X-ray to visible image converter with a cathode emission layer having non-uniform density profile structure

High quantum efficiency (.gtoreq.5%) ultrafast (.ltoreq.100 picosecond) and broad band (30 KeV to 80 KeV for medical application) X-ray photo-electron cathode, high gain X-ray real time image intensifier and portable low intensity real time projection type X-ray imagescope use new vacuum photo-electron devices. A new type X-ray photo-electron cathode has a specially designed alkali halide electron emission layer that makes possible high quantum efficiency, high speed and broad band X-ray photon detection. An X-ray photo-electron cathode followed by a direct coupled micro channel plate and an output phosphor display screen form a new type panel shaped direct view X-ray intensifier tube which can have both high spatial resolution (.gtoreq.10 lp/mm) and high gain (.gtoreq.10.sup.4). The thickness of this panel shaped disc X-ray intensifier can be less than 1 cm. A portable real time projection X-ray imagescope which employs this new type X-ray intensifier and other devices (small x-ray tube, small built-in high voltage power supply) can be widely used for medical, industrial and security applications.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to an image intensifying X-ray to visible 
converter and, more particularly, to such a converter utilizing a 
photo-electron cathode for the direct conversion of an X-ray image to an 
electron equivalent image which can be intensified to produce an enhanced 
real time visible image. A panel type direct view real time X-ray image 
intensifier with high spatial resolution and high gain has been made by 
using this new type X-ray photo cathode. A portable low intensity real 
time projection type X-ray imagescope has also been made by using the new 
type X-ray image intensifier. 
2. Description of the Prior Art 
Most of the early X-ray intensifiers are indirect conversion type, wherein 
the X-ray is converted to visible light in a scintillator, a visible light 
photo-electron cathode then converts the photons to electrons which are 
accelerated and multiplied by different techniques, as shown in U.S. Pat. 
No. 4,104,516, U.S. Pat. No. 4,140,900, U.S. Pat. No. 4,255,666 and U.S. 
Pat. No. 4,300,046. One commercially available X-ray imagescope employs an 
unstable radio active isotope X-ray source and an indirect conversion 
X-ray intensifier which consists of an X-ray scintillator, a pair of 
fiber-optic plates, a visible light photo-electron cathode, a micro 
channel plate, a phosphor display screen and a magnifier. There are 
numerous disadvantages in these prior art X-ray image intensification 
devices. The most obvious disadvantage being the use of a scintillator, 
wherein the response time is limited and unsuitable for use in ultrafast 
X-ray imaging. It is difficult to have both high spatial resolution and 
high sensitivity. In order to increase sensitivity, the thickness of the 
scintillator must be increased, which degrades the spatial resolution. 
Another disadvantage in using scintillators is that they usually are based 
on a fiber-optic plate which is the input window of a visible light 
intensifier. These fiber-optic plates must be specially manufactured for 
reducing the size of the image in order to improve the coupling and the 
viewing of the field. The complicated techniques for making fiber-optic 
plates and visible light photo-electron cathodes and the complicated 
structure of the intensifier itself, make these intensifiers very 
expensive. On the other hand, the use of these expensive fiber-optic 
plates also introduce further loss of light intensity and spatial 
resolution. It is further noted that because of the use of a visible light 
photo-electron cathode, the intensifier must be light shielded. Still 
another disadvantage is the inconvenience of using a radio active isotope 
as the X-ray source. The source not only supplies X-rays during the 
operation of the device, it also radiates constantly, day and night, and 
since the energy of the X-ray is not adjustable the isotope usually has a 
rather short life time. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a direct 
conversion type X-ray photo-electron cathode and intensifier which will 
provide an ultrafast and inexpensive X-ray imagescope with high spatial 
resolution and high gain. 
