Parent Gas Research Articles

Monte Carlo model for ion mobility and diffusion for characteristic electric fields in nanodosimetry

The quantification of the effects of space radiation for manned spaceflight can be approximated by nanodosimetric measurements. For the development of nanodosimetric detectors, a Monte Carlo model for ion mobility and diffusion for characteristic electric fields is presented.This model can be used to describe the interactions of ions in their parent gas based solely on commonly known input parameters, such as the ionization potential, kinetic diameter, molar mass, and polarizability of the gas. A model for approximating the resonant charge exchange cross section has been proposed, requiring only the ionization energy and mass of the parent gas as input parameters. The method proposed in this work was tested against experimental drift velocity data for a wide range of gases (helium, neon, nitrogen, argon, krypton, carbon monoxide, carbon dioxide, oxygen, propane). The transverse diffusion coefficients were compared to experimental values for helium, nitrogen, neon, argon, and propane gas.With the Monte Carlo code and resonant charge exchange cross section approximation model presented in this work, it is now possible to calculate an estimate of the drift velocities, transverse diffusion, and thus the ion mobility of ions in their parent gas. This is essential for further nanodosimetric detector development, as those parameters are often not well known for the gas mixtures used in nanodosimetry.

A CFD-based approach to study the deposition and distribution behaviour of 212Pb in a calibration chamber.

Among the several aspects of decay products behavior, deposition is of special significance because of its prominent role in the activity removal from the environment, which eventually results in the occurrence of decay product disequilibrium with the parent gas. This point is particularly important in case of thoron dosimetry where thoron progeny 212Pb accounts for the most of the radiological dose. The deposition depends on the size distribution of decay products and the structure of air turbulence at the air-surface interface. In the present work, the effect of varying air-flow (fan speed) and aerosol count median diameter (CMD) was studied on the deposition and distribution profile of 212Pb using computational fluid dynamics (CFD). The simulations have been carried out in a cubical calibration chamber of volume 8 m3, facilitated at RP&AD, BARC. Simulated results showed that the increase of total depositional loss rate of attached fraction of 212Pb due to increase of the fan speed was significant for CMD up to 400nm, beyond which this effect started becoming less prominent with increasing diameter. Besides, a minimum of the total depositional loss rate curve was seen to be shifted to the higher CMD with increase of the fan speed. CFD results were found to be in good agreement with experimental observations obtained in the controlled conditions with thoron source.

Molecular gas cloud properties atz≃ 1 revealed by the superb angular resolution achieved with ALMA and gravitational lensing

ABSTRACTCurrent observations favour that the massive ultraviolet-bright clumps with a median stellar mass of $\sim 10^7\, {\rm M}_{\odot }$, ubiquitously observed in z ∼ 1–3 galaxies, are star-forming regions formed in situ in galaxies. It has been proposed that they result from gas fragmentation due to gravitational instability of gas-rich, turbulent, and high-redshift discs. We bring support to this scenario by reporting the new discovery of giant molecular clouds (GMCs) in the strongly lensed, clumpy, main-sequence galaxy, A521-sys1, at z = 1.043. Its CO(4–3) emission was mapped with the Atacama Large Millimetre/submillimetre Array (ALMA) at an angular resolution of 0.19 × 0.16 arcsec2, reading down to 30 pc, thanks to gravitational lensing. We identified 14 GMCs, most being virialized, with $10^{5.9}-10^{7.9}\, {\rm M}_{\odot }$ masses and a median $800\, {\rm M}_{\odot }~\mathrm{pc}^{-2}$ molecular gas mass surface density, that are, respectively, 100 and 10 times higher than for nearby GMCs. They are also characterized by 10 times higher supersonic turbulence with a median Mach number of 60. They end up to fall above the Larson scaling relations, similarly to the GMCs in another clumpy z ≃ 1 galaxy, the Cosmic Snake, although differences between the two sets of high-redshift GMCs exist. Altogether they support that GMCs form with properties that adjust to the ambient interstellar medium conditions prevalent in the host galaxy whatever its redshift. The detected A521-sys1 GMCs are massive enough to be the parent gas clouds of stellar clumps, with a relatively high star formation efficiency per free-fall time of ∼11 per cent.

