Measurement of the 208 Pb ( 55 Mn , n ) 262 Bh Excitation Function

So-called “cold” nuclear fusion reactions have been successfully employed in the production of transactinide elements, most notably in the discoveries of elements 107-111 (see [1] for a review) and the reported production of elements 112 [2] and 113 [3]. In this series, the odd-Z members were first produced by irradiating Bi targets with even-Z projectiles because the lower effective fissility [4] of these systems is expected to lead to larger reaction cross sections relative to the analogous odd-Z-projectile reactions, which use Pb targets. Recently, our group has shown that an odd-Z-projectile reaction can have a cross section comparable to that of the corresponding even-Z-projectile reaction [5]. We continued this study by measuring the excitation function of the Pb(Mn, n)Bh (Z = 107) reaction using the Berkeley Gas-filled Separator (BGS) at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. The Mn + Pb reaction had previously been studied by Oganessian et al. [6] using a rotating-drum technique. Their experiments were unable to conclusively identify Bh so we chose to re-examine this reaction using the improved sensitivity of the BGS. The initial projectile lab-frame energy in the center of the 470-μg/cm Pb targets was 264.0 MeV, chosen using the “Optimum Energy Rule” proposed by Świątecki, Siwek-Wilczyńska, and Wilczyński [7]. Additional energies of 260.0 MeV and 268.0 MeV were also studied and the measured excitation function is shown in Fig. 1. Of the 33 total Bh decay chains observed at all three projectile energies, 22 were assigned to Bh and 11 were assigned to Bh. Production of ground state Bh was favored at all three energies. The observed decay chains are in good agreement with previously reported data on the decay of Bh [8]. A slightly improved half-life of ms was measured for 21 16 84− Bh. A new alpha group from Bh with energy 9657 keV was also observed. Neither Bh nor Bh was observed to decay by spontaneous fission (SF), leading to upper limit SF branches of < 11% and < 24%, respectively, and lower-limit partial SF half-lives of > 640 ms and > 30 ms, respectively (84% confidence level in all cases). At 268.0 MeV, two additional decay chains were observed and attributed to Bh, the 2n product of complete fusion. The observed magnetic rigidity of all Z = 107 evaporation residues was 2.16 ± 0.03 T m (statistical uncertainty only), corresponding to an average evaporation residue charge-state of approximately +8.0. Also shown in Fig. 1 is a theoretical prediction for the Pb(Mn, n)Bh excitation function, calculated with the “Fusion by Diffusion” theory described in [7]. The measured excitation function has a greater peak cross section and is shifted to higher projectile energies than the prediction. These observations are correlated and may be indicative of the late onset of second-chance fission in the fused system. This may be caused by the fission barrier in Bh being several hundred keV larger than expected. The maximum cross section of pb measured for the 180 150 540− Pb(Mn, n)Bh reaction at 264.0 MeV is much larger than that of 163 ± 34 pb reported for the analogous Bi(Cr, n)Bh reaction [8]. The Cr + Bi reaction may have been studied at projectile energies too high for optimum production of the 1n product, and the current results suggest that the complete excitation function of this reaction should be measured. The large cross section of the Mn + Pb reaction suggests that it may be possible to perform detailed nuclear structure studies of the decay of Bh using alpha-gamma correlation techniques.