Simulation of neutron spectrum and time distribution for long counters
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摘要: 针对长计数器基本结构，建立简化模型系统模拟了快中子脉冲经慢化体后进入正比管的能谱分布与时间分布，结果表明：中子能谱随时间的演化明显，31 μs以后，中子的能谱变化变小，趋于恒定；进入正比管的不同能量的中子时间分布不同，随着能量变低，时间分布变宽，热中子附近，时间分布可持续至上千μs；利用不同能量中子随时间的分布，计算了长计数器计数随时间的变化。慢化体半径超过20 cm后，进入正比管的中子通量与能谱基本不随慢化体半径变化而变化，此结果可为长计数器尺寸的优化设计提供参考。Abstract:
Background The long counters are widely applied among various types of neutron sources.Purpose In this work, neutron spectra in the long counters are specifically studied, in order to obtain a better understanding of the influences on the detection efficiency due to the size of moderators.Methods According to the basic structure of long counters, a simple model is built to systematically simulate the spectrum and time distribution of neutrons entering the proportional counter tube from a pulsed fast neutron source.Results The calculated results show that the evolution of the neutron spectrum is rapid at first, and becomes slower later. After 31 μs, the neutron spectrum almost no longer changes. The time distribution is different for neutrons of different energy. The lower the energy, the wider the distribution. For the energy of thermal neutrons, the time lasts more than1 m s. Utilizing the time distribution of different energy, the change of counts of the long counter over time is calculated.Conclusions Basically, the flux and spectra of neutrons which enter the long counters do not change with the variation of the moderator radius when it exceeds 20 cm. This result can provide a reference for the optimal design of the long counter.-
Key words:
- long counters /
- polyethylene moderator /
- neutron spectrum /
- time distribution
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图 2 穿过不同厚度慢化体后的中子能谱
Figure 2. Neutron spectrum passed through the moderator with different thickness
图 5 不同前端慢化体厚度下的中子能谱
Figure 5. Neutron spectrum under different front modulator thickness
图 8 两种初始入射脉冲宽度下2.45×10−8～8.09×10−8 MeV能段的时间分布比较
Figure 8. Compare of the time distribution for energy bin of 2.45×10−8 to 8.09×10−8 MeV between the two initial incident width
图 9 对数坐标下不同能量中子的时间分布
Figure 9. Time distribution of neutrons with different energy under logarithmic coordinates
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