A novel imaging readout with improved speed and resolution

A novel imaging readout with improved speed and resolution

Nuclear Instruments and Methods in Physit.'sResearch A310 (1991)299-3(14 North-Holland .~ ~ A A novel imaging readout with improved speed and resolu...

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Nuclear Instruments and Methods in Physit.'sResearch A310 (1991)299-3(14 North-Holland .~ ~

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A novel imaging readout with improved speed and resolution J.S. Lapington, A.A. Breeveld, M.L. Edgar and M.W. Trow Mullard Space St'it,nct. LaboratoD,, Department of Physics and Astronom.v Unicersio" Ctdlcge Londoll. London, UK

We describe a novel, charge centroiding, position readout, the spiral anode (SPAN) which combines excellent spatial resolution with very high count rate performance. SPAN is a planar structure of six electrically isolated electrodes. The ratio of the charges collected by the electrodes determines a two-dimensional position. The novel design allows the spatial resolution to be an order of magnitude better than the charge measurement accuracy. We present results from a prototype detector consisting of a microchannel plate stack in conjunction with the SPAN readout and discuss opportunities for the use of the SPAN readout with other photon counting detectors.

1. Introduction Imaging with a proportional counter or microchannel plate detector commonly requires a readout device able to determine the centroid position of a charge cloud, whether it be induced or actively collected. These devices can be divided into two general categories: those relying on charge division and measurement amongst a small number of electrodes [1-3], and those utilising charge detection alone but requiring a large number of independent electrodes and electronics [4,5]. In order to achieve high spatial accuracy, the former usually require precise charge measurement (in fact to a level higher than the position accuracy) which can be achieved only at the expense of speed, whilst the latter need complex electronics with many independent channels. We have designed a hybrid position readout combining the advantages of both of the above categories. The devices described below utilise charge division between several electrodes (from three to twelve) together with low accuracy charge measurement to determine the charge centroid position coordinate. Their novel design enables the spatial precision to be an order of magnitude greater than the charge measurement accuracy. The electrodes are deposited on a planar substrate with a repetitive structure and are electrically isolated.

2. The vernier anode The vernier anode uses six electrodes grouped into two triplets to encode each position axis. In each

electrode triplet, the electrodes vary sinusoidally along the axis of measurement and are 120 ° out of phase with each other. The charge collected on each electrode is measured, and from the three values, a position or phase calculated. This position coordinate is unique over a period of 360 °, representing the wavelength of the sinusoid. In ordcr that a unique position can be encoded beyond a wavelength, a second triplet of electrodes is used, being identical to the former in every respect except in wavelength. The wavelengths of the two triplets are slightly different and so a measure of coarse position can be obtained by looking at the diffcrence between the two calculated phases in a manner analogous to the operation of a vernier scale. A more accurate "'fine" position can be obtained by using the mean of the two calculated phases. The major advantage gained with this novel technique is the ability to measure position to a much higher accuracy than that to which the charge is measured. For example, if the charge on each electrode is known to one part in 64, the position can be determined to one part in 128 for each wavelength. In addition, 32 wavelengths can be uniquely distinguished using the phase difference be~',~een the two electrode triplets. Thus an overall position resolution to one part in 4096 can be achieved with electronics of much poorer resolution. Fig. 1 shows a schematic of a possible layout for a two-dimensional vernier anode. The periodic pattern structure has twelve electrodes per repeat pitch; six vary continuously in the vertical y axis and are used to encode this direction. The remaining electrodes are strips of constant width vertically whose widths vary from pitch to pitch such as to describe the six sinusoid ratios sampled at discrete intervals.

0168-9002/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved

IV. ASTRONOMY

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along the pattern axis plotted in the 3-D volume defined by the fractional electrode areas. This curve gives the spiral anode its name. The plane of the spiral is defined by A + B + C = constant. The arc length along the spiral gives the position along the axis.

