Studies of helium gas mixtures for low mass tracking detectors

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

Nuclear Instruments and Methods in Physics Research A315 (1992) 494-501 North-Holland

Section A

Studies of helium gas mixtures for low mass tracking detectors Presented by S.M. Playfer

S.M. Playfer, R. Bernet, R. Eichler and B. Stampfli Institut für Mittelenergiephysik, ETH Zürich, CH-5232 Vlligen PSI, Switzerland

Helium gas mixtures can improve tracking resolution for particles below 1 GeV/c because of the reduction in multiple scattering . We have measured the drift velocity and spatial resolution of several interesting mixtures including helium :DME and helium :C02 : isobutane. We discuss the advantages and disadvantages of using such a gas mixture for a detector at a 13-meson factory, and conclude that helium : DME (80 :20) and (70:30) mixtures will give significantly improved momentum resolution. The low drift velocity in such mixtures is a disadvantage in a high rate environment, but this is partly compensated by a low sensitivity to synchrotron radiation background.

1. Introduction For many years it was believed that helium was unsuitable as a drift chamber gas because of its high ionisation potential (24.6 eV), low primary ionisation (4.8 ion pairs per cm at STP), and large single electron diffusion (580 wm/ cm ). Zimmermann et al. [1] showed that with the addition of only 6% propane it was possible to achieve a resolution of 260 Wm for small drift cells with a maximum drift distance of 5 mm. There are several more recent measurements which also show that it is possible to obtain acceptable spatial resolution for gas mixtures consisting mostly of helium [2-4]. The reasons for this will be discussed in section 2 where we will be interested in choosing the optimal gas mixture for measuring charged particles between 50 MeV/c and 2 GeV/c . In sections 3-5 we describe a set of measurements of drift velocities and spatial resolutions for drift distances up to 5 cm. These have been made for the gas mixtures helium : isobutane, helium : DME and heli: C02 : isobutane, where the helium component um varies between 70 and 90%. In the final section these measurements are used as the basis for a discussion of the advantages and disadvantages of helium mixtures for tracking detectors for new high intensity facilities (4, T-charm and 13-factories). General considerations of the requirements for a central tracking detector at a 13-meson factory using e+e - collisions at the T (4S) have been given by Grancagnolo [5], who suggested helium : DME (95 :5) as a possible chamber gas. We reach the somewhat

different conclusion that suitable mixtures are helium : DME(70 : 30) or helium : isobutane(80 : 20). 2. Choice of helium mixture The intention of using a helium-based mixture at atmospheric pressure is to increase the radiation length Table 1 Radiation lengths [12] and primary ionization statistics [15] of chamber gases at STP Radiation length Xo [m] Helium 5299 Argon 110 C02 183 Methane (CH 4) 100 :0 649 Ethane(C2H 6) 340 Isobutane (C4H lo) 169 DME (C2H60) 222 Argon :ethane 50:50 166 90:10 181 C02 : isobutane Helium : isobutane 90:10 1313 80:20 749 70:30 524 Helium : C02 : isobutane 80-10 :10 776 70:20:10 550 Helium :DME 90 :10 1612 80 :20 951 70 :30 674 Gas mixture

0168-9002/92/$05 .00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

Ratio

Primary ions Np [cm -11 4 .8 24.3 36.5 26.5 43.5 89.6 66.2 33.9 40.1 13.3 21 .8 30.2 16 .5 19.6 10.9 17.1 23.2

