Proportional chambers for very high counting rates based on gas mixtures of CF4 with hydrocarbons

Nuclear Instruments and Methods in Physics Research A238 (1985) 249-264 North-Holland, Amsterdam

249

PROPORTIONAL CHAMBERS FOR VERY HIGH COUNTING RATES BASED ON GAS MIXTURES OF CF4 WITH HYDROCARBONS J . FISCHER, A . HRISOHOt, V . RADEKA and P . REHAK Brookhaven National Laboratory, Upton, New York 11973-5000, USA Received 24 January 1985

Very fast multiwire proportional chambers of low mass for very high counting rates were developed . The anode signals have an effective duration of 8 ns after pulse shaping and a time jitter of only 4 ns fwhm . This performance was achieved with gas mixtures of CF4 and hydrocarbons at atmospheric pressure in MWPCs with small interelectrode distances . Gas mixtures with high primary specific ionization density and low noise fast signal processing provide full efficiency at gas gains of a few times 10 4. At these conditions, the chambers can operate at rates of several times 10 7 s -1 cm -2 . Various gas mixtures and chamber geometries were explored and test results are reported .

1 . Introduction New experiments in high energy physics demand in some cases a higher performance than available from conventional gas detectors . For example, an experiment (no . E777) to search for very rare K + decays (K + ?T+P+e-) at the Brookhaven National Laboratory Alternating Gradient Synchrotron (AGS) requires the knowledge of particle trajectories in the incident beam to better than 1 mm at a flux of 5 X 10 $ particles/s within an area of a few dm2 , with a minimal beam interaction rate in the detector . The scintillation hodoscope, while known to be fast is excluded due to the unacceptably high interaction rate of beam particles in the scintillator . This would produce background particles which could simulate the rare events of interest in the high rate environment. The lowest mass beam detectors can best be realized with multiwire gas proportional chambers . The constraint of thin windows and a large area dictates the operation of such chambers at atmospheric pressure. The rate requirement of 5 X 10 8 particles/s with minimum chance coincidences, requires an effective time resolution of a few nanoseconds . To obtain such a time resolution and a spatial resolution below 1 mm, the electron drift distances should be short and the drift velocity high . The gas mixtures also have to produce sufficient ionization in thin layers at normal pressure for detection of mimimum ionizing particles with a high * This research was supported by the US Department of Energy : Contract No . DE-AC02-76CH00016. Present address : Laboratoire de L'Accélérateur Linéaire, Bâtiment 200, Orsay, France. 0168-9002/85/$03.30 © Elsevier Science Publishers B .V. (North-Holland Physics Publishing Division)

efficiency (>_ 99%) . In order to limit positive ion space charge effects in the detector at the rates of interest, the total avalanche charge produced by a minimum ionizing particle should be small (e.g ., less than 5 x 10 5 electrons). This study shows that to satisfy this condition and to achieve a high detection efficiency, the readout should be sensitive to signals of about 10 4 electrons produced by single ionization clusters . The processing of such small signals at high counting rates requires fast low noise electronics. Pulse shaping should perform the necessary shortening of the inherently long signals from proportional chambers. In this paper, developments leading to a detector satisfying such requirements are described . Sect. 2 introduces the general problematics of the fast chambers, time jitter, reduction of interelectrode distances, efficiency and ionization density in gases . Sect . 3 considers the fast gas mixtures based on CF, Sect . 4 discusses signal processing, detection sensitivity and avalanche gain. Sect . 5 describes the test chambers and the experiment set up . In sect . 6, the test results on time jitter, efficiency, and counting rate effects are presented and discussed. Sect . 7 concludes with an estimate of a fast chamber performance in a high energy physics experiment . 2. General considerations In order to reduce the resolving time of the detector, the interelectrode distance (cell size) has to be small and the electron drift velocity high. The time jitter for timing an event (timing resolution) is in our case caused

25 0

J. Fischer et at. / Proportional chambers for very high counting rates

by the spread in the arrival time of the first electron cluster to the multiplication region at the anode for all possible particle track locations. The cell size has to be small, also to reduce the drift time of positive ions from the avalanche to the cathode, in order to minimize space charge effects. There are, however, practical and theoretical limits to the reduction of the cell size. 2.1 . Practical limits of cell size reduction It is difficult to maintain a uniform anode to cathode gap below 500 pm in low mass chambers with an area of 10 2 to 10 3 cm2 . Variations in flatness and thickness of chamber frames, as well as wire deflections, due to electrostatic forces and gravity become significant. Because of the mechanical strength and nonuniformity of the anode wire diameter, wire diameters below 8 pint are not sufficiently reliable for high energy physics experiments, although 5 ,u.m wires have been used in small cell pressurized test chambers [1] . A large wire diameter is less desirable for the chamber performance. At a given gas amplification, a relatively larger wire diameter results in a higher average electric field outside the multiplication region near the anode, which can lead to instabilities and reduction of the attainable gas gain. 2.2. Limits of cell size reduction due to the ionization process A more fundamental limitation to the cell thickness reduction is given by the required counting efficiency of over 99% for the detection of minimum ionizing particles in a gas at atmospheric pressure. If the gas thickness is too small, then a minimum ionizing particle may occasionally traverse the chamber without producing any ionization, resulting in a loss of efficiency. Due to the statistical nature of the ionization process, the efficiency q and the average number of primary ionization events (clusters) N have the well known relationship iq - 1 - e -N for single cluster detection. Therefore, an average of N >_ 6 primary ionizing events (clusters) are needed for an efficiency of 99.8% . The minimum gas thickness is then a function of the average primary ionization density, expressed as N primary ionization events, or clusters, per centimeter of the chamber gas. N is assumed to be roughly proportional to the eletron density of the gas at least for gases with molecules of the lighter atoms. However, inner shells of heavier atoms contribute less to the primary ionization [2]. Molecules of equivalent electron density composed of several lighter atoms produce more clusters. From the ratio of electron densities and from conservative values for N, inferred from inefficiency measurements in some gases in proportional chambers, drift chambers and by direct cluster counting methods (e .g.,