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Consider the three-dimensional spate defined b~,, the fractional areas of the electrodes. The locus of the ctmrdinate representing the ~cariation in the electrode fractional areas along the axis de~ribes a curve in this space. This curve lies on a single plane bec~t~e the sum of the electrode fractional areas is equal to the width of the pitch which is a constant, if the electa~c structure were sinu~idal, as in the c a ~ of the vernier anode, the curve would be a circle. In the s~iral anode, the curve gradually spirals in towards the centre with the decreasing amplitude and wavelength (see fig. 3). This makes use of as many pixeis in the plane as is possible, each encoding a unique position coordinate. The equation of the curve is that of an Arehimedean spiral: r = kO. The form of thc spiral is further constrained by keeping the rate of change of arclength with respect to position, ~ s / ~ x , constant along the length of the spiral, so that the resolution will not vary along the axis. A one-dimensional anode of this form is currently being developed for use on the SOHO CDS satellite.

The spiral anode is a development of the vernier anode, with only three electrodes per axis rather than six, making it more suitable for applications where space and weight are important factors, it has a marked resolution advantage over the conventional wedge and strip anode [3], in which three electrodes measure the charge cloud position in two dimensions. For instance, a two-dimensional spiral anode {six electrodes) with 8-bit digitisation in each axis should give resolution better than 2000 × 2000 pixels. The pattern can best be explained by first considering a single axis or dimension. The three electrodes take a sinusoidal form as in the vernier anode. However, instead of distinguishing between one revolution and the next by means of a second triplet, in the spiral anode, the amplitudes and wavelengths of the three electrodes decrease along the length of the axis (see fig. 2). Thus each position along the axis has a unique electrode fractional area ratio.

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Fig. 4. Image of an array of 50 p.m pinholes, 1 by 1.7 m m apart. The maximum deviation in position is < 30 itm across the entire diameter of the MCPs. The histograms on the sides of the image are the integrated intensity across each axis. IV. A S T R O N O M Y

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The whole anode is symmetrical about the x = y axis which leads to some simplifications in the manufac-

The two,dimensional spind anode has been constructed in two different tbrms. The initial design used Iwo electrode triplets, one of whose electrodes varied continuously along its axis, while the other sampled the variation in th~ second, orthogonal axis, discretely. This se~md axis is composed of a triplet ot" strip electrodes of different widths, slotted between the pitches of the continuous axis and is similar to the scheme for the vernicr anode shown in fig. 1. The widths of the strips are chosen to vary along the discrete axis in such a way as to mimic a spiral. The t'ractional areas described by cach pitch would appcar as discrete points around the spiral in our thrcc-dimcnsional plot. However, bccausc or the ccntroiding nature of the device, whereby the the chargc cloud falls across several pitches, the actual point dcfincd by an event can exist anywhere around the spiral. The first successful two-dimensional anodes were of this form. The sceond design has both axes of continuous form. The pitches are once again interleaved, but this timc they are angled at 45 ° to each axis (sec fig. 2).

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(e.g. pitchwidth, amplitude, wavelength and rate ol" change of wavelength), the size and shape of the charge cloud, [6,7] must bc taken into consideration. The wavclcngth must be long enough, and the pitch small enough that thc circular charge cloud will encode its position with the correct fractional arcas defined by the measured chargc wdues. If the pitch is too largc, modulation will occur. This results from the charge cloud not avcraging across all electrodes and causes image distortion, If too small, the charge cloud smothcrs thc pattern so that the full range in amplitude during one wavc[cngth is not used, reducing the position resolution. Thus the pitch to wavclcngth ratio is crucial, If thc changc in amplitudc bctwccn one wavelength and the next is not largc enough, then the spiral arms will bc too closc together and will not bc distinguish-

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Fig. 5. (a) lalage of a resolution bar mask. The image is approximately 8 mm square. (b) Image of the set of small bar sizes in (a). The mast: has slit widths of 32, 38, 49, 60, 79, and 100 p.m. The 32 p.m bars are clearly resolved, corresponding to an FWHM resolution of better than 24 p.m. The series of parallel dots visible in the images are an artefact of the image display software.

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able, causing a ghosting effect. On the other hand, if too large, then the total arc length is diminished and the resolution reduced.

is used to interconnect the electrode elements. The entire manufacturing proccss uses dr5' procedures which are inherently cleaner than those used in lithographic replication.

