ИСТИНА

The principle of supersonic gas expansion is used in a wide range of applications in different areas as e.g. chemical reaction studies and nanoparticles production. Recently, it has been proposed to use cold gas jets for laser spectroscopy studies of radioactive isotopes1. In this so called IGLIS technique (In-Gas Laser Ionisation Spectroscopy) products of a nuclear reaction are stopped inside a gas cell filled with argon as a buffer gas at typically 500 mbar, carried by the gas through a sonic or a de Laval nozzle and stepwise ionised by laser radiation. In order to obtain an optimum frequency resolution it is crucial to achieve very low temperature to reduce Doppler broadening of the observed spectral lines. However, the necessary temperature and pressure near the nozzle are found in the unstable for gas phase region, thus argon is likely to condensate. The goal of this work is to investigate how this condensation affects the thermodynamic properties of gas phase and as well as estimate the minimal temperature that can be achieved after expansion into the low-pressure region. The consideration is based on 1D diabatic steady Euler equations for the gas phase, taking the release of latent heat of condensation into account. For the condensed phase (clusters formation) mean kinetics nucleation theory2 is used. This model was chosen since it allows to deal with small clusters accurately and to predict pseudospinodal decomposition. As especially the small clusters (dimmers, trimers) are responsible for the majority of condensed mass this fact is important. The Becker-Döring equation is used for modelling the cluster distribution function. The model is validated against results from experiments on argon condensation in a cryogenic nozzle3 were used. Good agreement between the calculated and measured values for the onset of condensation can be observed (fig. 1). The nucleation rate for the onset is of order of 1020±1 cm-3 s-1 which is above the experimental value reported3, however it was not measured directly. Using the parameters of the IGLIS setup, the gas conditions reach the limit for pseudospinodal decomposition, thus the minimal temperature that can be obtained for pressures above 0.1 mbar is about 28 K. These results are to be validated in the IGLIS laboratory at KU Leuven. This work has been funded by FWO-Vlaanderen (Belgium), by GOA/2010/010 (BOF KU Leuven), by the Interuniversity Attraction Poles Programme initiated by the Belgian Science Policy Office (BriX network P7/12), by the European Commission within the Seventh Framework Programme through I3-ENSAR (contract no. RII3-CT-2010-262010) and by a grant from the European Research Council (ERC-2011-AdG-291561-HELIOS).