TABLE OF CONTENTS When penetrating a matter of thickness d, a collimated beam of X-rays or ?-rays with initial intensity I 0 is attenuated exponentially:
Here is the ?-ray linear attenuation coefficient, usually measured in cm ?1 units. It is a sum of the interaction terms described in Chapter 6 of this Volume, hence it is also called total attenuation coefficient. Its inverse is called the mean free path, while the thickness reducing the photon beam by half is the half-thickness d 1/2; both are measured in cm. Frequently the mass attenuation coefficient / ? is used, because it does not depend on the physical state of the material. Its dimension is cm 2/g if the density ? is given in g/cm 3 units. Another important quantity is the mass-energy absorption coefficient en / ?, measured in the same units, which characterizes the energy deposition by photons. All these quantities, their units and usage have been defined by the International Commission on Radiation Units and Measurements (ICRU) in ICRU Report 33 (ICRU 1980), which has recently been superseded by two new ones (ICRU 1993c, 1998).
Tables and graphs of / ? and en / ? were presented for elements with Z=1 to 92, and for 48 compounds and mixtures of radiological interest (ICRU 1989b) for the energy range...
