Ⅰ. INTRODUCTION Critical current density J_c varies with structural imperfections in non-ideal type-Ⅱ superconductors, and it is sensitive to the defect concentration. Neutron irradiation has been established as a u...Ⅰ. INTRODUCTION Critical current density J_c varies with structural imperfections in non-ideal type-Ⅱ superconductors, and it is sensitive to the defect concentration. Neutron irradiation has been established as a useful tool to increase the defect concentration in superconducting materials. Many experiments have shown that neutron irradiation at a suitable fluence would展开更多
1 Introduction It is very important for practical application of HTSC to increase the critical cur-rent density （J<sub>c</sub>）by artificially introducing pinning centers to enhance flux pinning.Ex-perim...1 Introduction It is very important for practical application of HTSC to increase the critical cur-rent density （J<sub>c</sub>）by artificially introducing pinning centers to enhance flux pinning.Ex-periment results show that neutron radiation is 1000 times more effective than theelectron and proton radiation.So in recent years,the neutron radiation has becomepopular,theoretically and practically.After irradiation of fast neutrons in high flux,the magnetization ratio of polycrystal samples of Y and Bi Systems are M<sub>irr</sub>/M<sub>unirr</sub>≈2.8—3.1.But T<sub>c</sub> is descending with the neutron flux increasing and the decreasingratio is about（2.6—3.0 K）/(10<sup>18</sup> n/cm<sup>2</sup>).展开更多
基于MCNP程序模拟计算了不同孔隙度二氧化硅介质地层14 Me V中子慢化长度与中子迁移长度。首先,模拟计算了241Am-Be中子源条件下不同孔隙度Si O2介质地层的中子通量空间分布,并利用源距均方值公式计算得到了对应的中子慢化长度LS和中子...基于MCNP程序模拟计算了不同孔隙度二氧化硅介质地层14 Me V中子慢化长度与中子迁移长度。首先,模拟计算了241Am-Be中子源条件下不同孔隙度Si O2介质地层的中子通量空间分布,并利用源距均方值公式计算得到了对应的中子慢化长度LS和中子迁移长度LM。与文献结果对比:中子特征长度LS与LM结果相对误差均值分别为1.20%与-2.60%;该结果验证了源距均方值公式计算中子特征长度的有效性和可行性。同样地,计算得到了不同孔隙度Si O2介质地层14 Me V中子的中子特征长度LS和LM;计算结果表明:14 Me V中子的中子特征长度同样随地层孔隙度的增加而降低;其中水的14Me V中子特征长度为LS=12.73 cm、LM=13.00 cm,Si O2的14 Me V中子特征长度为LS=30.08 cm、LM=34.31 cm;中子慢化长度LS结果与文献结果的相对偏差≤±3.1%。展开更多
The irradiation effect of slow neutrons at low fluence on high-T_c super-conductors and its mechanism are presented.Based on an analysis and comparison of irra-diation effects by fast and slow neutrons from the viewpo...The irradiation effect of slow neutrons at low fluence on high-T_c super-conductors and its mechanism are presented.Based on an analysis and comparison of irra-diation effects by fast and slow neutrons from the viewpoint of nuclear physics,a novelmechanism in great contrast to that by fast neutron irradiation is proposed.It was bothexperimentally and theoretically shown that slow neutrons of low fluence and fast neutronsof high fluence have the similar irradiation effects on high-T_c superconductors.展开更多
基金Project supported by the National Center for Research and Development on Superconductivity
文摘Ⅰ. INTRODUCTION Critical current density J_c varies with structural imperfections in non-ideal type-Ⅱ superconductors, and it is sensitive to the defect concentration. Neutron irradiation has been established as a useful tool to increase the defect concentration in superconducting materials. Many experiments have shown that neutron irradiation at a suitable fluence would
基金National Centre for ResearchDevelopment of SuperconductivityFoundation for Doctoral Education of China.
文摘1 Introduction It is very important for practical application of HTSC to increase the critical cur-rent density （J<sub>c</sub>）by artificially introducing pinning centers to enhance flux pinning.Ex-periment results show that neutron radiation is 1000 times more effective than theelectron and proton radiation.So in recent years,the neutron radiation has becomepopular,theoretically and practically.After irradiation of fast neutrons in high flux,the magnetization ratio of polycrystal samples of Y and Bi Systems are M<sub>irr</sub>/M<sub>unirr</sub>≈2.8—3.1.But T<sub>c</sub> is descending with the neutron flux increasing and the decreasingratio is about（2.6—3.0 K）/(10<sup>18</sup> n/cm<sup>2</sup>).
文摘基于MCNP程序模拟计算了不同孔隙度二氧化硅介质地层14 Me V中子慢化长度与中子迁移长度。首先,模拟计算了241Am-Be中子源条件下不同孔隙度Si O2介质地层的中子通量空间分布,并利用源距均方值公式计算得到了对应的中子慢化长度LS和中子迁移长度LM。与文献结果对比:中子特征长度LS与LM结果相对误差均值分别为1.20%与-2.60%;该结果验证了源距均方值公式计算中子特征长度的有效性和可行性。同样地,计算得到了不同孔隙度Si O2介质地层14 Me V中子的中子特征长度LS和LM;计算结果表明:14 Me V中子的中子特征长度同样随地层孔隙度的增加而降低;其中水的14Me V中子特征长度为LS=12.73 cm、LM=13.00 cm,Si O2的14 Me V中子特征长度为LS=30.08 cm、LM=34.31 cm;中子慢化长度LS结果与文献结果的相对偏差≤±3.1%。
文摘硼中子俘获治疗(Boron Neutron Capture Therapy,BNCT)作为一种有效的治疗癌症的方法,在全世界范围内已被广泛认可。基于^(7)Li(p,n)反应的加速器中子源具有反应阈值低、中子产额高、平均中子能量低、易慢化等优点,是最具潜力实现BNCT的加速器中子源。首先采用蒙特卡罗程序Geant4(GEometry And Tracking)程序模拟了不同能量的质子轰击不同厚度锂靶的中子源特性,提出用3 MeV的质子轰击0.2 mm厚度锂靶的加速器参数设计方案。随后开展了中子慢化体的模拟和优化设计研究,提出了一种“26 cm MgF_(2)+10 cm TiF_(3)”的组合作为慢化层结构,采用0.08 cm的^(6)Li作为热中子吸收层、1.5 cm的Bi作为γ吸收层。在加速器质子束流15 mA的条件下,模拟获得的慢化体出口超热中子注量率达到1.16×10^(9)n·cm^(−2)·s^(−1),快中子与超热中子比率(Φ_(epi)/Φ_(fast))为21.64,热中子与超热中子比率(Φ_(epi)/Φ_(th))为125.23,γ成分(D_(γ)/Φ_(epi))达到1.82×10^(−13)Gy·cm^(2)·n^(−1)(epi),可以满足BNCT的研究需求。
基金Project supported by the National Center for Research and Development on Superconductivity of China and the Foundation for Doctoral Education.
文摘The irradiation effect of slow neutrons at low fluence on high-T_c super-conductors and its mechanism are presented.Based on an analysis and comparison of irra-diation effects by fast and slow neutrons from the viewpoint of nuclear physics,a novelmechanism in great contrast to that by fast neutron irradiation is proposed.It was bothexperimentally and theoretically shown that slow neutrons of low fluence and fast neutronsof high fluence have the similar irradiation effects on high-T_c superconductors.
