T.O. 33B-1-16-19SECTION IIIISOTOPE RADIATION SOURCES6.3 ISOTOPERADIATIONSOURCES.6.3.1 EnergySpectra.Radioactive nuclei emit gamma rays with discrete energy levels and a spectrum consists of a series of very sharplydefined energies. As the atomic nucleus of a particular radioactive isotope disintegrates, well-defined decay schemesare followed. Further, it is important to be able to express the source strength and rate of decay.6.3.2 SourceStrength.A new international unit for source strength is the Becquerel (Bq). The Becquerel is defined as one disintegration persecond. Therefore, 1 curie (Ci) = 3.7 x 1010 Bq. The unit Becquerel has no relationship to the source volume or thequantity or type of energy of the radiation emitted. This term only has meaning when the particular radioactive isotopeis known. For example, five Becquerels of cobalt-60 are not equivalent to five curies of iridium-192 because ofdifferent energy levels and decay schemes.6.3.3 FocalSpotSize.For isotopes the physical size of the radioactive source can be thought of as the “focal spot”. Since the Becquerel onlyrelates the number of disintegrations per second, this unit has no relationship to the volume of mass or size of theradioactive source. The term "specific activity" is used to define the quantity of radioactivity of one gram of thesubstance and is expressed as Becquerels per gram. For a particular number of Becquerels, the dimensions of theradioactive source are governed by its specific activity. For radiographic applications a small source size is desirable toproduce images with good resolution or sharpness, just as a small focal spot in an X-ray tube is required for highresolution 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 smallvolumes of material. Some special sources are stated as high specific activity, indicating a high radiation outputrelative to the source size. Nevertheless, in most isotopes the source size exceeds the focal spot size in X-ray tubes.6.3.4 DecayCharacteristics.As radioactive material decays, there are a fewer number of unstable atoms left to decay and as time passes theradioactive material is becoming less and less radioactive. Different isotopes have different decay rates. If a singleatom of an isotope existed, it would be impossible to predict at what moment in time it might disintegrate. But if largenumbers, of atoms exist, it is possible to measure the lapse of time required for one atom out of every two todisintegrate. This is called the half-life of an isotope. The half-life is defined, as the time required for an isotope todecay to one-half of its original radioactivity.6.3.5 IsotopeSensitivity.WARNINGThe radiation levels at the surface of the shielded container are hazardous topersonnel over prolonged periods of contact.CAUTIONUndeveloped film shall not be stored in the immediate area of the shieldedcontainer.Radiographic definition obtained with isotope sources is usually of lower quality than that obtained with X-rays becauseof:
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