1 Resistive Plate Chambers for Time-of-Flight P. Fonte LIP/ISEC Coimbra, Portugal. Compressed Baryonic Matter May 13 – 16, 2002 GSI Darmstadt/Germany

2 Main physical mechanisms Description of several. interesting devices High frequency front-end electronics Small segmented counters Bidimensional position-sensitive single-gap counters Large area Practical difficulties to be addressed for future applications Crosstalk Ageing Tails Background Rate capability Plan of the Presentation

3 Timing RPCs – General features Several (~4) thin (~ 0.3 mm) gas gaps Atmospheric pressure operation Both charge and time readout Offline correction for time-charge correlation. Time-charge correlation 1-2 ns  100 ps Electronics rise time Detector-related (not yet fully understood) Experimental

4 Basic physical mechanisms 2 main questions: How can we reach 50 ps  resolution in a gas counter? How is it possible to reach efficiencies of 75% in a 0.3 mm gas gap? cathode anode i i Detection level independent from the cluster position  exact timing N= average number of visible primary clusters Av. cluster density >> 1.4/0.3 = 4.6

5 Time resolution-principles cathode i i anode [Mangiarotti and Gobbi, 2001] n 0eff =i 0 d/(ev)  Monte Carlo i0i0

6 Time resolution-principles [Mangiarotti and Gobbi, 2001] tt ()KN  v  =  vt  =first Townsend coefficient v = electron’s drift velocity

7 Signal properties 1-Signal shape cannot be changed by any linear system Th 1 2 TDC t 1 t 2 2-  v can be easily measured Experimental Linear system Z(s) 

8 Time resolution-theory vs. measurements Single 0.3mm gap Applied Voltage (kV) Time resolution (ps  ) Time-charge corrected Uncorrected 0.75/(  v) Good agreement

9 The efficiency problem There should be at least one cluster in the efficient region of the gap d cathode anode i i d* Inefficient region (too small avalanches) Efficient region G min ~10 6 Experimental  Lmin=5/mm  Lmin=1.6/mm

10 F.Sauli CERN C 4 H 10 C2F4H2C2F4H CH 4 [Fisher, NIMA238] L(cm -1 ) The efficiency problem-cluster density data [Finck, RPC2001] 0.1 mm gap in C 4 H 10 Experimental Lmin No contribution from cathode (would also provide an effect to explain the t-q correlation) Calculation (HEED) Use this data [Riegler, RPC2001]

11 The efficiency problem (maybe solved) Large values of G 0 must be assumed (much above the Raether limit of 10 8 ). Possible by an extremely strong avalanche saturation effect (space charge effect). d cathode anode i i d* G min ~10 6 G0G0

12 Some hardware

13 Timing RPCs – Small single counters (9 cm 2 ) 33 Resolution of the reference counter [ Fonte 2000]  = 99.5 % for MIPs (75%/gap) -HV 4x0.3 mm gaps AluminumGlass (optimum operating point  1% of discharges)

14 Timing RPCs – Array of 32 small counters  = 88 ± 9 ps  = 97 ± 0.5 % [ Spegel et al, 2000 ] Crosstalk < 1% (not tested for multiple hits)

15 Very high frequency front-end electronics based on commercial chips Amplifier-discriminator-delay module Pre-amplifier based on the INA chip (HP/Agilent) 2.5 GHz bandwidth 20 db power gain 3 dB noise figure 10 ps  resolution above 100 fC Signal fast charge per channel (fC) Time resolution per channel (  2) (ps) RPC at Threshold=12.5 fC RPC at threshold=25 fC RPC at threshold=50 fC Pulser at threshold=25 fC Realistic tests using chamber-generated signals i=i 0 e st s =8.7 GHz Input signal (experimental) 10 ps  resolution above 100 fC [ Blanco 2000]

16 Double readout 4-gaps glass-metal counter [Finck, Gobbi et al, RPC2001] Edge effects

17 Timing RPCs – Large counter Active area = 10 cm  160 cm = 0.16 m 2 (400 cm 2 /electronic channel) 5 cm 4 timing channels 1,6 m Top viewCross section Ordinary 3 mm “window glass” Copper strips HV [Blanco 2001]

18 Timing RPCs – Large counter Charge distribution Time distribution Fast charge (pC) Events / 20 fC 0.5 pC   98 % Time resolution essentially independent from electrode size Time difference (ps) Events/20 ps  = 63.4 ps “300 ps tails”  1.5  fit

19 Timing RPCs – Large counter Time resolution (ps  )  = 50 to 75 ps Center of the trigger region along the strips (cm ) 93% 94% 95% 96% 97% 98% 99% 100% Time efficiency Strip A Strip B Strips A+B  = 95 to 98 % Center of the trigger region along the strips (cm ) No degradation when the area/channel was doubled to 800 cm 2 /channel [Blanco 2001]

20 Timing RPCs – Large counter [Blanco 2001] Position resolution along the strips Center of the trigger region along the strips (cm) Fit residuals (ns) Very good linearity (  1.4 cm)  X = 1.2 cm (0.75% of detector length)

