GB1204188A - Method for making image pick-up screens for x-ray image intensifier tubes - Google Patents

Abstract

1,204,188. Image intensifier tubes. VARIAN ASSOCIATES. 19 Dec., 1967 [27 Dec., 1966;(2) 10 Aug., 1967; 25 Aug., 1967 (2)], No. 57676/67. Heading H1D. The pick-up screen of an X-ray image intensifier tube is made by the evaporation in vacuum of an alkali metal halide, 16, e.g. CsI, NaI, LiI, KI, CsBr or RbI, on to a curved X-ray transparent substrate 6, e.g. of conductive glass or aluminium such that the evaporated screen has a density about the same as that of the bulk alkali metal halide. The alkali metal halide and its activator material, e.g. T1I, NaI or LiI when CsI is used, may be simultaneously evaporated, e.g. by flash evaporation on to the substrate either from separate or the same sources within the vacuum, Figs. 2, 3, 15 (not shown). If evaporated from the same source the mixture therein is so chosen that, taking into consideration the different rates of deposition due to the different vapour pressure of the two materials, the required final mole ratio is obtained (which may be different from that in the original mixture). Alternatively, the activator may be deposited over the alkali metal halide scintillator and subsequently diffused into the alkali halide layer. A photocathode layer 9, e.g. of Cs3Sb, Rb 3 Sb, K 3 Sb, Sb-K-Na-Cs, Sb-KNa or Cs-Te, may be deposited either directly over the X-ray sensitive scintillator layer 16 or over an intermediate buffer layer 8, e.g. of magnesium oxide, aluminium oxide or lithium fluoride. When buffer layer 8 is absent the photocathode/X-ray scintillator interface may be doped with a p-type dopant such as ThI or I to increase the quantum efficiency of photoemission, in particular in the blue region, Fig. 8 (not shown). As shown in Fig. 13 backscatter of photons produced in the scintillator 16 by incident X-rays (including γ-rays and other correspondingly high energy radiation) is prevented by forming a non-specular backing surface 321 at the substrate/scintillator interface by roughening the substrate surface. Alternatively a thin anodized aluminium film may be formed or the substrate coated with a light-absorptive layer, e.g. of carbon. In forming the photocathode in one embodiment, alternate K and then Sb layers followed by alternate Cs and Sb layers ending with Cs, which may be activated with O 2 , are laid down to optimize the photoemission. A potential gradient may be set up between the photocathode 9. and the conductive substrate 6 to further improve the efficiency, Fig. 9 (not shown).