Drift tube measurements on $Na+$ ions and clusters using the $24mNa+$ tracer

Mobilities of sodium ions and their clusters in different gases have been measured by gamma-ray spectrometry using the 20 ms isomeric state of $24Na$ as tracer. This isomer is obtained through the β-decay of $24Ne,$ which was produced by bombarding $22Ne$ with the 7 MeV triton beam of a cyclotron. This new technique of measurement allows us to determine the mobility of sodium ions in non-reacting gases of a pressure above 1 mbar. Measurements have been carried out in gases of pressures ranging from 6 mbar to 1000 mbar under an electric field up to 250 V/cm at room temperature. The measured mobility of $24Na+$ in pure neon gas at 1000 mbar was determined to be $3.2 cm2/Vs.$ This value is lower than the zero field mobility of about $8.5 cm2/Vs$ measured by Tyndall and Akridge. A small amount of polar molecules such as water or ethanol introduced into the drift tube causes their clustering with the sodium ions through monopole-dipole interaction, resulting in a strong decrease in the mobility. This clustering effect has been studied for various combinations of polar molecules and inert gases. The usable pressure range from 1 mbar to high pressures in the range of some hundred bars is interesting because most other methods for cluster studies cannot be done at such high pressures. With this method it is possible to measure the mobility change of sodium ions towards the condensation point and eventually even towards the critical point.