1 Multigap glass RPCs in HARP Design Running experience Results I. Boyko, G.Chelkov, D. Dedovich, A. Elagin, M.Gostkin, Y.Nefedov, K. Nikolaev, A. Zhemchugov (JINR Dubna), V. Ammossov, V. Koreshev (IHEP Protvino), F. Dydak, J. Wotschack (CERN Presented by: Joerg Wotschack (CERN)

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 2 RPCs in the HARP detector RPCs: -30 barrel (around TPC) -16 f/w (before 1 st DCH) Total number of readout channels: 368 Area covered: 8 m 2 Time-of-flight over 0.4– 2m for e/π separation below 300 MeV/c. Design goal: - Time resolution: 200 ps - High efficiency Hadron production experiment at CERN PS HARP data taking: 2001 & 2002

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 3 RPC design - glass stack 4-gap glass RPC Glass stack:1920 mm x 106 mm x 7.6 mm 6 glass plates: 0.7 mm; gap size: 0.3 mm (spacer: fishing line) HV: -6 kV (over two gas gaps) Central readout electrode for all four gas gaps

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 4 RPC design - layout barrel Glass stack Pre-amplifier 30 RPCs in two layers 100% coverage Looking upstream

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 5 Implementation in HARP Barrel: 30 RPCs in 2 layers Length: 2 m Width: 150 mm Thickness: 10 mm preamplifiers

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 6 RPC design - layout forward 4 x 4 = 16 RPCs perpendicular to beam distance to target: ~2 m RPCs identical to those in barrel same overlaps b/w RPCs as in barrel overlaps b/w horizontal and vertical RPCs Only central pads of RPCs in acceptance region looking upstream

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 7 RPC design - pad structure

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 8 RPC readout scheme 8 summing preamplifiers per RPC (on chamber) PA connected to 8 strips (strip = 30 x 104 mm 2 ) Splitter at 5 m distance Timing: discr. thr.: 5 mV Charge 80 m twisted pair cables TDC: CAEN V775 (35 ps) QDC: CAEN V792 (0.1pC)

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 9 RPC operating conditions Gas: C 2 F 4 H 2 :iCH 10 :SF 6 (90:5:5); ~1 volume change/hr HV: -6 keV over 2 gas gaps Random hits (noise + cosmics) Monitored over two years - stable Typical rates: 200–300 Hz/RPC (2000 cm 2 ) ≤ 0.1 Hz/cm 2 Low particle rates: ≤ 1 Hz/cm 2 Beam intensity: < per spill (400 ms) Typically 1000 interactions per spill (0.05 target) Average multiplicity: 4 (in barrel RPC acceptance) Rate: ~10 kHz/5 m 2 ≤ 1 Hz/cm 2 (barrel) Temperature: 20–35 º C in experimental area Barrel RPCs temperature stabilized: 27–30 º C (±0.5 º C ) Forward RPCs exposed to hall temperature

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 10 Data sets Scan of four RPCs exposed to 12 GeV/c π - beam Global time-slewing correction (time measured vs charge) Time & charge response vs impact position (x and y) Efficiencies and time resolution (Results presented earlier and not covered here, see RPC2003) Physics tracks with RPCs in HARP detector (2002) Corrections for electronics effects Corrections to global time-slewing correction t 0 for each pad Charge response as function of impact angle Time resolution & efficiencies in HARP

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 11 Steps from raw RPC time to TOF Convert TDC counts to picoseconds Correct for temperature effects Subtract arrival time of beam particle in target (measured by beam line instrumentation) Apply global time-slewing correction Correct for impact point dependence of timing Strip number Hit position along strip -> modification of global time- slewing correction Determine and apply pad specific t 0 constants

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 12 Temperature effects I Time response is strong function of temperature in experimental area Channel dependent day- night variations of t 0 of up to 900 ps (!) => Corr temp = 60 ±10 ps/  C Not a detector effect: barrel RPCs are temp.stabilized (±0.5  C) Threshold shift in splitter- discriminator electronics Barrel RPCs 600 ps +8.9 GeV Be

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 13 Temperature effects II Time response f/w RPCs Similar as in barrel  t/  T ≈ 54 ± 6.5 ps/  C. Forward RPCs are fully exposed to  T Suggests: small contribution from detector itself Charge response No temperature variation of charge for barrel RPCs Clear effect in forward RPCs  Q/  T≈ 3%/  C Charge (0.1 pC) Temperature Charge vs temperature f/w RPCs  t/  T slopes f/w RPCs  t/  T slopes

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 14 Time response vs pad strip Precise determination of systematic time shift as function of strip number + 92 ps in strips 1and ps in strips 2 and ps in strips 3 and 6 0 for strips 4 and 5 Uncertainty: < 10 ps Confirms older measurements Differences in transit times b/w strips and PA connections account for ≥50% as measured Relative time response (ps) Strip number Relative time response of the 8 strips of a pad normalized to strips 4+5 ‘Neutrals’ (hits from photon conversions)

