Objective: To investigate the scatter factor of Irdiuml92 HDR source during calibration in air. Methods: All measurements are performed with thimble 0.6cc Farmer type thimble chamber and PTW well-type chamber. The well type chamber has been placed 1 metre above the floor to guarantee negligible influence of the floor scattering. The distance of outer lateral surface of the chamber to room wall was varied in the range 5 cm to 200 cm. The sensitivity of the well-type chamber for scattering from room wall on different distance was investigated and then the reading at each distance had been normalized to the response for 200 cm distance.The Irl92 source and thimble chamber with build-up cap were positioned on the mobile stand and the distance between the source and chamber (SCD) was varied accurately from 5 cm~40 cm during the calibration. In the calibration procedure, the air Kerma has been measured according the multiple distance method(Goetsch), DGMP and shadow field method (SF) at several different distances from the center of the active core of the source and the reference point of the ionization chamber in a specially designed jig. During shadow field method(SF) the seven distances measurement was taken in two phases, the first phases was the measurement without shield, the second one with a 6 cm (three-tenth value layer)cone of lead block placed as close as possible the source and fully shield the chamber. Data was analyzed and fitted with Origin 8.0 software. All correction such as leakage, end-effect and chamber gradient was required prior to measurement. Results: For calibration with well type chamber the scatter is less influence from 1 meter distance from the lateral wall of the room .The result also shows that with Goetsch and DGMP method the scatter factor is independent of SCD and negligible. With the Shadow Field method the scatter factor is less but constant basically. Conclusions: The calibration has to carry out in minimum scatter environment with well type chamber at least 1 meter away from any wall or floor. For calibration in air with thimble chamber it is best to measurement the scatter in local and the SF likely will serve the better method of obtaining a scatter factor.%目的:研究铱192后装放射源在空气中刻度的散射因子.方法:PTW井形电离室放置在离地1m高的支架上,测量在电离室外缘离墙距离5 cm到200 cm处进行,读数以200 cm处读数归一.按多步法、DGMP法和遮挡法要求在空气中用Farmer指形电离室在不同源室距(SCD)上对铱192后装放射源进行测量,遮挡法测量中在放射源和电离室之间放置6 cm厚度的圆锥台型铅挡,测得数据用Origin8.0软件进行拟合处理分析.测量中读数均摈弃端效应、暗电流并做梯度修正.结果:井形电离室对铱192放射源刻度,读数在电离室和墙壁距离较近时受到墙面散射影响,电离室外缘离墙壁距离在1m以上时散射影响可以忽略,Farmer指形电离室在空气中测量结果表明用Goetsch法和DGMP法计算,散射因子恒定与测量距离(SCD)无关且可以忽略,SF法的散射因子略小.结论:井形电离室刻度后装放射源需在最小散射情况下进行,在空气中用指形电离室进行刻度应对散射进行实际测量,遮挡法是较为符合实际的测量方法.
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