Temperature dependence of ion formation in the nuclear track plasma

Abstract

Secondary ion emission from organic Van der Waals solids irradiated by 252Cf-fission fragments was studied as a function of sample temperature. The samples were produced by blowing a stream of gaseous ethylene and various alkanes onto a cold Cu-substrate having a temperature between 16 K and room temperature. Mass spectra of positive as well as negative secondary ions were measured using a linear time-of-flight (TOF) technique. The positive ion spectra are molecule specific, they contain cluster ions and above the molecular mass a strong pattern of reaction products. The negative ions ejected from ethylene and all alkanes investigated so far can hardly be distinguished, they consist mainly of CnHm− molecules with m=0,1,2,3. Remarkable is a strong temperature dependence particularly of negative ions. With increasing sample temperature, the rate of reactive collisions in the track plasma seems to increase. This temperature effect is not related to a common phase transition, – the change of yields with temperature is more gradual and is probably caused by a change of the sample density with temperature, which cannot be reversed.