中子在大气中产生氮俘获γ的蒙特卡罗模拟研究

为了精确计算早期核辐射,建立了中子及次级γ在大气中输运的蒙特卡罗计算模型,并利用几何分裂算法与时间分裂算法等减方差技巧提高计算效率,计算得到了距源点不同距离球面上中子与中子次级γ的信息,给出了不同位置不同时间的氮俘获γ能量释放率。开展了氮俘获γ能量释放率的规律性研究,并分析了中子能量对氮俘获γ的影响。结果表明,氮俘获γ能量释放率先随源点的距离增加而增大,在距源点约500 m达到峰值,而后随距离增加指数衰减。氮俘获γ能量释放率在时间上服从指数衰减规律,衰减时间在0.1 s左右。引入表征氮俘获γ辐射强度参数a和特征衰减时间参数τ,拟合得到了不同距离不同时间氮俘获γ能量释放率的快速计算公式。研究表明,氮俘获γ辐射强度、衰减时间及其空间分布均与中子能量密切相关。

Monte Carlo simulation of neutron capture γ-rays from nitrogen in the atmosphere

Neutron capture γ-rays from nitrogen is an important part of initial nuclear radiation. To accurately calculate the early nuclear radiation, the Monte Carlo computing model for the neutron and secondary γ-rays transport in the atmosphere is established. Some variance reduction techniques, such as geometric splitting algorithm and time splitting algorithm are used in the Monte Carlo method to improve computing efficiency. The neutron and secondary γ-rays information on the spherical surface at different distances are calculated. The energy release rates of capture γ-rays from nitrogen at different positions and at different times are given. The regularity of the energy release rate of capture γ-rays from nitrogen is studied, and the influence of neutron energy on capture γ-rays from nitrogen is analyzed. The results show that the energy release rate of capture γ from nitrogen increases with the increase of distance from the source at first, reaches the peak at about 500 m from the source, and then decreases exponentially with the increase of distance. The energy release rate of capture γ-rays from nitrogen obeys exponential decay law in time, and the decay time is about 0.1 s. By introducing the parameter of a representing the radiation intensity and the characteristic attenuation time parameter of τ, a fast calculation formula for the energy release rate of capture γ-rays from nitrogen at different distances and times is obtained by fitting formulas. Results show that the intensity radiation, the decay time scale of capture γ-rays from nitrogen and their spatial distribution are closely related to neutron energy.