On the excistence of triple-humped fission barriers in sup(231,233)Th

The neutron-induced fission cross sections, sigmasub(nf)(Esub(n)), for 230Th(n, f) and 232Th(n, f) have been measured from below to well above the fission thresholds. The excellent energy resolution available has been used for a detailed investigation of the 'fine structure' components of sigmasub(nf)(Esub(n)), appearing, more particularly, in the 230Th(n, f) and 232Th(n,f) resonances at Esub(n) approx.= 720 keV and 1.6 MeV and 1.7 MeV, respectively. Certain fission fragment anisotropies (FFA) for the 232Th(n, f) reaction were also measured. Using a simultaneous best-fit (SBF) procedure to all available sigmasub(nf)(Esub(n)) and FFA data, and, in particular, the 'fine structure' components mentioned above, pertinent fission barrier characteristics were then deduced. The resulting simultaneous presence of two rotational bands with opposite parties but the same quantum number K, together with the low value of the associated moment of inertia parameter, (h/2π)2/2F = 2.0 +- 0.2 keV, then constitute the experimental evidence for the presence of a third well in the fission barrier where the fissioning 231Th and 233Th nuclei have an asymmetric, pear-like octupole deformation. (orig.)