The effect of oxygen admixture on the properties of microwave generated plasma in Ar-O2: A modelling study

This work presents results of a self-consistent modelling analysis on microwave plasma generated in Ar-O2 mixtures at a frequency of 2.45 GHz at atmospheric pressure. The study focuses on how the plasma properties are influenced by the increase of the oxygen fraction in the gas mixture. The oxygen admixture is increased from 1% up to 95% in mass for values of the input microwave power of 1 and 1.5 kW. The results show that for a power of 1 kW and gradually increasing the oxygen admixture from 1% to 25% the electron density drops by a factor of more than four due to the energy lost by the electrons due to dissociation of oxygen molecules and the gas heating. An analysis of the number densities of species produced in the Ar-O2 plasma is presented. Oxygen admixtures of above 50% are considered along with an increase of the input microwave power in order to supply the discharge with electron number density values of the order of 1019 m-3. Gas temperatures above 3700 K are obtained in the plasma core along with a strong production of oxygen atoms with a number density of the order of 1023 m-3

On the Interaction of a Microwave Excited Oxygen Plasma with a Jet of Precursor Material for Deposition Applications

A plasma source based on a microwave discharge at atmospheric pressure is used to produce an oxygen plasma torch. An admixture of liquid precursor material is evaporated and injected into the torch through a nozzle, causing oxidization and deposition of doped silica at a nearby quartz substrate. The temperature generated inside the plasma source and in the plume, in the region of treatment, and at the substrate surface are key parameters, which are needed for process description and optimization of plasma-chemical reactions.Optical emission spectroscopy, high-speed imaging, and thermography were applied to observe and to characterize the jet behavior and composition. The experimental results are compared with self-consistent modeling

First Measurement of the He3+He3-->He4+2p Cross Section down to the Lower Edge of the Solar Gamow Peak

We give the LUNA results on the cross section measurement of a key reaction of the proton-proton chain strongly affecting the calculated neutrino luminosity from the Sun: He3+He3-->He4+2p. Due to the cosmic ray suppression provided by the Gran Sasso underground laboratory it has been possible to measure the cross section down to the lower edge of the solar Gamow peak, i.e. as low as 16.5 keV centre of mass energy. The data clearly show the cross section increase due to the electron screening effect but they do not exhibit any evidence for a narrow resonance suggested to explain the observed solar neutrino flux.Comment: 5 pages, RevTeX, and 2 figures in PostScript Submitted for publicatio

Multi-channel R-matrix analysis of CNO cycle reactions

The CNO cycle is the main process for hydrogen burning in stars somewhat more massive than the Sun. The reaction cross sections at Gamow energies are typically in the femto to pico-barn range and are consequently very difficult to measure experimentally. The CNO reaction rates are based on extrapolations of experimental data from higher energies. We are developing a multi-channel R-matrix code (AZURE) to provide a new and more comprehensive tool for fitting experimental data and making extrapolations to lower energies in all reaction and scattering channels. The 14N(p,γ )15O reaction is the slowest reaction of the CNO cycle and thus it determines the energy production rate of CNO burning. Furthermore, this reaction plays an important role in the determination of Globular Cluster age, since the position of the turnoff point, at which the GC stars escape from the Main Sequence, is powered by the onset of the CNO burning, whose bottleneck is the 14N(p, γ )15O. We have made a reanalysis of the most recent experimental data on the ground state and the 6.18 MeV transitions. The ratio of the cross sections of the 15N(p, γ )16O and 15N(p,α)12C reactions determines how much catalytic material passes to higher CNO cycles and has an effect on the production of heavier elements, particularly 16O and 17O. Simultaneous analysis of both reactions for all channels suggests that the ratio σγ/σα is smaller than previously reported

Comparison of the LUNA 3He(alpha,gamma)7Be activation results with earlier measurements and model calculations