S.M. Playfer et al. / Helium gas mixtures

of the gas in the detector significantly, and thereby reducing the multiple scattering of charged particles. Table 1 summarizes the radiation lengths and primary ionization statistics for a minimum ionizing particle for several gases and gas mixtures at standard temperature and pressure. The radiation length (Xo) and primary ionization Np are closely correlated. The primary ionization statistics are important for several reasons . The spatial resolution has a constant contribution due to ionization fluctuations which dominates for all gas mixtures at short distances . A second problem is the efficiency of the detector which depends on the ratio of signal to electronic threshold . For small statistics a larger gas amplification is required which amounts to a reduction of the length of the plateau for efficient operation . This is particularily true for short track lengths, and for tracks at large angles to the drift direction, which have a wide range of arrival times for the drift electrons. If the chamber is required to give particle identification information using d E/d x measurements, as is the case in designs for B-factory detectors, then a lower limit on the primary statistics is set by the required separation power. We have measured gas mixtures with Np >_ 10/cm, and conclude that Np >_ 15/cm is required for good spatial resolution anyway. A typical argon mixture has Np = 30/cm . The limit Np = 15/cm corresponds to a limit on the maximum radiation length Xo = 1000 m. As was mentioned in the introduction helium has a large single electron diffusion, giving poor spatial resolution at large drift distances. Fig . la shows the diffusion as a function of helium percentage for mixtures of helium : methane, helium : ethane, helium : isobutane, and helium : COZ as calculated by the WIRCHA program [6]. The diffusion improves significantly with only a few percent organic additive, and by 30% has almost reached the value for the organic gas alone. Fig. l b shows the drift velocity calculated with WIRCHA for the same mixtures as fig. la for a drift

495

field of 600 V/cm. A 20% organic additive increases the drift velocity from the 7.6 mm/Ws of pure helium to about 20 mm/ ps. Figs. la and lb suggest a range of 10-30% for organic components of helium mixtures, which agrees well with the limits from primary ionization statistics discussed above. There is one final argument for at least 10% organic admixture, which comes from our experience with operating chambers with these gases . Helium it, no quenching, and is very susceptiself has ble to high voltage breakdown and dark currents. This situation improves rapidly with the addition of an organic gas . 3. Description of apparatus

For our measurements we have built a small chamber with a uniform drift space defined by a cathode plate and a set of field-shaping electrodes, and with a wire collection plane containing seven anode wires and eight focus wires . The anode wire spacing of S mm is typical of chambers which would satisfy the dE/dx requirements of >_ 40 samples within a projected track length of 50 cm. The electrostatics of our chamber are shown in fig. 2. We wish to compare several possible helium gas mixtures, so our gas system allows us to change rapidly from one mixture to another, and to reproduce a given mixture . A mixture is made using partial pressures in a special mixing volume and the entire gas system including the chamber is evacuated to a pressure <_ 5 mbar between successive mixtures. An Oxisorb filter removes oxygen impurities from organic gases before mixing . The gas is flushed through the 3 1 chamber volume at 0.1-0.5 1/min. We estimate the accuracy of our gas mixing ratio to be 0.1%, in agreement with the reproducibility of our drift velocity measurements. Three separate sources of ionization in the drift region have been used. A nitrogen laser provides a

Fig . 1 . Single electron diffusion and drift velocity as a function of helium concentration .

IX . HIGH PRECISION DETECTORS

S.M. Playfer et al. / Helium gas mixtures

4%

defined beam at a given drift distance . These measurements determine the drift velocity and corrections for electrostatic nonuniformities iii the chamber . Cosmic ray particles provide tracks of minimum ionizing particles throughout the drift region, and over a finite range of angles of incidence . From these measurements we determine a spatial resolution which we believe is representative of conditions in a real experiment. An 55 Fe source is used to calibrate the gas amplification, which is adjusted to be about 1 .0 x 105 for all the mixtures, and to study the absorption cross section for X-rays. The anode wires are read out with a high-gain preamplifier and a 100 MHz 8-bit nonlinear FADC, both of which have been developed for the H1 experiment at DESY. The preamplifier has a gain of 110 mV/wA, a risetime of 10 ns, and a noise level of 2.5 mV. The FADC has a memory time of 2.56 Rs, which is read out via a VME interface to a Mac1I computer. A fast trigger is provided by a photodiode in coincidence with a signal from the middle wire of the chamber. Since the FADC window of 2.56 ws is shorter than the typical drift times that we measure, it is necessary to delay the diode signal by a variable amount depending on the drift distance. This delay is measured to an accuracy of 1 ns using an LRS9400 digital oscilloscope .