4. Manufacture 5. Measurement Spiral anodes of all the types described - one-dimensional and both versions of two-dimensional have already been manufactured and tested. Measurements taken with a two-dimensional anode of the 45 ° type, are presented below. The anode itself is made from a 2 mm thick quartz blank of 60 mm diameter, sputter coated with a very. thin layer of chrome for adhesion purposes followed by a 2 - 3 i.tm layer of aluminium. A 15 W, single mode, pulsed NdYAG laser is used to remove narrow (10 to 20 pLm) lines of aluminium to form the insulating gaps between the electrodes. A 150 x 150 mm computer controlled x - y coordinate table moves the substrate beneath the laser beam with a positional accuracy of 1 p,m. The pitch dimension used in the detector described here is 680 ixm, which is the width of the six electrode repetitive structure. Ultrasonic wire bonding

These anodes have so far only been used as the position readout behind an MCP stack consisting of a chevron configuration of double thickness MCPs. The charge collected on each electrode is measured with its own electronic channel consisting of a charge sensitive preamplifier, shaping amplifier and analogue to digital convertor before being read into a computer for image decoding and processing. Electronics are currently being developed so that the fast count rate of which the MCPs are capable, may be fully realized. With the present setup, the electronics restrict the count rate to 30 kHz randomly arriving events. The data processing software has been written to calculate the position of the event and to plot it on the screen in a variety of ways. When the three signal IV. ASTRONOMY

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values per axis have been read into the computer they must be normalized and converted, using simple algorithms, into X, Y coordinates on the spiral plane (see fig, 3). The position along the axis is then simply the are length alonll the spiral up to that point, Sophisticated fitting techniques are used to evaluate the optimum position decoding algorithm for a given anode. For a detector perlbrming at high count rate, the position decoding algorithms cannot be performed in real time. A ,%,stem is being developed whereby a lookup table can be used to provide the position given two ratios of signals for each axis,

mark: space ratio of 50%, for all sizes. Fig. 5b shows an image of the small bars of fig. 5a, The series of the sizes is 32, 38, 49, 60, 79, and 100 ixm. The 32 Ixm bars are clearly resolved. Assuming the mask has been illuminated with a parallel beam and the detector resolution has a Gaussian distribution, this corresponds to an FWHM resolution of less than 24 Ixm, We expect to gain a marked improvement in the resolution by varying the configuration of the MCP stack and the plate operating voltages.

Acknowledgements 6. Results The prototype anode had an active area of 32 mm and a pitch of 680 I~m, This was combined with a chevron pair of double thickness, 25 mm diameter MCPs with a pore diameter of 12,5 ~m and 15 p,m spacing, The two plates were butted together and the gap between the anode and the bottom MCP was 4.7 ram, The stack was operated with a voltage across the plates of 2,7 kV and a voltage across the anode gap of 300 V, Images were obtained by illuminating a series of masks, 3 to 4 mm in front of the MCP stack, with a collimated beam of soft X-rays. Fig, 4 shows the image obtained by illuminating an array of 50 p,m pinholes. The spacing of the array is 1 by 1,7 mm and the image covers the entire area of the MCPs. It is immediately apparent from this figure, that except for the regions at the extreme edge of the MCP, the spiral anode exhibits excellent linearity over the whole image. The maximum deviation in position is < 30 I~m across the whole active diameter. Fig. 5a shows an image of a bar mask which has a

We would like to thank Professor J.L. Culhane for his enthusiastic support and encouragement. This work is funded by the Science and Engineering Research Council.

References [I] M. Clampin, M., J. Crocker, F. Paresce and M. Rafal, Ray. Sci. Instr. 59 (1988) 1269. [2] M.B. Williams and S,E. Sobottka, IEEE Trans. Nucl. Sci. NS-36 (1989) 227. [3] C, Martin, P. Jelinsky, M. Lampton,R.F. Malina and H.O. Anger, Ray. Sci. Instr. 52 (1981) 1067. [4] J.G. Timothy, G.H. Mount and R.L. Bybee, IEEE Trans. NucL Sci. NS-28 (1981) 689. [5] W.E. McClintock,C.A. Barth, R.E. Steele, G.M. Lawrence and J.G. Timothy, Appl. Opt. 21 (1982) 3071. [6] M.L. Edgar, R. Kesscl, J.S. Lapington and D.M. Walton, Ray. Sci. Instr. 60 (1989) 3673. [7] J.S. Lapington and M,L. Edgar, Prec. SPIE 1159 (1989) 565.