B型超声诊断仪图像中的声像畸变

木文运用射线的方法,对于线扫和扇扫B超诊断仪中因圆柱形异物对声波的折射而导致柱后物体的图像畸变,导出了定量的理论公式,并对线扫公式作了少量理论和实验的图像对比。     

B型超声诊断图像中肿瘤后方的声场分布

本文分析了B型超声诊断图像中肿块后方的增强、阴影和侧后声影带等图像特征。分析中把肿块简化为圆柱,运用射线声学的方法,导出了脉冲入射情况下肿块后方组织的散射回波声压公式。考虑了组织的声衰减和仪器的时间增益补偿的影响,我们用计算机绘出给定参量的回声图像,又用组织仿体进行了定性的实验验证。理论和实验结果定性相符。     

Reduced Magneto-Resistivity of a Rare-Earth Crystalline and the Degeneracy Removals of Its Crystal-Field Levels

A theoretical approach is generalized and employed to calculate the magneto-resistivity of a rare-earth crystalline （CeAh） with degenerate ground crystal-field （CF） level in the presence of external fields. The calculated results show that when a magnetic field is applied in the c-direction, the magneto-resistivity may be reduced by more than 90% in certain cases in comparison with the pure CF contribution at the same temperature, demonstrating the strong effects of the degeneracy removals of the CF levels on the magnetic resistivity.     

The Magnetic Properties of TbNi2B2C Investigated with a Two-Sublattice Model

A two-sublattice model for rare-earth emtiferromagnets is employed to study the magnetic properties of TbNi2B2 （2. The theoretical susceptibility and magnetization curves obtained with the model show reasonable agreement with the experimental results.     

Magnetism of Rare-Earth Compounds with Non-Magnetic Crystal-Field Ground Levels

Among rare-earth compounds, there are many materials having non-magnetic crystal-field （CF） ground levels. To understand their magnetic behaviour at low temperatures, we study the effects of the CF levels and the Heisenberg-like coupling on the magnetic process of such a crystalline with mean-field and CF theory. It is found that the material can be magnetically ordered if the Heisenberg exchange is sufficiently strong. Additionally we obtain a condition for initial magnetic ordering, and derive a formula for estimating the Curie temperature if the ordering occurs.