11
Service
CAUTION:
Do not send a contaminated instrument for repair or calibration under any circumstances. There are no user
serviceable parts inside instrument.
If the In requires servicing, please contact your distributor or the manufacturer at the following address:
S.E. International, Inc.
P.O. Box 39, 436 Farm Rd.
Summertown, TN USA 38483-0039
Tel 931-964-3561, Fax 931-964-3564
E-mail: [email protected]
6 Basics of Radiation and Its Measurement
This chapter briefly tells what radiation is and how it is measured. This information is provided for users who are not
already familiar with the subject. It is helpful in understanding how the In works and in interpreting your readings.
Ionizing Radiation
Ionizing radiation is radiation that changes the structure of individual atoms by ionizing them. The ions produced in turn
ionize more atoms. Substances that produce ionizing radiation are called radioactive.
Radioactivity is a natural phenomenon. Nuclear reactions take place continuously on the sun and all other stars. The
emitted radiation travels through space, and a small fraction reaches the Earth. Natural sources of ionizing radiation also
exist in people and in the ground. The most common of these are uranium and its decay products.
Ionizing radiation is categorized into four types:
X-rays
are 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 are 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
are almost identical to X-rays. Gamma rays generally 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. It has more mass and less energy than a
gamma ray, so it doesn’t penetrate matter as deeply as gamma and X-rays.
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.
Decay:
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-ionizing, form. For example;
U-238 has 14 different stages of decay before it stabilizes.
An element may have several forms, or isotopes. A radioactive isotope of an element may be called “radioisotope”.
However, the more correct term is radionuclide.
Half-life:
Each radionuclide has a characteristic half-life, which is the time required for half of a quantity of the material to
decay.
Radiation Measurement Units
Several different units are used to measure radiation, exposure to it and dosage.
A
roentgen
is the amount of X-radiation or gamma radiation that produces one electrostatic unit of charge in one cc of dry
air at 0
°
C and 760 mm of mercury atmospheric pressure. The In displays in milliroentgens per hour (mR/hr).
A
rad
is the unit of exposure to ionizing radiation equal to an energy of 100 ergs per gram of irradiated material. This is
approximately equal to 1.07 roentgen.
A
rem
is the dosage received from exposure to a rad. It is the number of rads multiplied by the quality factor of the
particular source of radiation. The rem and millirem are the most commonly-used measurement units of radiation dose in
the U.S. 1 rem= 1rad.
A
sievert
is the standard international measurement of dose. One sievert is equivalent to one hundred rems. A
microsievert (
µ
Sv) is one millionth of a sievert.
A
curie
is the amount of radioactive material that decays at the rate of 37 billion disintegrations per second, approximately
the decay rate of one gram of radium. Microcuries (millionths of a curie) and picocuries (trillionths of a curie) are also
often used as units of measurement.
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