Isospin tracing: A probe of nonequilibrium in central heavy-ion collisions

Four different combinations of Ru-96(44) and Zr-96(40) nuclei, both as proj ectile and target, were investigated at the same bombarding energy of 400A MeV using a 4 pi detector. The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space usi ng two different isospin-tracer observables, the number of measured protons and the tritium to He-3 yield ratio. The experimental results show that th e global equilibrium is not reached even in the most central collisions. Qu antitative measures of stopping and mixing are extracted from the data. The y are found to exhibit a quite strong sensitivity to the in-medium (n,n) cr oss section used in microscopic transport calculations.