Observational signatures of massive black hole progenitor pathways: could Leo I be a smoking gun?

ABSTRACT Observational evidence is mounting regarding the population demographics of massive black holes (MBHs), from the most massive cluster galaxies down to the dwarf galaxy regime. However, the progenitor pathways from which these central MBHs formed remain unclear. Here, we report a potentially powerful observational signature of MBH formation in dwarf galaxies. We argue that a continuum in the mass spectrum of MBHs in (fossil) dwarf galaxies would be a unique signature of a heavy seed formation pathway. The continuum in this case would consist of the usual population of stellar mass black holes, formed through stellar evolution, plus a smaller population of heavy seed MBHs that have not yet sunk to the centre of the galaxy. Under the robust assumption of initial fragmentation of the parent gas cloud resulting in a burst of heavy seed production, a significant fraction of these seeds will survive to the present day as off-nuclear MBHs with masses less than that of the central object. Motivated by the recent discovery of an MBH in the relatively low central density Leo I galaxy, we show that such a continuum in MBH seed masses should persist from the lightest black hole masses up to the mass of the central MBH in contrast to the light seeding scenario where no such continuum should exist. The detection of off-centred MBHs and a central MBH would represent strong evidence of a heavy seeding pathway.

Collisional broadening and shift coefficients for (n+1)s[3/2]2 → (n+1)p[1/2]1 argon and neon lines in parent and foreign rare gases

Results of simultaneous measurements of pressure shift β and broadening ξ coefficients for 912.3 nm Ar and 703.2 nm Ne spectral lines of the (n+1)s[3/2]2 → (n+1)p[1/2]1 transition are presented. This is a lasing transition in a recently proposed optically pumped rare gas laser. Absorption line shapes fitted to Voigt profiles permitted to assess gas temperature by determining Doppler width components. These coefficients for the Ar 912.3 nm line with He and Ne as the collisional partners were determined for the first time. The values of shift and broadening coefficients in units of 10−10 s-1 cm3, reduced to 300 K are: βAr-Ar = -2.11±0.05, βAr-Ne = -1.21±0.05, βAr-He = 0.37±0.03, βNe-Ne = -0.78±0.05, βNe-He = 0.0±0.05; ξAr-Ar = 2.8±0.1, ξAr-Ne = 1.68±0.05, ξAr-He = 3.7±0.1, ξNe-Ne = 1.94±0.05, ξNe-He = 3.18±0.05.

Detecting Trace Gases at PPM Levels with Low Temperature Plasma Optical Emission Spectroscopy

Detecting Trace Gases at PPM Levels with Low Temperature Plasma Optical Emission Spectroscopy

The formation of the young massive cluster B1 in the Antennae Galaxies (NGC 4038/NGC 4039) triggered by cloud–cloud collision

Abstract The Antennae Galaxies, one of major mergers, are a starburst. Tsuge et al. (2020, PASJ, 73, S35) showed that the five giant molecular complexes in the Antennae Galaxies have signatures of cloud–cloud collisions based on the Atacama Large Millimeter Array (ALMA) archival data with 60 pc resolution. In the present work we analyzed the new CO data toward the super star cluster (SSC) B1 with 14 pc resolution obtained with ALMA, and confirm that two clouds show a complementary distribution with a displacement of ∼70 pc as well as connecting bridge features between them. The complementary distribution shows a good correspondence with the theoretical collision model (Takahira et al. 2014, ApJ, 792, 63), and the distribution indicates that the formation of SSC B1 with ∼106 M⊙ was consistent with the trigger of cloud–cloud collision on a time scale of ∼1 Myr, which is consistent with the cluster age. It is likely that SSC B1 was formed from molecular gas of ∼107 M⊙ with a star formation efficiency of $\sim\! 10\%$ in 1 Myr. We identify a few places where additional clusters are forming. Detailed gas motion indicates that the stellar feedback in the accelerating gas is not effective, while the ionization plays a role in evacuating the gas around the clusters at a ∼20 pc radius. The results have revealed the details of the parent gas where a cluster having a mass similar to a globular is being formed.

