ABSTRACT
This chapter discusses three conditions for which a detector can be considered to be a Bragg-Gray cavity. It describes briefly the characteristics of an ideal dosimeter, and presents the recommended properties of a cylindrical ionisation chamber for the purpose of reference dose calibrations based on the recommendations of the IAEA TRS398 Code of Practice. The chapter reviews the basic principle of the operation of an ionisation chamber. Plane-parallel ionisation chambers are designed so that they minimise scattering effects by the chambers. The thin front window samples the incident electron beam while the design of the side walls is such that there are only minimal electrons that enter the cavity from the side. Diodes are commonly used for relative dosimetry and in vivo dosimetry. The chapter also provides an overview of radiochromic film dosimetry. Small fields in radiotherapy are used in various treatment techniques such as intensity modulated radiotherapy, volumetric modulated arc therapy and stereotactic radiosurgery.
