The relativistic rise of the track density in bubble chambers

Abstract

The reasons for the high relativistic rise of the track density of fast charged particles in heavy liquid bubble chambers are considered. An approach is suggested which allows a quantitative description of the ionization-energy dependence for relativistic particles in bubble chambers filled with various liquids. This approach takes into account both the polarisation of matter and the energy threshold of nucleation in a superheated liquid. The expressions are given for a number of collisions per unit length with the energy transfer exceeding the threshold energy and for the specific energy loss in these collisions. It is shown that the ionization-energy dependence in bubble chambers can be described assuming the track density to be proportional to the number of collisions per unit length with the energy transfer exceeding the definite “effective” threshold energy but not to the specific energy loss in such collisions as usually assumed. This effective threshold energy is found to be several times greater than that calculated according to the thermal theory of nucleation and is close to 1 keV in heavy liquids. The observed relativistic rise of the track density must depend on both the liquid used and the operating conditions of a bubble chamber. Its maximum possible value for bubble chambers filled with H 2, He, C 3H 8, C 3F 8, SF 6, CBrF 3, Xe and a mixture of equal volumes of C 3H 8 and CBrF 3 is estimated to be 4;12;25;29;29;35;49 and 31 per cent of the minimum track density respectively. The low nucleation efficiency of fast charged particles and the possibility of identification of relativistic secondaries according to their ionization in bubble chambers are discussed.