||The first Proton Therapy Center in Taiwan will be soon in operation. The superior quality of the depth-dose distribution of protons than electrons or photons to the target volume is well known. Proton therapy can reduce the radiation dose to normal tissue surrounding the tumor due to high-energy proton beam energy can be adjusted so that after it penetrates the body, it stops in the tumor, and will not continue to penetrate to damage the rear of the organ.|
However, protons generate more ion pairs in gas filled ionization chamber than electrons or photons, therefore, higher current density in detector. From BOAG theory, recombination effects depend on the electric field applied, the gas width of electrodes and the current density. The purpose of this study is to investigate the recombination effects with parallel plate ionization chamber.
Pad Ionization Chambers were used to study the recombination effect. The experiment was done at Institute of Nuclear Energy Research with 30 MeV protons. We set the experimental steps: (1) two gas gap: 3mm and 6mm, respectively (2) four beam current intensity (1, 5, 10 and 15 nA) (3) scanning electric field between parallel plates (for 3mm, from 100V to 1500V in step of 100 volts; for 6mm, from 200V to 3000V in step of 200 volts).
Collection efficiency means the relations between its ionization current and collection by pad ion chamber due to recombination by ions. Using Faraday cup timely monitoring of their beam current changes, and it measured by subtracting the amount of current material to stop the current correction ionization chamber current actual incident, this amendment before the experiment is an important step before analyzing.
Finally, we verified BOAG theory and compliance with General Recombination (Volume recombination) effects as the dominate ingredient under the experimental conditions. In proper operating voltage region; the larger E field, the smaller electrode spacing and the lower current intensity, the recombination efficiency is less than 0.1%. In proton therapy, we can ignore this recombination phenomenon; the proton beam current means collection ionization current by ion chamber.
|| ”The Physics of Radiation Therapy”, Editor: Faiz M. Khan, p.56, (2003), 3rd edition, Lippincott Williams & Wilkins|
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