共查询到19条相似文献,搜索用时 171 毫秒
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用数值模拟的方法研究了FTO客体材料的线吸收系数随着穿透材料深度的变化关系,并拟合出材料有效线吸收系数与厚度之间的函数表达式。研究结果表明,在闪光照相中X光能谱发生了硬化,并随着穿透材料深度的增加谱平均线吸收系数会随之减小。FTO客体中钨的平均有效线吸收系数0.838(4.53%)cm-1,铜的平均有效线吸收系数0.297(4.96%)cm-1,能谱效应对有效线吸收系数的影响小于5%。在图像重建中利用上述的线吸收系数能够反演出精度达5%的材料密度。 相似文献
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研究闪光照相中影响材料有效吸收系数的两个主要因素,即X光的能谱效应和散射影响。用Monte-Carlo方法对闪光照相系统的客体各种组成材料的有效吸收系数进行数值模拟;求出不考虑散射而仅受能谱影响的有效吸收系数及其空间分布,发现该分布类似于医学CT数的分布规律,在高光程数对应的位置上该吸收系数较低;也得到了在20MeV闪光机情况下,客体各种组成材料的单个平均吸收系数。结果表明:钨、铝和铁的平均等效吸收系数分别近似为0.830,0.080和0.265cm
-1。 相似文献
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研究了能谱效应的影响,进行了理想的MC(Monte Carlo)数值闪光照相。能谱效应是由不同能量的X光与材料发生作用的微观截面不同引起的。能谱效应降低了密度提取的精度,因而在密度提取中需要考虑能谱效应,即使用反应能谱效应的有效吸收系数,而不是单能吸收系数。在闪光照相中能谱效应体现为:光路上面质量ρ(l)l越大,有效质量吸收系数μρ^-(l)越小;面质量越大,对应的自洽光程也越大,即μρ^-(l)ρ(l)l越大。 相似文献
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X光穿透物质时符合指数衰减规律,原则上,给定闪光照相光源能谱以及照相物质已知的情况下,可以计算出透射X光的照射量,但事实上X光光源能谱测量十分困难,因此,实际上照射量的计算无法实现。所谓的“等效单能模型”就是将上述全能谱照射量等效成某一“单能”的照射量,在实际实验测量中经常需要的物理量是数种材料等效质量吸收系数的比值,称之为相对质量吸收系数,一般情况下, 相似文献
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在高能闪光照相技术中,提高图像接收平面上直穿与散射的比值有利于有效信息的提取。利用蒙特卡罗方法对法国实验客体的照相过程进行模拟,得到图像接收区域直穿分量与散射分量的能谱分布,依据其分布的特点提出了利用能谱优化来提高接收区域直散比的方法,指出添加衰减屏可以达到能谱优化的效果。依据线吸收系数的变化趋势以及实际加工的难度选择钽作为优化能谱的材料,并根据最小照射量的限制得到了最大的衰减屏厚度。最后通过蒙特卡罗模拟,指出添加9 mm的衰减屏能够对照相结果的优化程度最好,照射量满足要求,而直散比上升到原来的3倍左右。 相似文献
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用傅利叶变换红外光谱仪研究了湖南晨溪地开石热分解前后的红外光谱,加热反应后原矿粉红外光谱在3620.73、3695.34cm-1处的地开石羟基伸缩振动吸收峰和在916.06、1009.33cm-1处的羟基弯曲振动吸收峰都消失,表明其质量失重为羟基脱水.在298~1123K,用TG DTA综合分析仪研究了地开石在程序升温下的热分解行为.结果表明,在600~1000K,其质量损失为13.7%,与理论计算的脱水量相近.根据TG DTG曲线上的基础数据,联合运用积分法(Coats Redfern方程、HM方程、MKN方程)和微分法(Achar方程)对地开石脱水反应过程进行了非等温动力学研究,结果显示,地开石热分解反应的机理函数为F2,求得反应的动力学参数:反应级数为2.1,表观活化能为131.62kJ/mol,指前因子为108.3s-1.作为一种验证方法,使用Ozawa方程对不同升温速率TG曲线上相同分解率处的绝对温度进行线性回归分析,求出的表观活化能值为133.07kJ/mol,此值不仅与上述各种方法的表观活化能值相近,也与确定机理函数时的表观活化能值相近.说明实验求得的动力学参数在很大程度上是可靠的. 相似文献
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乳腺癌病人单个细胞的Raman光谱 总被引:13,自引:3,他引:13
给出了乳腺癌病人正常乳腺细胞与癌细胞的拉曼光谱。从实验谱线中得到:癌变细胞的拉曼谱整体变弱, 就相对强度而言, DNA的2个磷酸根骨架峰782,1 084 cm-1和脱氧核糖-磷酸振动峰1 155及1 262 cm-1拉曼谱线明显的减少; 表征A型(DNA)构像的特征峰812 cm-1及谱线979,668 cm-1消失, 并有新峰1 175 cm-1出现, 905 cm-1的谱线增强并有6 cm-1的红移, 这说明DNA的磷酸根骨架有一定的断裂; 从而导致癌细胞的分裂繁殖失去有效的控制。在癌变组织细胞的拉曼谱中还发现了很强的一类与钙硬化密切相关的特征峰960 cm-1; 通过这些特征拉曼谱线的变化, 为癌症的诊断和治疗提供了有力的实验依据。 相似文献
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炸药爆轰法制备的纳米石墨粉的拉曼光谱 总被引:7,自引:2,他引:5
负氧平衡炸药爆轰法合成的纳米石墨粉,是一种新型的具有良好实用前景的纳米粉体材料。采用负氧平衡炸药梯恩梯(TNT),在分别充有氮气、氩气、二氧化碳等保护性气体、压力为0.25~2 atm的密闭容器内爆轰制备了纳米石墨粉。用激光拉曼光谱对制备的样品进行了测试,结果表明样品为石墨结构。纳米石墨粉的Raman峰与块体石墨相比,其峰位向高波数方向偏移了约5 cm-1。纳米石墨粉Raman峰的半高宽约为22 cm-1,由此可计算出纳米石墨粉的颗粒大小为2.97~3.97 nm。与高纯石墨Raman峰相比,纳米石墨粉的Raman峰由于尺寸效应出现了蓝移现象, 并对此现象进行了讨论。用X射线衍射仪(XRD)和透射电子显微镜(TEM)测定了纳米石墨粉的物相,并对其颗粒粒径进行了估算,其结果为2.58 nm(酸处理前)和1.86 nm(酸处理后),与Raman光谱的结果基本吻合。 相似文献
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Measurements of the attenuation of 1.5 to 7 MHz ultrasound in milks and creams, aqueous solutions of milk proteins and milk fat demonstrated that the attenuation due to the fat component was proportional to the fat concentration (at low volume fractions, less than 4%) and was caused by two effects: losses arising from its particulate nature and intrinsic attenuation (mass attenuation coefficient 1.1 cm2 g−1 at 7 MHz in pure milk fat). Comparison with theoretical equations showed that absorption by thermal conduction dominated the particulate losses and increased with decreasing fat globule size, over the range 1.5 to 7 MHz. This was confirmed by examining milks homogenized to different degrees. The losses in the non-fat components were mainly due to the protein components which also probably dominated the shape of the attenuation titration curves for skimmed milk, producing peaks in attenuation in the range pH 4–4.5 and about pH 11.3. 相似文献
