Avaliação do ponto efetivo de medida em camaras de ionização cilindricas usadas em feixes de eletrons com energias entre 6 e 20 MeV

AUTOR(ES)
DATA DE PUBLICAÇÃO

1984

RESUMO

When ionization chambers are used. in high energy electrons beams, a disturbance of the electron flux in medium is introduced. As observed in many experiments the response should be correlated to the electron flux at the effective point of measurement which is generally displaced from the geometrical center of the chamber. The radial displacement was determined in a water phantom for electrons beams at energies from 6 to 20 MeV for three commercial cylindrical ionization chambers of internal diameters varying from 3.5 to 9.0 mm. The chambers were irradiated with the main axis perpendicular to the direction of the beam. A 300 V bias voltage was applied and readings were taken with both polarities. It was observed that, with increasing depth in the water phantom, the radial displacement remains constant for the 8.9 MeV beam, it increases for the 12.6 MeV electrons and decreases for those of 16.8 and 19.7 MeV. A theoretical model was built in. order to calculate the displacement of the effective point of measurement. The Fermi-Eyges multiple scattering theory and a rectangular beam formalism developed by Jette (1983) for therapeutic electron beam are used. It was found that the radial displacement stays constant with increasing depth and it decreases with increasing average energy of the incident beam. The model also predicts that the displacement is dependent on the chamber radius. The experimental and theoretical results are compared. They show good agreement for 8.9 and 12.6 MeV electron, while for 16.8 and 19.7 MeV electrons they indicate that modifications in the theoretical model are necessary

ASSUNTO(S)

ionização camaras de ionização feixes de eletrons

ACESSO AO ARTIGO

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