Effect of the ohmic drop in a RPC-like chamber for measurements of electron transport parameters

Anna Raquel Petri

doi:10.15392/bjrs.v7i2a.637

Abstract

The main advantage of Resistive Plate Chambers (RPCs), applied, for instance, in High-Energy Experiments and Positron Emission Tomography (PET), is that it is spark-protected due to the presence of, at least, one high-resistive electrode. However, the ohmic drop across the latter can affect the charge multiplication significantly. In this work, we investigate this effect in a RPC-like chamber. The counter was filled with nitrogen at atmospheric pressure and the primary ionization was produced by the incidence of nitrogen pulsed laser beam on an aluminum cathode. The illumination area of the cathode was measured using a foil of millimetric paper overlaid on this electrode. In this way, the resistance of the glass anode could be estimated using the known resistivity of the glass (ρ=2×1012 Ω.cm). Therefore, the voltage drop across the dielectric was calculated by the product of the current across the gas gap and the anode resistance. In order to mitigate the effect of the resistive electrode, the laser beam intensity was limited by interposing metallic meshes between the laser and the chamber window. The dependence of the ohmic drop from the applied voltage was analyzed. The results obtained shown that, without the meshes, the ohmic drop corresponds up to 7% of the applied voltage, preventing the detection system to reach values of density-normalized electric fields in the gas gap (Eeff/N) higher than 166 Td. By minimizing the laser beam intensity and, consequently, the primary ionization, the ohmic drop represented only 0.2% of the applied voltage, extending the Eeff /N range up to 175 Td.

Highlights

INTRODUCTIONThe successful application of Resistive Plate Chambers (RPCs) in High Energy Experiments, as both trigger and time-of-flight detectors, is due to the good time resolution, down to 50 ps [1, 2], coupled to their relatively low cost and simplicity of operation, which make these detectors suitable for covering large areas [3,4,5]
The successful application of Resistive Plate Chambers (RPCs) in High Energy Experiments, as both trigger and time-of-flight detectors, is due to the good time resolution, down to 50 ps [1, 2], coupled to their relatively low cost and simplicity of operation, which make these detectors suitable for covering large areas [3,4,5]. These characteristics enable their use in Positron Emission Tomography (PET) with a performance comparable to scintillator detectors, but for a lower cost per channel [2, 6]
In order to minimize the influence of the ohmic drop across the dielectric, we investigate further this phenomenon in the setup for the measurement of the electron transport parameters in gases, employed previously in [10], aiming to extend the range of the electric field so far covered by restricting the laser beam intensity

Summary

INTRODUCTION

The successful application of Resistive Plate Chambers (RPCs) in High Energy Experiments, as both trigger and time-of-flight detectors, is due to the good time resolution, down to 50 ps [1, 2], coupled to their relatively low cost and simplicity of operation, which make these detectors suitable for covering large areas [3,4,5]. The difference in their behaviours was well explained by the ohmic drop across the resistive electrode in the avalanche mode [8] In another previous work [10], a RPC-like counter was adopted, where the primary ionization was produced by the incidence of a pulsed nitrogen laser beam on an aluminium cathode and the anode was made of a high-resistivity glass. This setup was developed for determining electron transport parameters in gases which have been scarcely studied in terms of these quantities, such as isobutane [10,11,12] and tissue-equivalent gas mixtures [13]. In order to minimize the influence of the ohmic drop across the dielectric, we investigate further this phenomenon in the setup for the measurement of the electron transport parameters in gases, employed previously in [10], aiming to extend the range of the electric field so far covered by restricting the laser beam intensity

EXPERIMENTAL SETUP

RESULTS AND DISCUSSIONS

CONCLUSION