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X射线质量衰减截面和光电截面的高精度测量 总被引:1,自引:0,他引:1
利用特征X射线源系统地测量了Si、Fe、Cu、Y、In、Sn等六种元素及SiH4的X射线质量衰减系数,实验误差为±1%. By using the characteristic X ray sources and the Si( Li) detector system, the X ray mass attenuation coefficients for Si,Fe,Cu,Y,In,Sn and SiH 4 have been systematically measured in the energy range of 1.486~29.109 keV. The accuracy of experimental data has been reduced to ±1% . 相似文献
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用杂化密度泛函B3LYP在6-31G*的水平上研究了Si3N4团簇可能结构的平衡几何构型和电子结构,得到了24个可能的异构体.Si3N4团簇的最稳定结构是由7个Si—N键和2个N—N键形成的3个四边形构成的三维结构.用自然键轨道方法(NBO)分析了成键性质,结果表明,Si—N键中的Si、N原子的净电荷较大,说明Si—N键中Si、N原子的相互作用主要是电相互作用.最强的IR和Raman峰分别位于1033.40 cm-1,473.63 cm-1处.并且讨论了最稳定结构的极化率和超极化率. 相似文献
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《Physics letters. A》2020,384(29):126757
The bending and torsional vibration of the periodic perpendicular cantilever beam-mass resonators (PCBMR) is idealized as translational and rotational oscillators attached to the main beam. In this paper, the effect of that torsional vibration of the PCBMR on the dynamics of an infinitely long Euler-Bernoulli beam is evaluated. The band-structure is explored by implementing the transfer matrix method in conjunction with Bloch-Floquet's theorem. The combination of the translational and rotational oscillator modifies the relative position of the coupling coefficient in the transfer matrix, which plays a pivotal role in the band-gap formation. The flexural band-structure is highly sensitive to the torsional vibration while the radius of gyration of the tip mass is considerably higher than the length of the PCBMR. Ill-tuning leads to split and reduction of attenuation band to 50%; whereas, around 38% elongation of the attenuation band in the low frequency regime can be achieved by proper tuning. 相似文献
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Kopechek JA Haworth KJ Raymond JL Douglas Mast T Perrin SR Klegerman ME Huang S Porter TM McPherson DD Holland CK 《The Journal of the Acoustical Society of America》2011,130(5):3472-3481
Ultrasound contrast agents (UCAs) are used clinically to aid detection and diagnosis of abnormal blood flow or perfusion. Characterization of UCAs can aid in the optimization of ultrasound parameters for enhanced image contrast. In this study echogenic liposomes (ELIPs) were characterized acoustically by measuring the frequency-dependent attenuation and backscatter coefficients at frequencies between 3 and 30 MHz using a broadband pulse-echo technique. The experimental methods were initially validated by comparing the attenuation and backscatter coefficients measured from 50-μm and 100-μm polystyrene microspheres with theoretical values. The size distribution of the ELIPs was measured and found to be polydisperse, ranging in size from 40 nm to 6 μm in diameter, with the highest number observed at 65 nm. The ELIP attenuation coefficients ranged from 3.7 ± 1.0 to 8.0 ± 3.3 dB/cm between 3 and 25 MHz. The backscatter coefficients were 0.011 ± 0.006 (cm str)(-1) between 6 and 9 MHz and 0.023?±?0.006 (cm str)(-1) between 13 and 30 MHz. The measured scattering-to-attenuation ratio ranged from 8% to 22% between 6 and 25 MHz. Thus ELIPs can provide enhanced contrast over a broad range of frequencies and the scattering properties are suitable for various ultrasound imaging applications including diagnostic and intravascular ultrasound. 相似文献