A micro-channel plate for use in focusing X-rays or particles of equivalent wavelengths has a radially packed array of square pores. For focussing parallel rays, such micro-channel plate may consist of two square-pore spherically curved and radially packed micro-channel plate elements having different radii of curvature and overlying one another, forming a concavo-convex compound plate. Such a plate is capable of providing a greater effective area than prior art square packed micro-channel plates at high X-ray energies.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to micro-channel plates (MCP's). The invention is 
concerned particularly with MCP's for use in imaging x-rays and particles 
having equivalent wavelengths. 
2. Description of the Related Art 
MCP's have been utilised to perform a lens function in x-ray and the like 
imaging applications. X-rays, or, particles reflected at grazing incidence 
from the internal glass walls of the channels, or pores, of the MCP can be 
brought to a focus. 
Square pore MCP's have been successfully applied in focusing X-rays or 
particles having equivalent wavelengths, for example neutrons, and have 
been used for example in X-ray telescopes. Other possible uses include 
X-ray lithography, flux concentration for X-ray scattering experiments, 
neutron focusing, X-ray microscopy and in diagnostic and therapeutic X-ray 
machines. 
The use of square pore MCPs in X-ray imaging is described in, for example, 
the paper entitled "X-ray focusing using micro-channel plates" by P. 
Kaaret et al published in Applied Optics vol. 31, No. 34, pages 7339 to 
7343, 1992. In an experimental arrangement described in this paper a flat 
(planar) MCP is utilised to focus diverging X-rays from a point source 
located at a finite distance from the MCP to an image. The pores of the 
MCP are parallel to each other and tilted relative to the surface by a 
bias angle and the MCP is orientated such that the pore axes are parallel 
to the optical axis. 
As is mentioned in this paper, square pore MCP's are considered to offer an 
improvement over MCP's having circular pores as they lead to a significant 
increase in the intensity of the focused beam which, it is said, is due to 
the fact that the angles of incidence and reflection are the same 
regardless of the point of reflection in the square geometry. 
Square pore MCP's for X-ray and the like imaging have also been produced in 
a spherically curved configuration in which the axis of each pore is 
aligned radially with respect to a spherical surface. By arranging that 
the axes of the pores extend normal to the spherical surface in this 
manner, parallel rays from a source at infinity can be imaged. The use of 
such an MCP is reported in the paper entitled "X-ray focussing using 
microchannel plates" by G. W. Fraser et al published in SPIE Proceeding, 
Vol. 1546, page 41-52, 11991. 
In these MCP's the pores are square-packed, that is to say, in 
cross-section, the pores are arranged in othogonal rows and columns, in a 
grid like pattern. 
We have found that improved results are achieved with a different 
arrangement. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a micro-channel plate 
comprising an array of square pores which is characterised in that the 
pores of the array are radially packed. 
The MCP may be curved, preferably spherically, for imaging, for example, 
parallel X-rays from a source at infinity, or flat for imaging diverging 
rays from a source at a finite distance. 
A radially packed, square pore, MCP has been found to provide improved 
performance compared with that of a square packed, square pore, MCP. 
Because of the so-called point spread function, a square pore MCP whose 
pores are arranged in a square grid of rows and columns of pores, gives an 
image in the form of a cross. With a radially packed, square pore array, 
the central focus is retained but the cross is lost. The radially packed 
square pore MCP leads also to a more useful effective aperture. 
In a preferred embodiment the micro-channel plate, suitable for use in 
focusing parallel X-rays and the like, comprises first and second 
spherically curved micro-channel plate elements of different radii of 
curvature overlying one another, the pores of the first element aligned 
with and communicating with the pores of the second element. More 
specifically, the plate may comprise a concavo-convex compound array 
having a first plano-convex element of radius R and a second plano-concave 
element of radius less than R, for example R/3. Such a plate will have a 
greater effective focusing area--a measure of its efficiency in focusing 
x-rays--than a square packed array, particularly at hard x-ray frequencies 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It should be understood that the Figures are merely diagrammatic and are 
not drawn to scale. Certain dimensions, in particular the size of the 
pores in relation to the overall MCP dimensions, and the degree of 
curvature have been greatly exaggerated. 
FIGS. 1 and 2 illustrate a prior art radially curved, square packed, square 
pore MCP 11 with a radius of curvature R which can for example be 5 or 10 
m. Being square packed the MCP has a grid like array of square section 
pores, or channels, 12 in which the individual pores 12 are aligned in 
orthogonal rows and columns. In the diagrammatic illustrations of FIGS. 1 
and 2 the pores are shown greatly enlarged for the sake of clarity. A 
typical diameter for such an array is 60 mm with each pore 12 being, say, 
12.5 .mu.m square and having a length of 8 mm. Because of the curvature, 
the pore size at the opposing sides may differ slightly. 
As can be seen in FIG. 2, the pores 12 of the spherically curved MCP 11 are 
stacked with their axes extending normal to the spherical surface of the 
MCP, these axes coinciding at the centre of curvature of the plate. 
For more details of square pore MCPs and their use in x-ray focusing 
applications and the like reference is invited to the aforementioned 
published technical papers, which are incorporated herein by reference. 
FIG. 3 and 4 illustrate an embodiment of an MCP in accordance with the 
invention which comprises a compound MCP 13 having a concavo-convex 
configuration and consisting of first plano-convex MCP element 14 and a 
second plano-concave MCP element 15 overlying one another in tandem. Each 
of the MCP elements 14, 15 comprises a radially packed, square pore MCP. 
