E.P. de Lima

Fano factors of rare gases and their mixtures

Abstract Measurements of FW (Fano factor × mean energy to make an ion-pair) for several non metastable Penning gas mixtures were made using the proportional scintillation process. Taking for W the values that correspond to 5.3 MeV alpha particles, upper limits of F were determined. The following results were obtained: 100% Xe, FW = 3.247, F ⩽ 0.15 ± 0.03 100% Ar, FW = 10.536, F ⩽ 0.40 ± 0.03 Ar + 80% Xe, FW = 4.635, F ⩽ 0.21 ± 0.03 Ar + 5% Kr, FW = 9.397, F ⩽ 0.37 ± 0.06 Ar + 24% Xe, FW = 5.093, F ⩽ 0.23 ± 0.02 Ar + 20% Kr, FW = 3.038, F ⩽ 0.12 ± 0.03 Ar + 20% Xe, FW = 3.582, F ⩽ 0.16 ± 0.02 Ar + 79% Kr, FW = 3.113, F ⩽ 0.13 ± 0.02 Ar + 5% Xe, FW = 3.204, F ⩽ 0.14 ± 0.03 100% Kr, FW = 4.584, F ⩽ 0.19 ± 0.02

Fano factors of krypton-xenon mixtures

Abstract Measurements of FW (product of the Fano factor and the mean energy to make an ion pair) for several mixtures of krypton and xenon were made using the proportional scintillation method. The following results were obtained: Kr 100% FW = 5.472 F ⩽ 0.23±0.01 Kr + 1.3%Xe FW = 4.426 F ⩽ 0.19±0.02 Kr + 20%Xe FW = 4.659 F ⩽ 0.21±0.02 Kr + 40%Xe FW = 4.828 F ⩽ 0.22±0.01 Kr + 60%Xe FW = 4.587 F ⩽ 0.21±0.01 Kr + 95%Xe FW = 4.558 F ⩽ 0.21±0.01 Xe 100% FW = 3.244 F ⩽ 0.15±0.01

Self-Quenching Streamers in Mixtures with Photoionization Vapours

The basic properties of self-quenching streamers in a proportional counter with a thick anode wire are investigated. In particular, the influence of mixing benzene and triethylamine to argon-isobutane mixtures is studied regarding thresholds for streamer production, pulse shapes and dead time. There is a lowering of the high voltage threshold for streamer production associated with low concentrations of triethylamine and benzene. Higher concentrations of benzene increase that thre¿ shold. Triethylamine leads to narrower pulses. Stable operation is observed for a much broader range of high voltages than in the absence of photoionization vapours.

Fragments and radicals in gaseous detectors

Abstract The need for detectors able to provide fast and reliable information, particularly in view of applications for high counting rates, requires a better knowledge of the physics of the processes involved, in order to master the after-effects resulting from the emission of photons. In turn, the identification of radicals and fragments formed in the avalanche is important to understand and eventually overcome the problems related with the ageing of the present gaseous detectors and those arising in the substrata of the new-coming microstrip gas chambers. Emission spectra, in the region from 120 to 450 nm, of the mixtures of argon with quenching gases (methane, isobutane and carbon dioxide) in the proportional and self-quenching streamer modes are reported. Especial attention is given to the carbon lines (7.94, 7.48 and 6.42 eV). For each mixture several fragments and radicals are identified.

Dead Region Time Evolution and Afterpulses Development in SQS

The dead length, 6, for t > 20 ¿s is well approximated by ¿=¿0 exp(-t/¿) for mixtures of argon-isobutane with benzene or TEA. ¿0 varies in an approximate linear way with the streamer charge and the systematics of ¿ regarding the high voltage is also presented. Afterpulses are associated not only with wall effects but also with photoionization in the gas mixture itself. Interpretation of the data is attempted.

Transient behaviour and rate effects in resistive detectors

Abstract The dependence of the detection efficiency (or charge gain) of a resistive detector on the counting rate has been considered by several authors. The influence of the resistivity of the electrodes, the counter geometry and the gas mixture composition have also been studied. Most of those measurements assume a stationary regime. In the present work we report on studies of the transient behaviour of detectors with a resistive cathode for various counting rates, charge gains and detector geometries. We show that in some cases the time-decay curves can be fitted by a single exponential plus a constant term, while in others (higher charge gains or higher counting rates) a sum of two or even three exponentials plus the constant term is needed to fit the experimental data. A study of the electric properties of the dielectrics used is also presented and a comparison is made between these results and the data obtained under irradiation conditions.

The charge distribution of self-quenching streamers

Abstract The induced charge method is used to study the SQS development in a single wire multicathode cylindrical counter using a mixture of argon with 48% isobutane. The experimentally measured induced charges associated to the electronic motion (gate width 150 ns) are compared to the calculated ones using a point-like charge model and realistic nonuniform charge distribution. Implications from the point of view of detectors as well as for the study of corona discharges are referred to.

The Gain Divergence on the Transition to the Self-Quenching Streamers

Based on the observed gain divergence characteris¿ tics occuring in a gaseous detector at the transition fran the proportional to the self-quenching streamers discharge (SQS), when adquate gas fillings are used and on the role played by photoionization, a mechanism is proposed to interpret quantitatively the process. The relevant intrinsic parameters, which determine the mechanism, are the number of photons emitted by electron per cm path near the anode, W, the mean free path of VUV radiation, L, the quantum efficiency, ¿, geometrical factors, g, which are interrelated. Fran the experimental data some of those parameters are computed for the mixtures argon-isobutane and argon-isobutane-triethylamine.

Photon breeding of self-quenching streamers (SQS)

A quantitative formalism for the photon breeding of the SQS is proposed and the number of breeding photons per electron is determined for argon-isobutane mixtures for concentrations of the quencher from 32% up to 60%. This number (probably in the region hv < 14 eV) is about 5 × 10−2.

Light emission associated with self-quenching streamers (SQS)

A general formalism associated with the light emission in the SQS mode is proposed and the number of photons per electron is experimentally determined for several argon-isobutane mixtures. This number is about 10−4 for 350 500 nm it varies from 5 × 10−3 to 1.5 × 10−3 when the quencher concentration goes from 20% to 60%.