CF4 Gas Mixture Research Articles

Study of long term stability of a 50 liters TPC, based on TRIPLE-GEM with optical readout, for the CYGNO experiment

The CYGNO project aims to study rare events, as low-mass (few GeV) Dark Matter (DM) particle or solar neutrino interactions, exploiting the approach of the optical readout of the scintillation light produced in the amplification in a multiple Gas Electron Multiplier (GEM) structure, of the primary ionization originated in large volume Time Projection Chamber (TPCs).The volume is filled with an He:CF4 gas mixture at atmospheric pressure. The 3D topology, and therefore direction of the recoils, is reconstructed thanks to the combined use of high-granularity, high sensitivity sCMOS cameras, for the precise tracking of the projection of the recoils on the GEM plane, and of fast light sensors in order to obtain the coordinate perpendicular to the camera plane.To conclude the R&D phase, the 50 L prototype, called Long Imaging ModulE (LIME), was moved underground at the Laboratori Nazionali del Gran Sasso (LNGS) in order to study the performance of the CYGNO experimental approach in a low background environment and to assess the contributions to the background from different sources, also comparing with Monte Carlo simulations.Stability studies and the effects of environmental condition on the light yield will be discussed, focusing on the effect of humidity on the detector response and stability amount and rate of self-sustaining micro-discharges. This is a crucial step towards the development of a large demonstrator.

Tuning of the Effective Refractive Index of Crystalline Si Thin Films with Controlled Modification of Nanohole Dimensions By Dry-Etching

Silicon thin films with holes are composite materials with noteworthy optical properties that can be controllably fabricated by state-of-the-art Si process technologies. For light with wavelengths larger than the hole sizes and spacing, typically by a factor of 10, the perforated Si thin films behave like an effective (refractive index) medium. Under these conditions, the optical properties of the films can be modelled by the effective medium approximation (EMA). By manipulating the volumetric air fraction (holes) of such films, the refractive index can be modified accordingly. Such a property can be exploited to tune the index contrast in various photonic devices such as filters and waveguides.This work presents a novel scalable method that combines charged sphere colloidal lithography (CSCL) and dry-etching to pattern Si thin films. We demonstrate controlled tuning of the effective refractive index by controlled lateral dry-etching of the holes. The fabricated nanoholes are spatially disordered in a 220 nm thick crystalline Si film. CSCL is performed using polystyrene particles of 60 (P60) and 100 (P100) nm diameter. The CSCL step results in a disordered arrangement of the particles with average spatial separations of 2-4 times their diameter. Using Cr as a mask, cylindrical holes are etched into Si by inductively coupled plasma-reactive ion-etching (ICP-RIE) using a pseudo-Bosch process. An additional isotropic ICP-RIE dry-etch step is introduced to widen the holes in a controlled manner, utilizing CF4 and O2 gas mixture. Importantly, this hole-widening step is time-controlled to ensure that the holes do not overlap laterally. Finally, the Cr mask is removed by wet-etching.By analyzing scanning electron microscopic (SEM) images, we estimate the lateral etch rates for the P60 and P100 samples are approximately 0.76 nm/s and 0.52 nm/s, respectively. Using this process, the average hole diameter can be increased systematically up to circa 68-97%, compared to their original diameters, while avoiding overlap. The effective refractive index of the perforated Si thin films is determined using the optical air fill factor obtained from modelling the spectroscopic ellipsometry data with Bruggeman effective medium approximation. The determined optical air fill factor varies from circa 20% to 50% with hole widening. The ellipsometry data provides the true air fill factor of the samples directly; SEM analysis could also be used, however, it is complex and cannot determine the actual porosity of the films. The ellipsometry results show that the determined refractive index is constant in the 1500 - 4000 nm wavelength range. The refractive index decreases from 3.42 for unstructured Si film to 2.9 and 2.7 for the as-etched perforated Si films of P60 and P100 samples, respectively. With controlled lateral etching of the holes, the effective refractive index systematically decreases further to approximately 2.1 for the P60 and P100 samples, which is a 39% reduction compared to Si refractive index. The determined effective indices represent averaged values accounting for possible structural birefringence, which can be used as a guideline to design photonic devices such as filters.The presented findings indicate that the effective refractive index of perforated Si thin films can be engineered by controlling the lateral dimensions of holes. While this conclusion relates specifically to crystalline Si thin films, the same concept may also be applied to other perforated dielectric thin films. Figure 1

