Isospin independence of the H–He double isotope ratio 'thermometer'

Light fragments (d, t, 3He,4He) were measured for central collisions of 112Sn+112Sn and 124Sn+124Sn at E/A=40 MeV. While individual isotope ratios depend strongly on the neutron-to-proton ratio of the entrance channel, the double ratio RH–He=[Y(d)Y(4He)]/[Y(t)Y(3He)] shows no such sensitivity. Within the assumptions of the Albergo thermometric technique, this independence is consistent with the attainment of full chemical equilibrium for each reaction at the same common temperature. However, the same independence is also predicted by calculations based on emission from an expanding emitting source, where the yield for the final double ratio is not obtained at a single well defined source temperature.