Table 1 Primary ionization cluster density Gas

Electrons per molecule

Average primary clusters/cm

Minimum gas thickness for 6 clusters

CH 4 C21-12 A+10%CH 4 C,H6 C02 C3 H s i - C4Hiu CF4 (alone)

10 14 =17 18 22 26 34 42 42

12 17 20,16 (refs. [3,4]) 21 26 30 40 (Ref. [3])' ) 50 (by electron density) 41 a)

5 mm 3 .5 3 ; 3 .6 2 .8 2 .3 2 .0 1 .5 (electron attachment losses) 1 .4

42

50 a.b)

1 .2

CF4 (in gas mixtures) C(CH 3 ) 4

a) From our measurements . b ) Also by extrapolation from ref. [3] .

refs. [3-5]), one may estimate the minimum gas thickness required for other gases of interest . Table 1 (an orientation guide only) shows the molecular electron density and the approximate primary ionization cluster density for several gases at atmospheric pressure, and the corresponding minimum gas thickness needed to produce six clusters . The values of the relevant gases are in reasonable agreement with our measurements . It appears that the last three gases in table 1 are candidates for efficient gas mixtures at normal pressure in small cell chambers with a gas thickness of 1 to 1 .5 mm . These gas mixtures must also have a high electron drift velocity and a low electron attachment. 3. Selection of fast gas mixtures To achieve the required timing performance, a gas mixture with a fast electron drift velocity has to be used . The drift velocity varies inversely with the product of the total scattering cross-section and the square root of the mean electron energy [6]. Traditionally, the velocity has been increased by lowering the mean energy of the drift electrons (preferably for a large range of E/p values) into the region of the Ramsauer-Townsend (R-T) minimum of the momentum transfer cross-section of the noble gases. The "cooling" of drift electrons is usually accomplished by collisions with molecules of a "quench" gas additive to noble gases. The quench gas also improves stability of operation of the counter by absorption of uv photons from the multiplication process, de-excitation of atoms, and after charge transfer it reduces electron emission from ion impact on the cathodes by predissociation of

J. Fischer et al. / Proportional chambers for very high counting rates quench gas ions . CO, CH,, C,H Z, CZ H 4, C,H6, NH 3, as well as more complex molecules, e.g . higher hydrocarbons, ethers and alcohols etc., are classical examples of quench gases. In small amounts, most quenchers also reduce the required electric field for a desired gas gain, and some of them increase the total ionization by the Penning effect. Recently, Christophorou et al. [6,7] reported that CF4 is a more effective additive to raise the electron drift velocity in noble gases which have a pronounced R-T minimum. In the same paper [6] they also reported a high electron drift velocity in pure CF4, rising from zero to a peak of = 15 cm/,us at E/p = 6 V cm -1 Torr -1 . The drift velocity declines to 12.8 cm/ps at E/p of 3 and 8 V cm -1 Torr -1 . Corresponding electron energies were not indicated. Even higher velocities were reported by Naidu and Prasad [8], increasing from - 15 to - 32 cm/,us for the E/p range of - 40 to = 90 V cm -1 Torr -1 , with respective (1?/Ft)= 4.3-5 .3 V. Drift velocities for the E/p region, from 8 to 40 V cm -1 Torr -1 , i.e . between those regions reported above, are expected to be lower than at the peak, but have not been reported . The fields in our small cell chambers (fig . 1) span all three E/p regions, with the major part of the drift volumes in the uncharted E/p region (e .g., fig. 2a). Our time jitter measurements (this paper), with 100%

(a)

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CF4 , seem to indicate only a moderate reduction of the peak drift velocity in the uncharted E/p region. For our purposes, noble gases with small additions of CF4 are not practical because of low primary ionization density or a lower drift velocity . Our choice was high purity CF4 with a minor quench gas addition . We found that the electron drift velocity in these new mixtures (averaged over the total drift path in our test chambers) is as high as the drift velocity in CF,. CF, alone is not sufficiently self-quenching as a counter gas for stable operation in our cell geometries . It also requires high electric fields and exhibits electron attachment . The addition of a quenching gas to CF4 should increase the primary ionization density of the mixture. A more important purpose of the additive, however, is to optimize the electron drift velocity by reducing the mean electron energy just to the region of the minimum of the scattering cross-section of CF, This reduction has to be accomplished for a wide range of E/p values, e.g ., 10-60 V cm -1 Torr -1 . An appropriate type and concentration of the additive has to be found by experiment since the relevant mean electron energies and scattering cross-sections are not known . A reasonable dense additive in our case is neopentane [C(CH3)4] at small concentrations (e.g ., 10-20%), since its cross-section, even at the R-T minimum, is relatively large. The upper electron energy level of its R-T minimum is fairly high (=0.5 eV) [9], which precludes cooling of the electrons more than is necessary. The less expensive isobutane, i-CH,,, also meets our requirements as the test results (sect. 6) indicate . The electron attachment in these mixtures must also be considered. In the typical fields of proportional chambers, CF, attaches electrons mainly via dissociatioe attachment processes occurring at electron energies above = 4.5 eV with cross-section maxima at 6 to 7.5 eV [6]. Therefore, by lowering the mean electron energy levels, one may reduce electron attachment, (section 6 .3) except at the very high fields near the anode wires, where also some dissociation into electronegative components and their products cannot be avoided.

