The analysis of a large body of heavy ion fusion reaction data with medium-heavy projectiles (6 < or = Z < or = 18) and actinide targets suggests a disappearance of the 3n exit channel with increasing atomic number of the projectile. Here, we report a measurement of the excitation function of the reaction (248)Cm ((26)Mg,xn)(274-x)Hs and the observation of the new nuclide (271)Hs produced in the 3n evaporation channel at a beam energy well below the Bass fusion barrier with a cross section comparable to the maxima of the 4n and 5n channels. This indicates the possible discovery of new neutron-rich transactinide nuclei using relatively light heavy ion beams of the most neutron-rich stable isotopes and actinide targets.

We report on an attempt to produce and detect 268 Hs and 269 Hs in the nuclear fusion reaction 25 Mg + 248 Cm using the gas phase chemistry apparatus COMPACT. No decay chains attributable to the decay of hassium isotopes were observed during the course of this experiment. From the nonobservation of 269 Hs we derive a cross section limit of 0.4 pb (63% confidence limit) for the reaction 248 Cm( 25 Mg, 4n) 269 Hs at a center-of-target beam energy of 140 MeV. The evaluated cross section limit for the 248 Cm( 25 Mg, 5n) 268 Hs reaction depends on the assumed half-life of unknown 268 Hs. Current systematics of the half-lives for even-even Hs isotopes suggests a value of 0.5 s, resulting in a cross section limit of 1.3 pb.Superheavy elements exist only because of nuclear shell effects, which stabilize them against spontaneous fission (SF). Nuclear stability in the region of elements with proton number 104 Z 108 is primarily influenced by the closure of deformed shells at Z = 108 and N = 162. Macroscopicmicroscopic as well as self-consistent mean-field calculations [1-4] predicted 270 Hs to be a deformed doubly magic nucleus. The nuclide 270 Hs was identified in one of our previous experiments and its observed properties confirm theoretical predictions of nuclear shell stabilization at Z = 108 and N = 162 [5]. To explore the strength of the shell stabilization in this entire region, it would be interesting to compare properties of 270 Hs to those of other even-even nuclei in its vicinity. However, most of the nearby even-even nuclides are presently unknown (see Fig. 1 in Ref.[5]), including both neighboring even Hs isotopes, 268 Hs and 272 Hs.In this Brief Report we report on an experiment aimed at identifying the unknown nuclide 268 Hs. In the nuclear fusion reaction 25 Mg + 248 Cm, the isotopes 268 Hs and 269 Hs were expected to be produced in the 5n and 4n evaporation channel. The HIVAP code [6] predicts maximum cross sections of 1.2 and 0.6 pb, respectively, for these two channels at beam energy of E lab = 140 MeV. HIVAP predictions of cross sections of the 3n, 4n, and 5n channels of the reaction 26 Mg + 248 Cm proved to be remarkably close to experimental values [7] and we expected a similar agreement for the reaction 25 Mg + 248 Cm.Various predictions for the decay of 268 Hs through emission of an α particle of by spontaneous fission (SF) exist in the literature, but these predictions have large uncertainties and vary considerably as shown below. These uncertainties result from the strong dependence of the half-life on the α decay energy, Q α , and shape of the fission barrier. Small differences in Q α , in height of the fission barrier, or an alternative path over the multidimensional fission barrier often yield half-lives that differ by orders of magnitude. A partial α decay half-life can be derived using a Viola-Seaborg-type formula from Q α values. Half-life estimates using the most recent set of parameters [8] are typically accurate to within a factor of two for even-even nuclei. Assuming a smooth trend of Q ...

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