Answer to Question #198 Submitted to "Ask the Experts"

Q

Can you tell me typical 'quality factors' for "real," complicated sources which emit more than one type of radiation at once?

A

By conventional definitions and as applied to practical radiation dosimetry, quality factors are applied to the absorbed dose (energy deposited due to radiation exposure) to provide a dose equivalent which reflects the biological risk associated with the radiation quality. In applied radiation dosimetry, the absorbed dose can be measured using a thermoluminescent or other radiation-sensitive detector and related to the radiation risk (dose equivalent) by the quality factor. As long as the radiation qualities (gamma, beta, neutron, x rays) associated with the source are constant, a spectrum- and radiation-average quality factor could theoretically be developed to relate the gross-measured absorbed dose to the effective biological hazard for the exposure. To develop such a radiation- and spectrum-average quality factor is relatively straightforward but could be complicated depending on the number of different radiation types comprising the incident field and the associated energy distributions. For a combination of monoenergetic sources of different radiation types, the effective quality factor could be determined by adding the products of the fraction of the total fluence for an exposure times the quality factor for each type of radiation. In practice, the different types of radiation will also be disributed in energy so the energy spectrum associated with each type of radiation will also have to be taken into account during the averaging process. For example, isotopic neutron sources emit not only neutrons but also gamma and beta radiation. For radiation protection purposes, an average quality factor reflecting the total hazard from all the types of emitted radiations could be estimated by adding the relative components of the neutrons, betas, and gammas as described above and accounting for energy dependence. This factor could then be applied to the absorbed dose measured for the source to estimate a total radiation hazard. The effective quality factor for such a source considering all types of radiation would be lower than that for the most hazardous type (neutrons in this case) and higher than that for the least hazardous (low-energy betas or photons). Since in practice radiation fields can consist of a variety of radiation energies, dosimeter calibrations are usually done only for radiation types expected to be encountered, and types and the fields vary as a function of time, such an approach is not routinely used. Theoretically, a source-average quality factor could be accurate for radiation risk estimates under appropriate circumstances.