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BackgroundThe direct simulation of the γ radiation field in a large space has a very low calculation efficiency.PurposeThis study aims to apply the global variance reduction (GVR) method to the calculation of the γ radiation field in a large space.MethodsFirstly, the volume correction factor was introduced for modifying the lower limit of the weight window wth to address the over-splitting problem caused by the volume difference between the counting cells/grids. The global quality factor (FOMG factor) calculated by the flux-based GVR method using the volume correction was 39 times higher than that obtained by direct simulation. Then, a non-counting area correction method was proposed to address the time-consuming problem encountered in non-counting area calculation while the FOMG factor was further improved by 40%. Finally, based on both the volume correction and non-counting area correction, the calculation of the γ radiation field were compared with that of seven GVR methods based on the particle error, weight, track, number, energy, collision and flux, respectively. The smoothing factor SI was introduced into the flux-based GVR method for results further analysis. [Results and Conclusions] The results show that the FOMG factor calculated by the seven GVR methods is about 2~3 orders of magnitude higher than that obtained by direct simulation, and the standard deviation σ is reduced by 2~3 orders of magnitude. The FOMG factor calculated by the weight-based GVR method is 2 304 times higher than that obtained by direct simulation; this value yields the best variance reduction effect among all GVR methods. As SI increases, the lower limit of the weight window wth of the simulation decreases, and the FOMG factor first increases and then decreases. When SI=0.8, the calculated FOMG factor has the largest value, which is 3 246 times higher than that obtained by direct simulation.

Background The debris motion is an important phenomenon of a high-altitude nuclear detonation, which is also a foundation for the study of the geophysical phenomena such as the ionosphere effect and artificial radiation belt. Purpose The study aims to clarify the debris motion characteristics and laws from a near-space nuclear detonation. Methods Firstly, a fluid dynamics model of debris motion from a near-space nuclear detonation was established. Many influence factors were considered, such as the variation of energy dissipation, air density varies with height, gravity, air temperature rise caused by X-ray depositions and radiation cooling. Then the parameters of debris motion within the explosion equivalent of 1 kt~10 Mt and the explosion height of 30~80 km were systematically studied. The evolutions of parameters such as center height, horizontal radius, expanding velocity, ascending velocity, and shape of debris were given. Finally, the variation laws of typical characteristic parameters such as maximum ascending height and expanding radius changing with explosion height and explosion equivalent were summarized. Results When the explosion height is 30 km, the maximum rising height and the maximum horizontal radius at 5 min for a kiloton-level nuclear explosion debris are about 13~16 km and 4~5 km, the maximum rising height and the maximum horizontal radius at 5 min for a megaton-level nuclear explosion debris are about 20~40 km and 15~30 km. When the explosion height is 80 km, the maximum rising height and the maximum horizontal radius at 5 min for a kiloton-level nuclear explosion debris are about 30~50 km and 20~40 km, the maximum rising height and the maximum horizontal radius at 5 min for a megaton-level nuclear explosion debris are about 200~400 km and 110~220 km. When the explosion equivalent is small and the explosive height is low, the debris evolves into a flat ellipsoid. When the explosion equivalent is large and the explosion height is high, the debris evolves into an inverted pear shape. Conclusions The results show that the maximum height, horizontal radius, and speed of the debris cloud increase with the increase in the explosion height and explosion equivalent. The changes of the height, the horizontal radius, the rising time, and the shape of the debris obtained from the study are in good agreement with the literature estimation method. Those obtained motion parameters of debris can provide delayed radiation source information for the study of the geophysical phenomena such as the ionosphere effect and artificial radiation belt of nuclear explosion in near-space.

Aiming at the simulation of the fine process of the electron transportation in gamma detectors, we calculate electron differential scattering cross-section (DCS) of several typical materials including Fe, ethylene and polyethylene. Based on two different calculation methods, which are partial wave methods based on Dirac equation and R-matrix theory, we find differences of the cross section at low energy region. The result indicates that both partial wave method and R-matrix theory associated with independent atom method (IAM) are not suitable for low energy electron impacting on strong coupled molecule, for example, electron-ethylene. For high energy electron interacting with atom and molecule, the result shows no critical difference because the kinetic energy of the incident electron is severely higher than the electron bound energy in molecule or the excitation energy of a certain atom.

