EXPERIENCE OF OUR NEW MONITOR FOR BETATRON THERAPY UNIT

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  • 電子線モニタ線量計の改造と使用経験
  • デンシセン モニタ センリョウケイ ノ カイゾウ ト シヨウ ケイケン

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The amount of high energy electron beams is instable owing to lots of factors. As a matter of course, we should be careful in controling the amount of beams and it is also necessary to improve the beam monitoring apparatuses to make them stable. We have improved a usual monitor and had experiments with it. Here is the report. (1) An ordinary monitor is influenced by humidity, air atmoshere induced noise from an apparatus and so on. We made a new monitor with due regard to these factors. When the count is set to 100 with a usual monitor, the standard dose meter (ionex) indicate is 280R in 16 MeV and the lowest one is 198R. Between standard dose meter (ionex) and usual monitor is 82R. With the trial monitor, the highest integrated dose is 103R and the lowest one is 89R. The difference is only 14R and we are satisfied with the result. We gaine this value after 16 months'trial. (2) The chamber is made air tight and has a double structure with a collection electrode sandwiched between high tension charging electrodes. In this way, we can eliminate the influences of humidity and induced noise from an apparatus itself and we can putup ion collection efficiency without wide the the distance between the electrodes. As for the indicator circuit, we can get an input signal with a big s/n ratio and so we need not use high impedance resistors in the input circuitry. There is little affect from humidity and induced noise, and the circuit is stable. The indicator circuit has a complete electronic system, therefore counting speed is greatly improved and we can measure a large amount of beams in a short time. The trial monitor has four measuring ranges and so measuring is easily done. Our monitor has a circuit tester, too, and we can test the measuring circuit on occasion.

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