21 Timing RPCs – single gap 2D-position sensitive readout X leftX right Time signal HV XY readout plane Y-strips (on PCB) RC passive network X-strips (deposited on glass) out left out right 10 strips for each coordinate at 4 mm pitch 4 cm 2 mm thick black glass lapped to ~1  m flatness Well carved into the glass (avoid dark currents from the spacer) 300  m thick high  glass disk (corners) metal box (no crosstalk) Precise construction (for small and accurate TOF systems) [Blanco 2002]

22 Charge distributionTime distribution Time resolution of single-gap and 4-gap counters may be similar! Time difference (ps) Events/10ps  = 66.6 ps (54 ps) 3  Tails=1.9 % 300ps Tails=0.36 % Events= 3  1.5  fit Timing RPCs – single gap Fast charge (a.u.) Events/bin  = 75 % ineficiency =54ps = =54ps  =54ps = =54ps 

23 Timing RPCs – Single gap Efficiency and time resolution  = 50 to 60 ps  = 75 to 80 % No influence from XY readout

24 Events/mm 2 Y(mm) X (mm) Timing RPCs – single gap Bidimensional position resolution edges  3 mm  resolution  3 mm FWHM (strips=4mm)

25 Timing RPCs – single gap Edge effects? Events/mm 2 Y (mm) X (mm) Time difference (ps) Events/4ps Sigma=74.0 ps (62 ps) 3-Sigma Tails=3.0 % 300ps Tails=1.5 % Center Essentially no edge effects Sigma=76.9 ps (66 ps) 3-Sigma Tails=2.9 % 300ps Tails=1.8 % Edge & Corner Time difference (ps) Events/4ps

26 Timing RPCs – multilayer 4 layers of single-gap chambers  = 50  = 99 % 3  Tails=1.5 % 300ps Tails=0.20% Single layer of 4-gap chambers Layer  = 75 %,  = 60 ps +  + 1% 300 ps 3  Tails=1.4 % 300ps Tails=0.0% Time difference (ps) Events/10ps Events=  = 32 ps  = 94.9 % Time difference (ps) Events/10ps Events/10ps Time difference (ps) tail-dominated M.C. Experimental

27 Open problems

28 The subtle crosstalk problem Time (ns) Current (µA) Threshold Few 100 ps jitter Realistic (measured) current shape Baseline shift due to crosstalk

29 The crosstalk problem – main approaches 1 – Do nothing and hope the problem is small or correctable offline 2 – Accept crosstalk and use many more channels than strictly needed. 3 – Shield channels and try to totally avoid crosstalk None of these approaches has been proven so far (as of Nov 2001) + Robust performance. + Testable in the lab. - Segmented mechanics. + Simple and economic + Being massively implemented (ALICE, STAR) -Possible fragile performance + Good 2D position resolution - Expensive -Effective granularity must be defined by testing - Possible fragile performance.

30 Ageing in streamer mode glass RPCs (BELLE) [Kubo et al, RPC2001] Problem water+freon+streamers  Fluoridric acid  Glass corrosion  Dark current  Ineficiency Freonless gas Freon gas

31 Ageing in avalanche mode glass RPCs? 3 glass cathode and 3 aluminium cathode counters Gas: (85% C 2 H 2 F 4 +10% SF 6 +5%C 4 H 10 ) + 10% rel. humidity Glass cathode Aluminium cathode Dark current (nA) Days Temperature (ºC) Integrated charge (mC  10 8 avalanches) Test in progress

32 Tails Time difference (ps) Events/10ps Events/10ps Time difference (ps) tail-dominated Background Theoretical time distribution has a tail Almost completely compensated by t-q correction Safe cure: redundancy (in a real application counters will be imperfect) Highly ionising background  more streamers  lower rate capability  lower resolution Problem was not studied so far. Severity unknown.

33 Rate capability – Standard RPCs Streamer mode < 300 Hz/cm 2 Avalanche mode < 3000 Hz/cm 2 Typically max. rate corresponds to an efficiency drop of a few percent (survey of recently published results)

34 Rate capability – Special RPCs Drift gap Amplification gap Resistive anode on a metal base  =10 7  cm Wire meshes (50  m wires at 0.5 mm pitch) 15 mm 3.5 mm microRPC [Fonte 1997] [Carlson et al, NSS2001] Si plate  =10 4  cm -HV mm PPAC microRPC PPAC 10 7 Hz/cm 2 Gain 5  10 4 Future GSI experiment

35 Main physical mechanisms – mostly understood except t-q correlation. Several interesting devices have been sucessfully tested with  <100 ps and very small tails or edge effects Small segmented counters. 2D position-sensitive single-gap counters. Large area counters. Practical difficulties to be addressed for future applications Crosstalk – several approaches proposed, none proven. Ageing – tests in progress, no problem so far. If problem: avoid water, freon or glass cathodes. Use chemistry to avoid formation of fluoridric acid. Tails – redundancy should solve any problem if really needed. Background – severity unknown. Rate capability – solutions should exist up to ~10 5 /cm 2. Conclusions