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/cm2

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 0.3-1 kHz/cm2)

The behaviour of RPCs at high rates and the DC model (I)

qdg

ERIg

EE oogap 11

The assumption that the RPC performances 'just' depend on the average field in the gap is often referred as the DC model.

)( gapEqq

(1)

(1) + )( dEE gapgap

d (glass thickness)Φ (particle flux)g (gap thickness)ρ (resistivity)

At high rates the average field in the gap Eo is modified

)1(

)(

dag

EEdaEE tho

ogap

thgap EEaq For instance:

)( gapT E

)( gapE

)( dT

)( d

[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.

)(0

1 refgap EEe

)(

)'( 0

gapT ES

nK

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:0901.4371

D. Gonzalez-Diaz et al., doi:10.1016/j.nima.2008.12.097

)1ln(

d

dV

dq

ddVdq

teq

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 fieldin the gap to fall by 1/e of the drop corresponding to the stationary value:

ddVdq

teq

B. Bilki et al., arXiv:0901.4371

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~1010 Ωcm.• εr < 50.• Ebreakdown> 0.5 kV/2 mm.

Molybdenum

• Atomic number 42

• Density 10.22 g/cm3

• High melting temperature 2623 °C

• Lowest linear thermal expansion coefficient of the engineering metals 

4.8 x 10-6 / K at 25°C

• High thermal conductivity 138 W/m K at 20°C

• Crystal structureBody centered cubicLattice constant a = 3.1470 Å

Molybdenite

Mullite

a bit of explanation of this!

Al2O3+SiO2

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. MoyaAdv. Mater. 2000, 12, No. 4 294

insulator metal

qco

pccer

ff

ff

)(

)(

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

Ebreak>1 kV/2 mm

Electrical permittivity

only few factors bigger than glass!

Summary of electrical properties

f(Mo) ρ[GΩ cm] εr(100 Hz) εr(1 MHz)

SPS

11% 23.3 39 32

12% 22.8 46 38

13% 10.5 145 100

HotPress

11% 19.8 25 -

13% 6.08 55 -

two different sintering methods have been tried (SPS and HotPress)

Stability with transported charge over CBM life-time

22 /2)/5.1(205.05/ cmCgappCkHz/cmyAQ

)/20/(8 2cmmCG

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 cm2 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

<Φ>=1200 Hz/cm2

<Φ>=580 Hz/cm2

by cutting the first 2 s of the spill the effect disappears.

measured rate in C@1GeV reactions (2003) at GSI-SIS (~8s time spill)DC limit

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! (σT)

Deviations from the DC model. The local fluctuations of the field

response to secondary particles from C@1GeV 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

N

qrms

EE

rmsq

gapo

Egap

2

1 2

2

AN (Average number of shots contributing per cell of area A)

2

22

2

222 1

2 q

rmsq

Ag

drms q

Egap

Campbel theorem for shot-noise

Quantitative description of the local fluctuations of the field (I)

222

2

22

gap

EgapoT E

rms

dE

dS

S

E

N

t

S

Krms gap

(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 mm2

D. Gonzalez-Diaz et al., Nucl. Phys. B (Proc. Suppl) 158(2006)111

T scan

HV scan

rate Φ(x) charge qp(x)

1 2

3 4

1 2

3

4

3 4

3

T=210C

T=210C

T=330C

3 4

3

T=210C

T=210C

T=330C

The DC model and the case of warm glass (III)

fit: DC model fit: DC model

Quantitative description of the stabilization time (I)

21 /2

/1

tt eAeAI

Measurement of the dielectric response function of float glass as the one used in HADES

Rate effects. Campbell theorem (analytical vs simulation)

2

22

222 1

2 q

rmsq

A

drms q

gVgap

Campbel theorem

+ )( thgap VVaq Campbel theorem with average drop (2)

(1)

Rate effects. Stabilization time (comparison with data)

The value of Vgap(t) from M.C. and the parameterization of to can be used for describing to as a function of the time within the spill

)(3

1DCrr 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