CATHODE STRIPS BETWEEN ANODES, CHAMBER 4 d

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SAME AS CHAMBER 3 0.6351 1,27

(d) 2 DIMENSIONAL

10

008

1

IDEA

READ EACH CATHODE PAD (TOP a BOTTOM

Fig. 1 . Test chamber geometries and interelectrode distances. Chamber 5 in (a) was selected for high counting rates.

4. Signal processing and circuits The signal currents from the anodes have a relatively long duration . The purpose of signal processing is to make the signals narrow and to obtain a good timing resolution . Narrow signals are required by the high rate of beam particles. A good timing resolution allows the selection of the beam particle belonging to a rare event detected by the other detectors in the experiment placed outside the beam . The shape of the signal current waveform at the anode is the convolution of two processes. The first process is the arrival and the multiplication in

J. Fischer et al. / Proportional chambers for very high counting rates

252

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--------------

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Fig . 2 . (a) Equipotential lines for a cell quadrant of chamber 5 at intervals of 5% of the anode to cathode potential difference . E/p values are averaged over the same intervals for an anode potential of 1400 V. (b) Electric field lines for a cell quadrant of chamber 5 . the anode region of electrons produced along the ionizing particle track . The second process is the drift of the positive ions produced in the avalanche away from the anode. The motion of the positive ions is directly seen by the preamplifier (connected to the anode) as the signal current . The first process is related to the production of the primary ionization clusters by the detected particle and it is therefore a stochastic process . (The fluctuations in the gas gain add to the total fluctuations in the charge but not in the time .) The second process can be considered as being deterministic for practical purposes . Positive ions created by a stochastic process are moving in the electric field of the chamber in a well defined way (fluctuations in the ion position vs time due to ion diffusion are negligible) . This motion induces a current of the form i(t)li . = (1 + t/t o ) -I , as the single electron response of the chamber. The adopted signal processing electronics cancels pulse tails due to the slow but deterministic motion of positive ions [11,12] . Thus, the remaining fluctuations in the shape of the signal are only due to the stochastic nature of the primary ionization in gases . In order to achieve a high detection efficiency single ionization clusters are de-

tected . The timing signal is produced at the arrival of the ionization from the closest (first) cluster to the anode. For tracks crossing the cell at different positions, the time resolution (time fitter) is then given only by the variation in the drift time of the "first" cluster to the anode. The minimum signal duration is the drift time difference between the first and last electron of the ionization track arriving at the anode . The signal processing circuits for each anode channel consist of a preamplifier, a shaping amplifier, a discriminator and a coincidence circuit. The preamplifiers are located at the chamber as close as possible to anode wire connections in order to minimize the total input capacitance and inductance of the connections . The preamplifier is based on a simple configuration with common base input and two cascaded emitter followers in the output (fig . 3). The do current return for the output emitter follower is provided at the receiving end of the cable in the shaping amplifier (680 Sl resistor in fig. 4) in order to reduce the power dissipation . The power dissipation of the preamplifier is = 18 mW . The preamplifier is realized in thick film hybrid technology. The rise time of the impulse response of this preamplifier is = 0 .8 ns, and the decay time constant as de-

25 3

J. Fischer et al. / Proportional chambers for very high counting rates .1-A

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Fig . 3. Fast low noise preamplifier circuit . R 9 = 39 12 for 50 12 cable, 91 12 for 100 12 cable. termined by its dominant pole (the collector resistance R 4 = 27 k S2 and the capacitance of the node at the collector of Q1 of = 1 .6 pF) is = 45 ns . The schematic diagram of the shaping amplifier is shown in fig . 4 . It consists of four stages of gain and two networks for tail cancellation. Each stage of gain has the same basic configuration as the preamplifier. A short delay line clipping of 1 to 2 ns (between transistors Q3 and Q4) is used to shorten the preamplifier response . The cancellation of the tail in the preamplifier response is achieved by attenuating the reflected wave in the delay line, which is adjusted by the potentiometer P1 . The (1 + t/t o ) -1 tail in the detector single electron response due to the slow motion of positive ions is cancelled with sufficient accuracy by the RC network between the third and fourth stage of amplification (transistors Q9 and Q10). Potentiometer P3 can be preset to about 100 to 200 Q, and the tail is then cancelled by adjusting potentiometer Pz ("pole-zero cancellation"). The tail cancellation is discussed and the circuits are described in more detail in ref. [12] . The discriminator is realized by using a fast comparator, AD9685BM (Analog Devices, Inc.). The overall gain of the preamplifier and the shaping amplifier provides a charge sensitivity of = 2 1.V/e, e.g ., a threshold signal of 1O 4e produces = 20 mV at the input to the discriminator .

