EVIDENCE FOR DIFFERENT INTERACTION POTENTIALS FOR $sup 4$He--$sup 4$He AND $sup 3$He--$sup 3$He FROM SCATTERING CROSS-SECTION MEASUREMENTS.

By means of low-energy cross-section measurements on ${\mathrm{He}}^{3}$-${\mathrm{He}}^{3}$, we derive an interatomic potential which has a minimum 3.4% deeper than that of ${\mathrm{He}}^{4}$-${\mathrm{He}}^{4}$. We demonstrate that the difference in the potentials is significant. The atomic-mass polarization effect is proposed as a possible explanation for the different potentials.