T.O. 33B-1-1
6-19
SECTION III
ISOTOPE RADIATION SOURCES
6.3
ISOTOPE RADIATION SOURCES.
6.3.1
Energy Spectra.
Radioactive nuclei emit gamma rays with discrete energy levels and a spectrum consists of a series of very sharply
defined energies. As the atomic nucleus of a particular radioactive isotope disintegrates, well-defined decay schemes
are followed. Further, it is important to be able to express the source strength and rate of decay.
6.3.2
Source Strength.
A new international unit for source strength is the Becquerel (Bq). The Becquerel is defined as one disintegration per
second. Therefore, 1 curie (Ci) = 3.7 x 1010 Bq. The unit Becquerel has no relationship to the source volume or the
quantity or type of energy of the radiation emitted. This term only has meaning when the particular radioactive isotope
is known. For example, five Becquerels of cobalt-60 are not equivalent to five curies of iridium-192 because of
different energy levels and decay schemes.
6.3.3
Focal Spot Size.
For isotopes the physical size of the radioactive source can be thought of as the “focal spot”. Since the Becquerel only
relates the number of disintegrations per second, this unit has no relationship to the volume of mass or size of the
radioactive source. The term "specific activity" is used to define the quantity of radioactivity of one gram of the
substance and is expressed as Becquerels per gram. For a particular number of Becquerels, the dimensions of the
radioactive source are governed by its specific activity. For radiographic applications a small source size is desirable to
produce images with good resolution or sharpness, just as a small focal spot in an X-ray tube is required for high
resolution radiographs. Large sources produce geometric distortion resulting in radiographs with poor definition.
Effort is constantly being devoted to producing radioactive isotope sources with high Becquerel strengths in small
volumes of material. Some special sources are stated as high specific activity, indicating a high radiation output
relative to the source size. Nevertheless, in most isotopes the source size exceeds the focal spot size in X-ray tubes.
6.3.4
Decay Characteristics.
As radioactive material decays, there are a fewer number of unstable atoms left to decay and as time passes the
radioactive material is becoming less and less radioactive. Different isotopes have different decay rates. If a single
atom of an isotope existed, it would be impossible to predict at what moment in time it might disintegrate. But if large
numbers, of atoms exist, it is possible to measure the lapse of time required for one atom out of every two to
disintegrate. This is called the half-life of an isotope. The half-life is defined, as the time required for an isotope to
decay to one-half of its original radioactivity.
6.3.5
Isotope Sensitivity.
WARNING
The radiation levels at the surface of the shielded container are hazardous to
personnel over prolonged periods of contact.
CAUTION
Undeveloped film shall not be stored in the immediate area of the shielded
container.
Radiographic definition obtained with isotope sources is usually of lower quality than that obtained with X-rays because
of:
