  A limiting factor in the ability to interpret isotope effect measurements in
cuprates is the absence of sufficiently accurate data for the whole phase
diagram; there is precise data for Tc, but not for the antiferromagnetic
transition temperature T_N. Extreme sensitivity of T_N to small changes in the
amount of oxygen in the sample is the major problem. This problem is solved
here by using the novel compound (Ca 0.1 La 0.9)(Ba 1.65 La 0.35)Cu 3 O y for
which there is a region where T_N is independent of oxygen doping. Meticulous
measurements of T_N for samples with oxygen 16 and oxygen 18 find the absence
of an oxygen isotope effect on T_N with unprecedented accuracy. A possible
interpretation of our finding and existing data is that isotope substitution
affects the normal state charge carrier density.
