Argon-isobutane Mixtures Research Articles

New Developments in Micromegas Microbulk Detectors

A new Micromegas manufacturing technique, based on kapton etching technology, has been recently developed, improving the uniformity and stability of this kind of readouts. Excellent energy resolutions have been obtained, reaching 11% FWHM for the 5.9 keV photon peak of 55Fe source and 1.8% FWHM for the 5.5 MeV alpha peak of the 241Am source. The new detector has other advantages like its flexible structure, low material and high radio-purity. The two actual approaches of this technique will be described and the features of these readouts in argon-isobutane mixtures will be presented. Moreover, the low material present in the amplification gap makes these detectors approximate the Rose and Korff model for the avalanche amplification, which will be discussed for the same type of mixtures. Finally, we will present several applications of the microbulk technique.

Micromegas prototypes with thermo-bond film separators

In this paper we report the results of Micromegas prototypes constructed by attaching micromesh to an anode using thermo-bond films. The excellent metal attaching ability and good dielectical property of this kind of film make it a promising material to be used as avalanche gap spacers. Several prototypes are successfully made. The electron transmission properties are first studied and then the gas gain is measured in argon-isobutane mixtures. The maximum gain of more than 104 is easily obtained. The energy resolutions for 55Fe 5.9 keV Kα ray can be better than 20% over one magnitude in gain for different operational gas mixtures and the best energy resolution of 13.7% (FWHM) can be achieved with the gas mixture of 94% argon concentration. The preliminary test results of the prototypes with sensitive area of 45 mm×45 mm without internal support show good uniformity across the sensitive area.

Development of Micromegas for neutrinoless double beta decay searches

The present paper is a summary of experimental tests performed with microbulk Micromegas for application in a neutrinoless double beta decay experiment based in a xenon TPC. No critical limitation related with high pressure has been detected for Micromegas, which has been tested up to 10 bar. No significant differences have been found between low energy X-rays and high energy alpha tracks regarding ionization yield and electronic transparency of Micromegas mesh. Using 5.5 MeV alphas in argon-isobutane mixtures, values as low as 1.8% FWHM have been obtained, with possible evidence that better resolutions are achievable. These results seems independent of pressure (probed up to 4.5 bar) or gas mixture (microbulks have run in xenon without quencher). The imperative necessity of high quality gas led to start on a new TPC and MPGD dedicated lab at Zaragoza.

Townsend ionization coefficients of some argon-based mixtures in strong nonuniform electric fields

The reduced Townsend ionization coefficient α/P is derived from the measurement of the mean gas amplification factor in a low-pressure (P ⩽ 50 kPa) proportional counter filled with argon–propane and argon–(dimethyl-ether) mixtures of various concentrations. The results are compared with our previous α/P values for argon–isobutane mixtures [I. Krajcar Bronić and B. Grosswendt, Nucl. Instrum. Methods Phys. Res. B 142, 219 (1998)]. The range of the reduced electric field strengths was 7×104 V m−1 kPa−1⩽Sa⩽4×106 V m−1 kPa−1. In such strong electric fields electrons do not reach the equilibrium with the electric field and the reduced gas gain depends on the gas pressure. For the analysis of the experimental gas gain data we used the field gradient model of Townsend–Williams–Sara–Segur. We determined the pressure dependent coefficients A* and B* of the Townsend ionization coefficient α/P=A*×exp(−B*/Sa). We found that the coefficients increase (i) if the admixture concentration increases at constant pressure, and (ii) if the pressure decreases at constant admixture concentration. In both cases the coefficients increase with the increase of Sa. The effective ionization potential of the mixtures, Vi*=B*/A*, at low admixture concentrations reaches the ionization potential I of the admixture as a consequence of the transfer of energy from the excited metastable states of argon to ionization of the admixture through the Penning effect. When α/P vs Sa curves for mixtures of different concentrations at constant total pressure are compared, crossings of the α/P curves at a certain range of Sa values are observed in all kinds of mixtures and for all pressures. We show the necessity of the crossings by taking into account the mathematical properties of the function describing the ionization coefficient.

Comparative study of gas amplification and energy resolution in some argon-based mixtures

A mean gas amplification factor M in mixtures of argon with propane and DME at various pressures ⩽50 kPa is studied. The deviations from the linear dependence of gas gain on high voltage in a semi-log plot are compared with those observed earlier in argon–isobutane mixtures. In all three kinds of mixtures, the magnitude M 0 of the gas gain where deviations start depends on the partial pressure of the admixture. Energy resolution at 5.9 keV has been measured in pure gases and in argon-based mixtures as a function of the gas gain. The best resolution has been obtained at low gas gains, ∼100–200. At these gains better resolution is obtained for mixtures with lower partial pressure of the admixture. At high gas gains energy resolution depends strongly on the admixture partial pressure, and above M 0 it degrades much faster due to avalanche chain formation.

