Delay line readout for drift chambers

NUCLEAR INSTRUMENTS AND METHODS 156 (1978) 347-351 ; © NORTH-HOLLAND PUBLISHING CO.

DELAY LINE READOUT FOR DRIFT CHAMBERS H. KREHBIEL, P. WARMING and E. ELSEN Deutsches Elektronensynchrotron Hamburg and H. Institut fiir Experimentalphysik der Universiti~t Hamburg, Hamburg, W. Germany

We report results on the readout of the second coordinate in a drift chamber by means of two fiat wire-wound delay lines as cathode electrodes running parallel to the anode wire. The lines were terminated at both ends by low-noise amplifiers based on the principle of "electronical cooling". With an amplitude-independent timing system a positional accuracy between 2.4 and 3.0 mm fwhm was obtained for a 1 m long line.

1. Introduction

2. Description of the delay lines

To see h o w g o o d a resolution o f the s e c o n d coordinate could be attained by delay line readout, a test c h a m b e r o f an o t h e r w i s e c o n v e n t i o n a l design was built~). It c o n t a i n e d f o u r 1 m long a n o d e wires, each c e n t e r e d in the m i d d l e o f a rectangular drift cell (size: 6 0 × 1 7 . 2 m m 2) f o r m e d by field s h a p i n g wires (fig. 1). Parallel to each a n o d e wire lwo flat delay lines were stretched in the Planes on the field s h a p i n g wires a b o v e a n d below the a n o d e wire.

T h e design o f the delay lines resembled in s o m e respects that o f R o t h e n b u r g et al.2). T h e two cond u c t o r s , w h i c h f o r m e d the delay line, were an inner straight copper strip and a thin insulated wire w o u n d tightly in o n e layer a r o u n d the strip (fig. 2). T h e inner strip was a 35 # m thick layer o f copper e m b e d d e d b e t w e e n two layers o f glassfibrereinforced epoxy. T h e w h o l e s a n d w i c h had a rect a n g u l a r cross-section and was 0.7 m m thick, 2.4 m m wide and 1 2 0 c m long. T h e o t h e r wire was

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anode wire

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delay line

Fig. 1. Cross-sectional view of one drift cell. epoxy.35

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Fig. 2. Schematic view of the delay line. V. ASSOCIATED ELECTRONICS

348

H. KREHBIEL et al.

wound around the inner strip by means of a special winding device, which was connected to a precision lathe. The two devices together allowed to feed the wire with constant tension and precise guidance to the strip. The wire spacing was very uniform and as narrow as was allowed by the wire diameter. Three types of delay lines were built, differing only in the wire diameter and consequently in the number of turns per cm. The measured electrical characteristics of the lines are listed in table 1. The characteristic impedance is between 400 and 65012, the propagation time between 175 and 285 ns/m.

3. Electronics Since the total induced charge is shared between all cathode electrodes and furthermore between both ends of the lines, special care for low noise in the amplifying electronics is important. With our test chamber we measured that the charge,

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Fig. 3. Basic diagram of the input stage of the charge sensitive amplifier.

detected at one end of a line, corresponds to only 15% of the total charge of the anode wire signal. Concerning the amplifier design we followed a suggestion by Radeka 3) and applied the principle of "electronical cooling". Such an amplifier has a definite real input impedance (necessary for proper delay line termination), which is achieved by electronical means, thus avoiding thermal resistor

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DELAY LINE R E A D O U T

TABLE 1 Delay line characteristics (length of each line: 1 m). De!ay line type

1 2 3

Wire Turns diameter (per cm) (mm)

0.10 0.12 0.15

80 62 50

Ohmic resistance of the winding (O/m)

Characteristic impedance (.Q)

Propagation time (ns/m)

Intrinsic rise time 5-50% after 1 m of line (ns)

de attenuation after 1 m of line

125 65 35

650 440 400

285 210 175

12.5 9.5 4.5

12 15 20

vert.

