The design of the new cylindrical drift chamber for the Mark II detector at the SLAC Linear Collider is described. Prototype tests to determine the working parameters of the chamber and to study possible gas mixtures are discussed

Burchat, P. R.

Hanson, G. G.

Sadrozinski, H. F. W.

UNT Digital Library

% * * •?W SLAC-PUB--3475 DEB5 005533 i ' / 1 / ' J „ SLAC-PUB-54 067)4-8^/^^7--^^ October IW4 THE NEW DRIFT CHAMBER FOR THE MARK U DETECTOR ^ AT THE SLAC LINEAR COLLIDER' PATRICIA R. BUHCHAT, GAIL G . HANSON, HARTMUT F.-W. SADROMNSXit Stanford Linear Accelerator Center, Stanford, California, USDS m m a *»•>»• e*" | \ MASTER 75 Abstract Tbe design of tbe new cylindrical drift chamber for the Mark II detector at tbe SLAC Linear Collider i« described. Pro-totype teat* to determine the working parameter! of tbe chamber and to atudy possible gas mixtures are discussed. 1. Introduction Tbe Ant detector to study Z° pbyaica at the SLAC Linear CoUider (SLC)1 will be an upgrade1 of the Mark D detector3 which has run successfully at both tbe SPEAR and PEP e+e" eturage rings at SLAC. With the existing Mark II drift chamber, precision for high momentum tracks h limited by the relatively small number of layers. Pattern recognition capability is limited by the inability to record more than one bit per « M wire. Con­sequently, a new cylindrical drift chamber is being constructed tor SLC. It will provide good momentum resolution with the Mark II 5 kG magnet, good solid angle coverage, and multi-hit capability tor eaie of pattern recognition &s4 high tricking effi­ciency. In addition, the sew drift chamber will measure charged particle energy lew due to iositatiOD [dB/dx] as an independent aid to calorimetry In eleetron-hadroD separation for particle mo­menta leas than about 10 CeVje, The design and construction of the sew chamber and prototype teats will be discuued. 3, Design and Construction 2.1 CELL DESIGN The design of tbe new drift chamber is based on a cell con-talLlng lix tense wires staggered ±380 jtm from the cell axis to provide local left-right ambiguity resolution. Figure 1 shows the basic dimension! of the cell. Tbe 30 nm diameter sense wires are spaced B.33 mm apart. There are two guard wires at each end of the cell for shaping tbe electric field and equalising the gain of all the tense wires. Potential wires are placed between the sense wires to reduce crosstalk between sense wires, to reduce electrostatic deflection o) tbe sense wires, and to allow control of the gain of the sense wires and the electric field in the cell independently. There we nineteen Held wires in each cell. The average cell half-width is 3.3 em. The cell just described has a very uniform electric drift field across the celt resulting in uniform iEf&x sampling regions and a simple time-distance relation. This uniformity can be seen in Figure 2 which shows the drift trajectories of electrons. 2.2 CHAMBER DESIGN The cells are arranged in 13 concentric layers with a mini­mum radial distance of 2.S cm between layer*. This results in 5B32 sense wire* and 72 drift-time and dE/dx measurements per track. Alternate layer* have their wires parallel to the axis or at ±3.6* to the axle to provide (tereo information. OfllFT CHiUBfR WIAE MTTERN 3BOj<ift OHSH >-SJo»— n Sense Wee Potential w.ie i Irstm Guars Yl.rf FiSitl * " t * Work supported by the Department of Energy, contract DE-AC03-76SF00SU. 1 Santa Cms Institute for Particle Physics, Vnivemity of Cali­fornia, Santa Crus, 95069. Presented at Ihe Nuclear Science Symposium, Orlando, Florida, October 31 - November 2,IM4. Fig. 1. Drift chamber eel] configuration. The active vnlume of the chamber has a length of 2.30 m and inner and outer radii of 19.2 em and IS].6 em respectively. The 36,936 wires are ttr:iig between two-inch thick aluminum end-pla.t« which are prestralned to maintain uniform wire tension during stringing. 2.3 FEEDTHROUGH DESIGN The nineteen wires in one raw of a cell are positioned by a sing)'; iDjection-moIded Delrin S0OAF feedthrough. These wires are located in machined notches along one nide of a slot in the feedthrough. The notches are machined at tbe same time aa three pin-boles which accurately locate the feedthrough on the endplate. The tingle open slot in the feedthrough allows nine­teen wires to be strung at once and allows access for wire re­placement, if necessary, and visual inspection of tbe interior. S. Prototype and Gas S t u d i o Two prototypej were built to study the characteristics of the cell design and to atudy possible gas mixtures to be need in tbe new drift chamber. In particular, we studied gases composed of argon, methane and COt in various proportions. Tbe primary motivation for studying these gases was to search for a gas with a low electron drift velocity as well as good position, double track and dJP/dx resolution and stability against breakdown. A mixture of StfSS argon - $0% ethane was alto studied. The advantages of a gas with low electron drift velocity fall into three categories aa listed below. 