The CYGNO experiment, a directional detector for direct Dark Matter searches

The CYGNO project aims at the development of a high precision optical readout gaseous Tima Projection Chamber (TPC) for directional dark matter (DM) searches, to be hosted at Laboratori Nazionali del Gran Sasso (LNGS). CYGNO employs a He:CF4 gas mixture at atmospheric pressure with a Gas Electron Multiplier (GEM) based amplification structure coupled to an optical readout comprised of sCMOS cameras and photomultiplier tubes (PMTs). This experimental setup allows to achieve 3D tracking and background rejection down to O(1) keV energy, to boost sensitivity to low WIMP masses. The characteristics of the optical readout approach in terms of the light yield will be illustrated along with the particle identification properties. The project timeline foresees, in the next 2–3 years, the realisation and installation of a 0.4 m3 TPC in the underground laboratories at LNGS to act as a demonstrator. Finally, the studies of the expected DM sensitivities of the CYGNO demonstrator will be presented.

CYGNO: a gaseous TPC with optical readout for dark matter directional search

The CYGNO project has the goal to use a gaseous TPC with optical readout to detect dark matter and solar neutrinos with low energy threshold and directionality. The CYGNO demonstrator will consist of 1 m3 volume filled with He:CF4 gas mixture at atmospheric pressure. Optical readout with high granularity CMOS sensors, combined with fast light detectors, will provide a detailed reconstruction of the event topology. This will allow to discriminate the nuclear recoil signal from the background, mainly represented by low energy electron recoils induced by radioactivity. Thanks to the high reconstruction efficiency, CYGNO will be sensitive to low mass dark matter, and will have the potential to overcome the neutrino floor, that ultimately limits non-directional dark matter searches.

Influence of microfabrication on superconducting properties of exfoliated thin films of layered superconductor NbSe2: reactive ion etching

We have experimentally investigated the influence of the reactive ion etching (RIE) on the superconducting properties of exfoliated thin films of layered superconductor NbSe2 through transport measurement. Wires with width of 2 μm and 0.5 μm were fabricated from a single NbSe2 film (thickness ∼ 65 nm) using RIE with CF4 and O2 gas mixture. While the 2-μm wires did not exhibit any distinguishable degradation in residual resistivity, residual resistance ratio, superconducting transition and current-voltage characteristics in the superconducting state, significant degradation was observed in the 0.5-μm wires.

Optimization of technological parameters in plasma chemical etching of quartz single crystals

Main technological parameters of the process of local plasmochemical etching of single-crystal quartz were optimized. The etching was performed in a gas mixture of CF4 and H2 under radio frequency (RF, 13.56 MHz) discharge excitation. The scientific experiment design by the Taguchi matrix method was used to examine the effect of chamber pressure, RF generator power, negative bias applied to the substrate holder, and hydrogen flow rate on the rate of the etching process. The experimental results made it possible to evaluate for the first time the influence exerted by the technological parameters on the etching rate. It was shown that the influence exerted by the technological parameters in the conditions under study decreases in the following order: pressure in the reaction chamber, bias potential, RF power, hydrogen flow rate.