For the cosmic ray measurements a plastic scintillator trigger replaces the photodiode . The window for the coincidence with the middle wire is set to 1.4 Ws, somewhat below the 2.56 Ws, because we will require *he neighbouring wires for the analysis. For each wire a search is made for the first pulse in the FADC memory above a software threshold on the integrated charge. This threshold corresponds to 1-2 primary electrons depending on the gas mixture . For this pulse the integrated charge Q and the leading edge time t are saved in a data file. The time information is obtained by a method of integration on the leading edge of the pulse . This method is insensitive to the exact shape of the leading edge, and does not use the information from the trailing edge, which is very uneven for the small statistics of helium mixtures. 4. Measurements of drift velocity A beam from a nitrogen laser is used to produce ionization for drift distances between 6 mm and 61 mm in steps of 5 mm. The ionization observed in the chamber is not a strong function of the gas mixture, and is typically about five times that of a minimum ionizing particle. In a measuring time of 500 s we

J7 m

I'IIIIIIIIIIIIIIII IIIIIIÏ'ÎÎIÏÎi ~IIIIIIIIIIIIIIIIIIIIIIIIIIIIII~IIIÎ `L71111lIIIIIIIIIIIIIIIIIIIIIIIIIIIII °~

inmimnni nii

III IIIIIIIIIIIIIIII IIIIII~Î~Î~ III IIIIIIIIIIIIIIIIIIIIIIIIIII 111111 ~nn1111111111111 Fig. 2. Electron drift and equipotential lines of the test chamber .

S.M. Playfer et al. / Helium gas mixtures

497

Table 2 Drift velocity measurements and calculated values [6] Gas mixture

Helium : isobutane

Ratio

90 :10 80 :20 70 :30

Helium : C02 : isobutane Helium : DME

80 :10 :10 70 :20 :10 90 :10 80 :20 70 :30

Drift field [V/cm]

Drift velocity Measured [mm/tis] 11.25±0.57 16.90±0.43 8.85±1.33 14.95±0.38 20.38±0.25 7.92±0.48 14.75±0.49 21.34+0.55 27.28±1.52 16.40±0.26 13.60±0.28 9.70+0.22 8.40±0.16 5.66±0.11

225.00 450.00 168.75 337 .50 562 .50 168.75 337 .50 562 .50 787 .50 562 .50 562 .50 393 .00 562 .50 562 .50

obtain an accuracy of 2 ns for the centre of the Gaussian beam profile, which is already smaller than the estimated alignment error of 100 Rm. The time-distance relationship is linear for drift distances greater than 10 mm. Table 2 summarises the drift velocities determined from the slope of this line. For helium : isobutane mixtures we have measured several drift fields. The agreement with values calculated from the WIRCHA program [6] is within the systematic uncertainties of the cross sections used as input for WIRCHA. Due to high voltage breakdown on our cathode plate we were unable to reach drift fields higher than 1 kV/cm . The drift velocity is expected to saturate at about 2 kV/cm at a value of 30 mm/Ws . For helium : C0 2 : isobutane and helium : DME mixtures the drift velocity is measured for a drift field of 560 V/cm, except for helium : DME(90 :10) (390 V/cm). These drift fields correspond to the diffusion minimum [6] . The addition of C0 2 or DME decreases the drift velocity compared to

Calculated [mm/ tLs] 11.15±0.45 16.25±0.65 9.67±0.39 15.63±0.63 20.75±0.83 9.39±0.38 16.03±0.64 22.08±0.88 27.23±1.09 18.18±0.72 13.63±0.55

helium : isobutane. For helium : C02 : isobutane the measurements are in agreement with WIRCHA, while the measurements of helium : DME(90 : 10) and helium : DME(70 : 30) are consistent with other measurements [2,4]. The laser measurements have been used to correct for nonuniformities in the electrostatics of the chamber. For each drift distance the time offsets of the six outer anodes relative to the central anode are determined with an accuracy of 3 ns. The corrections At are small except for the outermost wires where large electrostatic distortion in the amplification region can be seen in fig . 2. 5. Measurements of spatial resolution To simulate the conditions of a real experiment we have studied the spatial resolution for cosmic ray muons which are almost minimum ionizing, and are triggered

Table 3 Measured spatial resolutions of helium mixtures. The drift field is chosen near the expected diffusion minimum Gas Mixture

