Radiation detector

A radiation detector is assembled using spacers which contact one another through apertures in the electrode plates of the detector, so that the relevant thickness dimension of the dimensionally accurate spacers determines the spacing of the plates. Thickness variations of the plates can be compensated for at a supporting surface of the spacers being provided with depressable raised portions. The plates may be constructed as laminations for collimator beyond which a radiation-sensitive element is arranged, and can also form electrode plates for a gas ionization detector.

The invention relates to a radiation detector including a plurality of 
plates which are mounted at a distance from one another by means of 
intermediate pieces. 
Such a radiation detector in the form of a gas ionization X-ray detector 
for an X-ray scanner is known from U.S. Pat. No. 4,031,396; therein, the 
electrode plates are maintained at a distance from one another in the 
detector by stacking the plates on tensioning bolts with intermediate 
pieces. 
In high-resolution detectors, that is to say detectors in which a small 
distance exists between individual electrodes, it is difficult to prevent 
undesirable variations in the spacing of the plates. 
It is an object of the present invention to provide a radiation detector in 
which the spacing between the plates is very accurately maintained, 
notably between the electrodes of a gas-filled X-ray detector. 
To achieve this, a radiation detector of the kind set forth in accordance 
with the invention is characterized in that the intermediate pieces are 
formed by spacers which are mounted so that they contact one another 
through apertures in the plates. 
Because the spacing of the electrode plates in a detector in accordance 
with the invention is determined entirely by a relevant thickness 
dimension of the spacers, undesired variations in the spacing can be 
minimized by using spacers having a very high dimensional accuracy. 
Moreover, the consulative effect of the individual thickness variations of 
the constituent components, notably of the electrode plates, will be 
reduced. 
In a preferred embodiment, the spacers fit in the apertures of the 
electrode plates with a light clamping fit, so that during assembly each 
of the plates is first provided with preferably four spacers which form a 
unitary assembly therewith during further assembly. Moreover, successive 
spacers preferably mate with a snap connection effect, so that a coherent 
unit is obtained by stacking the electrode plates. In order to compensate 
for thickness variations as between the electrode plates and to eliminate 
the effect thereof on the spacing of the plates, a supporting surface of 
the spacers in a preferred embdodiment is provided with raised portions 
which respectively press the plates against a flat supporting surface of a 
preceding spacer. Depression of these raised portions is facilitated by a 
special shape of the spacers. For assembling a focusing detector, use is 
made of spacers of different thickness which preferably are of a different 
color to provide a visual distinction. Similarly, spacers of different 
thickness can be used in order to realize desired variations in the 
spacing between the electrodes. The spacers also can be used for 
assembling a radiation collimator comprising radiation opaque laminations 
beyond which, with respect to the radiation source, a scintillation 
detecting device is placed.

A multi-channel detector as shown in FIG. 1 comprises a housing 1 with 
sidewalls 2, an upper wall 3, a lower wall 5, a rear wall 7, an entrance 
window 9 which is transparent for the radiation 8 to be detected, and a 
series of electrode plates 11. The electrodes (also shown in FIG. 2) 
include anodes 13, which are, preferably metal plates, for example, a 
molybdenum lamination having a thickness of, for example, 0.3 mm, and 
cathodes 15 which consist of a carrier 17, for example, a printed circuit 
board, a first cathode 19 and a second cathode 21 from which respective 
signals can be derived individually, through terminals 23 and 25 and 
connections 26, by means of a signal read unit 20. By terminals 27, a high 
voltage can be applied to the anode plates by means of a high-voltage 
source 22. 
Between the electrodes 13 and 15 there are provided spacers 29 which are 
accommodated (as shown in FIG. 4) in apertures 31 in the electrodes. Each 
of the electrodes for assembling the detector forms an integral assembly 
unit with the spacers accommodated in the apertures. For a focusing 
detector, such as is customarily used in X-ray scanners, the thickness of 
the spacers 29 inserted in the bores 31 will be different from the 
thickness of the spacers 35 inserted in the bores 33. The difference in 
thickness determines the radius of curvature of a detector thus assembled. 
Similarly, spacers of different thickness can be used when the thickness 
of the anode plates and the cathode plates are different, and also when 
assembling a detector having a graded resolution, for example, a 
resolution which decreases towards the extremities. After stacking the 
detector, the overall length (measured along a circular arc for a focusing 
detector) is adjusted to a given value by compression. The mutually equal 
thickness of the spacers then ensures a mutually equal spacing of the 
electrode plates. The homogeneity of the detector can then be checked and, 
in the case of an error, the relevant electrode plate may be individually 
replaced. Similarly, spacers may be individually exchanged in respect of 
each electrode plate. 
A spacer 41 as shown in FIG. 3 comprises a central bore 43 having a 
diameter of, for example, 1 mm, and on one side, a cylindrical bush 44 
having an outer diameter which is adapted to the apertures 31, 33 in the 
electrode plates, for example, a diameter of 3 mm. The spacer is provided 
on its other side with a recess 45 which has a corresponding inner 
diameter of 3 mm. On the side of the recess 45, the spacer comprises, for 
example, 12 recesses 49 and 12 teeth 47. The recesses leave a part 51 in 
place and on this part there are provided raised portions 55. The raised 
portions have a height of, for example, 0.4 mm and the comparatively thin 
portions 51 enable the raised portions to be depressed. The height of the 
raised portions is chosen so that static thickness variations as between 
the electrode plates can be compensated for. The spacer has an outer 
diameter of, for example 6 mm and a thickness of, for example 2 mm, so 
that the spacing of the electrode plates to be mounted is defined as will 
be apparent from FIG. 4. 
FIG. 4 is a sectional view of the electrode plates in the form of anodes 
13, and cathodes 19 and 21 provided on printed circuit board 17 with each 
electrode plate being provided with at least one aperture 31. In each of 
the apertures there is situated a spacer 41 which is shown in a sectional 
view taken along the line IV--IV in FIG. 3a with each spacer comprising a 
central bore 43, a cylindrical bush 44, and a cylindrical recess 45. The 
bush 44 fits in the aperture 31 in the electrode plates and is inserted in 
the recess 45 of a next spacer with a snap-connection effect. The 
sectional view of the spacers illustrates the recesses 49 with the 
portions 51 on which the raised portions 55 are provided. During 
compression, the raised portions 55 press the electrode plates against 
supporting surfaces 57 of the spacers and are subsequently depressed into 
the recess 49. Thus, the distance between the electrode plates is 
determined only by the thickness dimension 59 of the spacer. 
After the checking and any correction of a detector thus stacked, it is 
connected to a lower support 60 (see FIG. 1) and an upper support 61 by 
means of adhesive, after which it is arranged in the housing. 
For a short-focus detector, that is to say a detector having such a radius 
of curvature that the fact that the spacers are not wedge-shaped is a 
drawback, wedge-shaped spacers are preferably used. 
The apertures 31 are then formed so that the spacers can be arranged 
therein in only one rotary position. The aperture 31 in the electrode 
plates in a preferred embodiment, and hence also the outer boundary of the 
bush 44, is shaped as an isosceles, non-equilateral triangle.