Alpha particles (symbol a ) 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.
Who discovered alpha particles?
Ernest Rutherford, an English scientist, discovered alpha
particles in 1899 while working with uranium. Rutherford's
studies contributed to our understanding of the atom and
its nucleus through the Rutherford-Bohr planetary model
of the atom.
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. You can find fact sheets for several
key alpha emitters at the Radionuclides page:
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.
Exposure to Alpha Emitters
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.
The currently used process generates large piles or "stacks"
of phosphogypsum, in which naturally occurring radium
is concentrated.
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.
Health Effects
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.
Is there a medical test to determine exposure
to alpha particles?
There are tests that can detect the presence of
alpha-emitting radionuclides in the body. However, they require
special equipment and testing is generally done by specialized
laboratories and facilities or hospitals.
Protecting People from Alpha Particles
How do I know I'm near alpha emitters and alpha
particles?
You must have specialized equipment to detect alpha radiation.
Generally, this equipment is expensive and requires an expert
to operate it.
The one alpha-emitting radionuclide that you can easily
measure yourself is radon. Inexpensive home test kits are
available to test your home for radon.
What is the government doing to protect people
from exposure to alpha emitters and alpha particles?
The U.S. Congress passes laws that authorize EPA and other
federal agencies, to protect public health and the environment
from radionuclides, including alpha emitters. EPA has issued
a variety of regulations that limit the release of radionuclides
to the environment. You can learn about limits set for individual
radionuclides by selecting alpha emitters from the Radionuclides
page.
How do I protect myself and my family from
alpha particles?
Protecting yourself from external exposure to alpha radiation
is easy, since alpha particles are unable to penetrate the
outer dead layers of skin or clothing. However, tissue that
is not protected by the outer layer of dead cells, such
as eyes or open wounds, must be carefully protected.
The exposure pathways of concern are inhalation or ingestion
of alpha emitters, which continue to emit alpha particles.
Alpha emitting radionuclides taken into the body release
alpha particles to sensitive tissues. Their high energy
transfers directly to tissue, causing damage that may lead
to cancer.
The most significant way people come in contact with alpha
emitters is in their home, school, or place of business.
Radon, is a heavy gas and tends to collect in low-lying
areas such as basements. Testing for radon in your home
and taking any corrective action necessary is the most effective
way to protect you and your family from alpha emitters.
Beta Particles 
