon the individual particle's energy.
What are the conditions that lead to alpha particle
emission?
When the ratio of
neutrons to protons in the nucleus is too low, certain
atoms restore the balance by emitting alpha particles.
For example: Polonium-210 has 126 neutrons and 84
protons, a ratio of 1.50 to 1. Following radioactive
decay by the emission of an alpha particle, the ratio
becomes 124 neutrons to 82 protons, or 1.51 to 1.
Alpha emitting atoms
tend to be very large atoms (that is, they have high
atomic numbers). With some exceptions, naturally
occurring alpha emitters have atomic numbers of at
least 82 (the element lead).
Which
radionuclides are alpha emitters?
There are many alpha
emitting radioactive elements, both natural and
manmade.
- Americium-241
- Plutonium-236
- Uranium-238
- Thorium-232
- Radium-226
- Radon-222
- Polonium-210
What
happens to atoms during alpha emission?
The nucleus is
initially in an unstable energy state. An internal
change takes place in the unstable nucleus and an
alpha particle is ejected leaving a decay product.
The atom has then lost two protons along with two
neutrons.
Since the number of
protons in the nucleus of an atom determines the
element, the loss of an alpha particle actually
changes the atom to a different element. For
example, polonium-210 is an alpha emitter. During
radioactive decay, it loses two protons, and becomes
a lead-206 atom, which is stable (i.e.,
nonradioactive).
What
uses do alpha emitters have?
The positive charge
of alpha particles is useful in some industrial
processes. For example, radium-226 may be used to
treat cancer, by inserting tiny amounts of radium
into the tumorous mass. Polonium-210 serves as a
static eliminator in paper mills and other
industries. The alpha particles, due to their
positive charge, attract loose electrons, thus
reducing static charge.
Some smoke detectors
take advantage of alpha emissions from americium-241
to help create an electrical current. The alpha
particles strike air molecules within a chamber,
knocking electrons loose. The resulting positively
charged ions and negatively charged electrons,
create a current as they flow between positively and
negatively charged plates within the chamber. When
smoke particles enter the device, the charged
particles attract them, breaking the current and
setting off the alarm.
How
do alpha emitters get into the environment?
Most alpha emitters
occur naturally in the environment. For example,
alpha particles are given off by uranium-238,
radium-226, and other members of the uranium decay
series. These are present in varying amounts in
nearly all rocks, soils, and water. However, human
activity, create or worsen the potential for
exposure of people and contamination of various
environmental media. For example, uranium mining
wastes, known as uranium mill tailings, have high
concentrations of uranium and radium. Once brought
to the surface, they can be become airborne or
enter surface water as runoff. A second example is
the mining and processing of phosphate for
fertilizer.
How
do alpha particles change in the environment?
Alpha particles don't
get very far in the environment. Once emitted, they
travel relatively slowly (at approximately
one-twentieth the speed of light) due to their
electric charge and large mass. They lose energy
rapidly in air, usually expending it within a few
centimeters. Because alpha particles are not
radioactive once they have lost their energy, they
pick up free electrons and become helium.
Alpha particles also
cannot penetrate most matter they encounter. Even a
piece of paper, or the dead outer layers of human
skin is sufficient to stop alpha particles.
How
can alpha particles affect peoples health?
The health effects
of alpha particles depend heavily upon how
exposure takes place. External exposure (external
to the body) is of far less concern than internal
exposure, because alpha particles lack the energy
to penetrate the outer dead layer of skin.
However, if alpha
emitters have been inhaled, ingested (swallowed)
or absorbed into the blood stream, sensitive
living tissue can be exposed to alpha radiation.
The resulting biological damage increases the risk
of cancer; in particular, alpha radiation is known
to cause lung cancer in humans when alpha emitters
are inhaled.
The greatest
exposures to alpha radiation for average citizens
comes from the inhalation of radon and its decay
products, several of which also emit potent alpha
radiation.
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Gross
Alpha