平行板电离室两种校准方法研究

目的 研究用γ光子线束(⁶⁰Co模体法)和高能电子线束(电子束法)校准平行板电离室吸收剂量因子方法。方法 电子束法:0.65cc指形电离室放在水中有效点深度2.88 cm (考虑电离室半径),平行板电离室(NACP02)放在水中有效点深度2.70 cm,都距监督指形电离室3 cm处,电子线束能量18 MeV,照射野15 cm×15 cm,SSD=100 cm,照射:300MU,测量;不加监督电离室,并按上述条件照射并测量;根据国际原子能机构(IAEA)381号报告,分别计算平行板电离室空气吸收剂量校准因子。⁶⁰Co模体法:水模体30 cm×30 cm×30 cm,0.65cc指形电离室放在水中深度5cm,照射野10 cm×10 cm,SSD=80 cm,照射时间60s;水模体25 cm×25 cm×25 cm,平行板电离室放在水中有效点深度5cm,其他条件相同,计算平行板电离室空气吸收剂量校准因子。最后将两种方法校准结果进行比较。结果 电子束方法校准平行板电离室结果为52.30 Gy/C·kg ^-1(不加监督电离室的值为52.27Gy/C·kg ^-1)。⁶⁰Co模体法校准平行板电离室结果为52.33 Gy/C·kg ^-1。结论 电子束法与⁶⁰Co模体法校准平行板电离室空气吸收剂量因子偏差仅为0.05%。因此,测量电子线束输出剂量,对平行板电离室的校准既可选择高能电子线束也可选择⁶⁰ Co光子γ线束。

Two Methods for Calibration on Plane Parallel Ionization Chambers

Objective Using two methods (The electron beam method and ⁶⁰ Co in-phantom method) for calibration plane parallel ionization chambers.Methods The electron beam method:The main recommendation in TRS-381 is to perform the determination of N_D,air^pp,_air using the electron beam method where the plane parallel ionization chamber(NACP02, Depth of the effective point of measurement 2.70 cm) to be calibrated is compared with a reference thimble chamber 0.65 cm³ (Depth of the effective point of measurement 2.88 cm). Electron beam energy is 18 MeV, The SSD should be 100 cm and the field size at the phantom surface 15 cm×15 cm,Dose rate of the accelerator 300 monitor units/min; or the same way but not use the chamber which is used to monitor. ⁶⁰ Co in-phantom method:The SSD should be 80cm and the field size at the phantom surface is10 cm×10 cm,measurement time 60 s, with a reference thimble chamber 0.65 cm³. The other condition is same with the electron beam method. Then to measure and compare the results of two methods. Results Absorbed dose to air factor using the electron beam method is 52.30Gy/C·kg ^-1 (with out monitor chamber 52.27 Gy/C·kg ^-1), where there are 0.05% deviation to ⁶⁰ Co in-phantom method which is 52.33 Gy/C·kg ^-1. Conclusion ⁶⁰ Co in-phantom method which can give the same results of calibration plane parallel ionization chambers may replace the electron beam method for absorbed dose to air factor.