DEVELOPMENT OF CALIBRATION GAS MIXTURES (CARBON DIOXIDE AND OXYGEN IN NITROGEN MATRIX) AT A TYPICAL CONCENTRATION RANGE OF MODIFIED ATMOSPHERE PACKAGING

Measurement of carbon dioxide (CO2), oxygen (O2), and nitrogen (N2) concentration in modified atmosphere packaging (MAP) food is critical to be carried out by the food industry. A slight variation in concentrations of CO2, O2, and N2 in food packaging may have a significant impact on product quality and safety for human health. Accurate and reliable measurement of CO2, O2, and N2 concentrations in food packaging is crucial, and it can only be achieved by calibrating the gas analyzer. This study aimed to develop gas mixtures for the calibration of CO2, O2, and N2 gas analyzers at a typical concentration range of modified atmosphere packaging. The calibration gas mixtures were prepared gravimetrically by following ISO 6142. The concentration ranges of CO2, O2, and N2 for calibration gas mixtures were set at 9-19% mol/mol, 1-5% mol/mol, and 74-88% mol/mol, respectively. Each parent gas was identified for its impurities using gas chromatography with a pulsed discharge helium ionization detector (GC-PDHID). The compositions of CO2, O2, and N2 in the mixtures were verified by evaluating the internal consistency within the prepared gas mixtures using gas chromatography with a thermal conductivity detector (GC-TCD). The short term stability of the prepared gas mixtures was evaluated using an equal division method. The result showed that good internal consistency was achieved between the gravimetrical and GC’s verification values, having linear regression coefficient (R2) ≥ 0.999. The t-test result has shown that CO2 has better short term stability than O2 and N2. In conclusion, the developed calibration gas mixtures at a typical concentration range of modified atmosphere packaging have shown satisfying results for CO2 component. However, further evaluation is still required to minimize the instability of O2 and N2 components.

Selective etching of SiN against SiO2 and poly-Si films in hydrofluoroethane chemistry with a mixture of CH2FCHF2, O2, and Ar

Gas chemistry has a significant impact on etch selectivity in semiconductor device fabrication, which is important for realization of atomic-scale removal and formation of high-aspect ratio features. To widen the controllable changes in the etchant composition in etching processes, our previous calculation showed the possibility of the controllable generation of CH2F and CHF2 related ions and radicals from a 1,1,2-trifluoroethane (CH2FCHF2) parent gas. The etch selectivity among silicon nitride (SiN), silicon dioxide (SiO2) and poly-Si films using CH2FCHF2 plasma mixed with O2 and Ar was investigated. The effects of the CH2FCHF2 and O2 partial pressures on the composition of CHF2+ and CH2F+ ions, which were measured with a quadrupole mass spectrometer, and on the possible reactions with respect to the CH2FCHF2 and O2 mixed gas phase were investigated using quantum chemical calculations. The etch selectivity was investigated through surface etching reactions for SiN, SiO2, and poly-Si films.

Modeling the Survival of Population III Stars to the Present Day

Abstract Recent numerical simulations have suggested the probability of a fraction of the primordial stars being ejected from the cluster of their origin. We explore the possibility that some of these can remain on the main sequence until the present epoch. We develop a semianalytical model guided by results of cosmological simulations to study the mass accretion by these protostars as a function of the original stellar mass and other parameters such as angular momentum and gravitational drag due to ambient gas. We also explore whether some of the protostars remain sufficiently low mass and long-lived to survive to the present day. This requires that the protostars are ejected from the star-forming region while their mass is less than . Assuming that the protostars gain mass via the spherical Bondi–Hoyle accretion from the ambient medium, we show that Population III protostars that initially form within a certain range of mass and are ejected with velocity larger than the escape velocity may survive to the present day on the main sequence. Thus, they may even be found in our Milky Way or its satellites. Our calculations also reveal that protostars that do not get ejected from the parent gas clump accrete a large amount of gas. We predict that these can become massive enough to be progenitors of black holes.