Low material budget floating strip Micromegas for ion transmission radiography

Floating strip Micromegas are high-accuracy and discharge insensitive gaseous detectors, able to track single particles at fluxes of 7MHz/cm2 with 100μm resolution. We developed low-material-budget detectors with one-dimensional strip readout, suitable for tracking at highest particle rates as encountered in medical ion transmission radiography or inner tracker applications. Recently we additionally developed Kapton-based floating strip Micromegas with two-dimensional strip readout, featuring an overall thickness of 0.011 X0.These detectors were tested in high-rate proton and carbon-ion beams at the tandem accelerator in Garching and the Heidelberg Ion-Beam Therapy Center, operated with an optimized Ne:CF4 gas mixture. By coupling the Micromegas detectors to a new scintillator based range detector, ion transmission radiographies of PMMA and tissue-equivalent phantoms were acquired. The range detector with 18 layers is read out via wavelength shifting fibers, coupled to a multi-anode photomultiplier.We present the performance of the Micromegas detectors with respect to timing and single plane track reconstruction using the μTPC method. We discuss the range resolution of the scintillator range telescope and present the image reconstruction capabilities of the combined system.

Characterization of a GEM-based scintillation detector with He–CF4 gas mixture in clinical proton beams

Accurate, high-spatial resolution dosimetry in proton therapy is a time consuming task, and may be challenging in the case of small fields, due to the lack of adequate instrumentation. The purpose of this work is to develop a novel dose imaging detector with high spatial resolution and tissue equivalent response to dose in the Bragg peak, suitable for beam commissioning and quality assurance measurements.A scintillation gas electron multiplier (GEM) detector based on a double GEM amplification structure with optical readout was filled with a He/CF4 gas mixture and evaluated in pristine and modulated proton beams of several penetration ranges. The detector’s performance was characterized in terms of linearity in dose rate, spatial resolution, short- and long-term stability and tissue-equivalence of response at different energies. Depth-dose profiles measured with the GEM detector in the 115–205 MeV energy range were compared with the profiles measured under similar conditions using the PinPoint 3D small-volume ion chamber.The GEM detector filled with a He-based mixture has a nearly tissue equivalent response in the proton beam and may become an attractive and efficient tool for high-resolution 2D and 3D dose imaging in proton dosimetry, and especially in small-field applications.

Photoelectron extraction efficiency from a CsI photocathode and THGEM operation in He−CF4 and He−CH4 mixtures

This work presents the experimental measurements obtained for UV-induced photo-electron extraction efficiency from a CsI photocathode into He with CF4 and CH4 gas mixtures. A 1000Å CsI photocathode was deposited on a gold plated THGEM for photo-electron conversion. Charge-gain measurements were obtained with a Single-THGEM detector operating in these gas mixtures using a continuous UV lamp for the extraction of photo-electrons. Charge-gains in excess of 105 were obtained for gas mixtures containing percentages of quencher higher than 20% while photo-electron extraction efficiency achieved ∼ 50% for He/CF4 and ∼ 30% for He/CH4. Single photon electron measurements were also performed to assess the maximal gains reached in this regime. A discussion for future GPM cryogenic applications is presented.

InAs-based metal-oxide-semiconductor structure formation in low-energy Townsend discharge

We developed and applied a method of InAs passivation in the low-energy plasma of Townsend discharge. The controlled interface oxidation in the Ar:O2:CF4 gas mixture under visualization of gas discharge plasma allowed growing thin homogeneous films in the range of 5–15 nm thickness. Oxidation with the addition of CF4 in gas-discharge plasma led to the formation of In and As oxyfluorides with a wide insulating gap and isostructural interface with unpinned Fermi level behavior. The metal-oxide-semiconductor structure showed excellent capacitance-voltage characteristics: small frequency dispersion (<15 mV), density of interface states (Dit) in the gap below 5 × 1010 eV–1cm−2, and fixed charge (Qfix) below 5 × 1011 cm−2.

Microsecond pulse driven Ar/CF4 plasma jet for polymethylmethacrylate surface modification at atmospheric pressure

A plasma jet driven by microsecond pulse was used for polymethylmethacrylate (PMMA) surface modification. In the paper, the effect of different proportions of Ar and CF4 gas mixture on PMMA modification was studied. After plasma treatment, PMMA samples were immersed in water to accelerate the aging effect. The morphology and elemental composition of the material surface were investigated by atomic force microscopy and X-ray photoelectron spectroscopy. In addition, the mechanism for plasma surface modification has been analyzed. Results showed that plasma treatment introduced hydrophobic groups (i.e. C–Fn and C–F) into material surface and increased their surface roughness. In the initial stage of plasma treatment, the contact angle of PMMA decreased and showed a hydrophilic property. After water immersion with different time, the oxygen-containing groups (CO) were gradually displaced by C–Fn and C–F and showed a hydrophobic property. The increased roughness and fluoride groups on the PMMA surface were responsible for hydrophobic behavior of PMMA.

