L. A. Diaz, J. A. Garzon, D. Gonzalez-Diaz, F. Guitian, L. Lopes, G. Mata, M. Morales, C. Pecharroman

1. Considerations on high rates

Simulated rate over the ToF wall 20 kHz/cm 2 Direction orthogonal to the magnetic kick Direction along to the magnetic kick Simulated rate on the TOF wall for Au-Au collisions at E=25 GeV/A (rate capability of ordinary tRPCs is kHz/cm 2 )

The behaviour of RPCs at high rates and the DC model (I) The assumption that the RPC performances 'just' depend on the average field in the gap is often referred as the DC model. (1) (1) + d (glass thickness) Φ (particle flux) g (gap thickness) ρ (resistivity) At high rates the average field in the gap E o is modified For instance:

[1] H. Alvarez-Pol et al., NIM A, 535(2004)277, [2] V. Ammosov et al. NIM A, 576(2007)331, [3] R. Kotte et al. NIM A(2006)155, [4] L. Lopes et al., Nucl. Phys. B (Proc. Suppl.), 158(2006)66. The behaviour of RPCs at high rate and the DC model (II)

Rate capability in the DC situation rate capability = particle flux for a 5% efficiency drop

Rate capability in the transient situation (pulsed irradiation) D. Gonzalez-Diaz et al., Nucl. Phys. B (Proc. Suppl) 158(2006)111 B. Bilki et al., arXiv: D. Gonzalez-Diaz et al., doi: /j.nima

Rate capability in the transient situation (pulsed irradiation) D. Gonzalez-Diaz et al., Nucl. Phys. B (Proc. Suppl) 158(2006)111 Equilibration time: time needed for the field in the gap to fall by 1/e of the drop corresponding to the stationary value: B. Bilki et al., arXiv:

2. Ceramic-metal composites

Ceramic-metal composites. what is it? Active field in material research. The high di-similarity of both materials allows to obtain an optimum combination of their properties. Main difficulty: an adequate procedure to obtain an homogenous mixture with small grain sizes. We have chosen mullite-molybdenum composites because they were expected to exhibit: Electronic conductivity. ρ~10 10 Ωcm. ε r < 50. E breakdown > 0.5 kV/2 mm.

Molybdenum Atomic number 42 Density g/cm 3 High melting temperature 2623 °C Lowest linear thermal expansion coefficient of the engineering metals 4.8 x / K at 25°C High thermal conductivity 138 W/m K at 20°C Crystal structure Body centered cubic Lattice constant a = Å Molybdenite

Mullite a bit of explanation of this! Al 2 O 3 +SiO 2

Electrical behaviour of ceramic-metal composites 'Experimental Evidence of a Giant Capacitance in Insulator-Conductor Composites at the Percolation Threshold' Carlos Pecharroman and Jose S. Moya Adv. Mater. 2000, 12, No insulator metal percolation

Optical-microscope picture after homogenization 11% Mb 12% Mb 13% Mb 0. 5 mm

Samples after sintering D=2 cm

Relaxation curves time [s] I [A]

Electrical conductivity High linearity and reproducibility E break >1 kV/2 mm

Electrical permittivity only few factors bigger than glass!

Summary of electrical properties f(Mo)ρ[GΩ cm]ε r (100 Hz)ε r (1 MHz) SPS 11% % % HotPress 11% % two different sintering methods have been tried (SPS and HotPress)

Stability with transported charge over CBM life-time 1 month of CBM operation at 50% duty cycle (HADES life-time!) puzzling! we attribute this to the absence of pasivation of the sample surface. T variations

Conclusions Five Mu/Mo samples customized for standing comfortably the highest CBM-TOF rates have been produced. Stability of the electrical properties within 25% was observed for 1 CBM month-equivalent. The observed decrease is likely to be produced through electrode-sample reaction due to the absence of sample pasivation. This is being studied under controlled conditions. The degree of reproducibility of the samples is very high, with 11%- and 12%-Mo samples produced both in SPS or HotPress. We considered the samples promising for RPC stable operation at high rates so several 1 and 4-gap RPCs with area ~3 cm 2 will be produced and its rate capability evaluated in a realistic situation.

with a bit of luck... rate capability = particle flux for a 5% efficiency drop

we are there! rate capability = particle flux for a 5% efficiency drop

appendix

Deviations from the DC model. The stabilization time =1200 Hz/cm 2 =580 Hz/cm 2 by cutting the first 2 s of the spill the effect disappears. measured rate in reactions (2003) at GSI-SIS (~8s time spill) DC limit

cell model Equivalent circuit M. Abbrescia, NIM A 533(2004)7 Quantitative description of the stabilization time (II). The cell model

Quantitative description of the stabilization time (III). Behaviour under X-ray irradiation

Fit to the DC model Not fitted! (σ T ) Deviations from the DC model. The local fluctuations of the field response to secondary particles from reactions (2003) at GSI-SIS (~8s time spill)

An approximate analytical calculation based on the Campbel theorem and the exact M.C. one, differ slightly but show similar scaling properties (Average number of shots contributing per cell of area A) Campbel theorem for shot-noise Quantitative description of the local fluctuations of the field (I)

(all the N=4 gaps are assumed to equally contribute to the time resolution) Quantitative description of the local fluctuations of the field (II) A>0.3 mm 2 D. Gonzalez-Diaz et al., Nucl. Phys. B (Proc. Suppl) 158(2006)111

T scan HV scan rate Φ(x) charge q p (x) T=21 0 C T=33 0 C 34 3 T=21 0 C T=33 0 C The DC model and the case of warm glass (III) fit: DC model

Quantitative description of the stabilization time (I) Measurement of the dielectric response function of float glass as the one used in HADES

Rate effects. Campbell theorem (analytical vs simulation) Campbel theorem + Campbel theorem with average drop (2) (1)

Rate effects. Stabilization time (comparison with data) The value of V gap (t) from M.C. and the parameterization of t o can be used for describing t o as a function of the time within the spill provides a better description of the data The result suggests a bias in the tRPC performances when extrapolating from short to long spills Drop at the end of the spill P. Colrain et al. NIM A, 456(2000)62