Argon-based Mixtures Research Articles

Charging up studies in thick Gas Electron Multipliers

Gaseous ionization detectors that have insulating media exposed to the active gas volume have issues related to charging up of the insulators during the course of its use[1]. The time-dependent variation of detector response in hole-based Micro-Pattern Gaseous Detectors (MPGDs), especially THick Gas Electron Multipliers (THGEMs), is one of the challenging problems that has been attributed to “charging up” and “charging down”. Experimental studies of stabilization of THGEM gas gain with time in argon-based mixtures under various experimental conditions will be presented here. Effects of different sources with varying irradiation rates on the gain saturation process have been studied. It has been observed that low-rate source shows two-step gain stabilization phenomena, one short-term saturated gain, another long-term saturated gain, whereas high-rate source shows just one-step gain saturation. While this two-step stabilization has been attributed to the charging up of the rim by earlier studies, its effect seems to be subdued for high-rate irradiation according to the present studies. The results provide an insight into the dynamics of gain saturation in THGEMs.

New shielding gas mixture for laser conduction welding of aluminum with a filler wire

It is common to use argon as the shielding gas for laser conduction welding of thin aluminum parts with a filler wire in the automotive industry. However, defects such as skips (i.e., discontinuities), holes in the weld, or a rough weld surface are commonly found when using argon as a shielding gas due to poor wettability. A new shielding gas mixture was developed in this study. With the addition of small amounts of active gases to an argon-based mixture, the wettability of the weld pool was significantly improved, yielding decreases in defectivity, improved penetration of the weldment, as well as a more stable welding process. The developed shielding gas mixtures could improve weld quality when compared to pure argon in laser wire conduction welding of different types of aluminum alloys in coach joint configuration. Energy dispersive spectroscopy (EDS) and wavelength dispersive spectroscopy (WDS) were used to detect the nitrogen and oxygen concentrations in the weld. The results show there is no noticeable increase in the oxygen or nitrogen concentration in the weld zone compared to the base metal when using the developed gas mixtures as shielding gas. Rising displacement testing was performed, it is shown that the weld strength obtained with the developed shielding gas mixtures is no less than that obtained with argon.

Investigation of UV laser ionisation in argon-based gas mixtures with a Triple-GEM detector

In the gas detectors, the ionisation mechanisms of the charged particles and X-ray/γ-photons are well studied by theory and experiment. However, on the experimental measurements of the ionisation ability of the lasers, publications only give the estimation results. This study aims to fill the gap and measures the laser ionisation precisely. A GEM detector and an optical system have been set up to measure the laser ionisation. The purity of each component of the gas mixtures is indicated in this study. Our research shows that a pulsed laser with a wavelength of 266 nm can generate 100–200 electrons per centimeter, which equals to the ionisation produced by 1–2 Minimum Ionising Particles (MIPs) in argon-based mixtures. Furthermore, this study gives an experimental reference to select the laser energy density in argon-based gas mixtures when applying lasers to calibration in the gaseous detectors. We have also studied the impacts of different vendors and gas flow rates on the ionisation created by UV laser pulses.

Gas mixture studies for streamer operated Resistive Plate Chambers

Resistive Plate Chambers operated in streamer mode are interesting detectors in neutrino and astro-particle physics applications (like OPERA and ARGO experiments). Such experiments are typically characterized by large area apparatuses with no stringent requirements on detector aging and rate capabilities. In this paper, results of cosmic ray tests performed on a RPC prototype using different gas mixtures are presented, the principal aim being the optimization of the TetraFluoroPropene concentration in Argon-based mixtures. The introduction of TetraFluoroPropene, besides its low Global Warming Power, is helpful because it simplifies safety requirements allowing to remove also isobutane from the mixture. Results obtained with mixtures containing SF6, CF4, CO2, N2 and He are also shown, presented both in terms of detectors properties (efficiency, multiple-streamer probability and time resolution) and in terms of streamer characteristics.

