Radiation Shielding and Radiological Protection

Abstract

This chapter deals with shielding against nonionizing radiation, specifically gamma rays and neutrons with energies less than about MeV, and addresses the assessment of health effects from exposure to such radiation. The chapter begins with a discussion of how to characterize mathematically the energy and directional dependence of the radiation intensity and, similarly, the nature and description of radiation sources. What follows is a discussion of how neutrons and gamma rays interact with matter and how radiation doses of various types are deduced from radiation intensity and target characteristics. This discussion leads to a detailed description of radiation attenuation calculations and dose evaluations, first making use of the point-kernel methodology and then treating the special cases of “skyshine” and “albedo” dose calculations. The chapter concludes with a discussion of shielding materials, radiological assessments, and risk calculations.  Radiation Fields and Sources The transmission of directly and indirectly ionizing radiation through matter and its interaction with matter is fundamental to radiation shielding design and analysis. Design and analysis are but two sides of the same coin. In design, the source intensity and permissible radiation dose or dose rate at some location are specified, and the task is to determine the type and configuration of shielding that is needed. In analysis, the shielding material is specified, and the task is to determine the dose, given the source intensity, or the latter, given the former. The radiation is conceptualized as particles – photons, electrons, neutrons, and so on. The term radiation field refers collectively to the particles and their trajectories in some region of space or through some boundary, either per unit time or accumulatedover some period of time. Characterization of the radiation field, for any one type of radiation particle, requires a determination of the spatial variation of the joint distribution of the particle’s energy and direction. In certain cases, such as those encountered in neutron scattering experiments, properties such as spinmay be required for full characterization. Such infrequent and specialized cases are not considered in this chapter. The sections to follow describe how to characterize the radiation field in a region of space in terms of the particle fluence and how to characterize the radiation field at a boundary in terms of the particle flow.Thefluence and floware called radiometricquantities, as distinguished from dosimetric quantities. The fluence and flow concepts apply both to measurement and calculation. Measured quantities are inherently stochastic, in that they involve enumeration of individual particle trajectories. Measurement, too, requires finite volumes or boundary areas. The same is true for fluence or flow calculated by Monte Carlo methods, because such calculations are, in large part, computer simulations of experimental determinations. In themethods of analysis discussed in this chapter, the fluence or flow is treated as a deterministic point function and should be interpreted as the expected value, in a statistical sense, of a stochastic variable. It is perfectly proper to refer to the fluence, flow, or related dosimetric quantity at a point in space. But it must be recognized that any measurement is only a single estimate of the expected value. Radiation Shielding and Radiological Protection   . Radiation Field Variables .. Direction and Solid Angle Conventions The directional properties of radiation fields are commonly described using spherical polar coordinates as illustrated in > Fig. . The direction vector is a unit vector, given in terms of the orthogonal Cartesian unit vectors i, j, and k by Ω = iu + ν + kω = i sin θ cos ψ + j sin θ sinψ + k cos θ. () An increase in θ by dθ and ψ by dψ sweeps out the area dA = sin θ dθ dψ on a sphere of unit radius. The solid angle encompassed by a range of directions is defined as the area swept out on the surface of a sphere divided by the square of the radius of the sphere. Thus, the differential solid angle associated with the differential area dA is dΩ = sin θ dθ dψ. The solid angle is a dimensionless quantity. Nevertheless, to avoid confusion when referring to a directional distribution function, units of steradians, abbreviated sr, are attributed to the solid angle. A substantial simplification in notation can be achieved by making use of ω ≡ cos θ as an independent variable instead of the angle θ, so that sin θ dθ = −dω.The benefit is evidentwhen one computes the solid angle subtended by “all possible directions,” namely,