A high-efficiency fast neutron detector prototype based on a triple Gas Electron Multiplier(GEM) detector, which, coupled with a novel multi-layered high-density polyethylene(HDPE) as a neutron-to-proton converter for improving the neutron detection efficiency, is introduced and tested with the Am-Be neutron source in the Institute of Modern Physics(IMP) at Lanzhou in the present work. First, the developed triple GEM detector is tested by measuring its effective gain and energy resolution with55 Fe X-ray source to ensure that it has a good performance.The effective gain and obtained energy resolution is 5.0×104and around 19.2%, respectively. Secondly, the novel multi-layered HDPE converter is coupled with the cathode of the triple GEM detector making it a high-efficiency fast neutron detector. Its effective neutron response is four times higher than that of the traditional single-layered conversion technique when the converter layer number is 38.
We theoretically investigate high-order harmonic generation in a two-color multi-cycle inhomogeneous field combined with a 27 th harmonic pulse. By considering a bowtie-shaped gold nanostructure, the spatiotemporal profiles of enhanced plasmonic fields are obtained by solving the Maxwell equation using finite-domain time-difference method. Based on quantum-mechanical and classical models, the effect of 27 th harmonic pulse, temporal profile of enhanced plasmonic field and inhomogeneity on supercontinuum generation are analyzed and discussed. As a result, broadband supercontinuum can be generated from our approach with optimized gap size of nanostructure. Moreover, these results are not sensitively dependent on the relative phase in the two-color field.
We theoretically investigate the contribution of the excited state to the ellipticity of the harmonics from H+2 at different orientation angles irradiated by a linearly polarized laser pulse. It is found that the first excited state has a significant influence to the ellipticity of the harmonics, and the contribution of higher excited states to the ellipticity can be neglected.Moreover, the conclusion is not dependent on the laser intensity.
We propose a theoretical scheme to generate a broadband supercontinuum using a modulated chirped polarization gating technique. By this technique, a broadband continuum of 155 eV can be obtained at microscopic level. The modulations on the supercontinuum can be eliminated after propagation. Then a smooth broadband supercontinuum with the bandwidth of 155 eV can be generated. As a result, isolated sub-100-attosecond pulses with tunable central wavelengths could be obtained straightforwardly. Our simulations also reveal that an isolated attosecond pulse can still be generated by using multicycle driving laser pulses, and the central wavelength of isolated attosecond pulse can be controlled by adjusting the delay τd (τd is the delay of the two counter-rotating circularly polarized pulses.).
本文采用第一性原理的方法系统研究了UO2的晶体结构、电子结构和弹性性质.在计算中采用广义梯度近似结合Hubbard U项描述电子的局域强关联效应.首先通过计算能带带隙大小并与理论值比较的方法,得到了合理的有效库仑相关作用能(Ueff)的取值,同时通过态密度的计算,进一步验证了Ueff取值的合理性.计算得到UO2中U原子的Ueff值为3.30 e V(Ueff=U-J,U=3.70 e V,J=0.40 e V).应用此参数计算得到的UO2晶格常数为5.54,带隙宽度为2.17 e V.该结果优于目前现有的研究结果,同时在同样的Ueff值条件下计算所得到的弹性常数与实验值也符合得较好.相较于之前的基于实验测量并分析得到的Ueff值,我们所采用的方法在对UO2性质描述上更为准确.不同的有效库仑相关作用能取值下的态密度结果表明,有效库仑相关作用能的大小可以影响铀原子5f电子轨道的分布.
