BaBar Experience with RPCs:A snapshot of our present knowledge

Henry BandUniversity of Wisconsin

May 11, 2001 Henry Band - U. of Wisconsin

Introduction

The BaBar detector at the PEPII B factory has operated a ~2000 m RPC detector since 1999

These RPCs operating in the streamer regime have proven very sensitive to temperature

Both RPC current and efficiency have suffered apparently irreversible changes (new results show hope of reversing changes)

Examination of opened RPCs show clear problems with the linseed oil coating

A few new RPCs with thinner oil coatings were installed in late 00

Although LHC RPCs operate in avalanche mode, our experience may be relevant to this next generation of RPC detector

IFR RPCs History of IFR construction Initial efficiency measurements Operational problems

Unstable temperature and currents

Declining efficiency Detector studies

BaBar Test stands Other

RPC Autopsy photos Inefficiency Model New RPCs

Characteristics Operation

BaBar plans

May 11, 2001 Henry Band - U. of Wisconsin

Instrumented Flux Return

Instrumented Flux Return 342 barrel RPCs 432 endcap RPCs 32 cylindrical RPCs

Electronics 3300 Front end cards

~ 40 mV threshold 16 channels connected to

alternate strips 1 TDC measurement 1 scalar

Gas 35% Freon 134a, 4.5% iso-

butane, 60.5% Argon 2-3 volume changes/day

High Voltage > 50 10 kV HV pods

May 11, 2001 Henry Band - U. of Wisconsin

IFR Collaboration

INFN Lab.-Frascati - F. Anulli, R. Baldini, A. Calcaterra, D. Falciai, G. Finocchiaro, P. Patteri,

I. Peruzzi, M. Piccolo, Y. Xie, A. Zallo, R. de Sangro INFN-Bari - A. Palano

INFN-Genova -S. Bagnasco, A. Buzzo, G. Crosetti, M. Lovetere, M. Macri, S. Passaggio, F. Pastore, M.G. Pia, E. Robutti, A. Santroni, S. Tosi

INFN-Napoli -T. Cartaro, N. Cavallo, G. DeNardo,F. Fabozzi, C. Gatto, , L Lista, P. Paolucci, D. Piccolo, C. Sciacca

LLNL - R. Bionta, V. Brigljevec, D. Lange, D. Wright

U. of Wisconsin- H. Band, A. Eichenbaum,J. Johnson U. Oregon - J. Brau, R. Frey, E. Grauges-Pous, M. Iwasaki, N. Sinev, D. Strom Yale U - T. Moore, H. Neal INFN-Roma - F. Ferroni, S. Morganti, G. Piredda, INFN-Pisa - M. Carpinelli, F. Forti

Consulting experts C. Lu -Princeton University, J. Va’vra -SLAC, M. Lazzari - Turin

May 11, 2001 Henry Band - U. of Wisconsin

IFR Construction

2RPC modules / chamber

3 chambers per layer

Endcap RPCs 0.9m 1.5-2.7 m

3 RPC modules / chamber

Barrel RPCs 1.25m 1.9-3.2 m

May 11, 2001 Henry Band - U. of Wisconsin

BABAR RPC

Operates in limited streamer mode Parallel high resitivity plates (Bakelite

1011 -1012 cm ) Graphite ~100 k/ According to the General Tecnica

factory the RPCs were filled 3 times with a mixture of 70% linseed oil and 30% n-pentane. Air was then flushed through the RPC for several days.

Strip pitch Barrel 20-33 mm (), 38 mm (z) Endcap 26 mm(y), 38 mm (x)

Changes from L3 RPCs Polycarbonate buttons with lip replaced

G10 cylindrical button Single layer with strips on both sides No mechanical envelope Nonflammable gas - low iso-butane %,

CBrF3 replaced by Freon 134a C2H2F4

Gas mixture in %

Date Argon Freon 134a isobutane99 45 50.2 4.8

Sep-99 56.7 38.8 4.5J un-00 60.6 34.7 4.7

Tests 65 30.3 4.7

May 11, 2001 Henry Band - U. of Wisconsin

Bakelite Surface Changguo Lu

Linseed oil coating smoothes surface imperfections

BaBar originalNew Babar RPCs

No oil

May 11, 2001 Henry Band - U. of Wisconsin

IFR History

RPCs chosen for BaBar 1/95 Mature technology Good L3 experience

RPC Production 7/96- 10/97 RPC QA tests - Frascati

I < 18 A and efficiency > 95%

Chamber assembly SLAC 12/96 - 12/97 Chamber tests at SLAC 5/97 – 12/97

Initial tests High Temperatures - 30 C High Currents ~100 A Efficiency > 95%

Assembly/test area air conditioned 7/97

Chamber insertion 6/97 -12/97 Cabling and Fecs installed from 10/97 Complete Barrel and Forward E.C. 11/98 Barrel cosmic ray tests 12/98 Sharply increasing current observed when

doors closed and LV on

Backward E.C. completed 4 / 99 First beam May 99

99% of RPCs working 96% of Fecs working

Heating from electronics & ambient hall raise steel T to 29-34

HV current limitations force reduction in # of RPCs

< I > 70 -210 A barrel ~70 Endcap Complete barrel cooling 10/99 Install Endcap cooling 1/00 - 4/00 <I> 60A barrel ~40 Endcap Average Efficiency remain ~85%

May 11, 2001 Henry Band - U. of Wisconsin

RPC tests at Frascati, 1997

HV modules shipped to LNF. Longitudinal strips were

applied. Modules packed into boxes Cosmic ray tests were made

to characterize each module Gas Ar 45%, Freon 50%,

isobutane 5%

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May 11, 2001 Henry Band - U. of Wisconsin