Etch selectivity during plasma-assisted etching of SiO2 and SiNx: Transitioning from reactive ion etching to atomic layer etching

Continued downscaling of semiconductor devices has placed stringent constraints on all aspects of the fabrication process including plasma-assisted anisotropic etching. To address manufacturing challenges associated with atomic-scale control, material selectivity, etch fidelity, and increasingly complex device architectures, reactive ion etching (RIE) is transitioning to plasma-assisted atomic layer etching (ALE). Even though the number of elements used in the semiconductor devices has increased several-fold over the last four decades, SiO2 and SiNx remain the most commonly used dielectric materials. In fact, fluorocarbon based, plasma-assisted ALE processes for SiO2 and SiNx have already been integrated into semiconductor manufacturing, including etching of self-aligned contacts for advanced transistors. However, several challenges remain in achieving ultrahigh etch selectivity of SiO2 over SiNx and vice versa. In this article, first, the authors provide a focused review on selective RIE of SiO2 over SiNx and contrast this with ALE. A particular focus is given to the etching mechanism, including the role of the mixing layer composition and thickness at the fluorocarbon-SiO2 interface, the F-to-C ratio in the fluorocarbon parent gas, H2 dilution, surface composition on the nonetched SiNx, ion flux and energy, Ar plasma activation duration in ALE, and chamber memory effects. Second, we discuss the reverse case of selectively etching SiNx over SiO2 with careful attention given to the role of novel hydrofluorocarbon gases and dilution of the primary feed gas with other gases such as CH4 and NO. In the second part of this review, we also discuss how novel surface chemistries are enabled by the introduction of ALE, which include selective (NH4)2SiF6 formation on the SiNx surface and selective surface prefunctionalization of SiO2 to enable ultrahigh selectivity. Through this review, the authors hope to provide the readers with an exhaustive knowledge of the selectivity mechanisms for RIE of SiO2 over SiNx and vice versa, which provides a basis for developing future highly material-selective ALE processes.

Approximation of the Mobility of Atomic Ions of Noble Gases in Their Parent Gas

The mobility of ions plays an important role in the theoretical analysis of many phenomena in low-temperature plasmas, such as ambipolar diffusion, the formation of electrode and surface sheaths in gas discharges, particle charging and the ion drag force in dusty plasmas. A modification of the semiempirical Frost formula for the mobility of positive atomic ions in their parent gases is proposed. The modified expression demonstrates excellent agreement with experimental results for various ionized rare gases in a very wide range of electric field strengths and temperatures.

EFFECT OF AIR VELOCITY ON INHALATION DOSES DUE TO RADON AND THORON PROGENY IN A TEST CHAMBER.

Inhalation doses due to radon and thoron are predominantly due to the inhalation of progeny of Radon and Thoron. The progeny/decay-products of radon and thoron are particulates unlike their parent gas and exhibit different physical properties like attachment to the aerosols and deposition on different surfaces. All these properties in turn depend on the environmental conditions such as air velocity, aerosol concentration, attachment rate, etc. The role of air velocity on deposition on surfaces decides the progeny particles left in the air for inhalation. Therefore, in the present work, we have studied the effect of air velocity on the inhalation dose due to radon and thoron progeny at the centre of a 0.5-m3 calibration chamber as well as on all surfaces. Hence, the studies were carried out at different air velocities, and inhalation doses were measured using deposition-based direct radon and thoron progeny sensors.

