"What quality factor"?

Abstract

This paper discusses the evaluation of dose-equivalent rates in mixed radiation environments such as are found, for example, around a high energy particle accelerator. It is shown that it is not possible to evaluate absorbed dose and dose-equivalent rates by any single measurement, in such complicated environments, with sufficient accuracy for purposes of radiation protection. This paper studies in detail the problems of evaluating dose equivalent from measurements of absorbed dose made with a tissue-equivalent ionization chamber. Our work supplements and is distinctly different from that of Shaw et al. They have shown that when a neutron spectrum is known one can use the ORNL (Oak Ridge National Laboratory) Monte Carlo calculation to calculate the dose equivalent and absorbed dose distributions in tissue and hence evaluate QF. On the other hand, in this paper we show that knowledge of the neutron spectrum is also necessary when one must choose an average “Quality Factor” to be applied to a single measurement of absorbed dose when only a table of (QF vs. neutron energy is available. Recently the U.S. Atomic Energy Commission has published a table of quality factors for monoenergetic neutrons as a function of neutron energy, and this has now appeared in National Bureau of Standards Handbook 107, but no values were given for the spectra spanning the large energy range typically found around high energy accelerators and reactors. By the use of the values of quality factor recommended for monoenergetic neutrons, the QF for a variety of typical neutron spectra was calculated. The neutron spectra studied include the Watt fission spectrum, the PuBe spectrum, the cosmic-ray neutron spectrum, and three typical accelerator spectra measured at CERN and the Lawrence Radiation Laboratory. In addition, calculations were made for a variety of spectra expressed in simple exponential form. These calculations indicate the quality factor to be an extremely sensitive function of neutron spectrum and maximum-energy cutoff in the range of practical interest-the average quality factor varies with these parameters by more than a factor of 3. This work indicates the potential errors in estimating dose equivalent when making absorbed dose measurements and assuming a single value of quality factor. Knowledge of such errors is a valuable guide to operational health physicists.