STUDIES OF HELIUM GAS MIXTURES IN DRIFT CHAMBERS*

Transcription

1 SLC-PUB-5626 SCIPP 91/25 September 1991 (1) STUDIES OF HELIUM GS MITURES IN DRIFT CHMBERS* Patricia R Burchat and John Hiser Santa Cruz Institute for Partice Physics University of Caifornia, Santa Cruz, C 9564 dam Boyarski and Don Briggs Stanford Linear cceerator Center Stanford University, Stanford, C 9439 bstract Some properties of gas mixtures, composed predominatey of heium and esser amounts of CO2 and isobutane, were measured in a prototype drift chamber The studies are motivated by the desire to reduce the mutipe scattering inside the drift chamber at future high-uminosity, ow-energy faciities such as 4 factories, T/charm factories, and B factories The ow atomic number of heium greaty reduces the mutipe scattering contribution to the momentum resoution compared to an argon-based gas mixture The position resoution, puse height, efficiency, and breakdown characteristics were measured for various gas mixtures mixture of 83% He, 1% CO2, and 7% isobutane gives comparabe resuts to that of an argon-based gas commony used in drift chambers Submitted to Nucear Instruments and Methods *Work supported by Department of Energy contracts DE-C3-76SF515 (SLC) and DE-M3-76SFOOOO (UCSC)

2 1 Introduction Traditionay, the nobe gas used in tracking devices in high-energy physics experiments has been argon because of its abundance, reativey ow atomic number, ow ionization potentia and high gain Now that we are entering the factory era of high-uminosity, ow-energy faciities (such as q5 factories, r/charm factories and B factories), where the sma statistica errors drive us to decrease systematic errors to unprecedented ow vaues, we consider using nobe gases with a ower atomic number, such as heium With these gases, the mutipe scattering contribution to the position resoution of tracking devices is significanty reduced In addition, the probabiity of beam-reated synchrotron photons interacting in the gas is significanty reduced, resuting in ess background in the chamber which reduces wire aging and probems in track finding and triggering Resoutions of 26 pm have been achieved in heium-propane mixtures [], and recenty measurements have been made on heium-dme and heium-coz-isobutane mixtures [2,3] In this paper, we describe our resuts on studies of heium-coz-isobutane gas mixtures in a prototype drift chamber The momentum resoution cp of a charged partice with momentum compo- nent p perpendicuar to the magnetic fied in a homogeneous tracking system can be parametrized as foows:* 2 * though drift chambers usuay do not have uniformy distributed materia or measurements, the parametrization is sti usefu for examining the genera dependence of the resoution on measurement errors and mutipe scattering 2

3 where is the contribution from spatia measurement errors and B/p is the contribution from mutipe scattering For a typica drift chamber, is in the range (5 to O)% GeV/c I n a chamber with argon-based gas, the mutipe-scattering I term is typicay around 7%/p Since the charged-partice momentum spectra at future high-uminosity, ow-energy faciities peak significanty beow 1 GeV/c, the resoution is dominated by the mutipe scattering term for most tracks if the drift chamber is fied with a traditiona argon-based gas By using a gas mixture with heium instead of argon, the radiation ength of the gas mixture can be increased by a factor of at east six, which decreases the mutipe-scattering contribution to the resoution by a factor of about 24 During a workshop for an asymmetric-energy B factory [4], Monte Caro studies were made on the effects of using a heium-based gas rather than an argon-based gas on four representative physics anayses The concusion in each case was that the physics measurements benefited significanty from the ower mutipe scattering in a heium-based gas, even if the intrinsic spatia resoution is a factor of two worse Heium gas might be expected to give a poorer spatia resoution because of the smaer number of primary (and tota) ion pairs aong the path of a charged partice The purpose of this study is to measure the resoution, as we as other properties, for a heium-based gas We consider mixtures of heium, CO2 (for imiting diffusion), and isobutane (for quenching) We measured the foowing

