Drift chamber for super-Belle Present performance and background Idea for upgrade Upgrade plan Summary Shoji Uno (KEK) Jan-20 th, 2003 at Super B-factory WS in Hawaii

Wire chamber Wire chamber is a good device for the central tracker.  Less material  Good momentum resolution.  Cheap  It is easy to cover a large region.  Established technology  Easy construction.  Many layers  Provide trigger signals and particle ID information. Can the wire chamber survive at S-KEKB?  Answer is ” Yes ”.  My talk and Senyo ’ s talk.

Momentum Resolution Thanks for filling He based gas and using Aluminum field wires. The resolution was calculated using cosmic ray events during a normal physics run. BaBar data was obtained from a talk in Vancouver WS.

dE/dx Resolution Good resolution was obtained. It is useful to identify hadrons and electrons. BhabhaMu pair M.I.P in Hadronic events 0.35<P<0.88GeV/c without any PID. Electron

Hit Rate ~200kHz for inner most layers.  Chamber itself was working.  No significant gain degradation was observed for four years operation.  Exceeds 0.2C/cm. 5-10KHz for most of layers.  Quite small.  The gas chamber can work at 20 times higher beam background. Inner most part was replaced with a new chamber in the last summer.  At S-KEKB, it will be covered with a silicon detector. Layer Hit rate/wire(kHz) CDC Exp 27 Run 206 HER 1.1A LER 1.5A L=9.6x10 33 cm -1 s -1 Main Inner Cathode

Total Current Draw of CDC Total CDC current(8400 wires) has kept about 1mA at the maximum beam current for each years. Vacuum condition is improving from year to year. Better masking system has been adopted for fixed masks near IP and movable masks in arc section.

Radiation Damage Test a: ’93 Plastic tube d: ’94 SUS tube b: ’93 Plastic tube + O2 filter e: ’94 SUS tube + O 2 filter c: ’94 Plastic tube f: ’94 Plastic tube Total accumulated charge on sense wire(C/cm) Gain degradation

Simulation Study for Higher Beam Background Detail will be reported by K.Senyo. MC +BGx1MC+BGx20

Idea for upgrade In order to reduce occupancy,  Smaller cell size More wires.  smaller hit rate for each wire. Shorter maximum drift length  shorter maximum drift time. A new small cell drift chamber was constructed((Maximum drift length is 2.5mm). Inner most three layers were replaced with the new chamber in the last summer.  Faster drift velocity Shorter maximum drift time. One candidate : 100% CH 4  Drift time simulation shows maximum drift time is similar to the present gas due to a large Lorentz angle.  More study

Small Cell Drift Chamber

Photo of small cell chamber Installation in the last summerJust after wire stringing

XT Curve & Max. Drift Time Small cell(5.4mm) Normal cell(17.3mm)

Drift Velocity Two candidate gases were tested.  CH 4 and He-CF 4 In case of He-CF 4, higher electric field is necessary to get fast drift velocity. In case of CH 4, faster drift velocity by factor two or more can be obtained, even in rather lower electric field.

dE/dx Resolution The pulse heights for electron tracks from 90 Sr were measured for various gases. The resolutions for CH 4 and He(50%)- C 2 H 6 (50%) are same. The resolution for He- CF 4 is worse than Ar- based gas(P-10).

xt curve for 7mm cell Distance from wire (cm) Pure CH 4 100nsec Distance from wire (cm) Drift time (  sec) He/C 2 H 6 = 50/50 100nsec

Wire configuration 9 super-layers : 5 axial + 4 stereo(2U+2V)  A 160*8, U 160*6, A 192*6, V 224*6,  A 256*6, U 288*6, A 320*6, V 352*6, A 388*8 Number of layers : 58 Number of total sense wires : Number of total wires : ~60000

Deformation of endplate Number of wires increase by factor 2.  Larger deformation of endplate is expected.  It may cause troubles in a wire stringing process and other occasions. In order to reduce deformation of endplates,  The endplate with a different shape is considered.  Wire tension of field wires will be reduced.

Curved Endplate Deformation of endplate due to wire tension was calculated at design stage of present Belle CDC. Deformation(mm) Present New

Baseline design CDC SVD

Main parameters Present Future Radius of inner boundary (mm) Radius of outer boundary (mm) Radius of inner most sense wire (mm) Radius of outer most sense wire (mm) Number of layers 5058 Number of total sense wires Effective radius of dE/dx measurement (mm) Gas He-C 2 H 6 Diameter of sense wire (  m) 30

Expected performance Occupancy  Hit rate : ~140kHz  ~7Hz X 20  Maximum drift time : nsec  Occupancy : 1-4%  140kHz X nsec = Momemtum resolution(SVD+CDC)  Pt /Pt = 0.19Pt  0.30/  [%] : Conservative  Pt /Pt = 0.11Pt  0.30/  [%] : Possible  0.19*(863/1118) 2 Energy loss measurement  6.9% : Conservative  6.4% : Possible  6.9*(752/869) 1/2

Summary Present CDC is working well.  Good momentum resolution is obtained even for low momentum range.  Energy loss measurement is also good.  Beam background is manageable range. Vacuum condition is still improving. New CDC  Smaller cell size for inner most layers.  New gas with a fast drift velocity is preferred.  Larger outer radius Better performance is expected.