Fig. 5a shows the preamplifier output in response to conversions of 5 .9 keV x-rays (practically a point ionization) in chamber 3 . The waveforms show induced charge as a function of time in the first 10 to 15 ns, before the onset of the decay in the preamplifier response becomes apparent . Fig. 5b shows the amplifier output for the preamplifier signals of fig . 5a, with only the preamplifier tail cancelled . The tail due to positive ions remains clearly visible . Fig . 5c shows the same signal after the second polezero cancellation network which cancels the (1 + t/to)-1 tail . The signal now returns to the baseline after about 10 ns, permitting a double pulse resolution of about 8 ns. The equivalent noise charge of the amplifying-shaping system is = 1 .5 X 10 3 rms electrons for a total input capacitance of the order of 10 pF . Such a low equivalent noise charge allows us to set the discriminator threshold down to the level of 10 4 electrons. The signal charge in the first few nanoseconds, due to a single cluster, is only a very small fraction of the total charge produced in the the avalanche by a minimum ionizing particle traversing the chamber. The ratio of these two charges is important because it determines the relation between the sensitivity of the readout and the actual operating conditions of the chamber . The avalanche charge determines the space charge effects at

254

J. Fescher et al. / Proportional chambers for very high counting rates

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high counting rates, and the gas gain is an inverse measure of the stability of operation of the chamber . The development of the charge signal induced by the positive ion motion as a function of time is discussed in ref . [12] . The fast pulse shaping used here (Gaussian, fwhm = 5 ns, fwtm = 9 ns) corresponds to an effective integration of = 5 ns . The positive ions drift from the anode wire only a very short distance in that time and the charge signal they induce is only about 1/5 of the total charge produced in the avalanche . Thus to produce a threshold signal of 10 4 electrons with a one-electron cluster, a gas gain of 5 X 10 4 is needed. The average ionization in a gas thickness of 1 .27 mm used in the smallest cells studied here is about 20 ion pairs. Thus the total avalanche charge produced for each detected track approaches 10 6 electrons . In actual operation, single electron sensitivity does not appear necessary, since the mean value of ionization per cluster has been found to be about 3 electrons with the gas mixtures chosen. Single cluster detection requires then a gas gain of only = 1 .7 X 10 4 and the avalanche charge for the whole ionization track (= 20e) is = 3 X 10 5 electrons . Thus the ratio of the total avalanche charge (number of ions) to the minimum useful signal is = 100 in the case of single electron detection, and = 30 for single cluster detection . The avalanche gain of 5 X 10 4 is still moderate, while the gain of = 2 X 10 4 allows conservative operating conditions . The measurement of the total avalanche charge is inconvenient due to a long ion drift time to the cathode (of the order of 5 to 10 jAs in the smaller chambers studied here) . The charge and gas gain measurements reported here have been performed with an effective integration time of 1 f.s (e.g ., with charge integration and 1 ps delay line clipping, such as in the Ortec 460 amplifier) . The charge thus measured represents about 4/5 of the total avalanche charge with these chambers . All the values referred to as the "gas gain" in figs . 7-10 were thus measured, and should be multiplied by 1 .25 to obtain the true avalanche gain .

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5. Experimental apparata The geometries of five test chambers of about 5 cm X 4 cm area are shown in fig. 1, with electrode configurations and dimensions indicated . Chambers 1, 2 and 5 are of the multiwire proportional type construction, with progressively smaller electrode distances, varying the anode to cathode gap over a range of 10. Chamber 5 has a cell quadrant size of only 635 X 395 t.m . Chamber 3 has potential wires centered between anode wires . In chamber 4, conducting partitions are substituted for the potential wires of chamber 3, thus dividing the chamber into isolated detector cells. The cathodes can be further subdivided longitudinally and read out individually if necessary.

255

J. Fischer et al. / Proportional chambers for very high counting rates

TIME TO AMPL CONVER

PHA "ig. 6. Experimental arrangement for the time jitter and countng rate tests, for all chambers.

Fig. 5. (a) Preamplifier output pulse in response to 5.9 keV X-rays (point ionization). (b) Shaping amplifier output pulse with preamplifier pole cancellation only. (c) Shaping amplifier output with cancellation of the tail due to positive ion motion.

In all chambers, a preamplifier outside the gas volume is connected directly (within 2-3 cm) to each anode. Two signal returns for each preamplifier are provided by short connections to the preamplifier ground from both cathode planes via distributed capacitive coupling . Anode wires are gold-plated tungsten alloy by Luma Metals [13] . Cathodes are films of Kapton with evaporated metal coatings .

The gas mixtures were flowing through the chambers it rates of about 10 cc (= 5 chamber volumes) per ninute after first being proportioned with flowmeters . 3as purities were as follows. CF4 (Freon 14) from vlatheson Gas Products [14], 99 .7% purity. C(CH 3 ) 4 ilso known as dimethylpropane or neopentane, "reearch grade" 99 .9% purity. The other gases were of `instrument grade" or equivalent, with over 99 .5% puity. All gases were used without further purification to imulate practical conditions in high energy physics :xperiments. The potential distribution for a quadrant of the `cell" of chamber 5, which has the smallest electrode listances, is shown in fig. 2a. Also indicated are some E/p values averaged over intervals of 5% of a practical anode to cathode voltage. The E/p values averaged over any drift path are 5-10 time higher than in "standard" MPWCs with larger cells. This is in part due to the cell size and in part due to the higher fields required for a given gas gain in these mixtures compared to P10 (as shown in fig. 10b) . Fig. 2b indicates the electric field lines for a quadrant of the same chamber. For most particle tracks, the first electron cluster to be detected would come from positions closely above or below the anode plane (near the top horizontal edge of this figure). The experimental arrangement for the time jitter and counting rate tests is shown in the block diagram of fig. 6. The system measures the distribution of the time

25 6

J. Fischer et al. / Proportional chambers for very high counting rates

differences between the passage of a ß-electron from a collimated 9° Sr/9° Y source and the detector signal . The time of passage of the minimum ionizing particles is determined by a fast plastic scintillator/photomultiplier and a constant fraction discriminator. The timing error of the discriminator is estimated to be less than-1 ns. A filter in front of the scintillator stops low energy electrons . The timing of the chamber signal is performed by a leading edge discriminator. The distribution of the time differences is recorded by a pulse height analyzer after time-to-amplitude conversion . For the counting rate tests, a high intensity collimated beam of 5.4 keV X-rays (from a chromium target X-ray generator) is directed through the same area of the chamber. The chamber current is monitored.

