A complex neutron spectrum generated along with a useful photon beam imposes an additional radiation protection risk around medical linear accelerators (linac). The thermal neutron component of this complex neutron spectrum formed during different photon modes of operation of Elekta Versa HD linac has been quantified using Indium foil activation technique. The thermal neutron fluence (Φ th ) at isocenter for 15 MV, 10 MV and 10 MV FFF beams was found to be 2.45 × 105, 4.35 × 104 and 3.2 × 104 neutrons cm-2 Gy-1, respectively. The analysis shows a reduction in the Φ th as the flattening filter is being taken out from the beam path. A negative correlation in Φ th with respect to field size has been observed with an average 18% reduction in Φ th per monitor units as field size changes from 10 cm × 10 cm to 40 cm × 40 cm. For particular field size and photon energy, Φ th was found to be uniform across the patient plane. From the measured gamma ray spectrum inside the treatment room six major isotopes have been identified which were 122Sb, 187W, 82Br, 56Mn, 24Na and 28Al.

中文翻译：

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不同光子能量下 Elekta Versa HD 直线加速器周围热中子注量的实验测定

与有用的光子束一起生成的复杂中子谱在医用直线加速器 (linac) 周围施加了额外的辐射防护风险。在 Elekta Versa HD 直线加速器的不同光子操作模式期间形成的这种复杂中子谱的热中子分量已使用铟箔活化技术进行了量化。发现 15 MV、10 MV 和 10 MV FFF 光束在等中心处的热中子注量 (Φ th ) 分别为 2.45 × 105、4.35 × 104 和 3.2 × 104 中子 cm-2 Gy-1。分析表明，随着从光束路径中取出展平滤波器，Φ th 会降低。已观察到 Φ th 与视场大小呈负相关，随着视场大小从 10 cm × 10 cm 变为 40 cm × 40 cm，每个监视器单位的 Φ th 平均减少 18%。对于特定的场大小和光子能量，发现 Φ th 在整个患者平面上是均匀的。从治疗室内测量的伽马射线光谱中，已经确定了六种主要同位素，它们是 122Sb、187W、82Br、56Mn、24Na 和 28Al。