Recently, the LUNA collaboration has carried out a high precision measurement on the 3He(alpha,gamma)7Be reaction cross section with both activation and on-line gamma-detection methods at unprecedented low energies. In this paper the results obtained with the activation method are summarized. The results are compared with previous activation experiments and the zero energy extrapolated astrophysical S factor is determined using different theoretical models.Comment: Accepted for publication in Journal of Physics

Measurement of the resonance at ER=1422 keV in Ar36(p,γ)37K

A new resonance at ER=1422 keV has been found in the reaction Ar36(p,γ)37K. From its measured resonance strength of ωγ=(6.0±1.5)×10-4 eV the proton partial width Γp can be calculated. Our data suggest a value of Γγ/Γp40 for the ratio of γ to proton partial width for the corresponding state at Ex=3241 keV in K37. We find that the corrected distribution of Gamow-Teller strength below the isobaric analog state as obtained from the β-delayed proton decay of Ca37 is in agreement with the results inferred from the Cl37(p,n)37Ar reaction

Activation measurement of the 3He(alpha,gamma)7Be cross section at low energy

The nuclear physics input from the 3He(alpha,gamma)7Be cross section is a major uncertainty in the fluxes of 7Be and 8B neutrinos from the Sun predicted by solar models and in the 7Li abundance obtained in big-bang nucleosynthesis calculations. The present work reports on a new precision experiment using the activation technique at energies directly relevant to big-bang nucleosynthesis. Previously such low energies had been reached experimentally only by the prompt-gamma technique and with inferior precision. Using a windowless gas target, high beam intensity and low background gamma-counting facilities, the 3He(alpha,gamma)7Be cross section has been determined at 127, 148 and 169 keV center-of-mass energy with a total uncertainty of 4%. The sources of systematic uncertainty are discussed in detail. The present data can be used in big-bang nucleosynthesis calculations and to constrain the extrapolation of the 3He(alpha,gamma)7Be astrophysical S-factor to solar energies

Status of the LUNA experiment

Luna is a pilot project initially focused on the 3He(3He, 2p)4He cross section measurement within the thermal energy region of the Sun (15–27 keV). A compact high current 50 kV ion accelerator facility including a windowless gas target system, a beam calorimeter and four detector telescopes has been built, tested and installed underground at Laboratori Nazionali del Gran Sasso. In these conditions, thanks to the cosmic ray suppression, we could attain a background level of less than 1 event per week, a rate similar to the one expected from 3He(3He, 2p)4He at the lower edge of the Sun thermal energy region

The 3He(alpha,gamma)7Be S-factor at solar energies: the prompt gamma experiment at LUNA

The 3He(alpha,gamma)7Be process is a key reaction in both Big-Bang nucleosynthesis and p-p chain of Hydrogen Burning in Stars. A new measurement of the 3He(alpha,gamma)7Be cross section has been performed at the INFN Gran Sasso underground laboratory by both the activation and the prompt gamma detection methods. The present work reports full details of the prompt gamma detection experiment, focusing on the determination of the systematic uncertainty. The final data, including activation measurements at LUNA, are compared with the results of the last generation experiments and two different theoretical models are used to obtain the S-factor at solar energies.Comment: Accepted for publication in Nucl. Phys.

Preparation and characterisation of isotopically enriched Ta2_2O5_5 targets for nuclear astrophysics studies

The direct measurement of reaction cross sections at astrophysical energies often requires the use of solid targets of known thickness, isotopic composition, and stoichiometry that are able to withstand high beam currents for extended periods of time. Here, we report on the production and characterisation of isotopically enriched Ta$_2$O$_5$ targets for the study of proton-induced reactions at the Laboratory for Underground Nuclear Astrophysics facility of the Laboratori Nazionali del Gran Sasso. The targets were prepared by anodisation of tantalum backings in enriched water (up to 66% in $^{17}$O and up to 96% in $^{18}$O). Special care was devoted to minimising the presence of any contaminants that could induce unwanted background reactions with the beam in the energy region of astrophysical interest. Results from target characterisation measurements are reported, and the conclusions for proton capture measurements with these targets are drawn.Comment: accepted to EPJ