The gas gain and the relative ionization are determined by comparison of pulse heights produced by 5.9 keV X-rays from a SSFe source and by minimum ionizing ß-rays. The observed signal charge is measured by comparison with the charge produced by a calibrated pulse injected into the preamplifier via a calibration capacitor . 6. Test results and discussion Time jitter, efficiency and space charge effects in relation to the gas composition, cell geometry, gas gain, and counting rates were studied.

Fig. 7. Time jitter vs gas gain in chamber 1, for common gas mixtures of argon with methane or ethane, and of CF4 with propane or acetylene. "Gas gain" in figs. 7-10 is based on the charge observed in 1 /As. The actual avalanche gain and the charge produced in the avalanche are about 25% higher than the values obtained by the measurements in 1 its with these chambers and gas mixtures . The signal charge obtained with the pulse shaping as m fig. 5c is about 1/4 of the signal charge in 1 ps.

J. Fischer et al. / Proportional chambers for very high counting rates

jitter to about one-half of that for the standard drift chamber gas mixture of 50% A + 50% CzH6, and to about one-quarter of the time jitter attainable with P-10. Chamber 1 could attain stable operation also for pure CF, for a limited gas gain range, resulting in a time jitter of about 11 ns fwhm. The mixtures of CF4 with 10-30% propane were almost as fast and more stable at higher gas gains with less electron attachment . Larger amounts of propane increased the time jitter, indicating a lower electron drift velocity . Other hydrocarbons can be substituted for special effects. The response to a mixture of 20% CZ HZ in CF4 is shown at a high gas gain to indicate the effectiveness of this quencher to lower the electron temperature. However, its primary ionization density is too low for our application . The selection of additives to CF4 is also influenced

First, the time jitter in various gas mixtures is explored and discussed for the various cells of fig. 1. The results are then considered as a function of the anode wire spacing from which an estimate of the drift velocity and cluster density is obtained. Efficiency tests and a cluster density estimate are next, followed by a discussion of the rate limitations, space charge effects and high rate results. 6.1 . Time jitter

A comparison of the time ,litter in gas mixtures containing CF, as a major component versus "standard" gas mixtures is shown in fig. 7 for the relatively large cell size of chamber 1 . As can be seen from the curves of the time jitter versus gas gain, the high electron drift velocity in the CF, mixtures strongly reduces the time

111

30 28 26 -

CHAMBER 2

4---

I mm -~

1

24 22 20 _ 18

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2

14

3 12

FWTM

10 8 6 4 2 0 10 3

2

3

4

5

104

257

2

3

4

5

GAS GAIN

Fig. 8. Time jitter vs gas gain in chamber 2 for various concentrations of isobutane in CF4.

258

J. Fescher et al. / Proportional chambers for very high counting rates

by the chamber geometry which varies the mean E/p values . This affects the electron energies and the corresponding electron drift velocities. Therefore, the mixtures should be optimized for the lowest time jitter for each geometry. Chamber 2 has half the anode spacing of chamber 1. Fig. 8 shows the reduced time jitter in this chamber for 100% CF, and for various concentrations of isobutane in CF, At the concentration of 20%, the time jitter of about 6 ns (fwhm) is almost as small as in pure CF, . Higher concentrations of isobutane reduce the drift velocity by dilution with the slower gas (increase in the scattering cross-section) . The upper two curves show the full width of the time jitter at one-tenth maximum, i.e ., near the base of the time jitter distribution . The measured time jitter in chambers 1 and 2 can be explained entirely by the fluctuations in the drift time of the first electron cluster arriving at the multiplication region around the anode. For a particle track with high ionization density, the maximum drift distance corre-

CHAMBER 3

635mm ~0

i

sponds to a track in the middle between two anodes . Moreover, the electric field in the "saddle point" regions centered between anode wires (fig . 2) is weak, which increases the time jitter further. The geometries of chamber 3 and 4 (fig. 1) were attempts to improve the time jitter not only by reducing the anode to cathode distance, but also to reduce or avoid these low electric field regions, while keeping the anode spacing the same as in chamber 2. This was accomplished in chamber 3 by insertion of field wires at cathode potential, positioned between the anode wires. In chamber. 4, these field wires were replaced by partitions at cathode potential, which resulted in an array of small rectangular proportional counter tubes. Such arrangements also provide good signal isolation between anodes. In addition, chamber 4 lends itself to readout of the induced charge via cathode pads as indicated in fig. 1 . This could effectively subdivide the length of each anode wire for increased readout rate capability, and/or for a two-dimensional fast readout.

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ALL MIXTURES

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H

1100

t

1150

i

1200

i

1250

VOLTS

i

1300

100% CF4

I

1350

i

1

1400

1

1450

Fig. 9. Time fitter and efficiency vs voltage in chamber 3 using CF, with additions of neopentane or isobutane.

