(Santiago de Compostela-Spain) High rate capability of ordinary glass RPCs Diego González Díaz (Santiago de Compostela-Spain) A. Blanco, P. Fonte, L. Lopes, R. Ferreira-Marques (LIP-Coimbra)

Detector used for tests Same detector layout than in HADES tests (April-2003). Gas mixture: 80% C2H2F4 + 20% SF6 . Normal window glass, 2 mm thick. 4-gaps, 0.3 mm thick. Aluminum electrodes, 2 mm thick. Shielding boxes. BGA2712 amplifier. Reference BICRON scintillator (110 ps for gammas). -Este es nuestro prototipo -Definir RPC, 4 gaps, electrodos de aluminio -Mezcla de gas

Sources used for tests Na22 (~ 0.1 mCi) (mainly used for calibration) Na22 (~ 1 mCi) Co60 (~ 1 mCi) 2 x 0.511 MeV collinear (positron annihilation) 1.274 MeV 1.173 MeV, 1.332 MeV (opposite directions but not perfectly collinear) Interaction occurs through Compton scattering in the electrodes. The electron scattered ionizes the gas and is detected. Charge distribution and time resolution are different from MIPS (!)

Temperature monitoring Heating system Temperature monitoring T1 T2 T3 RPC box is rolled in a coating that provides uniform heating Temperature is measured on-line through the voltage drop on PT100 sensors placed in front, behind and in the middle of RPC cell.

Trigger system used for tests Efficiency trigger Time resolution trigger self-trigger mode Coincidence with a 110 ps scintillator RPC scintillator RPC source source L Due to the high rate environment, coincidences by chance are dominant and must be suppressed with adecuate cuts. Cauchy distribution

Temperature dependence At constant Temperature Gain (<Vgap>) Efficiency ε(<Vgap>) Time resolution σT (<Vgap>) At non-constant Temperature - The gas density decreases linearly with T. - The glass resistivity decreases exponentially with T. Gain (<Vgap>*) Efficiency ε(<Vgap>*) Time resolution σT (<Vgap>*)

Temperature dependence Rate capability <q> is in principle a function of <Vgap>* only: If ΔV T/To is kept constant during measurements:

Temperature dependence As a first approach, if the reduced high voltage V* is kept constant ‘Rate capability’: If the rate capability κ(T) increases a factor n, the rate can be increased by the same factor and the performance of the detector remains unaltered.

+ + Temperature dependence: A model for the efficiency Average potential in the gap as a function of the corrent drawn through the glass Good aproximation if charge-space effect is high + Observed charge spectrum for γ primaries + d is an overall constant

Efficiency measurements Na22 Na22 + Co60 (2.5 cm further)

Efficiency measurements X12 Increasing T 33 degrees

Time resolution measurements 1-Tof window of 1 ns (cut random coincidences). 2-Events of good quality in reference scintillators (large amount of light collected). 3-Cut in the peak of the position distribution. T=21 deg Rate at center= 1.8kHz/cm2 After cuts 1 & 2

Time resolution measurements Time resolution for gammas stands within 90 ps without significative dependence on primary rate. So it is hard to get the κ(T) factor from timing measurements. No significative dependence [!]

Ageing T increase of 30 degrees

Geometric acceptance ≈ 87 % Conclusions (I) Results of april-03 at HADES Geometric acceptance ≈ 87 % HADES

Conclusions (II) When the Temperature increases?? Rate capability For 30 degrees increase no ageing effects observed in 80 days! measured expected Under study!

Geometric acceptance ≈ 87 % Conclusions (III) Geometric acceptance ≈ 87 % for κ(T) measured in 30 degrees increase