中子活化分析非弹性散射与俘获时间分布的Geant4模拟

利用蒙特卡罗程序包Geant4,模拟了14 Me V中子在水、聚乙烯、碳酸钙和氧化铁4种不同介质中的非弹性散射时间与俘获时间;通过分析脉冲宽度为10μs的14 Me V脉冲中子在这4种不同介质中的非弹性散射与俘获时间分布,得到非弹性散射与俘获时间门的宽度;利用得到的非弹性散射与俘获时间门,获得了14 Me V脉冲中子在聚乙烯介质中的净非弹性散射γ谱与净俘获γ谱。     

Stopping power of swift neon ions in dependence on the charge state in the non-equilibrium regime

We report on measurements of charge changing cross-sections and energy losses in dependence on the initial and final charge state of Ne ions at an incident energy of 2 MeV/u penetrating thin carbon foils. Different initial charge states could be separated in energy by applying a high voltage in front of the carbon foils; the final charges and their energy loss were measured with a high-resolution magnetic spectrometer. We derived a consistent evolution of the charge state distribution solving the corresponding rate equations for the measured cross-sections. Including the charge state dependent energy losses, ΔE(q_i,q_f), stopping power values for frozen charge states, S(q), could be extracted. The experimental data are compared with different theoretical predictions.     

Mean excitation energies extracted from stopping power measurements of protons in polymers by using the modified Bethe–Bloch formula

Recent stopping power measurements in thin polymeric films have been performed for protons of 0.4–3.5 MeV energies using the indirect transmission technique [H. Ammi, S. Mammeri, M. Chekirine, B. Bouzid, M. Allab, Nucl. Instr. and Meth. B 198 (2002) 5]. Experimental stopping data have been analyzed with the modified Bethe–Bloch formula and the mean excitation energies I have been then extracted from the data. Resulting values for each thin film are 76 ± 1 eV in Mylar, 70.8 ± 1 eV in Makrofol, 82.2 ± 1.2 eV in LR-115 and 55.4 ± 1 eV in Polypropylene. The I-extracted values are compared to those I_B calculated by using the Bragg’s rule.     

Influence of velocity difference of correlated particles on the stopping power and vicinage effect

We theoretically investigate the influence of the velocity difference of charged particles moving closely in a metallic target on the stopping power and vicinage function, as well as the energy loss of a pair of charged particles penetrating coherently in the target medium. These correlated charged particles are considered as a degenerate electron gas with appropriate density. Based on the dielectric formalism, the mathematical expressions for the stopping power, vicinage effects and related quantities for two correlated protons in an aluminium target are given. Plasmon-pole approximation without dispersion is used to describe the target response. We show that the energy loss is slightly enhanced with a small relative velocity difference. The interference between such protons exhibits a maximum at certain velocity, and exhibits an oscillating behaviour when product of the relative difference velocity and time is beyond a threshold value. The results can be applied to, for example, the energy deposition of high current density ion beams.     

Study on the gamma rays and neutrons energy response optimization of a scintillating fiber detector for EAST with Geant4

A new scintillating fiber detector inside magnetic shielding tube was designed and assembled for use in the next round of fusion experiments in the experimental advanced superconducting tokamak to provide D-T neutron yield with time resolution.In this study,Geant4 simulations were used to obtain the pulse height spectra for ideal signals produced when detecting neutrons and gamma rays of multiple energies.One of the main sources of interference was found to be low-energy neutrons below 10-5 MeV,which can generate numerous secondary particles in the detector components,such as the magnetic shielding tube,leading to high-amplitude output signals.To address this issue,a compact thermal neutron shield containing a 1-mm Cd layer outside the magnetic shielding tube and a 5-mm inner Pb layer was specifically designed.Adverse effects on the measurement of fast neutrons and the shielding effect on gamma rays were considered.This can suppress the height of the signals caused by thermal neutrons to a level below the height corresponding to neutrons above 4 MeV because the yield of the latter is used for detector calibration.In addition,the detector has relatively flat sensitivity curves in the fast neutron region,with the intrinsic detection efficiencies(IDEs)of approximately 40％.For gamma rays with energies that are not too high(＜8 MeV),the IDEs of the detector are only approximately 20％,whereas for gamma rays below 1 MeV,the response curve cuts off earlier in the low-energy region,which is beneficial for avoiding counting saturation and signal accumulation.     

Calculation of the power distribution in the fuel rods of the low power research reactor using the MCNP4C code

The Monte Carlo method, using the MCNP4C code, was used in this paper to calculate the power distribution in 3-D geometry in the fuel rods of the Syrian Miniature Neutron Source Reactor (MNSR). To normalize the MCNP4C result to the steady state nominal thermal power, the appropriate scaling factor was defined to calculate the power distribution precisely. The maximum power of the individual rod was found in the fuel ring number 2 and was found to be 105 W. The minimum power was found in the fuel ring number 9 and was 79.9 W. The total power in the total fuel rods was 30.9 kW. This result agrees very well with nominal power reported in the reactor safety analysis report which equals 30 kW. Finally, the peak power factors, which are defined as the ratios between the maximum to the average and the maximum to the minimum powers were calculated to be 1.18 and 1.31 respectively.     

Stopping power measurements for O, F and Si ions in Mylar by a transmission technique

Electronic energy loss of charged particles in materials is a fundamental process responsible for the unique response of materials in applications of advanced nuclear power, radiation detectors and advanced processing of electronic devices. In this study, stopping powers of ¹⁶O, ¹⁹F and ²⁸Si heavy ions crossing thin Mylar foils have been determined in transmission geometry. The energy loss was measured over a continuous range of energies from 1.6 to 5.5 MeV/n (MeV per nucleon) using the data that was tagged by a surface barrier detector (SBD) with and without stopping foils. We have compared the obtained stopping values to those predicted by SRIM-2008 computer code, ICRU-73 stopping data tables and MSTAR calculations. The effective charge values of these heavy ions have been also deduced from the experimental set of data.     

Electronic stopping power of hydrogen in KCl at the stopping maximum and at very low energies

The electronic energy loss of hydrogen ions in KCl was investigated in a wide energy range. Thin films of KCl were evaporated on an Au/Si substrate. Rutherford Backscattering Spectrometry (RBS) was performed with protons and deuterons at energies from 30 to 400 keV/nucleon. At lower energies experiments were performed by Time-Of-Flight Low energy ion scattering (TOF-LEIS) again with proton and deuteron projectiles. Experimental results are compared to calculated/tabulated values for the electronic energy loss. Whereas at energies beyond the stopping maximum very good agreement is found, at lower ion energies discrepancies between experiment and calculations increase. At very low ion velocities the extrapolated stopping cross section ε predicts vanishing electronic energy loss at energies below 100 eV/nucleon.