Low-loss silicon waveguides with sidewall roughness reduction using a SiO2 hard mask and fluorine-based dry etching

High optical field confinement-possible silicon waveguides are used in various fields. The performance of a silicon waveguide depends on its sidewall roughness, which is responsible for scattering loss. The fabrication process for a silicon waveguide with a smooth sidewall for low loss was studied. The propagation loss of an optical silicon waveguide was reduced by using a SiO2 hard mask and reactive ion etching (RIE) with fluorine gases. The sidewall angle was controlled by Si etching with a SF6 and CF4 gas mixture. The thickness of the SiO2 hard mask affects the sidewall smoothing treatment. The roughness of the sidewall was reduced in the SiO2 etching process with a CF4 and O2 gas mixture as the thickness of the SiO2 hard mask was increased. The minimum propagation loss (0.89 dB cm−1) of the fabricated silicon waveguide was achieved by using the fabrication method introduced in this paper.

Newly developed gaseous photomultiplier

A new micromesh gas (Micromegas) detector has been developed for a gaseous photomultiplier tube (PMT) with a bialkali photocathode. A basic performance test of the Micromegas detector was carried out for a Ne (90%) + CF4 (10%) gas mixture using an X-ray beam. We constructed gaseous PMTs with a bialkali photocathode and Micromegas detectors. The photoelectron collection efficiencies in several gases and the suppression of ion feedback were investigated.

Low-pressure plasma-etching of bulk polymer materials using gas mixture of CF4 and O2

In this study, we have proposed a low-pressure reactive ion etching of bulk polymer materials with a gas mixture of CF4 and O2, and have achieved precise fabrication of poly(methyl methacrylate) (PMMA) and perfluoroalkoxy (PFA) bulk polymer plates with high-aspect-ratio and narrow gap array structures, such as, pillar, frustum, or cone, on a nano/micro scale. The effects of the etching conditions on the shape and size of each pillar were evaluated by changing etching duration and the size/material of etching mask. The fabricated PMMA array structures indicate possibilities of optical waveguide and nanofiber array. PFA cone array structures showed super-hydrophobicity without any chemical treatments. Also, polystyrene-coated silica spheres were used as an etching mask for the pillar array structure formation to control the gap between pillars.

A study of Ar + C2H4 and Xe + CF4 gas mixtures

Mixtures of Ar + 40 ppm C2H4 and 50% Xe + 50%CF4 have been investigated. The spatial distributions of photoelectron clouds produced by primary scintillations on α- and β-particle tracks, as well as the distributions of photoelectron clouds due to photons from avalanches at the pin anode, have been measured for the first time. For a mixture of 50% Xe + 50%CF4, it has been shown for the first time that CF4 is a photosensitive dopant in a mixture with Xe. For a mixture of Xe + CF4 (1: 1), the maximum electron multiplication factors at the pin anode are K(β)max = 3 × 104 and K(α)max = 3 × 103 at a pressure of 1 atm (abs.) and K(β)max = 104 and K(α)max = 4 × 103 at a pressure of 10 atm (abs.)

Simulations and Experiments on O Density and Distribution in Ashing Process Using Surface Plasma Excited by Microwave

Simulation methods for the density and spatial distribution of O atoms have been developed to analyze high-density plasma (1011 cm-3) excited by two microwave sources. The density of O atoms, that react with photoresist, was calculated in a gas mixture of CF4 and O2, and the density has a maximum value at 10% CF4 partial pressure. For the distribution simulation, the rate of reaction between O atoms and photoresist was measured in a small cell, and the sticking coefficient was estimated to be 0.002. The O atom distribution on a glass substrate was calculated by the simulator, where the sticking coefficient was input, focusing on the density under a beam that connected the microwave sources and had no plasma source. The results of both the simulations are in good agreement with the experimental results. The simulations were applied to optimize the chamber configuration and process conditions. As a result, a high ashing rate of over 1430 nm with a uniformity of ±9.3% was obtained.

