Fission Barrier of Thallium-201

A new method involving the detection of fission fragments in mica has been applied to the measurement of the fission cross section of the compound nucleus ${\mathrm{Tl}}^{201}$ produced by bombardments of ${\mathrm{Au}}^{197}$ with helium ions. These data have been interpreted in terms of an expression for the ratio of fission to neutron-emission probabilities similar to those used conventionally, but modified to include the effect of quantum-mechanical barrier penetrability. In this way a height of 22.5±1.5 MeV was found for the fission barrier of ${\mathrm{Tl}}^{201}$ and a lower limit on the width could be established. The above value of the barrier, when interpreted on the basis of the liquid-drop theory, leads to an accurate determination of the ratio of the electrostatic to the surface energy of nuclei. This serves to establish the constant of proportionality between the "fissionability parameter" $x$ and the value of $\frac{Z^2}{A}$ as follows: $x=\frac{\left(\frac{Z^2}{A}\right)}{(48.4\mathrm{}\pm 0.5)}$. This measured barrier height, when added to the ground-state mass of ${\mathrm{Tl}}^{201}$, gives a saddle-point mass of this nucleus equal to 200.9949±0.0015 mass units (carbon scale).