WO1992007284A1

WO1992007284A1 - Direct digital position readout transducer

Info

Publication number: WO1992007284A1
Application number: PCT/GB1991/001773
Authority: WO
Inventors: George William Fraser
Original assignee: University Of Leicester
Priority date: 1990-10-12
Filing date: 1991-10-11
Publication date: 1992-04-30
Also published as: AU8669891A; GB9022256D0

Abstract

There is disclosed a direct digital position readout transducer for a 2m x 2n array of positions, adapted to assign coordinates uniquely to each of the 2m+n positions of the array, comprising readout elements arranged in m + n layers parallel to the array each ith layer (i = 1 to m) having 2i and each jth layer (j = 1 to n) having 2j readout elements, the elements of each layer occupying half of the 2m+n positions in that layer corresponding to the positions in the array and the ith and jth layers respectively assigning a '1' or a '0' to the ith and jth significant bits of the binary coordinates according as a signal incident at a position affects or does not affect a readout element in the ith and jth layer and the elements being so patterned within the layers as to yield thereby unique binary coordinate codes to the positions.

Description

DIRECT DIGITAL POSITION READOUT TRANSDUCER

This invention relates to position readout transducers such for example as may be used in connection with microchannel plate (MCP) electron multipliers.

Many forms of electronic image readout devices have been developed for use with MCP's. These devices, which sense the position of the MCP output charge pulse, and hence of the incident particle or photon which ςives rise to such pulse, include "analogue'^: and -'digital ^■' operat on devices. In analogue operation devices the MCP charge cloud may be divided (for example in resistive anode systems) or -shared (for example in wedge-and-strip anode systems) between a small number of electrodes, typically three or four for two-dimensional signal location. In digital operation systems, one conductor is associated with each pixel of the image field, as in a pin anode array. Each pixel then has associated with it a distinct amplifier and discriminator chain, which involves a high degree of electronic, complexity. However, one arrangement, the Multi-anode Microchannel Array (MAMA) has the charge collectors formed as two othogonal sets of parallel strips and uses coincidence electronics to locate the charge arrival position, which requires only 2N chains

2 for an N x N pixel array, instead of N .

Whilst these electronically complex arrangements can offer high spatial resolution, typically less than 100 microns full-width-at-half-maximum, they are expensive and, on account of the electronic complexity, are not particularly fast, reliable or easy to use. The speed and spatial resoution of analogue systems is ultimately limited by the operation of their signal processing electronics, which computes their charge ratios or signal rise times. Moreover, the precise shape of the charge cloud "footprint" emanating from the microchannel plate amplifier may significantly affect the spatial resolution and flat-field uniformity (differential non-linearity) of readout elements made uy of discrete electrodes, particularly in wedge-and-strip systems.

The present invention provides a position readout transducer which may be used with microchannel plate electron multipliers which does not suffer the disadvantages described above of prior art systems, and which may also have wider application.

In broad aspect, the invention comprise a direct digital position readout for a 2^m x 2ⁿ array of positions, adapted to assign coordinates uniquely to each of the 2 ⁿ positions of the array, comprising readout elements arranged in m + n layers parallel to the array each ith layer (i = 1 to m) having 2 and each jth layer (j = 1 to n) having 2-' readout elements, the elements of each layer occupying half of the 2^{m n} positions on that layer corresponding to the positions in the array and the ith and jth layers respectively assigning a "1" or a "0" to the ith and jth significant bits of the binary coordinates according as a 3ignal incident at a position of the array affects or does not affect a readout element in the ith and jth layers and the elements being so patterned within the layers as to yield thereby unique binary coordinate codes to the positions.

Of course, m and n can be equal, and m can be equal to 1 in the case of a linear array.

The array is said to be 2^m x 2ⁿ because such is the usual array format for digital readout; any other array format can of course be treated similarly if desired within the scope of the invention since it would be possible to regard any array which was not of 2^m x 2 format to be part of a larger array of that format, or to comprise a smaller array of that format with some extra positions. The elements may be patterned so that those corresponding to the "m" coordinate are in strips extending "n"-wise and vice versa. The elements in each layer may be patterned in strips of equal width spaced apart by the width of the strips.