Formation of secondary atmospheres on terrestrial planets by late disk accretion

Recently, gas disks have been discovered around main sequence stars well beyond the usual protoplanetary disk lifetimes (i.e., > 10 Myrs), when planets have already formed. These gas disks, mainly composed of CO, carbon, and oxygen seem to be ubiquitous in systems with planetesimal belts (similar to our Kuiper belt), and can last for hundreds of millions of years. Planets orbiting in these gas disks will accrete a large quantity of gas that will transform their primordial atmospheres into new secondary atmospheres with compositions similar to that of the parent gas disk. Here, we quantify how large a secondary atmosphere can be created for a variety of observed gas disks and for a wide range of planet types. We find that gas accretion in this late phase is very significant and an Earth's atmospheric mass of gas is readily accreted on terrestrial planets in very tenuous gas disks. In slightly more massive disks, we show that massive CO atmospheres can be accreted, forming planets with up to sub-Neptune-like pressures. Our new results demonstrate that new secondary atmospheres with high metallicities and high C/O ratios will be created in these late gas disks, resetting their primordial compositions inherited from the protoplanetary disk phase, and providing a new birth to planets that lost their atmosphere to photoevaporation or giant impacts. We therefore propose a new paradigm for the formation of atmospheres on low-mass planets, which can be tested with future observations (JWST, ELT, ARIEL). We also show that this late accretion would show a very clear signature in Sub-Neptunes or cold exo-Jupiters. Finally, we find that accretion creates cavities in late gas disks, which could be used as a new planet detection method, for low mass planets a few au to a few tens of au from their host stars.

Effect of gas temperature on CO2+ ion transport in CO2

The effect of gas temperature on the transport of the CO2+ ion in its parent gas CO2 is studied. For this purpose, a Monte Carlo (MC) solution technique of the ion transport equation developed in the past by the authors, which was formally proven to provide the exact solution of the transport equation, is used. The study is performed first by benchmarking against experimental data and another MC code, using cross sections from literature and then changing the gas temperature. The solution is characterized through its moments such as reduced mobility, coefficients of its expansion in Legendre polynomials and three-dimensional ion velocity distribution function. It is shown that the gas temperature has a significant effect on all these quantities. This finding has important consequences for plasma modeling which are discussed.

Young stars raining through the galactic halo: the nature and orbit of price-whelan 1

ABSTRACT We present radial velocities for five member stars of the recently discovered young (age ≃ 100−150 Myr) stellar system Price-Whelan 1 (PW 1), which is located far away in the Galactic Halo (D≃ 29 kpc, Z≃ 15 kpc), and that is probably associated with the leading arm (LA) of the Magellanic Stream. We measure the systemic radial velocity of PW 1, Vr = 275 ± 10 km s−1, significantly larger than the velocity of the LA gas in the same direction. We re-discuss the main properties and the origin of this system in the light of these new observations, computing the orbit of the system and comparing its velocity with that of the H i in its surroundings. We show that the bulk of the gas at the velocity of the stars is more than 10 deg (5 kpc) away from PW 1 and the velocity difference between the gas and the stars becomes larger as gas closer to the stars is considered. We discuss the possibilities that (1) the parent gas cloud was dissolved by the interaction with the Galactic gas, and (2) that the parent cloud is the high-velocity cloud (HVC) 287.5+22.5 + 240, lagging behind the stellar system by ≃ 25 km s−1 and ≃10 deg ≃ 5 kpc. This HVC, which is part of the LA, has metallicity similar to PW 1, displays a strong magnetic field that should help to stabilize the cloud against ram pressure, and shows traces of molecular hydrogen. We also show that the system is constituted of three distinct pieces that do not differ only by position in the sky but also by stellar content.

Modified Frost formula for the mobilities of positive ions in their parent gases

A modification to the semi-empirical Frost formula for the mobilities of singly charged positive ions in their parent gases is proposed. The modified expression demonstrates excellent agreement with experimental results for different ionized rare gases in a very extended range of reduced field strengths.

Molecular clouds in a Milky Way progenitor at z = 1

AbstractThanks to the remarkable ALMA capabilities and the unique configuration of the Cosmic Snake galaxy behind a massive galaxy cluster, we could resolve molecular clouds down to 30 pc linear physical scales in a typical Milky Way progenitor at z = 1.036, through CO(4–3) observations performed at the ∼ 0.2″ angular resolution. We identified 17 individual giant molecular clouds. These high-redshift molecular clouds are clearly different from their local analogues, with 10–100 times higher masses, densities, and internal turbulence. They are offset from the Larson scaling relations. We argue that the molecular cloud physical properties are dependent on the ambient interstellar conditions particular to the host galaxy. We find these high-redshift clouds in virial equilibrium, and derive, for the first time, the CO-to-H2 conversion factor from the kinematics of independent molecular clouds at z = 1. The measured large clouds gas masses demonstrate the existence of parent gas clouds with masses high enough to allow the in-situ formation of similarly massive stellar clumps seen in the Cosmic Snake galaxy in comparable numbers. Our results support the formation of molecular clouds by fragmentation of turbulent galactic gas disks, which then become the stellar clumps observed in distant galaxies.