High precision measurements of carbon disulfide negative ion mobility and diffusion

High precision measurements were made of reduced mobility, lateral and longitudinal diffusion of CS2(-) ions in 40 Torr CS2 and a 30-10 Torr CS2-CF4 gas mixture. The reduced mobility was found to be 0.4645 ± 0.0006 cm(2) atm/s V in CS2 and 0.523 ± 0.001 cm(2) atm/s V in the CS2-CF4 gas mixture at STP. The lateral diffusion temperatures for these two gases (295 ± 15 K and 297 ± 6 K) were found to be in good agreement with room temperature. By contrast longitudinal diffusion temperature was found to be slightly elevated (319 ± 10 (stat) ± 8 (sys) K and 310 ± 20 (stat) ± 6 (sys) K) though given the errors, room temperature diffusion cannot be ruled out. For lateral diffusion, significant capture distances (0.21 ± 0.07 mm and 0.15 ± 0.03 mm) were measured while for longitudinal diffusion the results were not conclusive.

Using an 80% CF4+20% CO2 gas mixture to recover aged anode wires in proportional chambers

A technique to recover a gas proportional counter with an aged anode wire using a glow discharge in an 80%CF4+20%CO2 gas mixture, has been developed and tested. Studies of aging effects were carried out under sustained irradiation by an intense 90Sr β-source of straw proportional counters operated with a 60%Ar+30%CO2+10%CF4 gas mixture. Special attention was paid to the aging mechanism of the anode wires. Our experience showed that using a given gas mixture the swelling of the anode wires is a typical mode of aging that leads to degradation of the gas gain. The proposed method restores the gas gain and the signal amplitude in the damaged zone of the wire. SEM/XEM analysis confirmed that the tungsten oxide deposits have been removed.

Primary scintillation characteristics of Ar+CF4 gas mixtures excited by proton and alpha particles

In this paper, we report how the concentration of carbon-tetrafluoride (CF4) affects the primary scintillation from Ar+CF4 gas mixtures excited by proton and alpha particles. The single photon counting method was used to measure the time spectra of the primary scintillation from Ar, CF4 and their mixtures at atmospheric pressure. Pure Ar exhibits a fast decay time constant, which is approximately 6ns, and a slow decay time tail. Initially, increases in the concentration of CF4 increase the decay time of the Ar+CF4 gas mixture. However, when the concentration of CF4 exceeds a certain threshold, the decay time decreases to that of pure CF4, approximately 8–9ns. We also report the photon emission spectra of Ar+CF4 mixtures excited by protons. The emission spectrum of the primary scintillation from Ar is improved by CF4 in both the ultraviolet region and the visible to near-infrared region.

Simulation program for multiwire-type two-dimensional neutron detector with individual readout

We are currently developing a multiwire-type two-dimensional neutron detector system for use at the Materials and Life Science Experimental Facility at J-PARC. This system can attain a high response time and a high spatial resolution using the individual line readout method, and the performance parameters of the system strongly depend on a gas composition and pressure of a fill gas that is a gas mixture of He-3 and CF4. A simulation program for our system was developed to determine this gas condition. In addition, a small detector prototype was fabricated to evaluate the simulation program. The program involves the following calculations for each gas condition: the probability of a reaction between a neutron and He-3, the ranges of secondary particles generated by the nuclear reaction, the ejection angle of the particles, the wall effect caused by the conversion gap, and the pitch of the multiwire element. The simulation results agreed well with the experimental results obtained using the small detector prototype. Thus, it was confirmed that the simulation program can be effectively used to determine the gas composition and pressure when the required spatial resolution and detection efficiency are known.