FIG. 3 shows the pore array geometry of the radially packed MCP. As can be 
seen from this figure, the pores 12 of square cross-section are arranged 
in a series of juxtaposed concentric circles, the number of pores lying 
side by side in each circle being determined by the circle's radius, with 
one side of each of the pores in each respective circle extending 
substantially tangentially of the circle. The flat sides of the MCP 
elements 14 and 15 face one another and the pores 12 of the element 14 are 
aligned with the pores 12 of the element 15 at a plane interface, 
referenced at 16, such that the pores of the element 14 communicate with 
respective pores of the element 15. 
As before, the pores of the arrays are shown greatly enlarged for the sake 
of clarity. 
The radius R of the plano-convex element 14 is typically 15 m, and that of 
the element 15 is R/3, typically 5 m. 
The radially packed array of the MCP 13 may have a typical diameter of 60 
mm with the pores in each element 14 and 15 having an overall length of 8 
mm and being 12.5 .mu.m square. 
With this MCP in use, for example, in X-ray imaging, rays reflected at 
grazing incidence from the internal walls of the pores 12 can be brought 
to a focus. Normally, when using an MCP, and considering parallel rays, 
e.g. from a source at infinity, only rays which suffer two reflections of 
adjacent walls are brought to focus. Single reflection rays produce an 
aberration in the form of a cross around the true image and those that 
pass straight through simply add to any diffuse background. 
In order to collect and focus parallel rays from a source at infinity using 
a square packed MCP having a grid-like pore geometry, as shown in FIG. 1, 
the array is curved at a radius of curvature R equal to twice the required 
focal length f. The grazing angle at the edge of the array is then 
determined according to the ratio of the diameter of the array to the 
focal length. To achieve high utilisation of the aperture at a given X-ray 
energy, it is necessary for the width to length ratio of the pores, and 
the grazing angle near the edges of the array, which should be close to 
the critical angle for the rays, to obey a certain relationship. 
Consequently, the collecting geometric area (aperture) of the array is 
small. Furthermore, only a fraction of this area is dedicated to the 
double reflection focused rays with the rest being blocked or lost to the 
single reflection or straight through rays. 
A much higher fraction of the aperture can be usefully employed using a 
radial packing arrangement for the pores of the array, as in the MCP 
elements 14 and 15 of FIG. 3 and 4. Then, unlike the MCP of FIGS. 1 and 2, 
the cross-section of the MCP is effectively the same for all azimuthal 
positions. Considering the element 14, for example, all the pores at a 
given radius provide the same projected single reflection area of on-axis 
rays and the rays are brought to a focus at f=R/2. Rays at an angle to the 
axis are not focused to a point and can lead to circular aberration. This 
aberration is corrected by introducing a second reflection in the same 
plane through the use of the second radially packed pore array of the MCP 
element 15 having a smaller radius of curvature, which, in the case of the 
embodiment of FIGS. 3 and 4, is one third that of the first. Paraxial rays 
are brought to a point focus at f=R/4 with a width corresponding 
approximately to the pure width. 
FIG. 5 illustrates the effective collecting, areas of three plates of like 
diameter, pore size and packing, at different energies of X-rays. Curves 1 
and 2 are for prior art square packed radially curved arrays as 
illustrated in FIGS. 1 and 2, of radii (focal length) 5 m and 10 m 
respectively. Curve 3 is for a tandem, radially packed configuration as 
illustrated in FIGS. 3 and 4 of focal length 5 m. The graphs show 
theoretical effective areas after pore surface roughness has been 
accounted for, and illustrate that the improvement brought about by the 
invention is particularly apparent at harder X-ray frequencies, that is, 
higher X-ray energy levels. At lower energies the improvement is less 
pronounced although still significant. 
The MCP elements are formed of lead glass, such as Corning 8161 glass, 
which can be reduced in hydrogen to give a high surface lead content for 
improved reflectivity. 
The MCP's, like those with circular channels used for electron 
multiplication purposes in image intensifiers and the like, may be 
fabricated by drawing, stacking and etching of glass fibres consisting of 
an acid soluble core glass and an acid resistant lead glass cladding. 
Square cross-section fibres are bundled, drawn and fused to form a boule 
with radially packed pore geometry and the required pore diameter. The 
boule is then sliced to produce a plate of the required thickness. 
Curvature corresponding to the desired radius of curvature can be achieved 
by heating the plate above its softening point between spherical mandrels 
prior to the final etching stage. For the MCP of FIGS. 3 and 4, consisting 
of tandem MCP elements, two plates may be cut from the same boule. Each 
plate is then curved to the required radius (R=2f and R=2f/3). Thereafter, 
the plates can be ground, lapped and polished on their joint plane to 
provide the necessary channel alignment, following which the two plates 
are cemented together in alignment. 
Although a square-pore, spherically-curved, radially packed MCP comprising 
two MCP elements in tandem has been described in particular, other 
embodiments are possible. Thus, for example, in another embodiment the MCP 
may instead comprise a single plate having a radially-packed array of 
square pores. Depending on whether the MCP is intended to be used for 
rays, or particles, which are parallel, as, for example, from a source at 
infinity, or diverging, as, for example, from a source located at a 
certain distance from the MCP, the MCP may be curved or flat. Moreover, if 
curved, the curvature may perhaps be other than spherical. 
From reading the present disclosure, other modifications will be apparent 
to persons skilled in the art. Such modifications may involve other 
features which are already known in the field of MCPs and which may be 
used instead of, or in addition to, features already described herein.