Helium : isobutane Helium :C02 : isobutane Helium : DME

Ratio

80 :20 80 :10 :10 70:20 :10 90:10 80:20 70:30

Drift field [V/cm] 562 .50 562 .50 562.50 393 .00 562.50 562.50

Spatial Resolution 010 [wm] 50±16 68±34 143±16 153±42 48±24 80±12

011

[wm/ cm ] 124±12 137±10 101±13 207±21 100± 8 73±10

Q(2) [wm] 205±14 195±10 190±12 343±15 137± 7 128± 7

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498

with a range of angles relative to the drift direction of ±20*. We have collected several 104 cosmic ray tracks for each of the helium mixtures given in table 3. In the analysis of the cosmic ray tracks we do not require all seven anode wires. Some mixtures with low ionization statistics have inefficiencies, while the 2.56 ps range of the FADC memory restricts the angular acceptance for gases with low drift velocities. We determine for each event the spatial resolution of the middle wire from a straight line fit to the two neighbouring wires plus at least two additional wires from

the remaining four. A X 2 cut is made on the straight line fit according to a preliminary estimate of the expected resolution. With this method we can measure drift distances down to 5 mm even if the cosmic ray track passes through the anode plane before the edge of the chamber giving no useful signals on the last two anodes . Note that points with drift distances < 4 mm could be on either side of the anode plane. For drift distances < 10 mm the nonlinear time-distance relationship has been corrected using predictions from the WIRCHA program . Corrections for the uncertainty in

500Helium :Isobutane

80 :20

E=562 V/cm

400

30043

100-

0 __j~

20 30 Drift Distance

(mm]

40

50

500jHelium :C02 :Isobutane

80 :10 :10

E=562 V/cm

400-

a0 a 0N Gl

a

200-

100-

0-

0

10

20 30 40 50 Drift Distance (mm] Fig. 3. Spatial resolution as a function of drift distance. (a) helium : isobutane(80 : 20), (b) helium : C02 : isobutane(80 :10:10), (c) helium : DME(70 : 30) .

499

S.M. Playfer et al. / Helium gas mixture--ç 500-

400-

0 0 0 0 N 41

200-

a

100 _~

20 30 Drift Distance

-T 40

50

Fig . 3 . (continued) .

the straight line prediction for the middle wire for the various possible wire combinations in the fit have been determined by a Monte Carlo study, and are between 0.835 and 0.935. The results of our cosmic ray measurements are presented in fig. 3 for the three mixtures helium : isobutane (80 :20), helium : C02 : isobutane (80 :10 :10) and helium : DME (70 : 30) . The data have been divided up according to the drift distance of the central anode so that each data point is the result of a Gaussian fit to the distribution of residuals (measured distance-straight line prediction) for - 300 events. Note that there are two independent measurements of helium : isobutane(80 : 20). The error bars are statistical. The measurements have been fitted with the empirical function :Q2(x) =QÔ +Qix,

where the ao term includes the effects of ionization statistics, electronics and systematic effects, and the Qi term includes the effect of single electron diffusion [7] . This parametrisation is only valid if the effects contributing to oo are really independent of x. The effect of ionization statistics is known to lead to worse resolution at drift distances less than the wire spacing. This effect, with the nonlinear time-distance relationship, and the problem of tracks passing through the anode plane, are all reasons for the deviation of our measurements from the parametrisation at short distances . This deviation is particularily noticeable for helium : isobutane(80 :20) for which the parametrisation is only

correct for x >_ 12 mm. In table 3 values are given for oo, al , and the spatial resolution Q(2) averaged between x=1 cm and x = 3 cm. The spatial resolutions of helium : isobutane and helium : C02 : isobutane mixtures are similar although one would have expected some improvement from the addition of C0 2 . The use of DME improves ai by - 30% . The helium : DME(70 : 30) mixture has slightly better resolution than (80 :20). The (90 :10) mixture has two times worse resolution than the (80 :20) mixture coming from larger values of both Qo and a, . The effect of al is expected from fig. la, although we measure a steeper loss of resolution than predicted. The pulse forms for the (90 :10) mixture are observed to be distorted by the lack of ionization statistics. We conclude that for small ionization statistics the spatial resolution deteriorates rapidly. The gas mixtures differ in their resolutions at short distances due to departures from the simple parametrisation . An explanation for the loss of resolution at short distances for helium : isobutane as compared to the other mixtures is the "time expansion" idea [8]. For slow gases, the large electric field near the anode wire produces a drift velocity in the collection region much larger than that in the drift region. This reduces the effect of the different drift path lengths on the distribution of electron arrival times. For helium : isobutane the drift velocities in the drift and collection regions are similar, and the electron arrival time distribution is not compensated for by a variation in drift velocity, as it is for the slower gas mixtures. IX. HIGH PRECISION DETECTORS