J. Fischer et al. / Proportional chambers for very high counting rates

The time jitter in chambers 3 and 4, i.e., using wires or partitions between anodes, was practically the same, about 5 ns fwhm (fig. 9), which is a slight improvement over the 6 ns measured in chamber 2 because of the improved electric field configuration . Chamber 4 was slightly less efficient (by 2-3%), attributed to particles passing through the dividing strips of 25 pm . The technical difficulties in construction of chambers 3 and 4, which practically double the number of closely spaced wires (or partitions) at high potential differences, overshadow the improvement in the time jitter . It was considered more practical and more effective to use the conventional MWPC geometry, but with more anode wires than chambers 2 or 3, and with anode to cathode

distances reduced to the expected limit for a high detection efficiency as in chamber 3. The best performance was indeed obtained with chamber 5 . It has the classical MWPC geometry of chamber 2, scaled down by a factor of - 1 .6, resulting in a cell quadrant size of 635 x 395 pm . The anode wire diameter was scaled down only by a factor of = 1.25 to 8 pm for practical reasons as mentioned in sect. 2. The electric field of chamber 5 was shown in fig. 2. Although the average E/p value for this chamber is about five or more times higher than in ordinary MWPCs, operation is stable with gas gains in the 10 5 region . The performance of chamber 5 is shown in fig. 10a for some binary and ternary gas mixtures . The depen-

TIME CHAMBER 5

a

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~8um DIA .

mm

15

x 20% i - C4Hlo 0 10/0 I - C4H,a+10%CZHZ + 20%CZHZ 0 20 0/. C(CH 3 ) 4 0 10%C(CH3)4+10%C Z H Z * 33%C(CH3)4

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a

+ --'IUoo ~ FWHM

GAS GAIN-10 4 3x10 4 1300

1350

1400

1450

VOLTS

1500

259

1550

1600

1650

Fig. 10(a). Time jitter vs voltage in chamber 5 for some single or double additives to CF4 .

J. Fischer et al. / Proportional chambers for very high counting rates

Fig. 10. (b) Gas gain as a function of applied voltage in chamber 5. dence of the gas gain on the applied voltage is shown in fig. 10b. The lowest measured time jitter with the fastest mixtures at suitable gains was below 4 ns fwhm and 8 ns fwtm . Of the various gas mixtures tried in chamber 5, the mixture of CF4 with 20% dimethyl propane C(CH 3 )4 is the fastest, i.e., it has the smallest time jitter. There is a slight increase in fwhm at higher or lower concentrations of C(CH 3)4. The performance with 20% isobutane in CF4 is almost as good . We have also tried additions of acetylene (C21-12) to see if this effective electron energy reducer would be useful, since our E/p ratios are beyond the maximum of the electron drift velocity in 100% CF, . 10-20% addition of C2 H 2 to CF, actually increases the time fitter, i .e., reduces the drift velocity . This additive may be too effective, thus lowering the electron energy too much for the maximum drift velocity in CF4 . Mixtures of 10% acetylene added to 20% dimethyl propane or to 20% isobutane in CF, were also tried for the same reasons. Also, in these cases, a small increase in the time jitter was noticed. 6 .2. Time jitter versus anode spacing

If one assumes that there is usually a cluster of electrons not far from the anode wire plane (for nearly normal incidence of charged particle tracks), then the

main part of the time jitter (fwhm) will only be due to the maximum distance of a track from an anode wire, i.e., half the anode spacing. This is indeed the case, as can be seen in fig. 11, where the time jitter (fwhm) is plotted versus half anode spacings, without regard to the chamber (gas) thickness. A linear relationship between the time jitter and the drift path length is apparent, even in the case of the small dimensions of chamber 5. The inverse of the slope of this line indicates an "average drift velocity" (averaged over the drift distance of half anode spacing) of wa = 12.5 cm/us (or = 8 ns/mm), which is similar for all three cases in spite of some increase of average E/p for the smaller cell sizes. This average wa is close to the peak in the drift velocity of = 15 cm/is reported [5] for pure CF, at E/p=6 V cm -1 Torr -1 . However, average E/p values are much higher here, which would increase the electron energy and may reduce the drift velocity. The quench gas additions cool the drift electrons back to a region of high velocity in CF,, but being slower gases, they also reduce slightly the maximum wa of the mixture as indicated by the measurements here . The graph of fig. 11 extrapolates to a residual time jitter of = 0.9 ns (fwhm) for the hypothetical case of s/2 = 0. This residual time jitter (t,) is ascribed to the average distance (A) between primary ionization clusters in the particle tracks, and it sets a lower physical limit for the time jitter . One may estimate an approximate value for A from the residual time jitter, and the "average electron velocity" wa considering that electron clusters drift to the anode plane from the front and the back of the chamber: A=2tr wa = 2x0.9nsx12.5 cm/lu s =225pm, 1/A = 44 clusters/cm .

A more direct confirmation of the cluster density, based on efficiency tests, will be discussed in the next section. One may note that the above timing resolution limit, based on the cluster density, also sets the limit for the spatial resolution if one were to use chamber 5 as a drift chamber . The time jitter for chambers 3 and 4, which have the same anode spacing as chamber 2, are below the graph of fig. 11, because of the more favorable electric field configuration . 6 .3. Detection efficiency

The detected number of electron clusters may be smaller than indicated in table 1 due to electron attachment processes in the gas, threshold effects in signal detection and statistical variations in gas gain . The electron attachment was studied by comparing the pulse height resolution for 5.9 keV X-rays (s5 Fe) in

J. Fischer et al. / Proportional chambers for very high counting rates

26 1

are due to the lower part of the cluster size distribution and the gas gain distribution . The above mixtures are, therefore, not only fast, but also efficient in small cells at atmospheric pressure . The efficiency in CF, alone (also fig. 9) was very low because of electron attachment and instabilities at the required high electric fields .

