Lindström J, Carlsson G A
Radiation Physics Department, Borås Hospital, Sweden.
Phys Med Biol. 1999 May;44(5):1353-67. doi: 10.1088/0031-9155/44/5/319.
The absolute efficiency of a phosphor screen is the ratio of the light energy per unit area at the screen surface to the incident x-ray energy fluence. Particle size is a critical factor in determining the absolute efficiency, but in most models its influence is not accounted for. To allow derivation of the particle size dependence, a model is proposed that describes the optical properties of the screen by means of a single parameter, the light extinction factor, xi, and assumes that the intrinsic efficiency (light energy/energy imparted to the phosphor material) is independent of particle size. The value of xi depends on the type of screen (phosphor, reflective backing, coating and binder) and has to be determined from measurements on at least two screens with known particle size and thickness. The absolute efficiency can then be calculated for an extended range of particle sizes and/or screen thicknesses. To test the model, experimental data from the literature were used to derive values of xi for screens of La2O2S:Tb, LaOBr:Tm and ZnCdS:Ag. The extinction factor was found to vary between -6 and +20%. The non-physical negative value for xi, found from one set of experiments on La2O2S:Tb screens, may be explained as resulting from a lack of accurate knowledge of the actual tube potential, influencing calculated values of the energy imparted to the screen. The results are promising but further well-controlled experiments (including improved dosimetric calculations to account forescape of K-radiation from the screen) are needed to confirm the model.
磷光屏的绝对效率是指屏表面单位面积的光能与入射X射线能注量之比。颗粒尺寸是决定绝对效率的关键因素，但在大多数模型中并未考虑其影响。为了推导颗粒尺寸的依赖性，提出了一个模型，该模型通过单一参数——光消光系数ξ来描述屏的光学特性，并假设本征效率（光能/赋予磷光体材料的能量）与颗粒尺寸无关。ξ的值取决于屏的类型（磷光体、反射背衬、涂层和粘结剂），必须通过对至少两个已知颗粒尺寸和厚度的屏进行测量来确定。然后可以针对更广泛的颗粒尺寸范围和/或屏厚度计算绝对效率。为了测试该模型，利用文献中的实验数据推导出了La2O2S:Tb、LaOBr:Tm和ZnCdS:Ag屏的ξ值。发现消光系数在-6%至+20%之间变化。在对La2O2S:Tb屏进行的一组实验中发现的ξ的非物理负值，可能是由于对实际管电压缺乏准确了解，影响了赋予屏的能量的计算值。结果很有前景，但需要进一步进行严格控制的实验（包括改进剂量计算以考虑屏发射的K辐射的前景）来证实该模型。
