A novel, two dimensional, fast, low cost and accurate readout system for MWPCs

Nuclear Instruments and Methods 217 (1983) 357-359 North-Holland Publishing Company

A NOVEL, TWO DIMENSIONAL, FOR MWPCs

357

FAST, LOW COST AND ACCURATE

READOUT

SYSTEM

H. v a n d e r G R A A F a n d J.P. W A G E N A A R Radiation Technology Group, Physics Department, Delft University of Technology, The Netherlands

We developed a readout system which produces binary words for both coordinates of an X-ray quantum within 300 ns after being absorbed in a MWPC. The spatial resolution depends on the photo-electron range and the chamber geometry. In our case we reach 150 t~m.

I. Introduction

A high pressure, xenon filled M W P C can replace photographic films with the main advantage of a higher efficiency. Whereas a film absorbs many quanta simultaneously, the readout of a M W P C has to process each quantum individually. A picture with a 512 × 512 grid has 250k pixels; therefore a reasonable image needs about 2 × 106 processed hits. In order to limit the exposure time the processing time for a quantum should be in the order of 400 ns or less.

2. Principle of the readout system Since a spatial resolution in the order of 150/~m is required in both directions, one has to make use of the induced charge signals from wires and strips, the latter

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being perpendicular to the wires [1,2]. We skipped the 'brute force' readout system in which each strip and wire had its own ADC, because the processing time per particle is far too long, and the price is very high. The block diagram of fig. 1 shows the basic circuit: the signals from the strips are fed, after amplification, into a unit which switches through the signals from three adjacent strips carrying the highest signals. The signals from the wires are fed into an identical unit; here the avalanche wire and both its neighbours are selected. The strip switch unit produces also a binary number for the strip carrying the highest signal, whereas the wire switch unit has a binary word for the avalanche wire number as an output. These codes form the most significant bits for the coordinate words. Both groups of three analog signals are fed in two almost identical processors which produce the less significant bits of the coordinate words.

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358

H. can der Graaf, ,I.P. Wagenaar / Nocel readout vrstem l}~r M [I'P('~

3. Chamber geomet~' and amplifiers Our c h a m b e r has three planes: one with 20 # m a n o d e wires ( W - A u ) spaced 2 mm: two cathode planes with 100 b~m cathode wires ( C u - B e ) spaced 2 mm. Spacing anode cathode plane: 6 m m . Two adjacent cathode wires form a strip of 4 mm. Gas: 47% Ar, 47% ethane, 6% methane. We noticed that simple, inexplosive gases are also usable. The preamps are based on a Radeka design [3] followed by a main amp stage. For this we applied the # A 733 video amplifier because of its low price and its option for an adjustable gain.

4. The switch unit We refer to the circuit diagram of fig. 2. Signals from wires or strips are fed into a c o m p a r a t o r stage, consisting of the very fast, low cost, two-in-a-chip N E 521. Each c o m p a r a t o r has a hysteresis of 100 mV to prevent oscillations. A positive signal from a main a m p will cause the connected c o m p a r a t o r to go high. The array consisting of N A N D gates and diodes is only i m p o r t a n t if the input signals are strip signals. If a gate goes low,

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than it disables the neighbouring gates to go low. Since all the strip signals are positive, a few neighbouring c o m p a r a t o r s will go high, but the gate, connected with the c o m p a r a t o r which had the biggest signal, will be the only one to go down, because this was the first one. In case of two equal strip signals one will 'win'. The next stage consisting of flip-flops acts as a second filter. Since only one or two neighbouring gates can be disabled, we have to eliminate negative signals from gates further away: the outputs of the gates are connected via diodes to an "event" line. This line will go low as soon as a c o m p a r a t o r has been activated. A short pulse is then created by the quad 'exclusive OR'. The rising edge of this pulse clocks all the D-type flip-flops. Therefore only the output of the flip-flop associated with the first activated c o m p a r a t o r will go high. Three analog switches are activated: the signals from three neighbouring strips or wires are connected with the outputs QL, Q M and QR. The connection is realized 65 ns after the activation of a comparator. A Priority Encoder provides for the digital n u m b e r for the strip carrying the highest signal, or, in case of the wire switch unit, the n u m b e r of the avalanche wire.

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The P R O M s must be p r o g r a m m e d according to the results of measurements or calculations of the induced charge. See ref. 5 for a calculation of these charges. Fig. 3. The signal processor. For strip signals the buffers are inverting; for wire signals only the middle buffer is inverting.

5. The signal processors Fig. 3 shows the principles of the signal processor. Here we use two F A D C s in the divide mode [4]. For the o u t p u t M of a 6 bit F A D C we can write: M = 63Vi/Vr~ f.

The signals are amplified by buffers and fed into the F A D C s in such a way that we get 6-bit words for Q L / Q M and Q R / Q M . In case of the strip signal processor these words contain information of the X-coordinate of an X-ray q u a n t u m ; in case of the wires, information about the angle of incidence of the avalanche [1,5]. This angle is a good measure for the Y-coordinate. Both 6-bit words together form a 12-hit code. This code adresses a fast P R O M which gives an 8-bit answer for the coordinate. The F A D C s produce within 75 ns their o u t p u t n u m b e r for r a n d o m events. T h e P R O M has a delay of 45 ns; so the total processing time of the signal processor is 120 ns. If one applies more signal processors in parallel the effective processing time may be reduced.

8. Results Fig. 4 is a shadow image of a steel M6 nut, with an outer diameter of 10 mm. The nut was irradiated with 6 keV X - q u a n t a from a SSFe source. The spatial resolution was limited by the dimensions of the source (9 mm). This caused a parallax error of 200 p.m. The preamps and buffers were only roughly calibrated. This causes the white bands. The total processing time for the readout system was 300 ns. After calibration and optimising the timing of the A D C s it is possible to reduce this to 200 ns. The total processing time can be smaller than 100 ns if we apply plain FETs instead of the F E T switches and if we use hybrid-build F A D C s instead of VLSI chips.

References [1] [2] [3] [4]

G. Charpak et al., Nucl. Instr. and Meth. 148 (1978) 471. J. Fisher et al., IEEE Trans. Nucl. Sci. NS-25 (1978). V. Radeka, IEEE Trans. Nucl. Sci. NS-21 (1974) 51. B. Hallgren and H. Verweij, IEEE Trans. Nucl. Sci NS-27 (1980). [5] H. van der Graaf and J.P. Wagenaar, these Proceedings, p. 330.

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