15

Fig. 11 . Time litter as a function of one-half anode spacing for chambers with 2056 isobutane in Cf4 . our gas mixtures with that in the P-10 mixture. Table 2 shows the results measured in chamber 5 at 1 ,us charge collection time and at low gas gains. We can see that the pulse height resolution with our fast gas mixtures is almost as good as with P-10 . Since the fluctuations associated with electron attachment would affect adversely the pulse height resolution, these results show that there is no significant electron attachment in these fast gas mixtures . Fig. 9 shows the measured detection efficiency for minimum ionizing particles in chamber 3. The efficiencies are also applicable to chamber 5 of the same thickness as chamber 3 (except for a different value of voltage for a given gas gain). For the gas gain region above 10 4 , the efficiency is above 99% (about 99 .4 for 20% C(CH 3 ) 4 ; and 99 .2 for 20% i-C 4 Hto ). The results indicate an effective value of 5 to 5'--, clusters in the chamber or about 40 to 44 clusters/cm depending on the mixture. From these results, the effective contribution from CF4 in these mixtures would be about 40 clusters/cm . One may note a small increase in the efficiency with gas gain due to signals, originally below threshold, now becoming detectable. These small signals

Table 2 Results measured in chamber 5 at 1 ps charge collection time and at low gas gains Gas mixture 100% CF4

Pulse height resolution (%)

80% CF, +20% i-C4 1110

-75 -22

80% CF4 +20% C(CH 3 ) 4 90% A+10% CH4

= 20

or

Fig. 12. (a) and (b) Shaping amplifier output signals in response to /3-rays, through chamber 3, showing the timing jitter relative to the time origin provided by (oscilliscope trigger) the scintillation counter behind the chamber. Gas 80% CF4 +20% isobutane . (c) The "Landau" pulse height distribution for B-rays in 1 .27 mm of the same gas.

262

J. Fischer et al. / Proportional chambers for very high counting rates

6.4. Output waveforms Figs. 12a and 12b show signal waveforms at the

shaping amplifier output due to the passage of 8-rays in chamber 3, filled with a 80% CF4 + 20% i-C,Hto mixture. The oscilloscope sweep was triggered by a scintilla-

tor behind the chamber (see fig. 6) . Sweeps without a

chamber signal are due to the larger area covered by the scintillator than by the anode wire . We see that the chamber signal starts at different positions. This spread

of the starting time is mainly due to the variation in the

drift time of the electrons with track locations and remains the dominant contribution to the time jitter. The other contribution, also visible in figs . 12a and 12b,

is a "walk" due to pulses of different pulse heights. A measured pulse height distribution of minimum ionizing

,ß particles passing through the 1.27 mm of the gas within the chamber is shown in fig. 12c. 6.5. Space charge effects The signal duration at the shaping amplifier output (10 ns at the base) allows rates of up to =100 MHz per anode wire. The presence of positive ions in the cell volume may pose a more restrictive limit. The space

charge due to positive ions reduces the elecric field at the anode, which results in a lower gas multiplication . To reduce the space charge, we should keep the

avalanches small, the drift distance short, and the ion drift velocity high . The avalanche charge required to achieve single cluster detection sensitivity was discussed in sect . 4. We have concluded that the total number of ions produced

for each detected track would be about 3 x 10 5 in normal operation, and it may reach 10 6 if single elec-

tron detection is desired. We have used the value of = 5 x 10 5 ions in the tests of the space charge effects. The set up shown in fig. 6 was used for the high

counting rate tests. The high rate of minimum ionizing

background particles was simulated by an intense X-ray source collimated over a small area of the chamber.

Knowing the energy of the X-rays (5 .4 keV) and the distribution of the ionization losses in the gas, e.g ., as shown in fig. 12c, one can present the results in an

equivalent rate of minimum ionizing particles. The rates

were also checked against total ion currents and by

inspection of the chance coincidence rates.

The performance at high counting rates was tested in

chambers 2 and 5. We were able to detect the degrada-

tion of the performance due to the space charge in mm gap), but did not have a strong

chamber 2 (1

enough radiation source to detect any degradation of the performance in chamber 5 with a gap of 0.635 mm.

Fig. 13 shows the measured space charge effects for

chamber 2 and an estimate for the onset of similar

effects in chamber 5. The upper part of fig. 13 shows the

v 0 FS (9 W 2 W N J a

W FQ W

N C 3

W F F W F Fig. 13 . Relative pulse heights and time jitter vs particle rate per cm of anode wire m chamber 2, and per cm 2 in chamber 2 and chamber 5, using CF4 with 20% isobutane. The mean total avalanche charge per count was - 5 x 10 5 electrons in these measurements .