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 15 Time response along strip I Time response is function of impact point distance to pre- amplifier (PA) Time difference b/w near and far ends of strip (  x ~ 90mm): 200 ps for large Q 0 ps for small Q Expect: 450–500 ps Effect explained by pulse reflection on not-terminated strip end and superposition of signals (simulation agrees with data) Requires charge-dependent modification of global time- slewing correction Charge (0.1 pC ) Impact point position along strip (mm) Slope (ps/mm) Time response (ps) PA Width of RPC Small charges Large charges Scan data

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 16 Charge response along strip Charge (0.1 pC) Vertical impact point position (mm) Active area of RPC PA Charge measured as function of distance to preamplifier within a single strip (scan) Confirmed by physics tracks comparing charge measured near and far from preamplifier Charge (0.1 pC)

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 17 pad ring far –– near Time-slewing correction - revisited Pragmatic approach: Measure difference in time response b/w far and near end of strips as function of charge Use results as effective correction for impact positions at strip ends For impact points along strip use an interpolation based on an analytical model calculation Results in modification of global time-slewing correction which is different for the eight pad rings

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 18 Pad specific t 0 constants t 0 normalization depends on cable delays and has to be determined for each pad individually from physics data Method: photon conversions Determine 50% point of rising edge of time spectrum (t 50% ) Calculate its relative position wrt time expected for hits in pad centre (t corr, analyt. simul.) Relate to nominal time of flight b/w target and centre of pad TOF(  ) = t 50% + t corr – t z0 – t 0 ( t z0 = beam arrival time in target) Estimated uncertainty: ~30 ps Typical time spectra for + neutrals (  conv.) – tracks Pad 151 (pad ring 7) t 50%

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 19 Stability of t 0 constants Coherent shifts of t 0 constants for different run periods 3 Ta Be:  t 0 = -250 ps H 2 O Be:  t 0 = +70 ps but: temperature slopes agree (!) Likely explanation: long- term threshold shifts in the discriminator/splitter electronics Requires t 0 calibration for each run period May 2002 (-8 GeV/c) August 2002 (+8.9 GeV/c) pad 24 t 0 (Ta) - t 0 (Be)t 0 (H 2 O) - t 0 (Be) Be(-) run:May 2002 Ta run: June 2002 Be(+) run: Aug 2002 H 2 O run: Sept 2002

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 20 Results Charge vs track length System efficiency Time resolution Particle identification

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 21 Charge response vs track angle Charge deposited in RPCs for charged particles as function of pad ring (= track impact angle) Charge deposited in RPCs for charged particles as function of path length in detector

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 22 System efficiency Intrinsic RPC efficiency was measured in scan (at high particle rate) to be 97–98% System efficiency is expected to be lower absorption in material in front of RPCs large energy-loss for low momentum protons Measured values in HARP: Eff = 97–98% Is a lower limit on intrinsic RPC efficiency  positive tracks + negative tracks

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 23 Time resolution - physics data  t from physics tracks through pad overlaps Same track is measured twice Independent of beam timing Peak position checks t 0 s and time-slewing correction Width measures convoluted resolution of the two pads Result (for all pads in barrel)  /√2 = 145 ps Narrow Gaussian (85%) on top of a wider distribution Barrel RPCs

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 24 Time resolution vs pad ring pad rings correspond to track inclination wrt RPC pad ring 2/3:  ≈ 90° pad ring 6/7:  ≈ 30°

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 25 Time resolution - the tails Noise: genuine low charge hits for which threshold is passed too early because of overlayed noise. Results in enhancement at low- charges. Knock-on: low-energy particles kicked out from RPC material and trapped in magnetic field; they move slowly in RPC adding charge some ns after genuine hit. Results in correct time signal but too large charge and therefore wrong time-slewing correction (effect only present in magnetic field)  t > 400 ps Data points normalized to same shape as  t < 400 ps spectrum Tracks through pad overlaps Charge spectra for peak and tails

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 26 Particle identification (I) +8.9 GeV/c 0.05 Be target – pad ring 5 (average  t ) positive tracks p π e Beta Momentum (GeV/c) negative tracks Momentum (GeV/c) p π e Pad ring 5

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 27 Electron enriched sample: photon conversion candidates two tracks with same origin and production angle Particle identification (II) +8.9 GeV/c on Be target (0.05 ) = ±  = 8.3%

RPC2005, 10–12 Oct. 2005Joerg Wotschack (CERN)HARP RPCs / 28 What have we learned? Multigap glass RPCs are great detectors: fast, precise, efficient, and robust Detector design OK, but … Strip termination would have made our life much easier. Threshold drifts of discriminators with temperature and ‘time’. Differences in signal transmission b/w strips and preamplifier. Small fraction of wrong time measurements Low threshold => noise correlated with hits (wrong time, early) Knock-on particles => right time, wrong charge = wrong TS corr. Overall: the system worked extremely well, final result  time = 145 ps; system efficiency ≈ 98%