4 properties of various heium-based mixtures and, for comparison, an argon-based gas composed of 89% argon, 1% CO2 and 1% methane, referred to as HRS gas: n drift veocity as a function of eectric fied, n puse size, n spatia resoution, n breakdown characteristics, and n efficiency 2 Properties of the Gas Components Properties of the individua gas components used in this study are isted in tabe 1, which shows the radiation ength, and the number of primary and tota eectron-ion pairs per cm produced by a minimum ionizing partice Since the radiation ength of heium is so much arger than that of the other gases, the contribution of heium to the mutipe scattering is amost negigibe However, the production of ion pairs is significanty ower in heium than in other gases The radiation ength, the number of primary ion pairs/cm, and the tota num- ber of ions/cm for the gas mixtures considered in this study are given in tabe 2 The number of ions/cm negects mechanisms such as the Penning effect which can increase the number of ions above the simpe weighted sum over the individua components

5 3 Measurements 31 Experimenta setup Measurements of heium-based gases were conducted in a prototype drift chamber previousy buit to study a drift chamber for Mark II at SLC [7] The prototype chamber is one-fifth of the fu ength of the Mark II chamber and consists of three super-ayers corresponding to the fourth, fifth and sixth ayers of the Mark II chamber The wires in the midde super-ayer are parae to the centra axis of the chamber, whie those in the outer ayers are incined at approximatey f38 to the axis The active ength of the wires is about 18 cm The drift ce has six sense wires spaced 833 mm apart, and a maximum drift distance of about 33 mm, as iustrated in fig 1 The sense wires are 3 pmdiameter, god-pated tungsten, staggered by f38 pm from the ce axis to provide eft-right ambiguity resoution uniform eectric drift fied in the drift region is provided by votages on a row of 19 fied wires at each edge of the ce Potentia wires interspersed with the sense wires reduce signa couping between the sense wires singe high-votage suppy feeds a resistor chain that provides the necessary votages for a the wires Throughout this paper, when we report the chamber votage, we are referring to the votage at the top of this chain, which is approximatey the same as the votage on the outermost fied wires in a superayer With this high-votage scheme, the eectric fied and the gain both change as the chamber votage is changed The nomina chamber votage of 46 V used for HRS gas provides 44 V at the centra fied wire, 144 V on the potentia wires, an eectric fied of 9 V/ cm in the drift region, and a charge density of 14 pc/m on the sense wires The drift chamber signas are ampified in two stages with the same preampifiers and postampifiers as used on the Mark II drift chamber [8] The time 5

6 between a cosmic-ray trigger (provided by scintiation counters above and beow the chamber) and the drift-chamber puse is digitized with LeCroy 2228, 11-bit TDC s The puse area is integrated during a -ps gate with LeCroy 2248W, 11-bit DC s tota of 24 wires are instrumented, but the measurements discussed here are based on the six sense wires in the centra ce The CMC modues are read out with a MC II computer running KMTM software pubished by Sparrow pproximatey 22 cosmic rays passing through the centra ce of the chamber were recorded in about six hours The heium-based gas mixtures were mixed in the aboratory with commercia mass fow controers, FoBoxR from Sierra Instruments Inc The gas-fow rate corresponded to about one chamber-voume change every two hours 32 Drift veocity In addition to the drift chamber, a separate sma drift ce and a p source were used to measure the drift veocity of the heium-based gas as a function of eectric drift fied The resuts of the measurements are shown in fig 2, for both a representative heium-based mixture and for HRS gas Note that the driftveocity curve for the heium-based gas is not saturated, and hence the pressure and temperature of a chamber using this gas must be monitored or controed to achieve high resoution For an eectric fied of 1 V/cm, the drift veocity is about 25 pm/cm, about haf that for HRS gas 33 Puse size The drift chamber signas were integrated with a LeCroy DC modue The median of the DC measurements is shown on a og pot as a function of chamber 6