Chamber Gas He(50%)-C 2 H 6 (50%)  Longer radiation length(680m).  Drift velocity is higher than other He-based gas. Average drift velocity : ~3.3cm/  sec in the chamber cell. Maximum drift time : ~400nsec for 18mm cell size.  Good dE/dx resolution. Gas system  Gas circulation( Flow rate : 3.0 liter/min).  Fresh gas( Input flow rate : 0.3 liter/min).  Keeping an absolute pressure constant.  O 2 contamination ~50ppm (with O 2 filter)  H 2 O contamination ~500ppm ( no control )

Momentum Resolution We could obtain a small constant term using He- based gas and aluminum field wires. Slop parameter is not so good as compared with expected value.  We had to change the electronics parameters to reduce the cross talk.  HV is slightly lower than the original value.  Alignment is not perfect.  More tuning to reject bad points.  Some effects from the beam background. Transverse Momentum(GeV/c)

dE/dx Measurement MQT chip (Charge to Time conversion) and multi-hit TDC. 80% truncated mean. Relativistic rise.  1.4 for electron. Good PID performance for lower momentum region. dE/dx information helps to separate high momentum K/ . log 10 P(GeV/c)

Comparison with Babar Momemtum resolution(SVD+CDC)  Pt /Pt = 0.19Pt  0.30/  [%] : Belle  Pt /Pt = 0.13Pt [%] : BaBar Mass(dE) resolution  Mass resolution for inclusive J/       9.6 MeV(Belle) vs 12.3 MeV(BaBar)  dE resolution for B 0  D   +, D   K +     13.8MeV(Belle) vs 19.0MeV(BaBar) We could obtain better resolution than BaBar.  BaBar has a CFRP support cylinder with 2mm thickness between SVD and CDC. Tracking efficiency for low momemtum particles is worse than BaBar.  D *+ D *   yield(slow  eff.) ~0.5(?) x BaBar  3 layers SVD (Belle) vs 5 layers SVD(BaBar) Energy loss measurement  5.6%(Belle) vs 7%(Babar) for Bhabar events

Pulse Height Variation Total accumulated charge.  >0.2 coulomb/cm at inner most layer for 4 years. No significant deterioration, so far Layer 49 Layer 1 Layer 3

CH 4 Faster drift velocity  Factor two faster than He(50%)-C 2 H 6 (50%) Long radiation length  Same as He(50%)-C 2 H 6 (50%) Good dE/dx resolution  Same resolution is expected from a test using electron from 90 Sr. Small radiation damage  Similar performance as He(50%)-C 2 H 6 (50%)

Electron Drift Simulation Electron drift simulation in gas volume was done by A.Sugiyama of Saga Univ., recently. He/C2H6=50/50(present gas) and pure CH4 under 1.5 Tesla. Small cell size(7mm) and normal cell size(18mm)  Maximum drift time for pure CH 4 is not shorter than the present gas due to large Lorentz angle. Simulation shows He/CH 4 =50/50 is slightly better than the present gas.  Smaller ionization loss  Worse spatial resolution and worse dE/dx resolution?

Drift line for 7mm cell He/C 2 H 6 = 50/50 Pure CH 4

Wire configuration Super-layer structure 6 layers for each super-layer  at least 5 layers are required for track reconstruction.  Even number is preferred for preamp arrangement on support board to shorten signal cable between feed- through and preamp. Additional two layers in inner most super- layer and outer super-most layer.  Higher hit rate in a few layers near wall.  Inner most layer and outer most layer are consider as active guard wire.

Problems and Cures Cross Talk  Coaxial cable between pin and preamp.  Better grounding is also effective.  We replaced all cables with coaxial ones in summer  Then, electronics parameter will move back original one. High background around inner most layer  Enlarge inner radius : 7.7cm  16cm  Smaller cell size : 8.6mm(64 in  )  6.8mm (160 in  )  One inner cylinder  No additional wall  Inner most layer should be considered as a guard wire(active). 3D Tracking(especially, low momentum tracks)  6(?) layers SVD  6 layers in stereo super layer  Better track reconstruction program.  Charge division?