Department of Radiation Sciences, Radiation Physics, University of Umeå, SE-901 85 Umeå, Sweden.
Phys Med Biol. 2012 Nov 7;57(21):7161-75. doi: 10.1088/0031-9155/57/21/7161. Epub 2012 Oct 12.
Radiation dosimetry of highly modulated dose distributions requires a detector with a high spatial resolution. Liquid filled ionization chambers (LICs) have the potential to become a valuable tool for the characterization of such radiation fields. However, the effect of an increased recombination of the charge carriers, as compared to using air as the sensitive medium has to be corrected for. Due to the presence of initial recombination in LICs, the correction for general recombination losses is more complicated than for air-filled ionization chambers. In the present work, recently published experimental methods for general recombination correction for LICs are compared and investigated for both pulsed and continuous beams. The experimental methods are all based on one of two approaches: either measurements at two different dose rates (two-dose-rate methods), or measurements at three different LIC polarizing voltages (three-voltage methods). In a comparison with the two-dose-rate methods, the three-voltage methods fail to achieve accurate corrections in several instances, predominantly at low polarizing voltages and dose rates. However, for continuous beams in the range of polarizing voltages recommended by the manufacturer of the LICs used, the agreement between the different methods is generally within the experimental uncertainties. For pulsed beams, the agreement between the methods is poor. The inaccuracies found in the results from the three-voltage methods are associated with numerical difficulties in solving the resulting equation systems, which also make these methods sensitive to small variations in the experimental data. These issues are more pronounced for the case of pulsed beams. Furthermore, the results suggest that the theoretical modelling of initial recombination used in the three-voltage methods may be a contributing factor to the deviating results observed.
高调制剂量分布的辐射剂量测定需要具有高空间分辨率的探测器。充液电离室（LIC）有可能成为此类辐射场特性描述的有用工具。然而，与使用空气作为灵敏介质相比，必须对载流子复合增加的影响进行修正。由于 LIC 中存在初始复合，因此一般复合损耗的修正比空气填充电离室更为复杂。在本工作中，比较并研究了最近发表的用于 LIC 的一般复合修正的实验方法，这些方法适用于脉冲和连续束。这些实验方法都基于两种方法之一：要么在两个不同的剂量率下进行测量（双剂量率方法），要么在三个不同的 LIC 极化电压下进行测量（三电压方法）。与双剂量率方法相比，三电压方法在几种情况下无法实现精确修正，主要是在低极化电压和剂量率下。然而，对于所用 LIC 制造商推荐的极化电压范围内的连续束，不同方法之间的一致性通常在实验不确定度范围内。对于脉冲束，方法之间的一致性很差。三电压方法的结果存在不准确性与求解所得方程组的数值困难有关，这也使得这些方法对实验数据的微小变化敏感。对于脉冲束的情况，这些问题更为突出。此外，结果表明，三电压方法中使用的初始复合的理论模型可能是观察到的偏离结果的一个因素。
