Institut des Nanotechnologies de Lyon INL, CNRS UMR5270, Université Lyon 1, Villeurbanne, France.
Phys Med Biol. 2013 Oct 7;58(19):6701-12. doi: 10.1088/0031-9155/58/19/6701. Epub 2013 Sep 9.
Gallium nitride (GaN), a direct-gap semiconductor that is radioluminescent, can be used as a transducer yielding a high signal from a small detecting volume and thus potentially suitable for use in small fields and for high dose gradients. A common drawback of semiconductor dosimeters with effective atomic numbers higher than soft tissues is that their responses depend on the presence of low energy photons for which the photoelectric cross section varies strongly with atomic number, which may affect the accuracy of dosimetric measurements. To tackle this 'over-response' issue, we propose a model for GaN-based dosimetry with readout correction. The local photon spectrum is calculated by convolving fluence pencil kernel spectra with the beam aperture fluence distribution. The response of a GaN detector is modelled by combining large cavity theory and small cavity theory for the low and high energy components of the local spectrum. Monte Carlo simulations are employed for determination of specific correction factors for different GaN transducer sizes and irradiation conditions. Some model parameters such as the cut-off energy and partitioning energy are discussed. The accuracy of the GaN dosimetric response model has been evaluated for tissue phantom ratio experiments along the central axis. These experiments have shown that calculated and measured GaN responses stay within ±3% at all depths beyond the build-up depth. The calculated GaN response factor is also in good agreement with measured data (±2.5%). The validated model with response compensation improves significantly the accuracy of dosimetric measurements: below 2.5% deviation as compared to 13% without compensation, for a 10 × 10 cm(2) field, at depth from 1.5 to 22 cm.
氮化镓（GaN）是一种直接能隙半导体，具有放射发光性，可用作换能器，能从小的探测体积中产生高信号，因此可能适用于小场和高剂量梯度。原子序数高于软组织的半导体剂量计的一个常见缺点是，它们的响应取决于低能光子的存在，而对于这些光子，光电截面随原子序数强烈变化，这可能会影响剂量测量的准确性。为了解决这个“过度响应”问题，我们提出了一种带有读出修正的基于 GaN 的剂量计模型。通过将注量铅笔核谱与束流孔径注量分布卷积，计算局部光子谱。通过将大空腔理论和小空腔理论结合起来，模拟 GaN 探测器对局部谱的低能和高能分量的响应。为了确定不同 GaN 换能器尺寸和辐照条件下的特定修正因子，采用蒙特卡罗模拟。讨论了一些模型参数，如截止能和分配能。在组织体模比实验中评估了 GaN 剂量响应模型的准确性，实验结果表明，在所有超出累积深度的深度处，计算和测量的 GaN 响应都在±3%以内。计算出的 GaN 响应因子与实测数据（±2.5%）也吻合较好。带有响应补偿的验证模型显著提高了剂量测量的准确性：与没有补偿时相比，在 10×10cm2 场、深度从 1.5 到 22cm 处，偏差低于 2.5%。
