1. BaBar Experience with RPCs: A snapshot of our present knowledge
Henry Band
University of Wisconsin

2. 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

3. 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
> kV HV pods
May 11, 2001
Henry Band - U. of Wisconsin

4. 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

5. Henry Band - U. of Wisconsin
IFR Construction
Endcap RPCs 0.9m  m
Barrel RPCs 1.25m  m
3 chambers per layer
3 RPC modules / chamber
2RPC modules / chamber
May 11, 2001
Henry Band - U. of Wisconsin

6. Henry Band - U. of Wisconsin
BABAR RPC
Operates in limited streamer mode
Parallel high resitivity plates (Bakelite   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 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 %
May 11, 2001
Henry Band - U. of Wisconsin

7. Henry Band - U. of Wisconsin
Bakelite Surface
Changguo Lu
Linseed oil coating smoothes surface imperfections
No oil
New Babar RPCs
BaBar original
May 11, 2001
Henry Band - U. of Wisconsin

8. 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/ /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 >  A barrel ~70 Endcap
Complete barrel cooling 10/99
Install Endcap cooling 1/00 - 4/00
<I> 60A barrel ~40 Endcap
Average Efficiency remain ~85%
May 11, 2001
Henry Band - U. of Wisconsin

9. 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% Hz A
May 11, 2001
Henry Band - U. of Wisconsin

10. Henry Band - U. of Wisconsin
Efficiency - LNF
Efficiency of barrel modules 11
10 
May 11, 2001
Henry Band - U. of Wisconsin

11. LNF data / Construction
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

12. 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

13. HV currents & temperatures 99
RPC current vs. time
Barrel temperatures vs. time
+10%/ day
@7800 V
Barrel face
+0.4 C / day
FEC power turned off
1 meter inside
@6000 V
May 11, 2001
Henry Band - U. of Wisconsin

14. 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 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

15. 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 ° in summer 99
Endcap temps °
May 11, 2001
Henry Band - U. of Wisconsin

16. Henry Band - U. of Wisconsin
Cooling / HV Currents 2000
May 11, 2001
Henry Band - U. of Wisconsin

17. Henry Band - U. of Wisconsin
RPC efficiency 00
Gas change
-1.4% /month
-0.56%/month
-1.4% /month
May 11, 2001
Henry Band - U. of Wisconsin

18. Henry Band - U. of Wisconsin
RPC efficiency 99-00
Barrel
Forward E.C.
Backward E.C.
 91%
 93%
June 99
 77%
 81%
 86%
 85%
Jan. 00
 81%
 75%
 71%
July 00
May 11, 2001
Henry Band - U. of Wisconsin

19. Henry Band - U. of Wisconsin
RPC efficiency 01
Efficiency loss is correlated with previous efficiency
May 11, 2001
Henry Band - U. of Wisconsin

20. Individual time histories
May 11, 2001
Henry Band - U. of Wisconsin

21. Henry Band - U. of Wisconsin
Detector studies
Realization that the RPC efficiencies did not recover spurred detector studies
During the 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

22. Henry Band - U. of Wisconsin
RPC radiography
D. Piccolo
The RPC efficiency was mapped in 2D showing irregular regions of reduced efficiency.
May 11, 2001
Henry Band - U. of Wisconsin

23. Henry Band - U. of Wisconsin
RPC radiography (2)
D. Piccolo
May V 56% Ar
May V 45% Ar
May 11, 2001
Henry Band - U. of Wisconsin

24. IFR efficiencies status
I. Peruzzi
May 11, 2001
Henry Band - U. of Wisconsin

25. IFR Construction Quality
I. Peruzzi
Quality distribution vs production box
Fraction of good chambers not uniformly distibuted in time
Production problems?
May 11, 2001
Henry Band - U. of Wisconsin

26. 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

27. SLAC/Wisconsin Test Stand
A. Soha
SLAC/Wisconsin Test Stand A
32 C.
28 C
36 C.
29 C.
May 11, 2001
Henry Band - U. of Wisconsin

28. 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.
Bakelite
Spacer
Graphite
FR4 frame
Linseed oil drops
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
Test RPC
May 11, 2001
Henry Band - U. of Wisconsin

29. Oil Stalagmite Formation
Changguo Lu
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.
May 11, 2001
Henry Band - U. of Wisconsin

30. Henry Band - U. of Wisconsin
Effect of Oil bridge
Changguo Lu
May 11, 2001
Henry Band - U. of Wisconsin