Production and decay of the heaviest odd-Z nuclei in the 249Bk + 48Ca reaction

Abstract

The reaction of ²⁴⁹Bk with ⁴⁸Ca has been investigated with an aim of synthesizing and studying the decay properties of isotopes of the new element 117. The experiments were performed at five projectile energies (in two runs, in 2009-2010 and 2012) and with a total beam dose of ⁴⁸Ca ions of about 9x10¹⁹ The experiments yielded data on a-decay characteristics and excitation functions of the produced nuclei that establish these to be ²⁹³117 and ²⁹⁴117 – the products of the 4n- and 3n-evaporation channels, respectively. In total, we have observed 20 decay chains of Z=117 nuclides. The cross sections were measured to be 1.1 pb for the 3n and 2.4 pb for the 4n-reaction channel. The new ²⁸⁹115 events, populated by α decay of 117, demonstrate the same decay properties as those observed for 115 produced in the ²⁴³Am(⁴⁸Ca,2n) reaction thus providing cross-bombardment evidence. In addition, a single decay of ²⁹⁴118 was observed from the reaction with ²⁴⁹Cf – a result of the in-growth of ²⁴⁹Cf in the ²⁴⁹Bk target. The observed decay chain of ²⁹⁴118 is in good agreement with decay properties obtained in 2002-2005 in the experiments with the reaction ²⁴⁹Cf(⁴⁸Ca,3n)²⁹⁴118. The energies and half-lives of the odd-Z isotopes observed in the 117 decay chains together with the results obtained for lower-Z superheavy nuclei demonstrate enhancement of nuclear stability with increasing neutron number towards the predicted new magic number N=184.