Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1991-01-23
1992-09-08
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
313103CM, H01J 3150
Patent
active
051460769
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an input screen for a radiological image intensifier tube.
Radiological image intensifier tubes allow a radiological image to be transformed into a visible image, generally to afford medical observation.
These tubes include an input screen, an electron optics system, and a screen for observing the visible image.
The input screen comprises a scintillator which converts incident X-photons into visible photons which then go on to excite a photocathode, generally constituted by an alkali antimonide, for example potassium antimonide doped with caesium. The photocathode thus excited generates an electron flux.
The electron flux issued by the photocathode is then transmitted by the electron optics system which focuses the electrons and directs them onto an observation screen consisting of a luminophore which then emits visible light. This light can then be processed, for example, by a television, cinema or photographic system.
In the most recent constructions the input screen comprises an aluminium substrate covered by the scintillator, itself covered by an electrically conductive and transparent layer, for example made from indium oxide. The photocathode is deposited onto this transparent layer.
The X-rays strike the input screen on the aluminium substrate side, and pass through this substrate so as to then reach the material constituting the scintillator.
The luminous photons produced by the scintillator are emitted somewhat in all directions. However, in order to increase the resolution of the tube, a substance such as caesium iodide, which has the property of growing in the form of crystals perpendicular to the surface on which they are deposited, is generally chosen as the scintillation material. The needle-shaped crystals thus deposited tend to guide the light perpendicularly to the surface, this being favourable to a good image resolution.
However, luminous photons are also emitted towards the rear, that is to say they spread towards the X-ray arrival side. These photons go on to strike the aluminium substrate, with an incidence which is random. They are reflected by the aluminium substrate towards the front, hence towards the photocathode, but the path of these photons is such that a loss of resolution results: a same X-photon incidence can result in the creation of electrons in the photocathode at different points.
FIG. 1 illustrates this loss of resolution by showing side by side the different paths followed by two luminous photons coming from the impact of a X-photon on the scintillator, resulting in the formation of electrons at different points of the photocathode. In FIG. 1 are recognised, the aluminium substrate 10, in general cambered, constituting the input face by which the X-rays arrive, the scintillator 12 made from caesium iodide, the crystals of which perpendicular to the surface tend to channel the luminous photons, the transparent conductive sub-layer 14 and the photocathode 16.
There could be envisaged the interposing, between the aluminium substrate 10 and the scintillation layer 12, a black, light-absorbing layer, so that there are no reflections on the aluminium surface and hence no creation of electrons in the photocathode from luminous photons coming from such a reflection.
However, this is difficult since it is not known how to adequately position such layers: if this absorbent layer is a metal layer, its absorption is difficult to control; depending on the conditions of deposition, the metal layer is either absorbent or reflective. If the absorbent layer is a black glass (glass loaded with metal particles) it is difficult to deposit; the fact that the screen is cambered does not make matters easier. If finally the absorbent layer is a layer constructed from organic matter, it will not be very compatible with the vacuum conditions which prevail inside the tube.
According to the invention, it is proposed to construct an input screen in which a thin anti-reflecting layer, transparent or slightly absorbent (at the wavelengths emitted by the scintillator
REFERENCES:
patent: 4855589 (1989-08-01), Enck et al.
Chareyre Francois
De Grott Paul
Raverdy Yvan
Davenport T.
Nelms David C.
Plottel Roland
Thomson Tubes Electroniques
LandOfFree
Input screen for radiological image intensifier tube utilizing a does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Input screen for radiological image intensifier tube utilizing a, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Input screen for radiological image intensifier tube utilizing a will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-135870