1. Effects depending on the temporal characteristics of the pulses are scaled by the drift velocity when their influence on the position resolution and double track resolution is evaluated. (a) The contribution of the 1/t tall to the multiple track separation it reduced in proportion to the electron drift velocity. OisWtw x in. 5 OT,.,I!'«!T 's rc^ira *'efc (b) Effect! of rise time, time dewing aad time walk a n reduced in proportion to the drift velocity. ,. .(c) Slower, mote reliable clock apeedi can be used is the , T D C system and differential non-linearities in the TDC system are leu significant for a low drift velocity. 2. The Lorentr. angle (the angle between the electric field lines and the electron drift trajectories) depend! linearly on'the electron drift velocity. .3 The gases with low electron drift velocity which we teated tended to be saturated with the f lope of the drift velocity vrrsua-electric Aeld being negative. Tbia ia advantageous for the following reason. The electron* with the longest drift trajectory are those which pass cloaeet to the poten­tial wires. The electric field ia lower in this region- An increasing drift ve<ciiy with decreasing electric field will partially compensate for the longer drift path Tbia pro­vides a focusing cfect which makes the pulae narrower. Fig. 2. Electron drift trajectories in drift chamber cell. a . i ELECTROS DRIFT VELOCITY Figure 3 shows the electros drift velocity a* a function of drift field for four of the gates considered. Thew curves are the results of measurement! made using a small drift chamber constructed for this purpose. 7 • ic r%£<. ( . i% : • • * .b i ' t , • Fig. 3. Electron drift velocity versus electric field. The four representative gases are: 1. 10% COt, \% methane, 89% argon which will be referred to aa HRS gas 4 and has an electron drift velocity of about SO pm/ns at the drift field in most of the cell; 2. 50% argon, 50${ ethane which was used in the old Mark D drift chamber and has a drift ve'ocity of 52 fm/ru; 3. 3% COi, 4% methane, 93% argon which has a drift veloc­ity of 33 /im/ns', 4. 0.4% C02, 4% methane, 95.6% argon which has a drift velocity of 21 um/ns. 3.2 POSITION RESOLUTION The position resolution was measured with a one-cell, full-length prototype using a LeCroy 2229A 8 channel TDC with 1 ns bin width. For each gat, cosmic ray data was recorded and analysed for at hast two different sense wire gains. The data was binned in 2 mm wide bins in s where * is the distance between the track and the sense wire. The position -..station was measured as a function of t by averaging the variance of the distribution of residual* of two triplets of wire*. The resolution was measured separately for track* passing to the left and right of the sense wire plane. II was assumed that c j = c j + el, where c« is the total resolution for an individual win, ff< is the resolution due to a dependent effects, and ffp ia the resolution with no t dependence. We then assumed that this x dependence is a result of diffusion, i.e., a = D\/x, where D is an effective diffusion coefficient. A least squares line fit win made In cj. versus i to determine D and on for each gas. These parameters were used to calculate the average resolution across the cell, (o), for each gas. Figure A shows the measured position resolution aa a func­tion of z for the four representative gases. The resolution cor­responding to the fit to these data is also shown. D, en and (IT) are shown in Tfible 1. The effective diffusion coefficients, D, range from 73 nmfy/em to M* fim/i/crr. and are thus a factor of 2 or 3 (mailer than the longitudinal diffusion coefficient for •Ingle electrons.' The ratio is a statistical factor which depends op the number of primary electrons collected and the number of primary electrons necessary to trigger the discriminator. c*.f" Cis:4i« Fig. 4. Position resolution versus drift distance. Table 1 POSITION RESOLUTION for ( pulse height} = 3.0 BlV at preamp input Gas Composition so D to % [lim) (um/v/em) {*"") COt methane argon ±5 Jim ±4 /jm/Vcm ±6 fitn 10.0 1.0 89.0 IDG 60 153 3.0 4.0 930 SO 134 189 0.4 4.0 95 6 111 144 Z!6 505? ethane, 50% argon 79 73 123 In Figure 5, (p) is plotted as a function of percent argon for all the gases studied. We observe that gases with large amounts of argon general;. have poorer resolution mainly due to the large diffusion contributions. Some of the contributions to tin resolution are: Electronics Resolution: The RMS resolution due to the 1 na liming bins for the TDC is (1 at/y/lt)vj e> IS «m for an electron drift velocity, ty, of 50 jun/ns. That due to noise is K . „ / V 1 * . t ) W W » 1 0 /»». Rise Time: Maximum changes in liming due to stalk l i e (Vis # «./V r M s)r R R «j aAOfim. Diffusion: Besides the independent diffusion contribution to the resolution, there k a constant contribution from the high field region around the wire.