The transducer may be adapted as a readout for a microchannel plate (MCP) electron multiplier by the elements comprising electrically conducting areas of thin layers separated by thin insulating layers and their positions corresponding to single channels of the MCP and being adjacent the output end of the MCP and being such that an output from a single channel of the MCP will affect all the elements in all the layers that are aligned with that channel, the elements of each layer being electrically connected together and to a single amplifier and discriminator circuit for the layer.

The readout thus operates on the charge induced by the moving MCP avalanche, that is to say as currents which flow while the MCP charge pulse is still propagating rather than on received charge actually entering the transducer.

The layers may be bonded together and to the output end of the MCP. The layers with more elements may be closer to the output end of the MCP.

Direct digital readout transducers according to the invention and in particular such a transducer adapted as a readout for a MCP electron multiplier will now be described with reference to the accompanying drawings, in which :-

Figure 1 is an exploded view of a 2 x 2 transducer;

Figure 2 is an element layout for the four layers of a 2 2 x 22 transducer;

Figure 3 is an element layout for the four

4 layers of a 1 x 2 , or sixteen- element linear array;

Figure 4 is a section through a MCP/transducer arrangement , ;

and Figure 5 is a graphical representation of signal voltage and lateral spread as functions of depth in the layer of the transducer of Figure 4. «

The drawings illustrate direct digital position readout transducers 11 for 2 x 2 arrays of positions. Figure 1 illustrates such a transducer for a

2 x 2, i.e. 2 x 2 array 12; Figure 2 illustrates one for a 4 x 4, i.e. 2 2 x 22 array; while Figure 3 illustrates one for a linear, 16 element, i.e. a 2 0 x 24 array. The transducers are adapted to assign coordinates uniquely to each of the 2 positions of the array, i.e. each of the four positions of the array of Figure 1, sixteen positions of Figure 2 and sixteen positions of Figure 3.

The transducers 11 comprise electrically- conducting readout elements 13 arranged in m + n layers LI, L2, etc etc parallel to the array 12. In Figure 1 there are two layers LI, L2 with an intervening insulating layer I, shown only in Figure 1, it being understood to be present between each pair of element- containing layers in Figures 2 and 3 also.

It will be realised that a 256 x 256 (2 8 x 28) array such as is commonly used in imaging devices will require 16 element-containing layers with fifteen intermediate insulating layers.

In the simple embodiment of Figure 1, the top layer LI comprises a single conductive element 13(1) occupying half of the layer while the bottom layer L2 also comprises a single conductive element 13(2) also occupying half the layer, but arranged differently to the element 13(1) so that, taken together in plan view, one quarter of the area is covered by both elements 13(1), 13(2), one quarter by element 13(1) only, another by 13(2) only while the fourth quarter is not occupied by either element.

Brief reference to Figure 4 shows how a larger- scale transducer operates as a readout device for a microchannel plate 41 of which a few channels 42 are shown in cross section. Each channel 42 is in effect a minute electron multiplier. A particle incident on the front face 41a gives rise to an electron charge cloud moving towards the bottom end of the 42. This induces currents in any conductor directly beneath that channel 42; the currents are induced as the charge cloud or avalanche moves towards the conductors and hence there is no delay. Thus the transducer can operate at high read-out rates and there is no position-dependent time constraint problem as is found in other MCP readouts.

There is some signal attenuation as between the top and bottom layers of the transducer but the layers are very thin, when produced by microfabrication techniques, and the attenuation can be compensated for by setting the discriminator threshold of each signal channel to a value appropriate to its layer level.

Referring back, now, to Figure 1, it will be seen that if the transducer 11 is considered as a readout for a MCP having four channels addressed by cartesian coordinates (0,0, (0,1), (1,0) and (1,1), then a charge at channel (0,0) will encounter no conducting element, a charge at channel (0,1) will encounter element 13(2), at channel (1,0.) element 13(1), and a charge at channel (1,1) both conductors 13(1) and 13(2). If conductor 13(1) is arranged to output (via fast amplifier 14 and discriminator 15, which gives a "1" output if the amplified signal level is high enough, a "0" otherwise) the "X" coordinate and conductor 13(2) the "Y" coordinate, then their outputs will directly correspond to the cartesian coordinates of the MCP channels.

