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 Alpha
rays
A particle ray consisting of two protons and two neutrons (namely, a nucleus
of helium). Alpha rays are produced following spontaneous decay of certain
radioactive atoms, such as radium, plutonium, uranium, and radon. Because
of its large mass and positive charge, an alpha ray can usually pass only
a short distance--less than 1 mm--in water. A single piece of paper can
stop an alpha ray effectively. Therefore, health effects of alpha-ray exposures
appear only when alpha-emitting materials are ingested (i.e., internal
exposure).
Beta
ray
A particle ray consisting of a fast electron whose mass is nearly 1/2000 of the mass of a proton or neutron. Beta rays are produced following spontaneous decay of certain radioactive materials, such as tritium (an isotope of hydrogen), carbon-14, phosphorus-32, and strontium-90. Depending on its energy (i.e., speed), a beta ray can traverse different distances in water--less than 1 mm for tritium to nearly 1 cm for phosphorus-32. As with alpha rays, the major concern for health effects is after their ingestion (i.e., internal exposure).
Gamma
ray
An electromagnetic wave, a gamma ray is similar to ordinary visible light
but differs in energy or wavelength. Sunlight consists of a mixture of
electromagnetic rays of various wavelengths, from the longest, infrared,
through red, orange, yellow, green, blue, indigo, and violet, to the shortest
in wavelength, ultraviolet. A gamma ray's wavelength is far shorter than
ultraviolet (i.e., it is far higher in energy). Gamma rays are produced
following spontaneous decay of radioactive materials, such as cobalt-60
and cesium-137. A cobalt-60 gamma ray can penetrate deeply into the human
body, so it has been widely used for cancer radiotherapy.
X
ray
X rays have the same characteristics as gamma rays, although
they are produced differently. When high-speed electrons hit
metals, electrons are stopped and release energy in the form
of an electromagnetic wave. This was first observed by Wilhelm
Roentgen in 1895, who considered it a mysterious ray, and
thus called it an X ray. X rays consist of a mixture of different
wavelengths, whereas gamma-ray energy has a fixed value (or
two) characteristic to the radioactive material.
Neutrons
Neutron particles are released following nuclear fission (splitting of
an atomic nucleus producing large amounts of energy) of uranium or plutonium.
In fact, it is neutrons that trigger the nuclear chain reaction to explode
an atomic bomb. The human body contains a large amount of hydrogen (a constituent
of water molecules that occupy 70% of the human body), and when neutrons
hit the nucleus of hydrogen, i.e., a proton that is positively charged,
the proton causes ionizations
in the body, leading to various types of damage. At equivalent
absorbed doses, neutrons can cause more severe damage to the
body than gamma rays. (Neutrons hardly damage cells because
they do not carry any electrical charge.) |
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How
much radiation do we receive from various sources?