* This effect increases the parameter de­fined as on. The contribution can be large ( » SO pen) and is gas dependent. Collection: Cluster statistics and non-uniform collection across the cell can be a large contribution with a dependence on the saturation of the drift velocity. 3.3 MULTIPLE TRACK SEPARATION The spatial width of the pulse from the chamber determines how far apart in space two tracks must oe before they can be resolved. For double track separation using the TDC, the first pulse must fall below the discriminator threshold before the is-rival of the second pulse. The main contribution to the pulse width comes from the distribution of drift times for the collected electrons. To first order, the spatial pulse width is equal to half the separation between sense wires which is 4.17 mm iu this cell. The pulse width can also be affected by the following factors. • The variation of drift velocity with deereasicu electric field will determine whether there .s a focusing or defoeuiing effect for electrons which drift close to the potential wires (see above). • The contribution of the l/i tail to the spatial width is proportional to the drift velocity. • Large diffusion coefficients result in broader pulses. The temporal width of the pulse a*, the TDC discrimina­tor threshold was measured For each gas, the mean tirr-e over threshold was multiplied by the drift velocity to determine the spatial separation needed to resolve two tracks. Figure 6 itiuwa the spatial pulse width as a function of electron drift velocity for the four gases. * 3--° X j «". C«# 9l\& 70 % ARGON * 1 • V * . r *. r .? ' Fig. S. Position resolution for average drift distance versus percentage of argon in the gas. The resolution for 50% argon • S0S6 ethane is about 75% that of HRS gas. o ic 20 JO 4C ;.* Fig. 6. Pulse width versus drift velot ity. 3 The 4% methane, 3% C 0 j mixture and the 4.0% methane, 0.-1% CO] mixture have good focusing properties (increasing drift velocity with decreasing electric field) and thus have small pulse widths. This is the dominant effect. The pulse width is also correlated with the drift velocity (because of the 1/f. tail) and the magnitude of the diffusion coefficient but these effects are not aa significant. Note tbat these measurements represent the putse width at the TDC threshold. With a sampling dE/dz system, the mini­mum separable distance between double tracks could be reduced by nearly a factor of two.' 3.4 GAIN The gala of the chamber depends on bot'i the chrrge per unit length on the sense wire and the amount of argon in the gas. The relative flains of different gas/high-vjltage configurations were measured. Figure 7 shows the cha:ge ID the sense wire which yields a mean pulse height of 3 m 1 ' at the preamp input for five gases. Tile position resolution measurements shown in Figures 4 .led 5 and the pulse width measurements ihowa in Figure *? .-orr^f^^ij t i this gain. r JC W bO ' 0 80 90 " 0 .. *.'0 aPGON Fig. 7, Charge per unit length on sense wires for equal pulse height as a function of percentage of argon in the gas. Since the gas with the lowest drift velocity (4% methane, 0.4% CO2) contains more argon than any other gas tested, it dries not need as much charge on the Bens'! wires to produce the same gain as Ibe other gases. The argon-ethane mixture, which coal-ins only 50% argon, requires 1C% more charge on the sense wire to produce the same gain aa HRS g-js. Consequently, the voltage on the field and potential wires mupt be higher. 3.5 dK/dx RESOLUTION To measure dE/dz resolution a S0-c>Ta-long chamber con­sisting of three layers of three celb each was operated in a 10 GtV positron beam. By combining events to get 72 dE/di mea­surements and discarding the 24 highest pulses (corresponding to the largest fluctuations on the Landau tail), we measure the following dE/dz resolution: OtC/Ji = 5.7% for HRS gas and "iE)i, = 4-7% f<" 3% COj, i% methane, 93% argon. The dE/dz resolution we actually obtain with the new drift cham­ber will depend on our ability to correct for systematic changes In gain due to saturation, stereo layers :rossing axial layers, temperature and atmospheric pressure changes, etc. 3 .6 GAS STABILITY All gases being considered were tested for stability against