自显影胶片剂量测量系统的测量性能

Measurement performance of Gafchromic film dosimeter

目的:通过黑化时间、剂量测量范围、剂量分辨率和剂量响应不均匀性等主要剂量测量性能的研究测试,确定基于EBT3系列胶片的自显影胶片剂量测量系统的适用条件和胶片读取时间。方法:对3张胶片分别授予不同的剂量,在辐照后不同时刻读取自显影胶片的光密度值,分析并确定黑化时间对剂量测量结果的影响及不同授予剂量对黑化时间的影响;通过吸收剂量-光密度值曲线确定自显影胶片剂量测量范围;通过两组剂量相差0.01 Gy的平均光密度值获得自显影胶片剂量测量系统的剂量测量分辨率;通过辐照整张胶片和裁剪胶片的方式评价自显影胶片的剂量响应不均匀性。结果:相对于辐照终止时刻的光密度值,4 h的光密度值变化分别为1.69%(2 Gy)、2.53%(4 Gy)和2.13%(8 Gy),24 h的光密度值变化分别为2.91%(2 Gy)、3.31%(4 Gy)和3.20%(8 Gy),48 h的光密度值变化分别为2.91%(2 Gy)、3.31%(4 Gy)和3.96%(8 Gy)。0.1~10.0 Gy内自显影胶片剂量测量系统给出的光密度值与水吸收剂量呈线性关系,10.0~30.0 Gy内自显影胶片剂量测量系统的光密度值与水吸收剂量呈非线性关系,但仍具有一一对应关系。辐照剂量相差0.01 Gy的两组光密度值平均值相差0.004;整张胶片方式不均匀性测量结果为2.2%,裁剪胶片方式不均匀性测量结果为2.8%。结论:辐照后24 h可作为自显影胶片EBT3的读取时刻,且授予剂量与黑化时间无关;自显影胶片剂量测量系统在0.1~10.0 Gy内可用于单点剂量测量和剂量分布测量,10.0~30.0 Gy内可用于单点剂量测量。自显影胶片剂量测量系统剂量测量分辨率好于0.01 Gy。为避免散射线影响,建议采用裁剪胶片的方式测试胶片剂量响应的不均匀性。

Objective To determine the applicable conditions and the film reading time of Gafchromic EBT3 film dosimeter through the research and test of the measurement performance such as blackening time,dosimetry range,dosimetry resolution and dose response non-uniformity.Methods Three Gafchromic films were exposure to different doses,and the optical densities were read at different moments after irradiation for analyzing and confirming the effects of blackening time on dose measurement and different doses on blackening time.The absorbed dose-optical density curve was used to determine Gafchromic film dosimetry range.The Gafchromic film dosimetry resolution was obtained by calculating the average optical densities of two groups with a dose difference of 0.01 Gy.The dose response non-uniformity of Gafchromic film was evaluated by irradiating the whole film and the cutting film.Results Relative to the optical density at the termination moment of irradiation,the optical density changes at 4 h were 1.69%(2 Gy),2.53%(4 Gy)and 2.13%(8 Gy);the changes at 24 h were 2.91%(2 Gy),3.31%(4 Gy)and 3.20%(8 Gy);and the changes at 48 h were 2.91%(2 Gy),3.31%(4 Gy)and 3.96%(8 Gy).The optical density given by Gafchromic film dosimeter within 0.1 to 10.0 Gy had a linear relationship with the absorbed dose,while the optical density of Gafchromic film dosimeter within 10.0 to 30.0 Gy had a nonlinear relationship with the absorbed dose,but there was still a one-to-one correspondence.The difference of the average optical densities of two groups was 0.004 when the inter-group dose difference was 0.01 Gy.The non-uniformity was 2.2%for the whole film and 2.8%for the cutting film.Conclusion Gafchromic EBT3 film reading time can be set at 24 h after irradiation,and the dose is independent of blackening time.Gafchromic film dosimeter can be used for point dosimetry and dose distribution measurement within 0.1 to 10.0 Gy,and point dosimetry within 10.0 to 30.0 Gy.The dosimetry resolution of the Gafchromic film dosimeter is better than 0.01 Gy.To avoid the effect of scattered rays,the cutting film is recommended for testing dose response non-uniformity.