7 votage in fig 3 for both HRS and the heium-based mixtures (Reca that the nomina chamber votage for HRS gas is 46 V) Note that the median puse size has the exponentia dependence on chamber votage, or charge density on the sense wire, as expected Generay, increasing the amount of heium whie decreasing the amount of CO2 resuts in a arger median puse size for the same chamber votage mixture of 83% heium, 1% CO2 and 7% isobutane has the same puse area as HRS gas, for the same chamber votage, despite the expectation that the tota number of ion pairs produced per unit track ength is about a factor of three ower (see tabe 2) It is unknown whether the puse height is being increased by higher gain aone, or by the Penning effect in which metastabe states in one gas (heium) transfer energy through coisions to other gases (CO2 or isobutane) which can ionize 34 Chamber breakdown To study the dependence of chamber breakdown on gas mixture, we measured the rate at which the signa from a singe sense wire fired a discriminator at a fixed threshod for different settings of the chamber votage The resuts are shown in fig 4 For the heium mixtures in which the fraction of isobutane is kept constant at 7%, the breakdown votages foow the same order as the chamber votages corresponding to constant puse size (see fig 3) Mixtures with ess heium and more CO2 break down at higher chamber votages When the amount of isobutane is decreased, the breakdown votage aso decreases The HRS gas breaks down at a chamber votage about 2 V ess than the 83% heium, 1% CO2, 7% isobutane mixture, even though they have the same median puse height for the same chamber votage

8 35 Efficiency To study the singe-wire efficiency, tripets of neighboring sense wires were considered We measured the probabiity that a singe sense wire has a signa above a fixed threshod when its two nearest neighbors have such signas This efficiency is potted as a function of chamber votage in fig 5 Note the suppressed zero on the vertica scae The chamber votage at which the efficiency approaches 1% is directy correated with the measured median puse size (fig 3), as expected Note that HRS gas and the 83% heium, 1% COz, 7% isobutane mixture have about the same efficiency and median puse size for the same chamber votage 36 Resoution For each track, the TDC timing information from the six wires in the ce was converted to spatia coordinates after resoving the eft-right ambiguity For HRS gas, which has reativey constant drift veocity in the drift region of the ce, a simpe time-to-distance reation of the form x =?J(t - to) was adequate, where x is the drift distance, 2) the drift veocity, t the TDC time, and to a constant reating the TDC start time of each channe to the trigger time for the event The constant 2, was determined from the data by requiring that the maximum drift distance match the size of the ce (33 mm), whie to was adjusted for each wire to center the residuas from track fits over many events For heium-based gases, the drift veocity is not constant but varies with the eectric fied E in the drift region of the ce, where E M 9 V/cm Even minor variations in E, such as that produced by the staggered sense wires, wi modify 8

9 the drift time sense wire that is staggered toward the right side of the ce has a sighty higher(ower) eectric fied on the right(eft) side of the ce, so the drift veocity is aso sighty higher(ower) in the right(eft) side of the ce The timeto-distance reation needs a correction term to account for the resuting eft-right difference in drift time We chose the foowing empirica form x = x IL S{ - exp[-(x/5)2]}, 2, = v(t - t,) ) where the added term ranges from near the wire to its maximum vaue 6 at arge distances, and is about 6 S at x = 5 mm The sign depends on the direction of stagger of the wire It was found that vaues of 6 up to 27 pm were needed for some gas mixtures The measured vaues of x from the six wires of each cosmic ray track were fit to straight ines Tracks which had missing hits on any wire, or tracks with any residua arger than four times the expected resoution, were discarded so discarded were tracks having an ange greater than &5 from the sense wire pane For the remaining tracks, the residuas were histogrammed, with a separate histogram for each miimeter interva in drift distance x These residua distributions were fit to Gaussian distributions to determine the mean and standard deviation for each drift interva We quote the resoution as the standard deviations from the Gaussian fits scaed by /at o account for the oss of two degrees of freedom for the two parameters when fitting the six drift distances to a straight ine The resoution c as a function of the drift distance 2 is shown in fig 6 for HRS gas and a He:COz:isobutane (83:1:7) mixture, both at a chamber votage of 48 V so shown as soid ines are the fits to a function containing a term for ion statistics near the wire 9

