Properties of the bakelite used for standard RPC chambers as a function of the operating temperature F. Bruni – G. Hull – S.M. Mari 4 digital thermometers ( ) precision ±1°C resolution 0.1°C 5 Transistor (a b c d e) linear output 10 mV/°C accuracy ±0.4°C Tests performed at: T=22, 30, 35, 40 °C 998 mb < p < 1016 mb 40 % < RH < 60 % The trigger is defined by the coincidence of 3 plastic scintillators Time accuracy better than 1 ns Plastic Scintillator A: 7.5  17  2.5 cm³ Plastic Scintillator B: 29  29  1 cm³ Plastic Scintillator C: 29  29  1 cm³ RPC1 Pad: 46  46 cm² (heated RPC) RPC2 Pad: 10  10 cm² (unheated RPC) Measures of current I Measures of counting rate Waveform acqusition: - Detection efficiency  - Time resolution  t - Charge distribution q 1 order of magnitude ! T=22 °C  t = 1.58  0.08 ns V a = 9800 V T=30 °C T=35 °C  t = 2.22  0.05 ns  t = 2.15  0.05 ns Time 22, 30, 35 °C The temperatures are continually monitored during all the tests. The measured values are the same within 1 °C University of “Roma Tre” Department of Physics INFN “Roma III” The “Temperature Box” Insulator materials are used for the construction of the box 4 tubolar lamps heat up the box 2 fan wheels keep the air movement caotic An elettromechanical thermostat switches the lamps on and off A worm-wheel of tubes is used for thermalizing the gas mixture before enternig the RPC The box can contain one 50  50 cm² RPC Experimental Set-Up Monitoring of the Temperature Frequency response analyzer Dielectric interface Solartron 1296 Solartron 1250 Cryostat Cryomech AL25 Compressor Cryomech CP510 Temperature controller Lake Shore 330 Temperature Probes Experimental system features: Blocking electrodes Electrically insulated cell and good heat exchange Frequency range : 10  Hz - 65 kHz Max. measured impedance > 100 T  Vacuum insulation T = 22 °CT = 30 °CT = 35 °C T = 40 °C (first acq.) Current (  A) Rate (Hz) Efficiency0.98 Time Resolution (ns) Broadband Dielectric Spectroscopy Study The bakelite frequency-dependent dielectric response is room temperature 6 round-shaped samples 30 mm diameter, 2 mm thickness 3 milled from the heated RPC (RPC1) and 3 from the unheated RPC (RPC2) Y m This method consists in the measure of the complex admittance (Y m ) as a function of the angular frequency (  ) of a sample sandwiched between two “blocking” electrodes Y m The measured admittance (Y m (  )) is related to the complex permittivity In terms of lumped circuit elements it can be described as the series of a Constant Phase Angle (CPA) element A constant 0  d  1 Where the admittance of the bakelite sample is unknown The Experimental Set-Up Teflon coated metallic electrodes j =  (-1)  0 = permittivity in free space S = sample surface h = sample thickness Measured Admittance Y m (  ) The low-frequency tails indicate the presence of electrodes polarization Estimation of the Samples Permittivity   = limiting high-frequency permittivity  = amplitude of the dielectric dispersion  = dielectric relaxation time  = microscopic conductivity of the sample  and  provide an empirical generalization of the ideal Debye relaxation (Havriliak- Negami relaxation) It is possible to evaluate the sample permittivity from the frequency response by performing a complex function fit procedure (solid lines in the figures) RPC2 untreatedRPC1 heated  5.10   0.04  18.3   0.2  (s) 0.20  0.03(6.9  0.1)   (  -1 m -1 ) (8.2  0.4)  (2.7  0.2)  Two orders of magnitude!!! Results form the fit analysis: Calculated imaginary component of the electric modulus M * =1/  * This way of represent the data offers the advantage of being more sensitive to the conductivity process, visible as a low frequency relaxation, and less sensitive to polarization phenomena. CONDUCTIVITY RELAXATION TIME  c = inverse of the low angular frequency corresponding to the maximum of M * It is an estimate of the length of time during which the bakelite of the RPC elementary cell can be considered as an insulator  c = s for the untreated RPC (RPC2)  c = 0.11 s for the heated RPC (RPC1)  c is linked to the time needed to recharge the elementary cell after a streamer process During the recharging time the bakelite of the heated RPC should be considered as conductor The RPC time resolution was measured as a function of the bakelite temperature. In 400 hours of operation at T = 40°C, we observed a time resolution worsening of about one order of magnitude (passing from the value of 2.5ns to 30ns) A dielectric spectroscopy study of the bakelite was performed: the real and imaginary component of the admittance was measured. From a fit procedure of the data, we estimated some material’s parameters and we observed that the heated bakelite is substantially different from the untreated bakelite The conductivity relaxation time was estimated; the low value obtained from the heated sample (0.11s), compared to the value obtained from the untreated one (273.34s), indicates that the bakelite of the RPC elementary cell can not be considered as an insulator during the streamer process. This study strongly supports a change in the structure of the bakelite affecting the RPC performances The Time Resolution Measurements Time T=40°C vs Time T=40°C vs Temperature Yb Ym Ycpa The total measured admittance Y m (  ) is due to the combined admittance of sample Y b (  ) and the admittance of blocking electrodes Y CPA (  ).