Identification of the new isotopeBa114and search for its α and cluster radioactivity

Abstract

Using the on-line mass separator at the Gesellschaft fur Schwerionenforschung Unilac we produced Ba through the Ni( Ni, 2n) Ba reaction and measured its production cross section to be 0.20+o 09 pb. The new isotope Ba represents the heaviest N = Z+ 2 nucleus known to date. With KE-E telescopes we measured the total (P-decay) half-life to be Tp = 0.43+o~s s and the partial o.-decay half-life to be T ) 1.2 x 10 s (1 Mev( E' ( 4 MeV) for Ba. With track detectors we found a half-life for spontaneous C emission T& & 1.1 x 10 s based on three carbon events. PACS number(s): 23.70. +j, 23.60.+e, 25.70.Jj, 27.60. +jCluster radioactivity is now a well established, although rare, decay mode of heavy nuclei. Intense experimental research in the last decade has led to the detection of 20 cases of spontaneous emission of clusters ranging from C to Si from trans-lead nuclei, with branching ratios relative to o, decay &om 10 down to 10 and partial half-lives from 10~u p to 10 s [1]. The general features of this new radioactive decay mode have been established; above all, its strong dependence on the barrier penetration factor and consequently on the Q value, which must be large in order to compensate for the small preformation factors typical of such complew clusters [2]. For this reason, all heavy residual nuclei resulting &om cluster emission have been found so far to differ &om the doubly magic Pb by three nucleons at most. Analogously, a new island of cluster radioactivity having residual nuclei close to doubly magic Sn has been predicted by Greiner et al. and Poenaru et al. [3,4]. They found the most favorable case to be C emission from Ba and calculated the decay rate using various mass predictions [5] for the then unknown Ba nucleus. The results of their predictions strongly depend on the adopted Q value: a 2-MeV spread in the Q values results in a factor 10 spread in the partial half-lives. More recently others [6 -10] have taken up the challenge of calculating the decay rate for C emission from~~Ba; their results for the same choice of Q value span more than eight orders of magnitude. This situation is in complete contrast with what is found in the translead region of cluster radioactivity, where Q values are generally known and all models predict half-lives for the same decay mode that typically agree within an order of magnitude [11]. It is clear that the possible island of cluster radioactivity in the trans-tin region would be interesting to ex-plore. Apart &om the possibility of discriminating among different theoretical approaches, the mere detection and measurement of the partial C decay rate of 4Ba could shed light on the behavior of cluster radioactivity in this region of the Chart of the Nuclides far below Pb, possibly providing information on the interplay with o. decay and on the role of the nearly double shell closure in a nucleus such as Sn far from the stability line. The first searches for

Direct observation of theBa114→Xe110→Te106→Sn

“…The BaF + ions were caught on a stopper foil at the centre of an array of ∆E-E silicon-plus-plastic-scintillator telescopes. In the initial experiment [9,10], the observation of three 12 C decay events was reported, but following the second experiment [16,17] with improved background subtraction, these claims were retracted. In that work, the half-life of 114 Ba was reported to be 0.43…”

“…Another interesting aspect in this region is the prediction of cluster radioactivity [7]. It has been widely reported that the most promising candidate for this type of decay in the region is the 12 C decay 114 Ba→ 102 Sn [8][9][10], and that the theoretical predictions of the partial half-life depend very sensitively on the Q value for 12 C emission (Q12 C ) [8]. The value of Q12 C is therefore an important quantity in the design of experiments to search for this novel decay mode.…”

“…Presently, the only barium isotope which has been shown to decay by α-particle emission is 114 Ba [14]. were performed using the on-line mass separator at the GSI UNILAC by Guglielmetti et al [9,10,16,17]. In that work, a 58 Ni beam with 4.9 MeV/A (284 MeV) was used, for which the cross section was reported to be 0.2…”

Microscopic description of cluster radioactivity in actinide nuclei

“…They range from 221 Fr up to 242 Cm. The emission of 14 C, 20 O, 23 F, [24][25][26] Ne, [28][29][30] Mg, and 32,34 Si has been observed. The common factor of all cluster radioactivity events is the heavy-mass residue which is in the neighborhood of the doubly-magic 208 Pb.…”

Discovery of the barium isotopes

“…In 1995 Guglielmetti et al announced in Identification of the new isotope 114 Ba and search for its α and cluster radioactivity the discovery of 114 Ba [15]. 114 Ba was produced at the Gesellschaft für Schwerionenforschung Unilac via the fusion evaporation reaction 58 Ni( 58 Ni,2n) 114 Ba at 4.2 MeV/u and identified using an on-line mass separator.…”