Siena 2002 Siena, 24/10/2002 G. Passaleva New results from an extensive aging test on bakelite RPC G. Passaleva INFN Firenze

Siena 2002 Siena, 24/10/2002 G. Passaleva Extensive aging test of RPCs at GIF 1.2 RPC prototypes under study since RPC A irradiated at GIF during 2001 – Q int  0.4 C/cm 2 RPC B not irradiated,used as a reference. GIF test setup  High flux position  Low flux position

Siena 2002 Siena, 24/10/2002 G. Passaleva RPC area 50x50 cm 2 1 meter  = 9x10 9  cm Gas mixture: 95% C 2 H 2 F 4 4% I-C 4 H 10 1% SF 6 source I, HV, T, P, gas, continuously monitored and recorded during the whole test. Particle rate: 1  3 kHz/cm 2 Test setup at GIF

Siena 2002 Siena, 24/10/2002 G. Passaleva RPC under high irradiation RPCs under high-irradiation flux show peculiar characteristics 1.An almost perfect linear dependence of the current on applied voltage 2.The current saturates when the particle flux is increased 3.A strong (exponential) dependence of the current on the temperature for a fixed value of the HV

Siena 2002 Siena, 24/10/2002 G. Passaleva Principle of RPC operation qiqi q=Gq i V0V0 gas Electrodes d,S,  particle G~ ; G  1 for V 0 < V T Due to space-charge effects (especially with SF 6 ): q  V gap -V T V gap =V 0 -IR I =  q(V gap ) (  = particle flux) If  low  I small  V gap  V 0

Siena 2002 Siena, 24/10/2002 G. Passaleva High flux conditions If    we must have q  0, i.e. V gap  V T and I max =(V 0 – V T )/R Linear dependence on HV Exponential temperature dependence of I via R: More generally: Saturation with flux

Siena 2002 Siena, 24/10/2002 G. Passaleva Current saturation X obtained fitting I at different  At GIF   1/Abs 0.7 where Abs is the filter setting (Abs=1,2,5, ) (1)

Siena 2002 Siena, 24/10/2002 G. Passaleva Measurement of R  R can be measured in a non-destructive way from the slope of I-V 0 curve  Its changes can be easily monitored  Precise knowledge of X is unimportant if X is large (an estimate is sufficient) We can obtain R by fitting I vs. V 0 curve

Siena 2002 Siena, 24/10/2002 G. Passaleva Cross check: R temperature dependence Temperature dependence is easily described in our model. Exponential fit to R measurements:  =0.126  Nice agreement with measurements on the bakelite slabs:  slab  =0.12  0.01

Siena 2002 Siena, 24/10/2002 G. Passaleva Cross check: temperature corrections on RPC current Daily temperature oscillation are corrected out using the proper temperature coefficient I 20 =I exp[-0.126(  -20°)]

Siena 2002 Siena, 24/10/2002 G. Passaleva Aging tests: irradiation Q int =0.4 C/cm 2 I  by a factor of 6 Interpretation: R  by a factor of 6 because of radiation effect Measurements of R with our model: Before irr.: R=10.9 M  After irr.: R=60.1 M 

Siena 2002 Siena, 24/10/2002 G. Passaleva New R measurements: RPC A RPC A - R 20 (1999) = 2.6 M  -  20 (1999) = 1.6x10 10  cm irradiation chamber in the lab Jan 01 Chamber again at GIF for tests. Q int  0 R  exponentially ! Oct 02:   =1.6x10 12  cm Did we reach the top ? May 02

Siena 2002 Siena, 24/10/2002 G. Passaleva New R measurements: RPC B Chamber at GIF for tests. Q int  0 R  exponentially ! RPC B - R 20 (1999) = 5.2 M  -  20 (1999) = 3.2x10 10  cm May 02 Oct 02:   =1.7x10 12  cm RPC B never irradiated ! Did we reach the top ?

Siena 2002 Siena, 24/10/2002 G. Passaleva Summary 1.As expected, R increases because of irradiation 2.Surprisingly, R increases spontaneously, much more than because of irradiation 3.We also see a saturation effect in the growing of R (to be confirmed) What are the effects on the RPC rate capability ?

Siena 2002 Siena, 24/10/2002 G. Passaleva Definition of rate capability The rate capability is defined by:  >95% (trigger) HV<11000 (streamers !) plateau  400V

Siena 2002 Siena, 24/10/2002 G. Passaleva GIF test October 1999 RPC A RPC B GIF Rate  3 kHz/cm 2 R A =1.8 M  R B =3.6 M  T=23.0 °C R  2  =2.6 M  R  2  =5.2 M  R A =1.8 M  R B =3.6 M  T=23.0 °C R  2  =2.6 M  R  2  =5.2 M  Rate max > 3 kHz/cm 2

Siena 2002 Siena, 24/10/2002 G. Passaleva GIF test August 2001 Rate max  1.15 kHz/cm  600 Hz/cm 2 After 6 months of irradiation (Qint  0.4C /cm 2) R A =31.6 M  T=25.1 °C R 2  =58.3 M  R A =31.6 M  T=25.1 °C R 2  =58.3 M  RPC A

Siena 2002 Siena, 24/10/2002 G. Passaleva Latest results: GIF test July 2002 Rate max  350 Hz/cm  200 Hz/cm 2 R=102.4 M  T=24.5 °C R 2  =175.7 M  R=102.4 M  T=24.5 °C R 2  =175.7 M 

Siena 2002 Siena, 24/10/2002 G. Passaleva Conclusions 1.We have developed and applied a method to measure RPC electrode resistivity, on-line, during chamber operations, in an easy and non destructive way. 2.This method allows to monitor the resistivity as well as to correct other physical quantities (e.g. the current) for temperature effects. 3.Using this method we have studied extensively the aging effects on bakelite RPCs: R increased by 2 orders of magnitude in 3 years. Rate capability dropped from a few kHz/cm 2 to less than 150 Hz/cm 2 (  no RPC in LHCb !!!) Pure spontaneous aging is the dominant effect