Performance of resistive plate counters with freonless gas mixtures for measurements of cosmic ray muons

Various operational features of bakelite Resistive Plate Counters with a bulk resistivity of 10 12 Ω cm are studied with different argon, methane and isobutane mixtures, but avoiding freon as counter gas contribution. The efficiency and the time resolution of the detector as well as the cross-talk behavior of neighbored read-out pads are investigated for different voltages by use of cosmic ray muons. The signals of the detector are classified by number and shape of the afterpulses. Relative abundances of these classes are measured and the timely appearance of the afterpulses is investigated. An efficiency of more than 90% is found for argon–isobutane mixtures and less than 27% for argon–methane mixtures. The time resolution differs between 1.27 and 4.19 ns, depending on the kind of quenching gas. A strong asymmetry of the time resolution spectra of the various gas mixtures has been observed. The shape of the signals depends on the used quenching gas.

Gas amplification and ionization coefficients in isobutane and argon–isobutane mixtures at low gas pressures

Measurements of the mean gas amplification factor in isobutane and in argon–isobutane mixtures are performed at total gas pressures of 10, 20, 30, 60 and 90 kPa over a range of reduced electric field strength 5 × 10 4 < S a < 1.4 × 10 6 V m −1 kPa −1. At low partial pressure of isobutane deviations from an exponential growth of gas gain with applied high voltage are observed. The gas gain at which this over-exponential effect occurs, increases as the isobutane partial pressure increases. The over-exponential growth of gas gain is explained by the contribution of secondary avalanches started by photons due to non-efficient quenching. The number of additional electrons produced per electron in an avalanche depends on the isobutane partial pressure. The dependence of the reduced gas gain on the reduced electric field strength in isobutane indicates that electrons are not in equilibrium with the electric field for S a values above about 3 × 10 5 V m −1 kPa −1. In mixtures, the S a value at which the non-equilibrium effect is observed decreases with decreasing percentage of isobutane in the mixture for the same total pressure. The gas gain data were fitted by an analytical model that takes into account non-equilibrium effects. A reduced first Townsend ionization coefficient of the form α/P=A ∗ exp(−B ∗/S a ) was derived. The fitting parameters A * and B * increase when: (i) the pure isobutane pressure decreases, (ii) the isobutane concentration in the mixtures increases at constant total pressure, and when (iii) the total pressure decreases at constant isobutane concentration. The α/ P for isobutane is compared with other experimental and calculated values which are all obtained for equilibrium conditions. An interesting behavior of the ionization coefficient curves for argon–isobutane mixtures is found: at low S a the α/ P increases when the isobutane concentration decreases, while at higher S a the opposite behavior is observed. Both the reduced electric field strength and the ionization coefficient where the multiplication starts ( M=2) increase with the decrease of the total pressure, as well as with the increase of isobutane concentration in the mixture at constant total pressure.

Ionization yield formation in argon-isobutane mixtures as measured by a proportional-counter method

By using the proportional-counter method, mean ionization yields produced by 5.9 keV photons in argon-isobutane mixtures were measured as a function of the mixture composition, total gas pressure, and gas gains between about 10 3 and 2 × 10 4. It was found that the yield for a given mixture is gain dependent: an initial increase of the yield at low gas gains is followed by almost constant yield values for gains between about 2 × 10 3 and 8 × 10 3; at still higher gains the yield decreases. Moreover it was observed that the ionization yield at constant gas gain depends on the total pressure of the gas mixture. This dependence leads to a rather clear correlation between yields and isobutane partial pressure. To explain the measurements a simple model of the mean energy W per ion-pair formed in Penning gas mixtures is also given. A direct comparison of calculated W values and measured data showed that the results of our proportional-counter experiments performed at high gas gains were strongly influenced by secondary effects induced during the formation of avalanches in the counter when using low isobutane partial pressures. Therefore, Penning mixtures with incomplete quenching should not be used as a counting gas in proportional counters if both high gas gain and good energy resolution are required, as for instance, in high-resolution low-energy (sub-keV) X-ray spectroscopy.

A Study of Argon-Isobutane Mixtures in a Proportional Counter: Gas Amplification, W Value, and Energy Resolution

The mean gas amplification factor, the single electron spectra, the mean energy required to form an ion pair (W), and the energy resolution were measured in argon-isobutane mixtures at various pressures over a wide range of the reduced electric field strengths in a cylindrical proportional counter. In mixtures with low isobutane partial pressure, gas gain is an over-exponential function of the applied high voltage. The relative variance of single electron spectra, the apparent W value and the energy resolution are functions of both gas gain and isobutane partial pressure in the mixture. A better energy resolution is obtained with mixtures containing less isobutance at low gas gains whereas at the higher gas gains usually applied in X ray spectroscopy the resolution is degraded.

A Study of Argon-Isobutane Mixtures in a Proportional Counter: Gas Amplification, W Value, and Energy Resolution

A Study of Argon-Isobutane Mixtures in a Proportional Counter: Gas Amplification, W Value, and Energy Resolution

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 < λ < 475 nm, and 2.5 × 10 −4 for the photons probably responsible for the afterpulses. For λ > 500 nm it varies from 5 × 10 −3 to 1.5 × 10 −3 when the quencher concentration goes from 20% to 60%.

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.

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.

Application of classical theory of electrons in gases to drift proportional chambers

We show that it is possible to understand both qualitatively and quantitatively, in the framework of the classical theory of electrons in gases, the behavior of drift velocities in gases commonly used in proportional drift chambers. We study in detail argon-isobutane mixtures and comment especially on the contribution of diffusion to the spatial resolution and the modification of drift velocities in magnetic fields.