(%)

I V/cm

horiz. 500 nsec/cm

a)

output signal of the charge sensitive amplifier

vert. horiz.

b)

20 nsec/cm

rising edge of the amplifier signal

vert. horiz.

c)

500 mV/cm

500 mV/cm 20 nsec/cm

pulse shape of the amplifier signal after double-differentiation

Fig. 5. Pulse shapes of delay line signals. V. A S S O C I A T E D

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H. K R E H B I E L

et al.

coincidence of scint,counters noise. The basic circuit elements are shown in fig. defining norrow be~rn ~ generQteri ~ 3 (a complete circuit diagram is given in fig. 4). / The circuit resembles an integrating operational IA de,oytio, . I I amplifier with a virtually grounded input. Because the field-effect transistor at the input has a finite forward transconductance, the input impedance of the amplifier is finite, In quiescent state the circuit is stabilized through the resistor R. A voltage signal at the input results in a current signal at the output. By capacitive current division part of the current signal is fed back to the input. Thus the input impedance is given approximately by S -I(CR+Cv)/CR, where S is the forward transconductance of the FET, C, the feedback capacIcobtel q"""l ity and Cv is a trimming capacitor needed to make the input impedance adjustable to the line impedance. Since the forward transconductances of the available FETs were too small to give the right in- Fig. 6. Block diagram of the electronics for the resolution meaput impedance, two matched FETs were connect- surements. ed in parallel. Because of the capacitive feedback the amplifier the delay line signals, a "double-differentiation" has integrating properties. Thus the output signal technique was employed resulting in bipolar sigrepresents the integrated incoming charge divided nals of less than 150 ns length (fig. 5). The pulse by C,. This signal was passed on by a source-fol- shaping circuitry also added the two signals, which lower to the second amplifier stage, which was a were propagated simultaneously on the two paralconventional amplifier of gain 6 and capable of lel delay lines above and below the anode wire driving a terminated 50 12 line. To obtain ampli- (fig. 6). This resulted in a better signal-to-noise ratude-independent accurate time information from tio and thus in an improved resolution. The bipoi

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18

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time difference of left and right signa Fig. 7. Delay line resolution (spectrum).

DELAY

LINE R E A D O U T

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TABLE 2 Delay line resolution (length of each line: 1 m). Delay line type

1 2 3

Characteristic impedance (-Q)

650 440 400

Propagation time (ns/m)

(ns)

Resolution fwhm in the middle 6 c m from the end of the line of the line (ns) (ram) (ns) (mm)

12.5 9.5 4.5

1.3 1.0 0.8

Intrinsic rise time 5-50% after 1 m of line

285 210 175

lar signals were fed into a zero-crossing discriminator which yielded the start or stop signals for the propagation time measurements. The time intervals were measured by means of a time-to-amplitude converter in connection with a multi-channel pulse-height analyser. 4. Resolution measurements The drift chamber was tested in a beam of 4 GeV electrons of DESY. The width of the beam, defined by a coincidence of four properly adjusted scintillation counters, was less than 0.5 mm fwhm. The TAC was gated by this coincidence. The coordinate of an incident electron along the delay line was obtained from the measured time intervals between the corresponding signals arriving at the left and right end of the delay lines. An example of these measurements is shown in fig. 7. Here the propagation time differences were recorded for five different chamber positions with respect to the beam. The resolution of the delay line was obtained from these spectra. The results of the resolution measurements are listed in table 2. The positional accuracy in the middle of the line is nearly the same for all the lines tested: 2.5 mm fwhm. The increasing rf attenuation is

2.4 2.4 2.5

2.0 1.5 1.3

3.0 3.4 3.7

compensated to first order by the better time resolution of the slower lines. Towards the ends of the lines the accuracy degrades slowly due to the signal attenuation. The relative decrease of the time resolution is nearly the same for all lines (50%) and consequently the slower lines have better positional accuracy towards the ends (3.0 mm fwhm 6 cm from the end). 5. Conclusion Using two flat wire-wound delay lines with lownoise amplifiers and amplitude-independent timing electronics in a test setup we have demonstrated that a positional accuracy between 2.4 and 3.0 mm fwhm for the second coordinate in a 1 m long drift chamber is attainable.

We are indebted to Messrs. J. Bech, P. Liathke and V. Masbender for their technical assistance in building the drift chamber and the electronics. References 1) p. Warming, Thesis (DESY-F22, unpublished). 2) A. Rothenberg, private communication; cf. these proceedings. 3) V. Radeka, IEEE Trans. Nucl. Sci. NS-21 (1974) 51.

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