According to the invention, there is provided a high quantum efficiency 
direct conversion type X-ray photo-electron cathode which consists of a 
cathode substrate made of a light metal, such as aluminum, and a cathode 
emission layer made of an alkali halide, such as CsI or CsBr. The emission 
layer has a high density sub-layer coated on the cathode substrate, and 
the low density sub-layer is coated on the surface of the high density 
sub-layer, with the low density sub-layer having a decreased density 
profile from the interface with the high density sub-layer to its emission 
surface. The high density sub-layer functions to produce direct X-ray 
photo-electrons, and the low density sub-layer excites secondary 
electrons. 
The high quantum efficiency X-ray photo-electron cathode (5%-10%) is 
achieved by selecting suitable material for the cathode substrate and the 
emission layer and by selecting the correct thickness and density profile 
for high density and low density sub-layers. 
This direct conversion type X-ray photo-electron cathode has an ultrafast 
(.ltoreq.100 picosecond) response time and a broad response band (30 KeV 
to 80 KeV for medical application). This X-ray photo-electron cathode is 
not sensitive to visible light and does not need any light shielding. 
The X-ray image intensifier consists of an X-ray photo-electron cathode, a 
micro channel plate (MCP) and a phosphor display screen. Two alternative 
types of intensifiers have been made. The first design employed the above 
X-ray photo-electron cathode coupled with an MCP. The second design simply 
coated the X-ray photo-electron cathode emission layer directly on the 
input surface of the MCP. A phosphor display screen was then provided to 
be responsive to the electrons eminating from after the MCP. This approach 
yields not only a higher spatial resolution, but also a compact 
intensifier. The response time of the intensifier is rather fast (in the 
order of one hundred picosecond), because of the use of real X-ray 
photo-electron cathode instead of a combination of X-ray scintillator and 
visible light photo-electron cathode. 
The X-ray photo-electron image intensification is performed by a 1 mm thick 
MCP, the gain can be as high as 10.sup.4 for one MCP and 10.sup.7 for two 
MCPs in series. The gain can be adjusted by biasing the MCP. The electrons 
released from the MCP are accelerated by high voltage potential and 
bombarded onto a phosphor display screen. The final intensified X-ray 
image may be directly observed or recorded by a camera. The spatial 
resolution is around 10 lp/mm-15 lp/mm. The time jitter of the image can 
be less than 100 picoseconds for a 50 mm to 100 mm diameter clear aperture 
intensifier. 
This X-ray intensifier can be operated in a gated mode by using a short 
high voltage pulse to bias the MCP, which may be important for many 
ultrafast image applications. 
A portable low intensity real time projection type X-ray imagescope 
consists of a small X-ray tube, an X-ray intensifier and a built-in high 
voltage power supply. The intensity and energy of the X-ray source as well 
as the gain of the intensifier can be adjusted. The image of the object 
can be directly observed or recorded by camera. The X-ray energy can be 
adjusted from 30 KV to 80 KV (for medical application) and the current is 
about 100 .mu.A adjustable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1A, there is 
shown (FIG. 1 is) a schematic diagram of the new type X-ray photo-electron 
cathode. The cathode substrate, shown as a metal foil 6, is made of 
aluminum or similar light metal. The thickness is chosen to assure that 
the attraction force from the nearby static electric field does not pierce 
and damage the metal foil and so as not to attenuate the X-ray intensity 
significantly within a selected band width. We have found in the instant 
case that a 50 .mu.m thick aluminum foil functions effectively for the 
band width from 30 KeV to 80 KeV. Cathode emission layer 7 is coated on 
the light metal cathode substrate to a thickness of about 250 .mu.m and is 
an alkali halide material, such as CsI or C.sub.3 Br, having a high 
secondary electron emission coefficient. The cathode emission layer has a 
high density sub-layer, which is about 1-2 .mu.m with a varying density of 
50-100%, i.e., 2-4 g/cm.sup.3. The low density sub-layer has a decreased 
density profile, as shown in FIG. 1B, extending from the interface with 
the high density sub-layer to its emission surface. The density 
distribution profile, as shown in FIG. 1, starts from a 50% density at the 
interface and decreases to about 2% at the emission surface. The average 
density is about 10%, i.e., 0.4 g/cm.sup.3. The X-ray photons are absorbed 
in the high density sub-layer to excite high energy photo-electrons, 
which, in turn, excites many low energy secondary electrons in the low 
density sub-layer. The density control of the low density sub-layer is 
very important, as it determines the efficiency of the secondary electron 
emission. As these secondary electrons are forced by the electric field 
across the emission layer, they are emitted from the cathode surface and 
yield electron current 8 shown in FIG. 1A. The electrons emitted from the 
cathode then are accelerated and multiplied by an MCP. As an alternative, 
it is also possible to coat the cathode emission layer directly on the 
input surface of the MCP without having the aluminum foil substrate. 
