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.
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.
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.
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.
Isotope Sensitivity
WARNING
CAUTION
- Focal Spot Size. Usually isotope sources have a larger focal spot size than X-ray tubes in order to have a sufficient quantity of radiation to prevent very long exposure times.
- Fixed Radiation Energy. Isotopes emit radiation with an energy characteristic of that particular radioactive material. Therefore, the operator has no choice of radiation energy, and it is not always possible to select the radiation energy compatible with the absorption characteristics of the part being inspected.
- Exposure Techniques. Exposure times are important and often isotopes are weak in radiation output, and source-to-film distances must be decreased to reduce exposure times. This leads to poor definition.
Isotope Cameras. Isotopes emit their radiation continuously and cannot be shut off or stopped like an X-ray generator. These isotopes are stored in a radiation shielded container to reduce the radiation to a level safe for unprotected personnel when not making radiographic exposures. The shielded container must be designed so that the radioactive isotope can be remotely positioned for the radiographic exposure. Many schemes have been devised for remote handing of isotopes. Source holders, commonly called isotope cameras, generally are of two typical designs. The simplest cameras are designed for direct beam radiography, and the source is only allowed to produce a restricted conical direct beam. The container itself is used to absorb radiation that in not emanating from the window or port. Some units are designed so that opening and closing can be accomplished at a remote distance. The other types of isotope cameras are normally used for circumferential radiography. These are devised to move the source from its shielded container to a point some distance away and then, upon completion of exposure, return the source to the container. In the latter types of cameras the radiation is being emitted in all directions.