The breakdown voltage characteristics, the effective secondary emission coefficient and the ionization coefficient of the argon-based mixtures

In this paper, the direct current gas breakdown voltage characteristics in mixtures of argon with molecular gases were studied experimentally in a discharge tube with coplanar metallic electrodes separated 100μm. The obtained breakdown voltage curves exhibit U-shaped form and at the right-hand side the curves (high values of pd product) show increase with admixture of a small amount of H2 or N2. Based on the measured breakdown voltage curves the effective secondary emission coefficient γ for argon-based mixtures have been estimated. In addition, Two Term Approximation has been used to calculate the ionization coefficients for pure gases and the mixtures. It was shown that addition of a small amount of molecular gases affects more the values of the secondary electron yield than the ionization coefficients.

Performance of Pixel-Readout Micro-Pixel Chamber with Analog-Readout System Used as X-ray Polarimeter

We developed an analog-readout system for a pixel-readout micro-pixel chamber (µ-PIC) to be used as an astronomical X-ray polarimeter, and demonstrated that the sensitivity of the new system reached up to that predicted by a simulation. A pixel-readout µ-PIC is a micro-pattern gaseous detector with a fine position resolution and good stability at sufficient gain operation, and is suited for astronomical X-ray polarimetry. However, as shown by Katagiri et al. (2007), the sensitivity to X-ray polarization was found to be statistically lower by a significant amount than that expected from the simulation of Ueno et al. [Nucl. Instrum. Methods Phys. Res., Sect. A 525 (2004) 28] because of the readout system and background produced by the scattering of the X-ray beam in air. We therefore developed a new readout system and carried out a beam test with aluminum tubes that reduced the background. As a result, we demonstrated that for collimated beams, the modulation factors, which are indicators of the sensitivity to X-ray polarization, were 0.24±0.08 at 8 keV and 0.18±0.07 at 15 keV in a neon-based gas mixture, and 0.18±0.04 at 15 keV in an argon-based mixture. These values are consistent with those predicted by the simulation within errors.

Development of Pixel-Readout Micro-Pixel Chamber for X-ray Polarimetry

We developed a pixel-readout micro-pixel chamber (µ-PIC) as an astronomical X-ray polarimeter and demonstrated its ability to achieve a higher sensitivity than a strip-readout one by a beam test. The µ-PIC is one of the micropattern gas detectors that have been developed so far. However, its readout was of the strip type. This limits the sensitivity to polarization because it cannot determine the polarization angle uniquely without rotating the detector. Thus, we started developing the pixel-readout type of µ-PIC. A beam test at a synchrotron-radiation facility was carried out to examine the performance for polarimetry. Without rotating the detector, we successfully detected the polarization angle. The modulation factor for 8 keV X-rays in a neon-based mixture was 0.12±0.03, which is at the same level as that of the strip-type µ-PIC, and 0.07±0.01 at 15 keV in an argon-based mixture. These results indicate the ability of the new µ-PIC to achieve a higher sensitivity to X-ray polarization than that obtainable with the strip-type µ-PIC.

Application of micro-pixel chambers for X-ray polarimetry

We measured the X-ray polarization using our micro-pixel chamber with a strip readout, while changing the X-ray energy (5– 18 keV ) and gas mixtures (neon- and argon-based). As a result, modulation factors of 0.18 (at 9 keV ) and 0.12 (at 14 keV ) were obtained for neon- and argon-based mixtures, respectively. We also simulated this experiment utilizing a Monte-Carlo code, EGS4, which reproduced the experimental results quite well. According to the Monte-Carlo simulation, the modulation factor should be improved by a pixel-readout detector. The simulated modulation factors are 0.25 (for X-ray with energy at >6 keV ) with a neon-based gas mixture and 0.18 (for X-ray at >10 keV ) with an argon-based one.

