Units of radioactivity
For the quantitative measurement of different aspects of radioactivity, there are a substantial number of units of radioactivity. The number is relatively high because the aspects include the energy of a radiation source, as well as the effect of ionizing radiation in air, absorbed in arbitrary materials, and specific effects on biological systems. Doubling the number of units is the reality that they are defined both for the International System of Units (SI) and in “traditional” or “customary” units.
Properly interpreting the units can be quite complex. A given quantum of ionizing radiation will have different effects not only due to the type of radiation, which is considered in the computation of the units of biological effect, sieverts and rems, but also due to the energy level within a radiation type. Fast, medium speed and slow neutrons, for example, have different biological effects.
On arriving at an accident site, the Incident Commander tells the radiation survey specialist that a package of medical isotopes is on a crashed truck. It is labeled to contain 0.2 Ci or 7.4 x 109 Bq of Cs 137. She wants to know the risks it presents, and get advice on how to handle it.'
This is a practical example how single units do not fully characterize a hazard. The basic quantitative measurements define the amount of potential radioactivity in the container: two tenths of a curie, or 200 millicuries, of cesium (Cs). This is equivalent to 7,400,000,000 Bq, or 7.4 GBq.
To assess hazard, you must consider the isotope involved. Cesium 137, widely used in medicine and industry, and also a likely contaminant from a nuclear reactor accident, is a beta and gamma emitter with a half life of 30 years. This half life means it will decay to 0.1 curie in 30 years.
Risk assessment requires a knowledge of the ionizing radiation emitted, and also the physical form of the isotope. Assume the package is intact, although some of the outer shielding may have torn off; there is no dispersal hazard. Beta and gamma radiation have different biological effects. They also have very different penetrating power. Beta particles are principally an internal hazard and a slight hazard to the outer skin. As long as the package contents are not dispersed, the beta aspect is relatively safe. Beta particles cannot travel, in air, for long distances.
Even though the biological effect of gamma rays is less, the radiation can travel long distances, penetrate light shielding, and, as an external source, is a hazard to living things.
“Practical steps that can be taken to reduce your internal risk
to Cs 137 would include wearing anti contamination
clothing complete with face mask or respirator (if the
responder is trained and respirator fitted.) Your exposure
to the gamma emitter in Cs 137 can be reduced by relying
on the exposure control methods of:
“Time spent in the radiation field may be lessened
by rotating the crew. Unless you have a designated
function, stay out of the radiation field. Put as much
shielding between you and the radiation source as possible.
The denser the material the better the shielding. For
example, a fire truck may provide better shielding than a
concrete block wall.”
As opposed to most of the other units, the SI unit becquerel (Si) and the common unit curie (Ci) deal with the activity of the source, not the effects on radioactivity reaching its destination. Linked with area or volume measurements, these units are useful in giving a quantitative measurement of contamination of areas of the ground or volumes of water. When the source contains multiple isotopes, as with a reactor accident, it is most useful to state activity of each isotope.
1 Bq = 1 event of radiation emission per second. Since this is a very small unit, common measurements are:
1 kBq = 103 Bq
1 MBq = 106 Bq
1 GBq = 109 Bq
The older unit, the curie (Ci), is equal to 37 GBq. Curies are large units, so common representations are