FIG. 2 is a schematic diagram of the new type X-ray intensifier 10 which 
consists of an input window 5, an X-ray photo-electron cathode 6 and 7, an 
MCP 9, an aluminum foil 11, a phosphor display screen 12 and an output 
window 13 within a light shield 14. Two different kinds of intensifiers 
have been made. In the first case, an X-ray photo-electron cathode, as 
shown in FIG. 1A, is followed by an MCP with an 0.4 mm spacing, and in the 
second case, the X-ray photo-electron cathode emission layer is coated 
directly on the input surface of the MCP. The other part of the 
intensifier is the same. The distance between the output surface of the 
MCP to the aluminum foil 11 is approximately 3 mm. The output surface of 
the MCP is grounded (V=O). The voltage of the cathode substrate is about 
-1600 V to -1800 V. The operation potential across MCP is about -900 V to 
-1000 V for the first case and -1000 V to -1200 V for the second case. 
Aluminum foil 11 has an adjustable positive potential from +6 KV to +7 KV. 
20 Cd/m.sup.2 brightness can be achieved on the output screen. The 
intensifier is panel shaped with diameter from 50 mm to 150 mm and less 
then 10 mm thickness. It is air shielded and vacuumed to 8.times.10.sup.-7 
torr. 
FIG. 3 is a schematic diagram of the portable low intensity real time 
projection type X-ray imagescope. The X-ray source 1 uses a small X-ray 
tube with a focus spot of about 0.3.times.0.3 mm.sup.2, with a current 
about 100 .mu.A and a voltage in the range of 30 KV to 80 KV (for medical 
application). The X-ray tube is shielded by a lead protection cover. A pin 
hole is prepared in the shield for releasing the X-ray. The size of the 
pin hole is determined by the size of the input face of the intensifier 
and the distance from the X-ray source to the input window of the 
intensifier. The X-ray cone released from the pin hole must be stopped by 
the intensifier 10 completely. The distance between the X-ray source and 
the input window of the X-ray image intensifier is about 15 cm to 25 cm 
depending on the specific application. The intensifier is mounted on one 
end of an aluminum light shield 14 for easy observation of the image. The 
image of the object can be directly observed or recorded by a camera from 
another end of the light shield 14. 
FIG. 4 is a block diagram of a typical high voltage power supply unit, 
comprising separate high voltage supplies 15 and 16 for the intensifier 14 
and X-ray tube 1. An additional power supply 17 is provided for the X-ray 
tube 1. In order to reduce the size and the weight of the power supply, a 
high frequency oscillator is employed in the high voltage supplies 15 and 
16. The high voltages for the X-ray tube and X-ray intensifier are 
generated by high frequency transformers associated with rectification 
circuits in a conventional manner. The high voltage power supply unit is 
built in with the imagescope and shielded with silicon rubber. A 2 A 12 
VDC power pack is used for supplying DC current to the high voltage power 
supply unit. The 12 VDC can be selected from a rechargeable battery or a 
120 VAC to 12 VDC adapter. 
While the invention has been described in terms of alternate preferred 
embodiments, those skilled in the art will recognize that the invention 
can be practiced with modification within the spirit and scope of the 
appended claims.