The mobility of positive ions in helium. Part I.―Helium ions

Abstract

In a previous paper we gave an account of some experiments on the mobility of positive ions in helium gas. It was shown that minute traces of impurity profoundly affect the speed at which the charge is carried through the gas, so that very little significance can be attached to the values of the mobility of ions recorded in the literature. In particular we studied the behaviour of carefully purified helium gas when admitted at a pressure of 360 mm. of mercury into a baked-out apparatus. In our method of determining the mobility, a single group of ions gives rise to a peak in a current frequency curve, and it is easy to follow the changes produced by an impurity. Initially the ions in the helium had a high mobility and in one case we obtained a group with a mobility of the order of 17 cm./sec./volt/cm. at 760 mm. As the gas became contaminated by impurities gradually evolved from the glass walls and metal parts of the apparatus, peaks corresponding to groups of smaller mobility appeared, and eventually the charge was all carried by a group with mobility rather less than 8. The ions in these experiments were produced by means of α-particles from polonium. Traces of impurity in helium may affect the results in several ways. Firstly, they may lead to the formation of ionic clusters, particularly if the molecules of the impurity have a permanent dipole. In our experiments it seemed unlikely that molecules capable of forming clusters were present. Secondly, although with small concentrations of an impurity a negligible number of its ions will be produced by the direct action of the α-rays, they may be formed indirectly in either of the two following ways:—