2 2

Figure 2 illustrates a 2 x 2 array 12 having coordinates (00,00) to (11,11) and the conducting element layer configuration to give a direct digital readout thereof. It will be seen that in layers LI and L2 the conducting elements are strips.

An 8 x 8 or 2 3 x 23 array would require only two further layers, like LI and L2 but with twice as many equally spaced strips on each. And so on. Figure 3 illustrates a 16 element linear array 12 and the corresponding layer configuration. Here L4 gives the most significant bit, LI the least significant.

It will be appreciated that the 0 or (0,0) element in the above described arrays produces no signal even if energised by a particle or photon. This clearly reduces the usefulness of the Figure 1 arrangement by 25%, since only three out of the four positions of the array will give rise to any signal. Of course, if one knows in some other way that a signal has been received and yet the transducer gives no output, one may deduce that the signal was received at location (0,0). In general, however, the transducers will have so many positions in their array that the loss of one position can be easily tolerated, particularly since that position will be or can be arranged to be at a corner or an end, in the case of a linear array.

Figure 5 is a graph showing (a) signal voltage and (b) lateral spread of the induced signal in the 'Z' direction, i.e. through the layers. Signal voltage decreases from top to bottom of the layer stack (the top being closest to the MCP, of course) as already mentioned, and this can readily be compensated for by setting the discriminator thresholds To reduce the effects of lateral spread, the layers with the finest _* conductor pitch may be placed nearest the MCP, as shown in the other Figures. Aside from that consideration, however, the layers may be arranged in any order, it being only necessary properly to assign their outputs correctly for the digital readout to correspond to the array addresses. Figure 2 shows how the connections are made for the 4 x 4 array transducer. The number 1, 2, 3, or 4 after the amplifer 14 for each layer LI, L2, L3, L4 indicates which digit, namely 1st, 2nd, 3rd or 4th of the position coordinate (ab, cd) of the array 12 the layer is connected to. It will be seen that the layer position in the stack does not have to correspond to the significance of the bit of information it supplies.

Whilst particular reference has been made to an MCP readout transducer, the facility for direct: digital encoding afforded by the invention will clearly be of more general application. Thus a direct digital encoding keypad using pressure-sensitive keys could clearly be constructed, and here it may be mentioned that the "array" need not be neatly laid out as so far described and illustrated in a square or rectangle or in-line arrangement, but distributed in any convenient or conventional style such for instance as the "QWERTY" typewriter keyboard outputting ASCII codes directly.

Claims

1. A direct digital position readout transducer for a 2^m x 2ⁿ array of positions, adapted to assign coordinates uniquely to each of the 2 positions of the array, comprising readout elements arranged in m + n layers parallel to the array each ith layer (i = 1 to ) having 2^X and each jth layer (j = 1 to n) having 2-¹ readout elements, the elements of each layer occupying half of the 2 ⁿ positions in that layer corresponding to the positions in the array and the th and jth layers respectively assigning a "1" or a "0' to the ith and jth significant bits of the binary coordinates according as a signal incident at a position affects or does not affect a readout element in the ith and jth layer and the elements being so patterned within the layers as to yield thereby unique binary coordinate codes to the positions.

2. A transducer according to claim 1, in which m = n.

3. A transducer according to claim 1, in which m = 1.

4. A transducer according to claim 1, in which the elements are patterned so that those corresponding to the "m" coordinate are in strips extending "n"-wise and vice versa.

5. A transducer according to claim 1, in which the elements in each layer are patterned in strips of equal width spaced apart by the width of the strips.

6. A transducer according to claim 1, adapted as a readout for a microchannel plate (MCP) electron multiplier by the elements comprising electrically conducting areas of thin layers separated by thin insulating layers and their positions corresponding to single channels of the MCP and being adjacent the output end of the MCP and being such that an output from a single channel of the MCP will affect all the elements in all the layers that are aligned with that channel, the elements of each layer being electrically connected together and to a single amplifier and discriminator circuit for the layer.

7. A transducer according to claim 6, in which the layers are bonded together and to the output end of the MCP.

8. A transducer according to claim 6, in which the layers with more elements are closer to the output end of the MCP.