Matsumoto M, Kubota H, Ozaki Y, Kanamori H
School of Allied Health Sciences, Faculty of Medicine, Osaka University, Japan.
Med Biol Eng Comput. 1995 Jan;33(1):48-51. doi: 10.1007/BF02522945.
It has been generally expected that, as X-ray tube-voltage ripple increases, the X-ray spectrum shifts to the low photon-energy side, and therefore the mean energy decreases, i.e. the beam quality is softened. This is the normal order. Previous calculation (Birch et al., 1979), however, showed that the beam quality generated by 100% ripple was harder than that by 50% ripple. This is the reverse order, against general expectation. To verify the reverse order, X-ray spectra are measured using a germanium detector system. The measurements are performed with various thicknesses of aluminium object. To obtain large ripples at small tube currents, small-capacity high-voltage cables are made. To perform efficient measurements, the homogeneous sensitive region of the germanium crystal is determined, and detector-collimators with various diameters are prepared. As a result, experimental verification of the reverse order is performed for various thicknesses of object. The reverse order also appears in photon spectra and mean energies calculated using three models from previous work (Kramer, 1923; Birch and Marshall, 1979; Tucker et al., 1991) by inserting the tube-voltage waveforms measured with the authors' system. The experimental results agree well with the results using the Birch-Marshall model. Mean energies reach the minimum levels for the three models at 20-60% ripple, as do the experimental results. The reason for the phenomenon is clarified by qualitative illustration.
一般认为，随着X射线管电压纹波增加，X射线谱向低光子能量侧移动，因此平均能量降低，即光束质量变软。这是正常顺序。然而，先前的计算（Birch等人，1979年）表明，100%纹波产生的光束质量比50%纹波产生的光束质量更硬。这与一般预期相反，是反向顺序。为了验证这种反向顺序，使用锗探测器系统测量X射线谱。测量在不同厚度的铝物体上进行。为了在小管电流下获得大纹波，制作了小容量高压电缆。为了进行高效测量，确定了锗晶体的均匀敏感区域，并制备了不同直径的探测器准直器。结果，对不同厚度的物体进行了反向顺序的实验验证。通过插入作者系统测量的管电压波形，在先前工作（Kramer，1923年；Birch和Marshall，1979年；Tucker等人，1991年）的三个模型计算出的光子谱和平均能量中也出现了反向顺序。实验结果与使用Birch-Marshall模型的结果吻合良好。对于这三个模型，平均能量在20%-60%纹波时达到最低水平，实验结果也是如此。通过定性说明澄清了该现象的原因。