Efficiency - LNF

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May 11, 2001 Henry Band - U. of Wisconsin

LNF data / Construction

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Neither the measured current or singles rate was found to depend on bakelite resistivity

Singles rate < 1kHz /m2

Current 3-20 A RPCs air shipped to SLAC HV modules assembled 2 or 3

to a chamber 2nd view strips applied SLAC cosmic rays tests Completed and tested

chambers inserted into BABAR steel

After erection of BABAR steel - installed

DAQ cabling HV cabling Gas connections

May 11, 2001 Henry Band - U. of Wisconsin

June 1999

By May 99 IFR cabling complete

RPC efficiencies good But - warning signs

Some RPCs with > 100 A were disconnected

Limited HV current capability FECs inside steel produce

2.3 W of heat each Total power into barrel ~2.6

kW Poor heat transfer to outside

With collision data Barrel heats up ~.4 C/day IR hall temperature climbs

during summer

77%

91%

93%

May 11, 2001 Henry Band - U. of Wisconsin

HV currents & temperatures 99

RPC current vs. time

@6000 V

@7800 V

FEC power turned off

+10%/ day

+0.4 C / day

Barrel temperatures vs. time

1 meter inside

Barrel face

May 11, 2001 Henry Band - U. of Wisconsin

RPC Efficiency 99

Climbing temperatures forced the disconnection of more RPCs

Cooling system being prototyped Average currents were

70 A(sextant 4) -210 A (sextant 0)

65 A Forward endcap 70 A Backward endcap

Because of voltage drops across a ~1.5 M resistor in the HV distribution network, the voltage on working RPCs was reduced by 100-300 volt

Additional inefficiency from more conventional sources HV shorts 3% Gas leaks <2% Dead front end electronics <2%

May 11, 2001 Henry Band - U. of Wisconsin

Water cooling

Tests of prototype cooling loops in sextant 1 started in mid June

Additional loops installed in August

Full barrel installation of copper loops brazed into BABAR steel- Oct. 99

Completed endcap cooling system - Jan. 00

Remove ~ 10kW heat Stabilized temperature

and RPC currents Barrel temps. ~19-21°,

were 29-33° in summer 99

Endcap temps 22- 24°

May 11, 2001 Henry Band - U. of Wisconsin

Cooling / HV Currents 2000

May 11, 2001 Henry Band - U. of Wisconsin

RPC efficiency 00

-1.4% /month

-1.4% /month

-0.56%/month

Gas change

May 11, 2001 Henry Band - U. of Wisconsin

RPC efficiency 99-00

June 99

Jan. 00

July 00

Barrel Forward E.C. Backward E.C.

77% 91% 93%

81% 86% 85%

71% 81% 75%

May 11, 2001 Henry Band - U. of Wisconsin

RPC efficiency 01

Efficiency loss is correlated with previous efficiency

May 11, 2001 Henry Band - U. of Wisconsin

Individual time histories

May 11, 2001 Henry Band - U. of Wisconsin

Detector studies

Realization that the RPC efficiencies did not recover spurred detector studies During the 99-00 data run access to

the RPCs was limited Physical removal of any RPC would

require > 1 month access Limited tests were carried out during

accesses or on a few chambers Lowered barrel temp. to 17 C. Reversed HV Increased gas flow Lowered discriminator threshold Put weights over inefficient regions

No effect Gas composition was regularly tested

No evidence of water vapor

A wide spread of plateau curves were measured.

May 11, 2001 Henry Band - U. of Wisconsin

RPC radiography

The RPC efficiency was mapped in 2D showing irregular regions of reduced efficiency.

D. Piccolo

May 11, 2001 Henry Band - U. of Wisconsin

RPC radiography (2)

May 99 8100 V 45% Ar

May 00 8100 V 56% Ar

D. Piccolo

May 11, 2001 Henry Band - U. of Wisconsin

IFR efficiencies status I. Peruzzi

May 11, 2001 Henry Band - U. of Wisconsin

IFR Construction Quality

Quality distribution vs production box

Fraction of good chambers not uniformly distibuted in time

Production problems?

I. Peruzzi

May 11, 2001 Henry Band - U. of Wisconsin

Test stands

Test stands were restarted or started at several locations Frascati Napoli SLAC U. of Oregon

SLAC test stand utilized 9 post BABAR production RPCs which had been used in background tests

The RPCs were subjected to a heating cycle at 36 C.

Permanent changes in both current and efficiency were seen

Several RPCs were opened

Large oil drops spanning the 2 mm gap were found.

May 11, 2001 Henry Band - U. of Wisconsin

SLAC/Wisconsin Test Stand

36 C. 29 C.32 C.

28 C

A

A. Soha

May 11, 2001 Henry Band - U. of Wisconsin

IFR inefficiency model

Linseed oil surface treatment under heat and HV has beaded up into drops.

Some drops span the RPC gap making a high resistance short which reduces the local E field below that required for streamers.

Both HV current and efficiency are irreversibly changed

The new shape of the buttons may be more likely to trap oil

The number and location of the blobs explains both the RPC to RPC variation and the efficiency holes

Gas leaks, HV shorts, and point discharges also contribute

FR4 frame

Bakelite

Linseed oil drops

SpacerGraphite

Test RPC

May 11, 2001 Henry Band - U. of Wisconsin

Oil Stalagmite Formation

In lab tests using freshly painted bakelite (uncured) the application of 4-5 kV resulted in the movement of the oil and the formation of stalagmites within hours.

In BaBar RPCs the oil is more viscous and the time scale is presumably much longer.

Changguo Lu

May 11, 2001 Henry Band - U. of Wisconsin

Effect of Oil bridge Changguo Lu