散射中子对带屏蔽体的252Cf同位素中子源辐射场的影响

Effects of scattered neutrons on the neutron radiation field generated by Cf-252 neutron source with a shield

同位素中子源和中子刻度室在实际使用中通常带有屏蔽体。本文采用蒙特卡罗计算与实验结合的方法,研究了散射中子对带屏蔽体的252Cf同位素中子源辐射场的影响。通过计算带屏蔽体的252Cf中子源和裸252Cf中子源的中子能谱、中子注量率和中子周围剂量当量率,分析了屏蔽体产生的散射中子对辐射场的影响。依据蒙特卡罗计算所获得的有、无源室墙壁、地面和屋顶条件下的中子注量率和中子周围剂量当量率,分析了源室墙壁、地面和屋顶所产生的散射中子对辐射场的影响。计算结果表明,该中子辐射场的平均中子通量-周围剂量当量转换因子由裸252Cf中子源的385 pSv·cm2降至280 pSv·cm2,源室的墙壁、地面和屋顶产生的散射中子对中子注量率与中子周围剂量当量率的影响与距中子源的距离平方成正比。中子周围剂量当量率仪的实验测量结果与理论计算结果符合良好,验证了计算结果的可靠性。本研究为利用非标准同位素中子源中子参考辐射场进行辐射防护仪表的校准提供了一种实践可行的方法,具有很好的实用性。

Background： Shields are commonly constructed for a radionuclide neutron source in an actual calibration room in practice. Purpose： Monte Carlo （MC） calculation and experiments were applied to evaluate the effects of scattered neutrons on the neutron radiation field generated by Cf-252 neutron source with a shield. Methods： The effects of scattered neutrons caused by the shield of Cf-252 neutron source were evaluated by calculating the neutron spectra, neutron flux rate and neutron ambient dose equivalent with MC simulation. Similarly, the effects of scattered neutrons caused by the walls, ground and roof of source room were analyzed. Results： The calculation results show that the neutron flux-ambient dose equivalent conversion factor changes from 385 pSv-cm2 of a bare Cf-252 radionuclide from an idealized situation to 280 pSv.cm2 with the shield. The contribution of scattered neutrons from the walls, ground and roof is proportional to the square of distance between wall and source. The experimental data on dose rate are consistent with the calculated results and indicate the reliability of this method. Conclusion： This study provides a practical and feasible way to calibrate the radiation protection instruments using a non-standard radionuclide neutron radiation field.