breakdown. Stability tests for HRS gas were done with a single wire strung in a 3/4 inch diameter tube. The diameter of the wire was varied and the tube was operated with the wire acting either as an anode or a cathode. The electric field on the surface of the wire when breakdown of the gas occurred is (hown in Tible 2 for different wire diameters. Note that the gat around a sense wire generally breaks down at a higher voltage than around a field wire of the same diameter. The wire diameters and maximum surface fields for the new Mark ff drift chamber are 30 pm and -187 kV/em for the sense wires, 100 pm and -76 kVJcm for the guard wires, 178 pm and 21 fcV/cm for the field wires, and 304 nm and 20 kV/cm for the end field wires. However, the charges on the wire at breakdown given in Table 2 cannot be used aa limits for safe running in the final chamber since feedback between sense and field wires with gain results in a lower breakdown threshold. ill 3/4 Table 2 GAS STABILITY Electric field on surface of iach tube at breakdown in wire HRS gas. Field W diameter ire * ! (kV/cm) ( Sense Wire diameter [pm] E. [kV/em) 20 190 1 20 280 30 147 1 30 220 50 UB 50 ll>4 100 S3 100 98 177 69 177 :o 305 54 In order to study this feedback effect, a small chamber with a replaceable sense wire and Tour 175 pm field wires was COD-structed. Negative voltage was applied to the field wires and the sense wire WHJJ '•nnnrctod to ground. The conducting chamber walls were connected to ground or negative voltage in order to control the charge on the sense and field wires independently. The diameter of the sense wire was varied between 20 /im and 175 fim. It was found that breakdown occurred in HRS gas when .he totil gain from sense and field wires8 is ibeut ID7, This etfe.'t is shewn in Fig. B where the gain of the field wires is plotted against the gain of the tsense wire at breakdown. Needles of hydrocarbon polymers formed on th-e field wires when the amount oi methane in the gaa was greater than *s S% almost independent of the amount of CO^. The 50% argon - 5056 ethane mixture behaved very differently. The voltage could initially be raised quite high before current was drawn. But then the gas polymerized rapidly around the field wires and breakdown occurred at much lower voltages, 3.7 SUMMARY OP GAS STUDIES We conclude that of the gases tested, those most suitable for use in the Mark II drift chamber at SLC axe HRS gas (vj = 50 /im/i.j, [a) = 153 jim, pufse width = 4.7 mm, CJB/JM ~ 5.7%) and 3% CO*, 4% methane, 93% argon [vd = 33 /*ro/i», {a) = 189 /im, pufse width = 3.4 mm, oAEfAx = 4-7%). The 4 4. Conclusion SOW argon, 50% ethane mixture will not be used became of gat breakdown problems. o 3 JEN5E iVlRE GAIN Fig. 8. Gain if the Geld win versus gain of the sense wire at breakdown (or BRS gas. The solid line represents a combined gain of 10T. The chamber i« presently being strung at a rate of ft 30OQ wires per week with completion of stringing expected in early December, 19M. The upgraded detector will '. e rjoved into the interaction region at PEP for checkout. The data taken at PEP will be used for final development of tracking programs and algorithms for dE/dx measurements before the move to SLC beginning in January, 1986. Acknowledgements The authors would like to acknowledge the support of the Mark II collaboration (Caltech-Colorado-U.C. Santa Crui-Hawaii-Johns Hopkins- LBL-Mkhigan-SLAC), and in particular, the contributions of A. Boyarski, D. Briggs, F. Bulos, E. Fernanda, A. Gioumousis, D. Hutchinson, W. Rowe, A. Seiden, P. Sheldon and S. Weies to the prototype tests. References 1. SCAC Linear Collider Conceptual Design Report, 5LAC-229, Jure, 19B0. 2. Proposal for the Mark 11 at SLC, CALT-68-101S, April, 1983. 3. R.H.Schindter, '•• ai.,Phys. Rev. D 24, 78 (1981). 4. This mixture has been used successfully in the High Reso­lution Spectrometer at PEP and the Mark HI detector at SPEAR. 5. F.Pius, NIM 205(1983), 425. 6. Dimethyletber: A Low Veokity, Low Diffusion Drift Cham­ber Gas, F. Villa, SLAC-PUB-3037, January, 1983. 7. Review of dE/dx System for Mark n/SLC, Mirk 17/SLC Note 45. 8. Principles of Operation of Multiwire Proportional and Drill Chambers, F. Sauli, CERM.77-09, May, 1977, pp. 35-44. DISCLAIMER employee* matc !„ „ ^ t Z I T " " ' *"* ' ^ ^ ' ^ " r M > ° f *'" bililv f„r ihr . . I P implied, or assumes any legal liability or rssoensi-procevs d ^ d ^ d . „, r ep r in t s .ha. ils U M w o u W n o l M t i a g e pr^iyTZ,ZT^ 

New drift chamber for the Mark II detector at the SLAC Linear Collider

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New drift chamber for the Mark II detector at the SLAC Linear Collider

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