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Book review |
This Report is No 3 in the new series of the Journal of the ICRU. It provides guidance for the operational dose equivalent quantities for neutrons, and is complementary to Report 47 which dealt with the corresponding measurements for external photon and electron radiations.
The Executive Summary with which it begins gives a clear and concise indication of the areas to be covered. After a very brief Introduction, the first main section discusses quantities and their relations. Much of this will be familiar to anyone experienced in the field of radiation protection, but it is always helpful (particularly to newcomers) to have these basic concepts conveniently brought together. The question of the relationship between the operational quantities, the basic physical quantities and the protection quantities is addressed in some detail.
The next sections describe the principles of measurement and the neutron field characterization. Neutron fields in work-places span energies from a fraction of an electron volt to tens of MeV and thus a monitoring technique must be used which is appropriate for the energies concerned. The direction distribution is a further confounding factor, and therefore only a spectrometer with an isotropic response can give correct information on the energy distribution of the neutron fluence. Due to its wide-spread use for this purpose, the multi-sphere type of spectrometer is explained in some detail. Several other measuring systems which have been reported over the years are also touched on briefly.
A large number of methods exist for routine monitoring, considerations of which form the next section. The factors to be considered when selecting a method are listed, with a distinction between those suitable for personal monitoring and those more appropriate for area monitoring. The advantages and disadvantages of all the systems generally available are helpfully explained, and so this is the longest single section of the Report. This is followed by a brief consideration of the special case of high-energy (>20 MeV) neutrons. Attention is then drawn to the difficult situation where only extremities are likely to be subject to neutron irradiation.
Since all methods of measuring neutron fields measure fluence and the required quantity is dose equivalent a means of converting one to the other is essential. The difficulties and potential pitfalls of such a calibration form the penultimate section, and the last two pages provide a useful set of Conclusions.
Helpfully, the ancillary material is collected together in a set of appendices. The first of these, "The Role of Computational Methods in the Determination of Operational Dose Equivalent Quantities for Neutrons" points out that, for the last two decades, improvements in neutron dose measurements have not actually been derived from the development of new devices, rather from a better knowledge and characterization of existing ones. These improvements are largely due to the enhanced support of computational dosimetry, and the way in which this has been developed and applied is described in some detail. The conversion coefficients that relate neutron fluence to the operational quantities are listed in Appendix 2 for both a wide range of monoenergetic neutrons and for spectra of commonly used sources such as 252 Cf, while Appendix 3 is a literature review of response calculations for multi-sphere spectrometers.
It needs to be borne in mind that that the operational dose equivalent H* is actually a substitute for the effective dose E. Since it is the latter quantity to which risk estimates are related, it is important to know the ratio of H*/E. This, of course, will vary with neutron energy, but recent calculations have shown that, in most cases, the ambient dose equivalent will over-estimate the effective dose. The current radiation weighting factors for neutrons therefore ensure more than adequate protection.
Noting that there are nearly 400 references, it is difficult to imagines that any significant and relevant information has been over-looked. The Report is very readable, key points and definitions being repeated and emphasised as required. It can therefore be highly recommended to all those interested in neutron dosimetry.
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