Accuracy of neutron self-activation method with iodine-containing scintillators for quantifying 128I generation using decay-fitting technique
Accuracy of neutron self-activation method with iodine-containing scintillators for quantifying 128I generation using decay-fitting technique
复制标题
使用含碘闪烁体的中子自激活方法使用衰变拟合技术量化 128I 产生的准确性
DOI：
10.1016/j.nima.2015.08.005
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发表时间：
2015
影响因子：
1.4
通讯作者：
G. Wakabayashi
中科院分区：
文献类型：
--
作者：
A. Nohtomi;G. Wakabayashi
我们评估了用含碘闪烁体的自激活方法对激活探测器中的128I产生量进行量化的准确性；自激活方法是最近提出的，用于X射线放射治疗机周围的光中子在线测量。在这里，我们考虑了确定初始计数率R0的准确性，该计数率是在激活探测器的中子辐照终止后观察到的。R0值与入射中子产生的活度直接相关；在进行这种评估时，应考虑到从活度发出的辐射的探测效率。用计算机程序数值模拟了不同初始计数速率(R0)和本底速率(RB)以及计数统计起伏等不同条件下12 8I活度的衰变曲线。用非线性最小二乘拟合程序对每隔一分钟采样并在同一时间段积分的数据点进行拟合，得到R0值作为具有相关不确定度的拟合参数。在相同的拟合程序中同时计算相应的本底速率R、B。相同的数据集也被用于传统激活方法的众所周知的积分算法评估，并将结果与所提出的拟合方法的结果进行比较。当我们固定RB=500cpm时，对于具有20个数据点的R/0RBσ2 0，其相对不确定度为R0/R0≤0 0 2，从用于拟合的中子辐照终止后的1~2 0min，对于具有相同数据点的R/0 RB≥5 0，获得了σR0/R0≤0.0 1。利用实际抽样数，用衰减拟合法得到了评估初始计数率的合理的相对不确定度。这些结果澄清了拟合法的理论局限性。积分方法被发现可能容易受到背景水平的短期变化的影响，特别是尖峰噪声造成的瞬时污染。这种拟合方法可以很容易地检测到并消除这种尖峰噪声。
We evaluated the accuracy of a self-activation method with iodine-containing scintillators in quantifying 128 I generation in an activation detector; the self-activation method was recently proposed for photo-neutron on-line measurements around X-ray radiotherapy machines. Here, we consider the accuracy of determining the initial count rate R 0, observed just after termination of neutron irradiation of the activation detector. The value R 0 is directly related to the amount of activity generated by incident neutrons; the detection efficiency of radiation emitted from the activity should be taken into account for such an evaluation. Decay curves of 128 I activity were numerically simulated by a computer program for various conditions including different initial count rates (R 0) and background rates (R B), as well as counting statistical fluctuations. The data points sampled at minute intervals and integrated over the same period were fit by a non-linear least-squares fitting routine to obtain the value R 0 as a fitting parameter with an associated uncertainty. The corresponding background rate R B was simultaneously calculated in the same fitting routine. Identical data sets were also evaluated by a well-known integration algorithm used for conventional activation methods and the results were compared with those of the proposed fitting method. When we fixed R B= 500 cpm, the relative uncertainty σ R 0/R 0≤ 0.02 was achieved for R/0 R B≥ 20 with 20 data points from 1 min to 20 min following the termination of neutron irradiation used in the fitting; σ R 0/R 0≤ 0.01 was achieved for R/0 R B≥ 50 with the same data points. Reasonable relative uncertainties to evaluate initial count rates were reached by the decay-fitting method using practically realistic sampling numbers. These results clarified the theoretical limits of the fitting method. The integration method was found to be potentially vulnerable to short-term variations in background levels, especially instantaneous contaminations by spike-like noise. The fitting method easily detects and removes such spike-like noise.