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

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. 2005 Joerg Wotschack (CERN) HARP RPCs / 2

RPCs in the HARP detector

RPCs:-30 barrel (around TPC)-16 f/w (before 1st DCH)

Total number of readout channels: 368Area covered: 8 m2

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 PSHARP data taking: 2001 & 2002


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


RPC design - layout barrel

Glass stack Pre-amplifier

30 RPCs in two layers100% coverage

Looking upstream


Implementation in HARP

Barrel: 30 RPCs in 2 layers• Length: 2 m• Width: 150 mm• Thickness: 10 mm

preamplifiers


RPC design - pad structure


RPC readout scheme

QDCTDC

Trigger

RPC modulesPA

(12 QDCs)(12 TDCs)

Splitter

8 channels

16 channels80 m twisted pair

80 m twisted pair

5 m coax8 channels

8 summing preamplifiers per RPC (on chamber)

PA connected to 8 strips (strip = 30 x 104 mm2)

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)


RPC operating conditions Gas: C2F4H2:iCH10:SF6 (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 cm2) ≤ 0.1 Hz/cm2

Low particle rates: ≤ 1 Hz/cm2

Beam intensity: < 20000 per spill (400 ms) Typically 1000 interactions per spill (0.05 target) Average multiplicity: 4 (in barrel RPC acceptance) Rate: ~10 kHz/5 m2 ≤ 1 Hz/cm2 (barrel)

Temperature: 20–35 ºC in experimental area Barrel RPCs temperature stabilized: 27–30 ºC (±0.5 ºC ) Forward RPCs exposed to hall temperature


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 t0 for each pad Charge response as function of impact angle Time resolution & efficiencies in HARP


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 t0 constants


Temperature effects I Time response is strong

function of temperature in experimental area

Channel dependent day-night variations of t0 of up to 900 ps (!)

=> Corrtemp= 60 ±10 ps/C Not a detector effect:

barrel RPCs are temp.stabilized (±0.5C)

Threshold shift in splitter-discriminator electronics

Barrel RPCs

600 ps

+8.9 GeV Be


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

Ch

arge

(0.

1 p

C)

Temperature

Charge vs temperaturef/w RPCs

t/T slopesf/w RPCs

t/T slopes


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 QExpect: 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)

Slo

pe

(ps/

mm

)T

ime

resp

onse

(p

s)

PAPA

Width of RPC

Small charges Large charges

Scan data


pad ring 6

--- 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


Pad specific t0 constants t0 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 (t50% ) Calculate its relative position wrt

time expected for hits in pad centre (tcorr, analyt. simul.)

Relate to nominal time of flight b/w target and centre of pad

TOF() = t50% + tcorr – tz0 – t0

( tz0 = beam arrival time in target)

Estimated uncertainty: ~30 ps

Typical time spectra for + neutrals ( conv.) – tracks

Pad 151(pad ring 7)

t50%


Stability of t0 constants Coherent shifts of t0

constants for different run periods3 Ta - 8.9 Be: t0 = -250 ps

H2O - 8.9 Be: t0 = +70 ps

but: temperature slopes agree (!)

Likely explanation: long-term threshold shifts in the discriminator/splitter electronics

Requires t0 calibration for each run period May 2002 (-8 GeV/c)

August 2002(+8.9 GeV/c)

pad 24

t0(Ta) - t0(Be)t0(H2O) - t0(Be)

Be(-) run: May 2002

Ta run: June 2002

Be(+) run: Aug 2002

H2O run: Sept 2002


Results

Charge vs track length System efficiency Time resolution Particle identification


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


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


Time resolution - physics data t from physics tracks

through pad overlaps Same track is measured

twice Independent of beam timing Peak position checks t0s 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


Time resolution vs pad ring

Time resolution vs pad ring

pad rings correspond to track inclination wrt RPCpad ring 2/3: ≈ 90°pad ring 6/7: ≈ 30°


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

t > 400 ps

Data points normalized to same shape as t < 400 ps spectrum

Tracks through pad overlapsCharge spectra for peak and tails


Particle identification (I)+8.9 GeV/c 0.05 Be target – pad ring 5 (average t)

positive tracks

pπ

e

Bet

a

Momentum (GeV/c)

negative tracks

Momentum (GeV/c)

p

π

e

Pad ring 5 Pad ring 5


Electron enriched sample: photon conversion candidatestwo tracks with same origin and production angle

Particle identification (II)

+8.9 GeV/c on Be target (0.05 )

<> = 0.995 ± 0.003 = 8.3%


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%

Documents

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