Calorimetry of high energy photon and electron beams
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1975Author
Tedman, Brian M.
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Abstract
The past two decades have seen a very dramatic rise in the production
of megavoltage X-ray and electron accelerators for use in therapeutic
medicine. Concomitant with this increase has been the need to develop
accurate and reliable methods for measuring the deposition of radiation
energy within the irradiated medium. Frequently, the ionometric methods
used for assessing the absorbed dose in materials irradiated by X-ray
beams generated at a few hundreds of kV have been extended to the megavoltage
region, but, whilst the precision and reliability of such methods have
generally been unaffected by this extension, the interpretation of such
measurements and their relationship to the absorbed dose has become
extremely complex. In this situation the technique of calorimetry
offers the only direct and unambiguous approach for measuring the
absorbed dose and this thesis deals with the design, construction and
operation of such an instrument with the particular intention of
calibrating a number of secondary dosimeters.
The production of a stable temperature environment, which enables the
small temperature changes encountered in calorimetry to be measured
accurately, is discussed and it was concluded that a multi-jacket system,
operated in the quasi-adiabatic mode, was the most suitable. The actual
temperature measurements were made with a Wheatstone Bridge Network, using
a thermistor transducer, and it is shown that if a particular criterion
is adopted for the sensitivity of the system then the bridge can be so
designed to maximise this sensitivity. The practical construction of
a bridge system using a phase sensitive detector and capable of a
temperature resolution of 2 x 10 5 K is also described.