CHAPTER 17 17.3. FREE-FIELD DATA NECESSARY FOR ASSESSMENT OF NUCLEAR RADIATION EFFECTS 17.3.1 General Introduction An assessment of nuclear-radiation effects on personnel (presented in Chapter 18) requires a knowledge of the total nuclearradiation exposure, measured by the nuclear-radiation exposure dose or a time integration of the dose rate received at the point of ex- posure. The total radiation exposure from a water-surface or under- water nuclear detonation may include contributions from some or all of the following: neutrons, gamma-rays, and beta particles. These different radiations emanate at various times from the fireball or from radioactive materials that result from the detonation. While directional and energy characteristics of the radiations should be understood to permit accurate estimation of the total exposure, it is frequently possible to estimate nuclear-radiation exposures by ecaling field-test dose-rate or dose data. However, in some cases exposures mist be calculated with theoretical techniques, primarily ry in situations where the exposures are reduced by shielding materials (as when below-decks spaces are shielded by a ship's structure). Theoretical calculation of such nuclear-radiation exposures requires knowledge of the nuclear radiation characteristics, such as source strengths, energy spectra, and energy degradations that occur between the source and exposure point as well as of the radiation source and ship geometries. Each component of the total radiation exposure has, in general, a broad energy spectrum that changes with time as the radioactivity decays. Moreover, the decay rate itself differs slightly for different situations, depending on fractionation of the radioactive debris. 17.3.2 Measurement of Nuclear Rodiation The ionization produced during the passage of nuclear radiations through any medium is used both for detection and measurement of the radiation. The amount of ionization produced can be measured, and, depending on the kind of radiation involved, either of two units. can be expressed in Gamma radiation measured in units of roentgens is termed an exposure dose, which measures the quantity of gamma radiation in terms of the ionization produced in air. One roentgen of gamma radiation produces 1 esu per ce of air, which is equivalent to the release of about 88 ergs of energy per gram of dry air. Instruments have been developed that measure gamma dose rate (the number of roentgens delivered per unit time) and gamma dose (a time-integration of the dose rate during the exposure period). Exposure-dose gamma measurements provide free-field measurements of gamma radiation. 17-13