Monte Carlo simulation research on reference neutron radiation of 241Am-Be radionuclide
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摘要:
研究用于校准场所中子剂量监测仪表的241Am-Be中子参考辐射场计量特性。采用蒙特卡罗方法模拟了空气自由中子参考辐射（FRNR），GB/T 14055规定的最小尺寸中子参考辐射（SRNR）和实际中子参考辐射（ARNR）中不同检验点处中子周围剂量当量率、散射中子占比和能谱分布特征。研究结果表明，空气对FRNR中的剂量率和能谱分布影响小，近似为理想中子参考辐射；采用5%含硼聚乙烯作屏蔽的最小尺寸SRNR可减少热中子，降低散射中子占比，影锥法不适用于小尺寸中子参考辐射中对散射中子的修正；ARNR中的散射中子更少、占比更低，影锥法所得散射中子占比与理论值基本一致。
Abstract:This paper presents the study on the 241Am-Be radionuclide sources reference neutron radiation field metrology characteristics for calibrating area neutron dose monitoring instruments. The Monte Carlo method was used to simulate the neutron ambient dose equivalent rate, scattering neutron proportion and neutron energy spectrum distribution at different points of test in the air free-field reference neutron radiation (FRNR), the minimum size of standard reference neutron radiation (SRNR) specified in GB/T 14055 series standards and the actual reference neutron radiation (ARNR). The results show that air has little effect on the dose rate and energy spectrum distribution in FRNR, which is approximately an ideal reference neutron radiation; the minimum size SRNR shielded by polyethylene containing 5% boron can decrease thermal neutrons and reduce the proportion of scattered neutrons. The shadow-cone method is not suitable for the correction of scattered neutrons in small-sized reference neutron radiation; scattering neutrons in ARNR are fewer and have a lower proportion, and the proportion of scattered neutrons obtained by shadow-cone method is basically consistent with the theoretical values.
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图 4 不同条件下FRNR中各检验点中子能谱分布
Figure 4. Neutron energy spectrum distribution at each point of test under different FRNR
图 5 SRNR中各检验点剂量率
(A：polyethylene containing 5% boron，B：concrete；solid line：half-cubical type，dotted line：cubical type)
Figure 5. Dose rate of each point of test under SRNR
图 6 半立方体SRNR中各检验点中子能谱分布
Figure 6. Neutron energy spectrum distribution at each point of test under half-cubical type SRNR
图 7 三种中子参考辐射中各检验点剂量率（屏蔽材料：混凝土）
Figure 7. Dose rate of each point of test under three kinds of reference neutron radiation (shielding material：concrete)
图 8 三种中子参考辐射中0.75 m处检验点中子能谱分布（屏蔽材料：混凝土）
Figure 8. Neutron energy spectrum distribution at 0.75 m point of test under three kinds of reference neutron radiation (shielding material：concrete)
表 1 FRNR和空气FRNR中各检验点剂量率、空气散射贡献占比
Table 1. The dose rate of each point of test and the contribution proportion of air scattering in FRNR and air FRNR
distance/m dose rate/(μSv·h−1) ④the contribution proportion of
air scattering obtained by
shadow-cone method(③/①)/%⑤the theoretical contribution
proportion of air
scattering[(②—①)/①]/%①FRNR ②FRNR＋air ③FRNR＋air＋
shadow cone0.75 52.01 52.24 0.70 1.35 0.44 0.85 40.49 40.69 0.52 1.28 0.49 0.95 32.42 32.59 0.42 1.30 0.52 1.05 26.53 26.69 0.36 1.36 0.60 1.15 22.12 22.26 0.32 1.45 0.63 1.25 18.72 18.85 0.28 1.50 0.69 1.35 16.05 16.17 0.25 1.56 0.75 
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表 2 SRNR中各检验点散射中子剂量率贡献占比（影锥法）
Table 2. The contribution proportion of scattered neutron dose rate at each point of test under SRNR (shadow-cone method)
type contribution proportion at different distance/% 0.75 m 0.85 m 0.95 m 1.05 m 1.15 m 1.25 m 1.35 m A1 8.29 10.03 11.93 14.00 16.16 18.42 20.77 A2 8.56 10.73 13.19 15.89 18.70 21.33 22.90 B1 17.11 21.27 25.80 30.70 35.94 41.52 47.57 B2 18.62 23.89 29.77 36.23 43.16 50.18 55.99 Notes：A—polyethylene containing 5% boron，B—concrete；1—half-cubical type，2—cubical type
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表 3 SRNR中各检验点散射中子剂量率贡献占比（理论值）
Table 3. The contribution proportion of scattered neutron dose rate at each point of test under SRNR (theoretical value)
type contribution proportion at different distance/% 0.75 m 0.85 m 0.95 m 1.05 m 1.15 m 1.25 m 1.35 m A1 7.52 9.35 11.32 13.39 15.59 17.91 20.37 A2 8.09 10.38 13.02 16.02 19.45 23.36 27.85 B1 16.59 20.81 25.37 30.32 35.64 41.35 47.45 B2 18.70 24.05 30.21 37.24 45.34 54.72 65.89 Notes：A—polyethylene containing 5% boron，B—concrete；1—half-cubical type，2—cubical type
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表 4 三种中子参考辐射中各检验点散射中子剂量率贡献占比（屏蔽材料：混凝土）
Table 4. The contribution proportion of scattered neutron dose rate at each point of test under three kinds of reference neutron radiation (shielding material：concrete)
type contribution proportion at different distance/% 0.75 m 0.85 m 0.95 m 1.05 m 1.15 m 1.25 m 1.35 m FRNR＋air shadow-cone method 1.35 1.28 1.30 1.36 1.45 1.50 1.56 ARNR shadow-cone method 10.97 13.65 16.57 19.69 22.99 26.46 30.07 SRNR(1) shadow-cone method 17.11 21.27 25.80 30.70 35.94 41.52 47.57 SRNR(2) shadow-cone method 18.62 23.89 29.77 36.23 43.16 50.18 55.99 FRNR＋air theoretical value 0.44 0.49 0.52 0.60 0.63 0.69 0.75 ARNR theoretical value 10.48 13.12 16.01 19.07 22.36 25.76 29.37 SRNR(1) theoretical value 16.59 20.81 25.37 30.32 35.64 41.35 47.45 SRNR(2) theoretical value 18.70 24.05 30.21 37.24 45.34 54.72 65.89 Notes：1—half-cubical type，2—cubical type
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