Investigation into nitrogen–inert gas interactions in d.c. diode glow discharges

Neon–nitrogen and argon–nitrogen d.c. diode discharges have been investigated using spatially resolved optical emission spectroscopy (OES) to enable sampling inside the cathode sheath and plasma regions. The discharges were operated at −2 kV cathode bias and at 6.0 Pa total pressure with cathode current density and sheath thickness measurements evaluated for all gas mixtures. The results show that a greater proportion of ions reaching the cathode are nitrogen when neon is used as the primary gas instead of argon. Penning ionisation of nitrogen by neon metastables is the suggested mechanism and the effect is maximised at low nitrogen partial pressures. For 10–50% nitrogen in neon, the current density is higher than that achieved from neon or nitrogen alone or the equivalent argon-based mixtures. Moreover, information from cathode sheath thickness studies suggest that the dominant ionic species reaching the cathode is N + rather than N 2 + or Ne + at low nitrogen partial pressures. From OES studies, the production of nitrogen atomic species is attributed to N 2 +–N 2 0 dissociative charge exchange collisions in the sheath and since this mechanism depends on the availability of N 2 +, the effect is significant in argon-based mixtures only at high nitrogen partial pressures. The implications for plasma processing are also discussed.

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 a Resistive Plate Chamber operated in avalanche mode under 137Cs irradiation

A 2 mm gas gap Resistive Plate Chamber with bakelite plates has been operated in avalanche mode with a 137Cs source. We report measurements of the efficiency for a cosmic ray detection and the charge developed in the gap, performed for two different gas mixtures: the first containing a high percentage of environmental friendly “freon” C 2H 2F 4; the second one having a high percentage of argon. We show that the freon-based mixture exhibits a wide full-efficiency plateau with low streamer probability, while the argon-based mixture allows a reduction in the detector power consumption.

Ａｒ‐Ｏ２，Ａｒ‐ＣＯ２およびＣＯ２‐Ｏ２雰囲気における鋼溶接金属の酸素吸収について ガス・メタル・アーク溶接における鋼溶接金属の酸素吸収について 第１報

In an oxidizing gas welding atmosphere of Ar-O2, Ar-CO2 or CO2-O2 gas mixture, oxygen absorption behavior by steel weld metal has been investigated metallurgically, using a popular Fe-Si-Mn electrode wire and two types of base metal, that is, pure iron and rolled steel for welded structure. As an addition of oxygen to argon-based mixture, the oxygen content in steel weld metal extremely increases and the losses of alloying elements such as silicon and manganese steadily increase. As an addition of carbon-dioxide to argon-based mixture, the oxygen content in steel weld metal increases somewhat and limited amount of alloying elements losses. As an addition of oxygen to carbon-dioxide-based mixture, only a little amount of oxygen is absorbed into the steel weld metal. Despite the active gas of CO2 instead of Ar adds into the welding atmosphere, unexpectedly, the oxygen content of steel weld metal under CO2-O2 is lower than that of the weld metal under Ar-O2 for the same oxygen partial pressure over the range of 0.01 MPa. The losses of alloying elements are not so significant in this atmosphere.

Ａｒ‐Ｏ２，Ａｒ‐ＣＯ２およびＣＯ２‐Ｏ２雰囲気における鋼溶接金属の機械的性質について ガス・メタル・アーク溶接における鋼溶接金属の機械的性質について 第１報

In an oxidizing gas welding atmosphere of Ar-O2, Ar-CO2 or CO2-O2 gas mixture, mechanical properties of steel weld metal such as ultimate tensile strength and Charpy impact property have been investigated, using a popular Fe-Si-Mn electrode wire (YGW 12) and rolled steel for welded structure (SM 400B). As an addition of oxygen to argon-based mixture, both the tensile strength and toughness tend to decrease. As an addition of carbon-dioxide to argon-based mixture, the tensile strength decreases and then improves, and impact property reveals complex behavior. As an addition of oxygen to carbon-dioxide-based mixture, changes in atmospheric gas composition have a minor effect on the mechanical properties of steel weld metal.