4. Radiation units
Contents
4. Radiation Units
With the introduction of the Ionising Radiations Regulations 1999, it has been obligatory to adopt the International System of Units (SI) for use with ionising radiations. These will be used throughout the local rules, and in all information or data collection for compliance with legislation. Conversion factors to the old units will be found in Appendices 35 and 36.
Note: SI units must be used in drain records to summarise monthly cumulative totals and for storage records.
4.1 Activity
The SI unit of activity is the Becquerel (Bq), and is equal to one disintegration per second (dps). This is a very small unit in relation to the Curie, which is the activity of lg of radium, equivalent to 3.7 x 1010 dps.
4.2 Absorbed Dose
When ionising radiations pass through matter, some or all of the energy they possess is given up, and the absorbed dose is a measure of this energy deposition. In the SI units, 1 Gray (Gy) is defined as the energy deposition of 1 joule per kilogram.
4.3 Equivalent Dose
For the same absorbed dose of radiation, some types of ionising radiations have a greater biological effectiveness. To allow for this radiation weighting factors have been introduced which reflect the ability of different ionising radiations to cause damage, and if we multiply the absorbed dose by the appropriate factor, we arrive at what is called the Equivalent Dose. This is measured in Sieverts. For work with beta, gamma and X-ray emissions, this factor has been assigned a value of 1 and, therefore, will be of no concern to most people. A summary of radiation weighting factor values is given in Appendix 36.