S.M. Playfer et al. / Helium gas mixtures

500

6. Advantages and disadvantages of helium mixtures In this section we discuss the advantages and disadvantages of helium gas mixtures for a specific application in a main tracking detector for an experiment at an e + e - B-factory. We assume that such a detector will have an active radial length of 50 cm, a total of 40 measuring layers, and will operate in a magnetic field of 1.0 T. The choice of a jet chamber geometry with typical drift distances of 2 cm corresponds to the measurements we have made with our test chamber. The above assumptions do not differ significantly from The momentum resolution is given by the formulae of Gluckstern [9] : (~p,lPt)2

=

2 (aQ, Pt) + b/Xo,

with a = 6.0 x 10 -5 Wm- ' GeV- ' and b = 6.7 x 10 -3 m - '. Fig. 4 shows the resolution for six helium mixtures using the spatial resolutions ax = Q(2) in table 3, and the radiation lengths Xo in table 1 . Also shown are the resolutions for argon : ethane(50 : 50), as used in the CLEO II Detector [10], and for C02 : isobutane(90 :10), for which we have measured a value of ar(2) = 130 pm. All the helium mixtures have better resolution for pt below 500 MeV/c. The reduced multiple scattering of a (90 :10) mixture only becomes important for transverse momenta below 100 MeV/c, which is already below the "curl-up" momentum for

our geometry . At higher transverse momenta the spatial resolution becomes the dominant factor, and the helium : DME(90 :10) mixture gives poor results . The mixture helium : DME(70 : 30) has a spatial resolution comparable to C02 : isobutane, and gives the best momentum resolution for high transverse momenta . A disadvantage of helium mixtures for use in a high luminosity collider is the low drift velocity. Whereas the CLEO II tracking detector with typical drift distances of 5 mm has drift times - 100 ns, our geometry has drift times of several ps. If the drift time is an important constraint (e.g. if the drift chamber information is required for triggering), then helium : DME mixtures are not suitable, and a better choice is helium : isobutane(80 : 20). With a higher drift field, it . The is possible to reach drift velocities of 30 mm/Ls typical drift distance of 2 cm can be reduced by going to a minicell geometry, but then the material of the field-shaping wires makes an important contribution to the scattering in the chamber . Longer drift times increase the sensitivity to background processes such as beam-gas events and synchrotron radiation . Evidence from the recently installed vertex drift chamber at the ARGUS detector [11] suggests that at present luminosities background hits are mostly due to synchrotron radiation which produces a clearly identifed copper fluorescence line. We have measured the absorption rates of our gas mixtures for the 5.9 keV gammas from an 55 Fe source (table 4), and our results agree with the expected

Fig. 4. Comparison of the momentum resolution of various gas mixtures . Detector parameters as in table 4 . AE55: argon : ethane(50 : 50), CI91 : C02 :isobutane(90 : 10), HD91 : helium : DME(90 : 10), HD82: helium : DME(80 : 20), HD73: helium : DME(70:30), HI82: helium : isobutane(80 : 20), HIC811 : helium : isobutane :002(80 :10:10), HIC712 : helium : isobutane : C02(70 :10 :20).

