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What
are beta particles?
Beta particles are
subatomic particles ejected from the nucleus of some
radioactive atoms. They are equivalent to electrons.
The difference is that beta particles originate in the
nucleus and electrons originate outside the nucleus.
What
are the properties of beta particles?
Beta particles have an
electrical charge of -1. Beta particles have a mass of
549 millionths of one atomic mass unit, or AMU, which
is about 1/2000 of the mass of a proton or neutron.
The speed of individual beta particles depends on how
much energy they have, and varies over a wide range.
While beta particles
are emitted by atoms that are radioactive, beta
particles themselves are not radioactive. It is their
energy, in the form of speed, that causes harm to
living cells. When transferred, this energy can break
chemical bonds and form ions.
What
conditions lead to beta particle emission?
Beta particle emission
occurs when the ratio of neutrons to protons in the
nucleus is too high. Scientists think that an excess
neutron transforms into a proton and an electron. The
proton stays in the nucleus and the electron is
ejected energetically.
This process decreases
the number of neutrons by one and increases the number
of protons by one. Since the number of protons in the
nucleus of an atom determines the element, the
conversion of a neutron to a proton actually changes
the radionuclide to a different element.
Often, gamma ray
emission accompanies the emission of a beta particle.
When the beta particle ejection doesn't rid the
nucleus of the extra energy, the nucleus releases the
remaining excess energy in the form of a gamma photon.
The decay of
technetium-99, which has too many neutrons to be
stable, is an example of beta decay. Scientists think
that a neutron in the nucleus converts to a proton and
a beta particle. The nucleus ejects the beta particle
and some gamma radiation. The new atom retains the
same mass number, but the number of protons increases
to 44. The atom is now a ruthenium atom.
Other examples of beta
emitters are phosphorous-31, tritium (H-3), carbon-14,
strontium-90, and lead-210.
Which
radionuclides are beta emitters?
There are many beta
emitters.
- tritium
- cobalt-60
- strontium-90
- technetium-99
- iodine-129 and -131
- cesium-137
What
happens to beta particles in the environment?
Beta particles travel
several feet in open air and are easily stopped by
solid materials. When a beta particle has lost its
energy, it is like any other loose electron. Whether
in the outdoor environment or in the body, these
electrons are then picked up by a positive ion.
How
are people exposed to beta particles?
There are both natural
and man-made beta emitting radionuclides. Potassium-40
and carbon-14 are weak beta emitters that are found
naturally in our bodies. Some decay products of radon
emit beta particles, but its alpha-emitting decay
products pose a much greater health risk.
Beta emitters that
eject energetic particles can pose a significant
health concern. Their use requires special
consideration of both benefits and potential, harmful
effects.
- Key beta emitters
used in medical imaging, diagnostic and treatment
procedures are technetium-99m, phosphorus-32, and
iodine-131. For example, people who have taken
radioactive iodine will emit beta particles. They
must follow strict procedures to protect family
members from exposure.
- Radioactive iodine
may enter the environment during a nuclear reactor
accident and find its way into the food chain.
- Industrial gauges
and instruments containing concentrated
beta-emitting radiation sources can be lost,
stolen, or abandoned. If these instruments then
enter the scrap metal market, or someone finds
one, the sources they contain can expose people to
beta emitters.
At one time, strontium-90
was the major man-made beta emitter in the
environment. Fallout from atmospheric nuclear testing
from the 1950's to the early 1970's spread
strontium-90 worldwide. However, most of the
strontium-90 from these tests has now decayed away.
Testing also released
large amounts of cesium-137 into the environment.
Although, cesium-137 emits beta radiation, its gamma
radiation is of greater concern. Some cesium-137 from
fallout remains in the environment, but most of it has
decayed as well.
Does
the way a person is exposed to beta particles matter?
Yes. Direct exposure to
beta particles is a hazard, because emissions from
strong sources can redden or even burn the skin.
However, emissions from inhaled or ingested beta
particle emitters are the greatest concern. Beta
particles released directly to living tissue can cause
damage at the molecular level, which can disrupt cell
function. Because they are much smaller and have less
charge than alpha particles, beta particles generally
travel further into tissues. As a result, the cellular
damage is more dispersed.
How
can beta particles affect people's health?
Beta radiation can
cause both acute and chronic health effects. Acute
exposures are uncommon. Contact with a strong beta
source from an abandoned industrial instrument is the
type of circumstance in which acute exposure could
occur. Chronic effects are much more common.
Chronic effects result
from fairly low-level exposures over a along period of
time. They develop relatively slowly (5 to 30 years
for example). The main chronic health effect from
radiation is cancer. When taken internally beta
emitters can cause tissue damage and increase the risk
of cancer. The risk of cancer increases with
increasing dose.
Some beta-emitters,
such as carbon-14, distribute widely throughout the
body. Others accumulate in specific organs and cause
chronic exposures:
- Iodine-131
concentrates heavily in the thyroid gland. It
increases the risk of thyroid cancer and other
disorders.
- Strontium-90
accumulates in bone and teeth.
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Gross
Alpha