Dependence of the Average Charge of an Ion on the Density of the Surrounding Medium

Abstract

The experiments of Lassen revealed the existence of two "density effects" expressing a dependence of the average charge of a moving heavy ion on the density of the surrounding medium. These are as follows: (1) the "pressure effect" characterizing gases and (2) the "state of condensation effect" characterizing solids and liquids. According to Bohr and Lindhard, these two effects are phenomenologically analogous since they involve a relationship between the times separating successive collisions of the moving ion and the times required for the excited orbital electrons to readjust themselves either by radiation or by redistribution of their excitation energy. It is conjectured that the phenomenological similarity may not be complete since in determining the state of condensation effect one also has to take into account the effect of a polarizing field due to the reaction of the perturbed medium on the moving ion. This field causes spontaneous emission of electrons carried by the ion, thus increasing the effective charge of the ion. A relationship is established between various values of the polarizing field and the corresponding average charge of an incident heavy ion in which the orbital electrons are continually maintained in an excited state due to the recurrent collisions with the atoms in the surrounding medium. The value of the polarizing field is determined for a particular case of a heavy fission fragment in gold and by applying this value to the above relationship it is shown that the contribution of the autoionization is substantial. Therefore, this contribution should be taken into account in determining the cause of the "state of condensation effect." The "autoionization" is due to "distant collisions," i.e., to the portion of the medium relatively far removed from the particle track, whereas the effect of Bohr and Lindhard is due to "close collisions" with atoms adjacent to the track.