10 (which we approximate with an exponentia), a constant term oo, and a diffusion term added in quadrature in the form u(x)2 = 2e-2bx + a,2 + u$x The exponentia term is comparabe for a gas mixtures studied, despite the fact that the estimated number of ion pairs per centimeter is significanty smaer for the heium mixtures (see tabe 2) Tabe 3 ists the measured vaues of cro and ad for a the gas mixtures The dependence of resoution on drift distance is very comparabe for HRS and for severa heium mixtures The resoutions were aso avera,ged over two different ranges of drift engths: 8 mm and 24 mm These ranges of drift distances were chosen because they correspond to the two ce sizes we were considering for an asymmetric B factory for sma-ce and jet-ce geometries, respectivey The resoution is potted as a function of chamber votage in fig 7 for HRS gas and for the heium-based mixtures For a gases, the resoution for an 8-mm ce is sighty better than that of a 24-mm ce The resoution improves with increasing chamber votage unti it reaches a minimum t higher chamber votages, the resoution degrades again The vaues of CT, and ad in tabe 3 correspond to a chamber votage at which the resoution is minimized t these votages, the median puse height is about the same for a gas mixtures 37 Lore&z ange Because the drift veocity is reativey ow for the heium-based mixtures, the Lorentz ange in a magnetic fied is aso expected to be sma The expected Lorentz ange was cacuated for the gas mixtures considered in this study with a computer 1

11 program [9] in which the Botzmann transport equations are soved using the known eastic and ineastic cross sections for a number of pure gases This program has given good agreement with experimenta measurements for many gas mixtures This program predicts that a mixture of 83% heium, 1% CO2, and 7% isobutane wi have a Lorentz ange of 15 for a magnetic fied of 1 Tesa and an eectric fied of 1 V/ cm HRS gas is predicted to have a Lorentz ange of 33 for the same conditions smaer Lorentz ange is particuary desirabe for a sma-ce drift chamber where a arge Lorentz ange can ead to spiraing eectron drift trajectories and, therefore, degraded resoution and poor efficiency for charged partices which traverse the edges of the ce 4 Concusions Our measurements on a mixture of 83% heium, 1% CO2, and 7% isobutane indicate that the efficiency, median puse size, position resoution, and effects of diffusion are simiar to those seen in the commony used argon-based mixture (89% argon, 1% CO2, 1% methane) for the same charge density on the sense wire We aso find that the maximum operabe high votage setting for this heium-based gas is about 35% higher than that of the argon-based gas These resuts on heium-based gases are reevant for a ow-energy faciities where partices of reativey ow momenta are being measured, and the position resoution of tracks is dominated by mutipe scattering The radiation ength of this heium mixture is amost eight times greater than the argon mixture, which significanty reduces the mutipe scattering error heium-based gas is an obvious aternative to the traditiona argon-based mixtures used in drift chambers 11

12 References [] W Zimmermann, V Hepp, R Keogg, M Schmitt, Skuja, Backer, C Grupen, H Suhr, G Zech, N Magnussen and H Meyer, Nuc Instr Meth 243 (1986) 86 [2] V Cindro, H Koanoski, Lange, D Lauterjung, F Muer, T Siegmund, W Soder and H Thurn, DO-E (1991) [3] S M P a yf er, R Bernet, R Eicher, B Stampfi, ETHZ-IMP-PR-91-3 (1991) [4] Workshop on Physics and Detector Issues for a High-Luminosity symmetric B Factory, SLC-373 (1991) [5] Review of Partice Properties, Phys Lett B239 (199) [6] F Saui, CERN Report 77-9 (1977) [7] G brams et a, Nuc Instr Meth 281 (1989) 55 [8] D Briggs et a, IEEE Trans on Nuc Sci NS-32, No 1 (1985) 653 [9] Pashca Coye, LORENTZ program, UC Santa Cruz, unpubished 12

