1 TPC Basics H. Wieman LBNL Oct. 18, 99

2 Ann. Rev. Nucl. Part. Sci : 217

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4 Introduction l The STAR TPC is a 3D 70 mega pixel, digital camera in a magnetic field »records path and ionization density of charged particles from the collision, providing: –momentum from track sagitta –particle ID from dE/dx –full  coverage for -1.7<  <1.7

5 Introduction l will describe how it works l will explain some design choices

6 Simple and Cheap l The recording media, the film, is an empty volume of gas »Can’t get 70 million reusable active elements for fewer dollars l Only 140,000 electronics channels (digital oscilloscopes) are required to readout the 70 mega pixel image »still the cheapest solution even including electronics

7 Active gas volume in the STAR TPC

8 The Gas Volume Sets the Limits l dE/dx truncated mean resolution limited by track length, 1/  L l  p/p limited by transverse diffusion l Two track resolution limited by diffusion

9 Design Philosophy l Don’t try to make anything better than limits set by TPC gas »Improve one thing and it messes up something else, examples: – increasing gas gain to improve position resolution degrades dE/dx dynamic range –reducing shaping time to improve two track resolution increases noise »Plus it costs more and leads to aggravation = Screw up Factor detector analog of the uncertainty principle

10 Back to Electron Diffusion in Gas l Drift velocity and diffusion depend on mean time between collisions and inelastic cross sections (see Blum and Rolandi and Biagi’s Magbolz Code) E P10 drift velocity vs E field at 1 atmosphere Just a sketch 5.6 cm/  s 130 volt/cm STAR operating point velocity

11 Electron Diffusion in Gas l Measured diffusion parameters, note dependence on magnetic field for P10 »pressure improves but.. more voltage and more problems »other gases but slower l Electron attachment forming negative ions is another issue »all TPC materials tested »limits on O 2 (Purdue, Stringfellow)

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13 Inner and outer field cage plus central membrane

14 Field Cage l Uniform field »less than 200  m drift error in azimuthal direction

15 Calculating drift distortion radial distortion error potential evaluating the coefficients

16 Distortion error from 2 shorted stripes (mm)

17 Field Cage Requirements l Alignment of ends and central membrane must be good to mm level l voltage matching of endcap and field cage must be good to 5 volts (central membrane at 31 kV) leakage resistance between stripes must be > Gig  or uniform

18 The Readout, Outer and Inner sector

19 Outer Sector Corner

20 Sector Wire Geometry

21 MWPC Pad Readout l Boosts signal from ~45 electrons to 20,000 electrons at the preamp-shaper l Multi-Wire Proportional Chamber +pads is ideal for TPC readout »well matched to inherent resolution in position and dE/dx »robust and dependable

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27 Pad current measured with fast amp Measured I 1/(t + t 0 ) t (ns) t 0 = 10 ns

28 Measuring Gas Gain l DC method with source current measurement doesn’t work for pads l Practical method: calibrate pre-amp shaper with known step pulse and test capacitor - compare pulse height with signal from Fe 55 X-ray source (220 electrons) Must correct for shaper width for different fractions of signal integration.

29 Additional Measurements l Pad response function l Ratio of pad signal to wire signal (30% for STAR) l Time spread l Method - focused UV on a photo cathode to get repeating point source of electrons »See Wayne Betts Masters Thesis and STAR note

30 Front-End Electronics on Pads l preamp shaper »reduce noise »remove random structure in electron cloud »suppress late current tail l switched capacitor array »rapid 512 analogue sample and hold for more leisure digitization preamp shaper chip Switched array + ADC chip 16 ch in 16 ch fiber optic

31 Shaper Amplifier Demonstration t (ns) (simulation) Signal + series noise Signal Unfiltered Filtered with shaper amp Signal + series noise Signal shaper width chosen equal to electron diffusion width 45 e most probable for min I spread due to diffusion for 2 m drift

32 TPC Laser System l Project head: Alexei Lebedev (BNL) l UV beams generate ionized tracks in the TPC drift volume for spatial calibration and check of drift distortions l Two Nd-Yag Lasers frequency quadrupled

33 Internal Beams

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35 Gating Grid l volts on alternate wires acts as shutter for electrons. Stops positive ions. Opens in a ~ 1  s l Designed to reduce charge injection into amplifiers

36 Sector Testing with Cosmic Rays 10/19/98-10/30/98 Drift distance ~ 80 cm Z drift changes by ~ 45 cm as shown in panels Pedestals at 3 , Zero B field run: track width < 1 cm Slide from Jim Thomas

37 Cosmic ray event

38 Run July-99 Beam + something

39 Run July-99 Beam + something

40 dE/dx vs rigidity From David Hardtke 11% resolution

41 99 Engineering Test Results Iwona Sakrejda has been encouraging an extensive effort to analyze data taken during the engineering run. Check out her collection of results at:

42 Performance, first results Position resolution ~ 800  m, expect 500  m »should improve now that electronic noise has been reduced to spec value - will be able to run with lower threshold l Momentum resolution 80% larger than goal »should improve when use B field map

43 Performance, first results l dE/dx resolution 11%, expect 7% »improve with reduced threshold? l Front-End electronic noise now ~1100 e rms, where it should be l Test with high multiplicity awaits first collisions