J. Fischer et al. / Proportional chambers for very high counting rates

relative pulse height as a function of the equivalent minimum ionizing particle rate (given both in rates per cm of anode wire and in rate per cm2 of the chamber area). In chamber 2, the pulse height decreased by 10% at a rate of over 10 7 s -1 cm -2 and by = 16% at 2 x 10 7 _ s t cm-2 . The lower part of fig. 13 shows the increase of the time jitter versus the equivalent rate of minimum ionizing particles. The time jitter for minimum ionizing particles is somewhat less sensitive to the space charge than the pulse height . It shows an increase at the rate values where the pulse height has already decreased by 15-18% . For a uniform flux of particles, the upper limit of the flux density for a given decrease in pulse height, due to the space charge, is a strong function of the chamber dimensions . It scales inversely with nearly the fourth power of the cell dimensions for the following reasons. i) A decrease by a factor of s d in the chamber cell size would decrease the total drift time of the positive ions by sâ . It has been shown [15] for cylindrical electrode geometry that the effect of positive ions on the electric field at the anode wire scales nearly with the square of the cathode radius . As a consequence the gas gain is reduced and there is a decrease in pulse height which scales approximately with a power of 1.8 of the cathode radius . The same scaling applies to other geometries from dimensional considerations . ii) There is an additional power of 2 in the overall scaling if the rate per unit area is considered . One factor of Sd comes from the decrease of the gas thickness and a correspondent decrease in the ionization charge for each particle . The remaining factor of Sd is due to the presence of more anode wires per unit area so that the particle flux per wire is decreased. The scaling factor, sd, of 1.6 between chambers 2 and 5 gives the rate capabilitiy improvement of 1 .6 38 = 6. A more conservative prediction for the rate limit of chamber 5 is indicated in fig. 13 . From this one could expect chamber 5 to operate at the rates of up to about 10 8 s-1 cm -2 . Thus for a beam size of 300 cm2, with a uniform flux density, the space charge effects would allow the total counting rate of over = 10 1° s-1 . 7. Conclusions We have developed a low mass position sensitive proportional gas chamber operating at very high counting rates with a good timing resolution . The chamber has small interelectrode distances and uses a new, fast gas mixture based on CF, with a hydrocarbon quencher. The mixture provides sufficient primary ionization in = 1 mm of the gas at atmospheric pressure for efficient detection of minimum ionizing particles. Fast, low noise, pulse shaping circuits were devel-

26 3

oped to match the detector capabilities of a time resolution of 4 ns fwhm, double pulse resolution of = 8 ns per anode, space charge limited counting rates of over 10 8 s -1 cm -2 and position resolution below 1 mm (fwhm). A chamber with 400 anode wires is intended to be used as a beam chamber in an AGS experiment at a total rate of 5 x 10 8 s-1 . The resolving time of the chamber leads to four random coincident tracks per event on the average. If one assumes that these are distributed uniformly over all 400 wires of the beam chamber, and that the position accuracy of the beam particle reconstructed from its decay products in the rest of the experimental apparatus is about 1 mm, then such a beam chamber would effectively reduce the background by a factor of one hundred. The limit in the flux density of particles in this case is given by the timing resolution which is determined by the electron drift time in the chamber cell . With the small cell dimensions and a low gas gain the space charge is not a limiting factor. The electron drift time scales linearly with the cell dimensions, but the maximum flux density (i .e., the counting rate) due to space charge scales inversely with nearly the fourth power of the cell dimensions . Thus the relative importance of the two effects changes steeply as a function of the cell size . With the cell size chosen here there is ample reserve in the gas gain before the onset of space charge effects. However, moderate values of gas gain are preferable to reduce the possibility of deposit formation, and to provide wider margins in the parameters affecting the stability of operation of the detectors. References [1] P.A. Souder, J. Sandweiss and A.A . Disco, Nucl . Instr . and Meth . 109 (1973) 237. [21 V.K . Ermilova, L.P . Kotenlso, G.I . Merzon and V.A . Checkin, Sov. Phys . JETP 29 (69) 861. [3] W. Farr, J. Heintze, K.H . Hellenbrand and A.H. Walenta, Nucl. Instr. and Meth . 154 (1978) 175. [4] A.H . Walenta, 1983 SLAC Summer Institute on Particle Physics, Stanford, CA, preprint Si-83-23, (UniversityGHS-Siegen, Siegen, W. Germany, 1983). P. Rehak and A.H. Walenta, IEEE Trans. Nucl . Sci. NS-27 (1980) 54 . [6] L.G . Christophorou, D.L . McCorkle, D.V. Maxey and J.G . Carter, Nucl . Instr. and Meth . 163 (1979) 141. L.G . Christophorou, D.V. Maxey, D.L. McCorkle and J.G . Carter, Nucl . Instr. and Meth . 171 (1980) 491. M.S . Naidu and A.N . Prasad, J. Phys. D 5 (1972) 983. D.L . McCorkle, L.G. Christophorou, D.V. Maxey and J.G . Cartner, J. Phys. B 11 (1978) 3067 . [10] K.H . Valentine, M.K. Kopp, C.C . Guerrant and J.A. Harter, IEEE Trans. Nucl . Sci. NS-31 (1984) 264. [11] R.A . Boie, A.T . Hrisoho and P. Rehak, Nucl . Instr. and Meth. 192 (1982) 365.

264

J. Fischer et al. / Proportional chambers for very high counting rates

[12] V. Radeka, A. Hrisoho and D. Stephani, Fast Signal Processing for Gas Proportional Detector (to be published) . [13] Gold plated tungsten alloy wires type 861, No. 60 finish,

made by Luma Lampan AB, Kalmar, Sweden ; Distributoi: SAES Getters/USA, Inc. [141 Matheson Gas Products, Inc., East Rutherford, NJ 07073. [151 R.W. Hendricks, Rev. Sci. Instr. 40 (1969) 1216 .