JINST Instrumentation Theses Archive

Ph.D. degree thesis
accepted by Universidade de Santiago de Compostela, Spain, in 2006

Diego Gonzalez Diaz

Supervisors: Juan Antonio Garzon Heydt and Paulo Jorge Ribeiro da Fonte

Research and developments on timing RPCs. Application to the ESTRELA detector of the HADES experiment at GSI.

 Keywords:

• Instrumentation and methods for time-of-flight (TOF) spectroscopy
• Timing detectors
• Gaseous detectors
• Detector modelling and simulations II (electric fields, charge transport, multiplication and induction, pulse formation, electron emission, etc)

 Abstract:
A novel design based on shielded tRPCs was developed for covering the inner region of the High Acceptance Di-Electron Spectrometer (HADES), placed at GSI-SIS (Darmstadt). The foreseen tRPC wall covers 8 square meters, corresponding to a polar angle region between 18 and 45 degrees. Due to the presence of two layers, the design keeps the geometrical losses at the level of 1%, providing also the required redundancy for the self-calibration of the device. Measurements made on a realistic prototype for Carbon collisions at 1 GeV/A showed cross-talk levels below 1%, mainly triggered by streamers. The concept showed time and space resolutions at the level of 80 ps and 6 mm times 6-15 mm (sigma) respectively, with efficiencies above 95%, and high homogeneity. The very low levels of cross-talk guarantee good performances in a multi-hit environment.

In order to further extend the rate capability of the standard tRPC technology based on float glass, the moderate warming of the detector was studied. Systematic measurements were performed under gamma illumination with a Na source allowing to prove, consistently, that the rate capability of the device can be increased in one order of magnitude by just a modest increase of the temperature in 25 degrees. The behavior of the avalanche gain, efficiency and the maximum of the time response are well described within a simple DC model. This fact allows to relate the observed increase in the rate capability with the measured decrease in the resistivity of the glass (that was measured independently). At operating temperatures above 55 degrees the detector performances become unstable.

The prospects for high rate applications have been explored by extending (theoretically) the DC model to include the local fluctuations of the field that arise in a dynamical situation. Comparison with data indicates a minor role of the fluctuations of the field and relates it to a fundamental quantity (the influenced area per avalanche) that ultimately rules this phenomena. In the particular case of glass tRPCs, a lower limit for the influenced area of 0.3 squared mm was obtained. The effect of the charging time of the plate is also discussed quantitatively in the context of a spill duration smaller than the relaxation time of the material.

Finally, different gas mixtures were explored and their timing properties evaluated, suggesting that a gas based on pure iso-butane or with a high presence of SF6 results in a slower mixture.