11Christian Lippmann15.11.2002; IEEE NSS

Detector Physics of Detector Physics of Resistive Plate ChambersResistive Plate ChambersChristian Lippmann (CERN), Werner Riegler (CERN)

We simulate We simulate Timing RPCs in one and in one and four gap four gap configurations as configurations as in:in:

P. Fonte et. al., NIM A449 P. Fonte et. al., NIM A449 (2000) 295-301(2000) 295-301A. Akindinov, P. Fonte et. A. Akindinov, P. Fonte et. al., CERN-EP 99-166al., CERN-EP 99-166P. Fonte and V. Peskov, P. Fonte and V. Peskov, preprint LIP/00-04preprint LIP/00-04

0.3 mm gap(s); glass 0.3 mm gap(s); glass resistive plates (resistive plates (=8=8, =2x1012 cm))

CC2 2 FF4 4 HH2 2 / i-C/ i-C44HH10 10 / SF/ SF66 (85/5/10)(85/5/10)

HV: 3(6)kV, E: 100kV/cm

Resistive plate chambers are gaseous parallel plate avalanche detectors with

time resolutions down to 50 ps, making them attractive for trigger and

time of flight applications.

22Christian Lippmann15.11.2002; IEEE NSS

Simulation InputSimulation Input

Primary ionization: Primary ionization: HEEDHEED (Igor Smirnov)(Igor Smirnov)

Townsend, attachment coefficient: Townsend, attachment coefficient: IMONTE IMONTE (Steve Biagi)(Steve Biagi)

Diffusion, drift velocity: Diffusion, drift velocity: MAGBOLTZ 2 MAGBOLTZ 2 (Steve Biagi)(Steve Biagi)

Avalanche fluctuations: Avalanche fluctuations: Werner LeglerWerner Legler ( (Die Statistik der Elektronenlawinen in elektronegativen Gasen bei hohen Feldstaerken und bei

grosser Gasverstaerkung,1960,1960))

Space Charge Field: Space Charge Field: CERN-OPEN-2001-074CERN-OPEN-2001-074, "Static electric fields in , "Static electric fields in

an infinite plane condensor with one or three homogeneous layers“an infinite plane condensor with one or three homogeneous layers“

33Christian Lippmann15.11.2002; IEEE NSS

Gas ParametersGas Parameters

Primary Ionization: Primary Ionization: Number of Clusters per mmNumber of Clusters per mm

pp00=1013mbar, T=1013mbar, T00=296.15K=296.15K Measurements (lines): F.Rieke et Measurements (lines): F.Rieke et

al., Phys.Rev. A6, 1507 (1972)al., Phys.Rev. A6, 1507 (1972)

Townsend CoefficientTownsend Coefficient Attachment CoefficientAttachment Coefficient pp00=1013mbar, T=1013mbar, T00=296.15K=296.15K

44Christian Lippmann15.11.2002; IEEE NSS

Efficiency and Time Efficiency and Time ResolutionResolution

At threshold crossing: At threshold crossing: space charge effectspace charge effect has no has no influence (for very fast amplifiers)influence (for very fast amplifiers)

Measurements: open symbolsMeasurements: open symbols 7GeV pions (9.13 clusters/mm), 20fC threshold, 200ps amplifier 7GeV pions (9.13 clusters/mm), 20fC threshold, 200ps amplifier

peaking time, 1fC noise, T=296.15K and p=970mbpeaking time, 1fC noise, T=296.15K and p=970mb

Single gapSingle gap Quad gapQuad gap

55Christian Lippmann15.11.2002; IEEE NSS

Include Space Charge EffectInclude Space Charge Effect

Divide gas gap into Divide gas gap into several stepsseveral steps

Electrons multiply Electrons multiply and drift towards and drift towards the anodethe anode

Calculate Calculate dynamically the dynamically the field E of the field E of the space charge at space charge at each step where each step where electrons are electrons are multipliedmultiplied

(E), (E), (E), v(E), vDD(E)(E) DiffusionDiffusion No photonsNo photons

66Christian Lippmann15.11.2002; IEEE NSS

Example AvalancheExample Avalanche

77Christian Lippmann15.11.2002; IEEE NSS

Charge SpectraCharge Spectra

Simulated avalanches at HV=3kV, p=1013mb, single Simulated avalanches at HV=3kV, p=1013mb, single gap RPCgap RPC

Supression Factor Supression Factor 10107 7 !!!!

88Christian Lippmann15.11.2002; IEEE NSS

Reminder: Wire Tube/ Wire Reminder: Wire Tube/ Wire ChamberChamber

1/r field geometry1/r field geometry Space charge region very short (<100V) 1.5 orders of magnitude jump to limited streamer region

From NIM 200, 345 (1982)

Space Charge Region

99Christian Lippmann15.11.2002; IEEE NSS

Timing RPC: Long Space Timing RPC: Long Space Charge ModeCharge Mode

Homogeneous (applied) electric fieldHomogeneous (applied) electric field Proportional Region is below ThresholdProportional Region is below Threshold Very long space charge Region (>700V)Very long space charge Region (>700V) Charge grows first exponentially, then linearly with HV Charge grows first exponentially, then linearly with HV

(which is also an experimental fact)(which is also an experimental fact)

Space Charge Region

1010Christian Lippmann15.11.2002; IEEE NSS

Ratio QRatio Qindind/Q/Qtottot

For avalanches For avalanches where no space where no space charge effect is charge effect is present we expect:present we expect:

Indicator for a Indicator for a strong space charge strong space charge effect present for effect present for E > 7.5kV/mmE > 7.5kV/mm

1111Christian Lippmann15.11.2002; IEEE NSS

ConclusionConclusion

We have applied We have applied standardstandard detector physicsdetector physics simulations simulations to Timing RPCs and find to Timing RPCs and find goodgood agreementagreement with with measurements for efficiency, time resolution and measurements for efficiency, time resolution and charge spectra.charge spectra.

The operational mode of timing RPCs is strongly The operational mode of timing RPCs is strongly influenced by a space charge effect.influenced by a space charge effect.

The suppression factor is huge (10The suppression factor is huge (1077).).

Details on our work:Details on our work: CERN-EP-2002-024 NIM A481(2001) 130-143 CERN-EP-2002-046 CERN-OPEN-2001-074