S. M. Playfer et al. / Helium gas mixtures

50 1

Table 4 . Comparison of properties of gas mixtures. We assume a tracking detector at a B-factory with 40 measuring layers over a radial distance of 50 cm in a magnetic field of 1 T. The drift velocity is measured for a drift field of 560 V/cm The momentum resolution is calculated using the measured spatial resolution for a mean drift distance of 2 cm [9] . The dE/dx resolution is calculated [13] . The measured y absorption rates are given in arbitrary units Gas mixture

Helium : isobutane Helium : C02 : isobutane Helium : DME C02 : isobutane Argon : ethane

Ratio

80 :20 80 :10 :10 70 :20 :10 80 :20 70 :30 90 :10 50 :50

Drift velocity [mm/ p.s]

100 MeV/c v[%]

1 GeV/c

Q[%]

dE/dx resolution ai%]

5 .9 keV Y absorption [a .u .]

20.4 16.4 13 .6 8.4 5 .7 4.6 50.9

0.30 0.29 0.34 0.25 0.30 0.55 0.65

1 .28 1 .12 1 .13 0.84 0.77 0.88 1 .10

6.4 7.1 6.9 7.2 6.6 5 .6 6.3

59 120 210 67 93 825 7500

photoattenuation lengths [12]. The very low absorption cross-section of helium is an important argument for the use of helium mixtures in a high luminosity environment . Particle identification is very important for the physics program at a B-factory. The main tracking detector should provide dE/dx information to separate ir : e and ir : K up to 2 .5 GeV/c . This separation may be supplements by additional detectors such as an electromagnetic calorimeter and a ring-imaging Cherenkov . Helium mixtures are expected to have worse d E/d x resolution than argon mixtures because of the large ionization potential and low primary ionization . We have not made any measurements relevant to the d E/d x resolution, but hope to do so in the future . For completeness we present in table 4 the results of a calculation of the anticipated resolution based on the formulae of Allison and Cobb [13] . There is one measurement by Onuchin and Telnov [14] for helium : C02(95 :5) which is significantly worse than the prediction, but the theory is not expected to be valid for small samples (defined by 6 .83ZxP/1 < 0.5, where Z is the average molecular charge, xP is the sample thickness in cm atm, and I is the mean ionisation potential) . The helium mixtures in table 4 have sample sizes above this limit (6 .83ZxP/I - 1 .0) . Our conclusion from table 4 is that helium mixtures are very attractive for use in a tracking detector at a B-factory or a similar facility . If the low drift velocity is

Momentum resolution

no problem then helium : DME(70 : 30) gives the best momentum resolution . A higher drift velocity of 20-30 mm/ps can be achieved by using helium isobutane(80 : 20) with some loss of momentum resolution above 500 MeV/c .

Acknowledgements We would like to thank O. Knecht for technical assistance, and G . Viertel for lending us the nitrogen laser and parts of the gas system .

References [1] W. Zimmermann et al, Nucl. Instr . and Meth. Ez243 (1986) 86 . [2] K. Maartens, Diplomarbeit, Heidelberg (1989). [3] S .Playfer, Contibution to European B Factory Meeting, Paris (1990); P. Burchat, Contribution to Workshop on the Design of a Detector for a High-Luminosity B Factory, Stanford (1990). [4] H . Thurn, Diplomarbeit, Dortmund (1990). [5] F. Grancagnolo, Nucl. Instr. and Meth . A277 (1989) 110. [6] J . Fehlmann, J .A. Paradiso, G. Viertel, WIRCHA a program package to simulate drift chambers, ETH Zürich Internal Report (1983). J [7] . Va'vra, Nucl. Instr. and Meth . A244 (1986) 391 . [8] V . Commichau et al, Nucl. Instr. and Meth . A235 (1985) 267. [9] R . Gluckstern, Nucl . Instr. and Meth . 24 (1963) 381 . [10] D. Cassell et al, Nucl . Instr. and Meth . A252 (1986) 325 . [11] H. Kapitza, ARGUS pVDC working group, private communication (1991). [12] Review of Particle Properties, Particle Data Group, Phys. Lett. B239 (1990) 1 . [13] W.W . Allison and J .H . Cobb, Ann . Rev. Nucl . Part . Sci. 3 0 (1980) 253 . [14] A.P . Onuchin and V.I . Telnov, Nucl . Instr. and Meth. 120 (1974) 365 . [15] F. Rieke and W. Prepejchal, Phys . Rev. A6 (1972) 1507 .

IX . HIGH PRECISION DETECTORS