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Ionizing radiation is categorized into four types:
X-rays
are usually manmade radiation produced by bombarding a metallic target
with electrons at a high speed in a vacuum. X-rays are electromagnetic radiation of
the same nature as light waves and radio waves, but at extremely short wavelength,
less than 0.1 billionth of a centimeter. They are also called photons. The energy of
X-rays is millions of times greater than that of light and radio waves. Because of this
high energy level, X-rays penetrate a variety of materials, including body tissue.
Gamma rays
occur in nature and are almost identical to X-rays, but have a shorter
wavelength than X-rays. Gamma rays are very penetrating; thick lead shielding is
generally required to stop them.
Beta radiation
. A beta particle consists of an electron emitted from an atom. Beta
particles penetrate matter less deeply than gamma or X-rays, but they are
biologically significant because they can be more effective than gamma radiation at
disrupting cellular material.
Alpha radiation
. An alpha particle consists of two protons and two neutrons, the
same as the nucleus of a helium atom. It generally can travel no more than 1 to 3
inches in air before stopping, and can be stopped by a piece of paper.
When an atom emits an alpha or beta particle or a gamma ray, it becomes a different
type of atom. Radioactive substances may go through several stages of decay before
they change into a stable, or non-radioactive, form.
An element may have several forms, or isotopes. A radioactive form of an element is
called a radioisotope or radionuclide. Each radionuclide has a half-life, which is the
time required for half of a quantity of the material to decay.
Electron
Proton
Neutron
A hydrogen atom has one electron
and one proton. The most common
isotope has no neutrons and is stable.
Tritium is a radioactive isotope of
hydrogen. It has two neutrons in
its nucleus.
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The following chart shows the complete decay chain for Uranium 238, which ends
with a stable isotope of lead. Notice that the half-life of the radionuclides in the
chain range from 164 microseconds to 4.5 billion years.
Isotope Emits
Half-life
Product
U-238 alpha
4.5
billion
years Th-234
Thorium
Th-234
beta
24.1 days
Pa-234
Proactinium
Pa-234 beta 1.17
minutes
U-234 Uranium
U-234 alpha
250,000
years
Th-230
Thorium
Th-230 alpha 80,000
years
Ra-226 Radium
Ra-226 alpha 1,602
years
Rn-222 Radon
Rn-222 alpha 3.8
days
Po-218 Polonium
Po-218 alpha 3
minutes
Pb-214 Lead
Pb-214 beta 26.8
minutes
Bi-214 Bismuth
Bi-214 beta 19.7
minutes
Po-214 Polonium
Po-214 alpha 164
micro-seconds Pb-210 Lead
Pb-210 beta 21
years
Bi-210 Bismuth
Bi-210 beta 5
days
Po-210 Polonium
Po-210 alpha 138
days
Pb-206 Lead
Measuring Radiation
Alpha, beta, gamma, and x-rays ionize material they strike or pass through. The
amount of radiation is generally determined by measuring the resulting ionization.
The Geiger tube used in the Digital Radiation Monitor consists of an anode (positive
electrode) positioned in the center of a tubular cathode (negative electrode) filled
with a mixture of argon, neon, and either chlorine or bromine gases. The cathode is a
thin-walled metallic cylinder sealed at each end with an insulating disk to contain the
gas. The anode is a wire that extends into the cylinder. A high voltage is applied to
the electrodes to create an electrical field within the chamber. When radiation passes
through the chamber and ionizes the gas, it generates a pulse of current. The Digital
Radiation Monitor electronically processes these pulses to display the radiation
level.
Alpha
Window
Cathode
(Side Wall)
Anode
