[Study of the effectiveness of collection of cylindrical ionisation chambers irradiated in the pulsed radiation beams of a linear accelerator (author's transl)].
The difference of potential of supply of ionisation chambers is often inadequate to collect all the ions formed under the effects of a pulsed ray. The authors thus decided to determine the effectiveness of the collection of ions in one of the types of cylindrical ionisation chambers used most commonly at present in calibrating teleradiotherapy beams : Nuclear Enterprises chambers type 2505/3 A and 3 B. They determined essentially the value of effectiveness of the collection of ions using lithium borate, a thermoluminescent dosimeter the response of which is independent of dose up to very high flow rates. In the case of a pulsed ray emitted at constant frequency, it is easy to evaluate the dose delivered by impulsion and then compare the values obtained experimentally with those calculated using theoretical formulae such as Boag's formula. By contrast, in the case of an accelerator equipped with an electromagnetic sweep system of the beam in order to ensure a homogeneous dose of electrons, instantaneous flow doses (dose/impulsion) are difficult to evaluate by virtue of the sweep and variables from one type of energy to another. The authors thus sought an empirical method making it possible to evaluate the effectiveness of the collection of ions, knowing the energy of the incident electron beam and the dose flow indicated by the ionisation chamber receiving its usual supply tension. The results of this study as a whole show not only that it is essential to take into account the recombination of ions when ionisation chambers are used to calibrate the irradiation beams of new accelerators for medical use, but also that the effectiveness of the collection of ions is dependent upon the electronic equipment associated with the chamber.
D008433:Mathematics; D011867:Radioisotope Teletherapy / Q000379:methods*; D013819:Thermoluminescent Dosimetry
Etude de l'efficacité de collection des chambres d'ionisation cylindriques irradiées dans les faisceaux de rayonnement pulsé d'un accélérateur linéaire