Equipment used for beam callibration in Raditaion therapy
...e of equipment used for beam calibration is the parallel plate chamber. They are also known pancake chambers due to the two plates and are also called extrapolation chambers. They are used for different applications to thimble chambers. They operate according to the same principles as other ionisation chambers, but are configured differently. The parallel plate chambers have two electrodes usually with a 0.2mm air gap between them. The upper electrode is made from a thin metal such as beryllium and has a carbon coating. The lower electrode is known as the collecting electrode. The collecting electrode attracts free electrons that are released when gas molecules in the air are ionised, and this can then be registered as a measurable current (C.Opie: Personal Communication, 2004). Parallel plate chambers can be calibrated in many different ways, including the use of beams from cobalt 60 machines (uwrcl.medphysics.wisc.edu, 2002). Some type of build up material is usually found under the lower collecting electrode. A diagram of the parallel plate chamber can be found in the appendix under diagram 2. Application To calibrate the beams certain physical conditions must be maintained. There are certain materials that radiation beams travel through and distribute dose in the same way it does in human tissue. Water is often used as a tissue equivalent material or phantom material. Another type phantom material used is lucite which is also known as solid water due to its similar properties to water. Water tanks made out of Perspex are often used. They usually have a capacity of about 300 litres. Thimble chambers are often placed in these tanks to measure exposure when megavoltage X-ray beams are passed through the water. Beam flatness, symmetry and energy are some of the qualities that can be derived from water tank measurements. In Australia, beam calibration protocols are administered by ARPANSA. Measurements taken from thimble chambers are accurate and precise at any beam energy and direction. No ion recombination takes place in thimble chambers. Ionisation of radiation inside thimble chambers is meant to represent the same amount ionised in a free-air ionisation chamber (C.Opie: Personal Communication, 2004) . This is achieved by making the thickness of the chamber casing equivalent to the thickness of air in a free-air ionisation chamber (Khan 2003, p82). Although most ionisation chambers come with small errors that must be corrected mathematically, thimble chambers have been designed in such a way to minimise these errors. Since different beam energies require differing thicknesses of plas...