The visual spectrum of Jupiter's Great Red Spot accurately modeled with aerosols produced by photolyzed ammonia reacting with acetylene

We report results incorporating the optical properties of the red-tinted photochemically-generated aerosols of Carlson et al. (2016, Icarus 274, 106–115) in spectral models of Jupiter's Great Red Spot (GRS). This material - created in laboratory GRS simulations from acetylene reacting with photolytic products of ammonia produced by ~0.2-μm radiation, similar to solar radiation on Jupiter within and above the upper troposphere, provides an excellent match to 0.35–1.05-μm spectra of the core of the Great Red Spot obtained by the Visual Infrared Mapping Spectrometer (VIMS) during the 2000–2001 Cassini-Huygens flyby. Radiative transfer models of GRS spectra acquired near the central-meridian (CM) and limb by the visual channel of the Cassini/VIMS near closest approach on December 31, 2000 and January 2, 2001, respectively, show remarkable agreement for model morphologies where the following conditions all exist: (1) most of the optical depth of the Carlson et al. (2016) chromophore resides near the top or above the main cloud layer, rather than being uniformly distributed within it, (2) the chromophore consists of relatively small particles in the 0.1–0.2 μm range, and (3) the 1-μm optical depth of the chromophore layer is small, of the order of 0.1–0.2. For such models, the chromophore layer mass abundance is 32–40 μgm cm−2. Consideration of the availability of the acetylene and ammonia parent gas material near the observed chromophore layers gives powerful support for the chromophore residing at the top of the main cloud in the upper troposphere rather than residing as a detached layer in the stratosphere. Under steady-state formation/loss conditions, consideration of plausible eddy diffusion coefficients pertaining to the relatively quiescent Jovian upper atmosphere yield untenable vertical transport times of more than several centuries required to supply the acetylene needed to form the chromophore layer, with stratospheric chromophore models requiring more than half a millennium. Consideration of the convective nature of the GRS and possible presence of acetylene-generating thunderstorms yields upper-troposphere chromophore layer formation times ranging from 11 years to 1.5 months for (1) plausible eddy diffusion coefficients ranging from 104 to 106 cm2 s−1 and (2) efficient conversion of C2H2 and NH3 into the Carlson et al. (2016) chromophore. Thus, the enhanced red coloring of the GRS may be due to the combined effects of (1) the relatively high, 0.2-bar cloudtop where ammonia ice and gas are delivered to prime photo-dissociation altitudes, by (2) powerful convection, which also delivers acetylene from depth created by (3) lightning, which is itself a by-product and indicator of powerful convection, and aided by (4) the vortex nature of the GRS, which helps to confine and concentrate the chromophore as it forms over time within the GRS anticyclone.

Cross-sections, transport coefficients and dissociation rate constants for molecular ion interacting with Kr

A hybrid method based on quantum mechanical formalism for electrons and a classical treatment for nuclei was used to study the dynamics of electronically ground state krypton dimer in collision with its parent gas. Two sets of collision cross-sections, namely non-reactive scattering and collision induced dissociation, were calculated from this method by using different sets of atom–atom interaction potentials (neutral–neutral, ion–neutral). These cross-sections were injected in Monte Carlo simulations for the determination of mobility, diffusion and dissociation rate constant. It is noteworthy that we also used an inverse method based on JWKB approximation to calculate a global momentum transfer collision cross-section without discerning the inelastic processes as dissociation. Thus, the transport coefficients obtained are also in a pretty good agreement with experimental data. All Monte Carlo simulations were performed at ambient pressure and temperature over a broad range of reduced electric field. Moreover, effects of rovibronic excitations were also studied thoroughly in order to improve correlation between theoretical mobility calculations and experimental data available in the literature.