X-ray or gamma ray systems or devices – Beam control – Collimator
Patent
1997-04-17
1998-03-10
Porta, David P.
X-ray or gamma ray systems or devices
Beam control
Collimator
2505051, 378 84, G21K 102
Patent
active
057270449
DESCRIPTION:
BRIEF SUMMARY
This application is the national phase of international application PCT/GB95/01711, filed Jul. 19, 1995 which designated the U.S.
This invention relates to microchannel plates (MCPs).
Recently, it has been demonstrated by Fraser et al (Fraser, G W, Brunton, A N, Lees, J E and Emberson, D L Nucl. Instr. and Meth. A 1993, 334, 579) that spherically slumped MCPs may be used as focussing and collimating X-ray optics. Spherically slumped MCPs have been shown to obey the well known "thin lens" equation: ##EQU1## where 1.sub.s and 1.sub.f are the source and focal distances respectively, f is the focal length of the MCP and R is the radius of curvature of the MCP (taken to be positive when the source is positioned on the concave side of the MCP). If a point source of X-rays is positioned at the MCP focus (on the concave side of the MCP at a distance R/2) then 1.sub.s =R/2 and 1.sub.f =.infin.. In other words, X-rays passing through the MCP should, ideally, emerge from the MCP, after grazing incidence reflection, as a collimated beam travelling parallel to the optical axis defined by the line joining the point source and the centre of the MCP.
In Fraser et al a spherically slumped MCP with constant cross-sectional thickness was employed. In FIG. 1, a MCP 10 of this type, with cross-sectional thickness L, is illustrated. In the coordinate system used the x axis is defined as the optical axis. It was noted in Fraser et al that whilst a substantially parallel beam of X-rays 12 could be generated from a point source 14 at a distance R/2 from the MCP, the intensity distribution of this beam in a plane perpendicular to the optical axis was highly non-uniform. FIG. 2a shows such a non-uniform two dimensional X-ray image 20 generated with radiation of wavelength 44.7 .ANG.. FIG. 2b shows the X-ray intensity distribution 22 in an axial cut through image 20 with a slice width of 6 mm. The non-uniform intensity distribution is due to the presence in the beam of a mixture of those rays experiencing one grazing incidence reflection in the channels, and those rays which pass through the channels without being reflected.
The non-uniformity of the X-ray beam is unfortunate, since a parallel X-ray beam of uniform intensity (a "flat field") is highly desirable in a number of applications such as X-ray lithography. Conventionally, a parallel or quasi-parallel beam can only be produced by maximising the separation between the source and the plane of interest, with an attendant drop in the intensity of the X-ray beam.
The present invention is based upon a novel MCP configuration which greatly improves the uniformity of the parallel X-ray beam.
Particles which have equivalent de Broglie wavelengths to X-rays, such as thermal neutrons, are within the scope of the invention.
According to the present invention there is provided a tapered microchannel plate.
The MCP may be spherically slumped and may be used to collect X-rays emanating from a point source and generate a collimated beam thereof. The length of the capillary channels may vary as a function of the distance of the channels from the optical axis so as to ensure that the probability of X-rays reflecting only once from the interior of the channel is high. The capillary channels may be circular in cross-section and the length of the channels (and hence the cross-sectional thickness of the tapered MCP) may be substantially described by the equation: ##EQU2## where D is the diameter of the channels, R is the radius of the curvature of the MCP, y is the perpendicular distance of the channel from the optical axis and L(y) is the length of a channel at y.
The tapered MCP may be fabricated by grinding a MCP with a numerically controlled grinding machine.
A tapered microchannel plate according to the present invention will now be described with reference to the accompanying drawings, in which:
FIG. 1 is a cross-sectional view of a prior art spherically slumped MCP;
FIG. 2 shows a prior art X-ray image and an axial cut through said image;
FIG. 3 is a cross-sectional view of a tapered MCP;
F
REFERENCES:
patent: 4748328 (1988-05-01), Chang et al.
patent: 5479469 (1995-12-01), Fraser et al.
Fraser et al: "Production of quasi-parallel X-ray beams using microchannel plate X-ray lenses", Nuclear Instruments And Methods In Physics Research -vol. 334, No. 2-3. (1993) pp. 579-588.
Database WPI, Sec. PQ, Week 7851, Derwent Publications Ltd., London Class P61, An 78-L1281A & SU.A.505 558-May 15, 1978, see abstract.
Wilkins et al: "On the concentration, focusing, and collimation of x-rays and neutrons using microchannel plates and configurations of holes", Review of Scientific Instruments, vol. 60, No. 6, Jun. 1989, pp. 1026-1036, XP000035867.
Brunton Adam North
Fraser George William
Medley Adam
Metcalf Carl Jonathan
Porta David P.
University of Leicester
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