13 Tabe 1 Radiation ength, and primary and tota ion-pair production [5,6] from a minimum ionizing partice for individua gas components used in mixtures in this study at atmospheric pressure and 2 C Gas Name Symbo Z Primary Tota, (m) Ions/cm Ions/cm heium He methane CH argon r carbon dioxide CO isobutane C4H

14 Tabe 2 Radiation ength, and primary and tota ion pair production for gas mixtures considered in this study at atmospheric pressure and 2O C, cacuated using tabe 1 Gas Mixture x b-4 Primary Ions/cm Tota Ions/cm 89% r, 1% CH4, (HRS gas)% CO % He, 2% CO2, 7% C4H % He, 15% COz, 7% C4H % He, 1% CO2, 7% C4Hro % He, 5% CO2, 7% C4H % He, 15% COz, 3% C&H % He, 1% COz, 1% C4Hro

15 Tabe 3 Resuts of a fit of the resoution versus drift distance IC to a function of the form CT(x)2 = cl2e-2bx + ; + nix Gas Mixture Chamber Votage (V) 89% r, 1% CH4, 1% CO2 (HRS gas) 48 95&4 19 f 5 73% He, 2% CO2, 7% C4H k8 19 f4 78% He, 15% COz, 7% C4H f7 19 -f4 83% He, 1% CO2, 7% C4H1 48 9&8 11 &4 88% He, 5% CO2, 7% C4H f9 131f5 82% He, 15% COz, 3% C4H f zt6 8% He, 1% CO2, 1% C4H f12 15f5 15

16 Figure Captions 1 Ce configuration i; the prototype drift chamber used in this study The ce has six sense wires spaced 833 mm apart, and a maximum drift distance of about 33 mm 2 Measured drift veocity as a function of eectric drift fied for a heium-based mixture and for HRS gas 3 Measured median puse size as a function of chamber votage for the heium- based gases and for HRS gas 4 Rate at which the signa from a singe sense wire fires a fixed threshod discriminator as a function of chamber votage for heium-based mixtures and for HRS gas The symbos are defined in fig 3 5 Singe-wire efficiency as a function of chamber votage for heium-based mixtures and for HRS gas The symbos are defined in fig 3 6 Measured resoution as a function of drift distance for HRS gas and for a heium-based mixture 7 Resoution as a function of the votage on the chamber for HRS gas and severa heium-based mixtures The average resoutions for drift distances up to 8 mm and up to 24 mm are shown 16

17 DRIFT CHMBER WIRE PTTERN e 38pm &3*3cm Offset I - : 833mm -r : : 75 1 cm x Sense Wife Guard Wire 3-83 Potentia Wife Fied Wire 4499 Fig 1

18 ELECTRIC FIELD (V/cm) 699 Fig 2

19 Heium CO2 sobutane v 88% 5% 83% 1% x 78% 15% 7o1o 73% 2% 1 I 82% 15% 3% + 8% 1% 1% rgon CO2 Methane 89% 1% 1% 1 I I IT - p/ 5 1 I Chamber Votage (V) 6992 Fig 3

20 7 N I 6 ai 75 [r t g Chamber Votage (V) 6993 Fig 4

21 1 oo z - 2= LLI Chamber Votage (V) 6994 Fig 5

22 3 25 I I I - HRS ( a > I t s 1 t3 25 I He: CO*: isob (83:1:7) I w x mm> 6995 Fig 6

23 +z s 2 - 'i + ' t 4 $ iis 2 - s 15 - I HRS Gas 4 vv V vv I (a) I I I 24mm v 8mm I I I He: CO2: isob (73:2:7) (b) He: CO2: isob (78:15:7) (c) v V VV rv v v $ ;- vvvv V 1 I I I I I I He: CO2: isob (83:1:7) (d) He: CO2: isob (88:5:7) (6 P 25 - E- + s 2 -, V o 6 b bv V 15 - s e vvvv 1 - I I I I I I He: CO2: isob (82:15:3) (f) He: CO2: isob (8O:O:O) (g) 25-4 ' 2 -?$ O 15 - vv v ' VV $ I I I I I I 1 * Chamber Votage (V) 6996 v v v V Fig 7