To study the effect of temperature dependent neutron cross section on the neutron transmission performance of typical metallic spherical shells, this paper generated the temperature dependent neutron cross section data by using NJOY program, and established the computational model of interaction of neutron with iron and aluminium spherical shell. The temperature evolution of energy spectrum of transmitted neutron and secondary gamma ray, and the number ratio of transmitted neutrons to secondary gamma rays has been studied at various neutron energies. The results show that only when the energies of the incident neutron are in a certain range, the temperature dependent cross sections can have a significant effect on the energy spectrum of the rays penetrating the spherical shell. For different metal materials, the transmitted neutron spectrum and the secondary gamma ray spectrum will be significantly affected when the temperature increases to different high value. Generally, the transmitted neutron number decreases with the increase of temperature, and the number ratio of transmitted neutron to secondary gamma ray also decreases with the increase of temperature, while the changing trend of number of secondary gamma ray with temperature is related to the type of shell material.

To obtain the transient current response law of the metal component irradiated by pulsed gamma rays, the pulsed gamma ray irradiation experiment of the iron plate was carried out on “Qiangguang-I” accelerator. The transient current of iron plate generated by pulsed gamma rays was measured and analysed, and the relationship between the amplitude of pulse current and the dose rate of gamma rays was obtained. The results show that the current response sensitivity of the iron plate is about 5.7×10-7(A/m2)/(Gy/s) when the gamma rays with the energy of 0.8 MeV irradiate the iron plate. The charge deposition rate in the iron plate can be obtained by Monte Carlo simulation, and then it can be converted to gamma ray induced current of the metal component irradiated by gamma rays.

To study the influence of the electron beam radius on the deposition parameters in dielectric under high energy electron irradiation, a model of dielectric irradiated by plane electron beam source was established. Monte Carlo simulation method was used to calculate the distribution of the injection current density, injection charge density, dose rate and radiation induced conductivity in the dielectric layer of Teflon after electron irradiation with different energies and different beam radii. The results show that the radius of the electron beam source has little influence on the distribution trend of the injection current density in the dielectric layer, as well as the peak position of the injection charge density and the dose rate, but it will significantly affect the peak value of injection charge density and the dose rate. When the electron beam radius is smaller than the dielectric layer radius, the calculated peak is higher than that of the electron beam completely covering the dielectric layer. The higher the electron energy is, the greater the relative deviation will be.

This study aims to get the optimization neutron shielding design of iron/polyethylene combined shield structure. The neutron transmission coefficient with various energies for different thickness of iron and polyethylene combined shield structure were calculated by using Monte Carlo method. The simulation results show that the optimization effect of iron/polyethylene combined shield is not obvious when the neutron energy is low or the shield is thin, there is an optimal thickness ratio of iron to polyethylene adopted to get the best neutron shielding performance when the energy of neutron source is above 2 MeV and the total thickness of combined shielding structure is more than 20 cm. The optimal thickness ratio of iron to polyethylene increases with the increasing energy of neutron source; with the increasing of neutron source energy ranging from 4 MeV to 14 MeV, the optimal thickness ratio of iron to polyethylene trends from 0.11 to nearly 1.6.

Research works on high-altitude electromagnetic pulse (HEMP) have attracted considerable attentions for its ability to disturb or damage the electronic and power systems. HEMP can be divided into the following three parts: early-time (E1), intermediate-time (E2), and late-time (E3). Most of the studies of HEMP are focused on E1, and few investigations have been done on E2 in the open literatures. However, the amplitude of E2 is about 10–100 V/m, and it is able to couple into the cables whose lengths are several kilometers. Therefore, E2 is also a threat to power systems. The simulation of E2, which can provide more accurate environmental parameters for research works on HEMP interactions with systems, is an important supplement to HEMP simulation. In this article, a numerical simulation method of E2 is presented which combines the Monte Carlo transport code and the 2-D EMP code to simulate both E1 and E2. In addition, the simulation results of E2 are given which are in qualitative agreement with the standard waveform. This work is helpful to fill the gap in the simulation of E2 at present and to simulate HEMP environment more accurately and completely. [ABSTRACT FROM AUTHOR]