Comparison of complementary reactions in the production of Mt

Author(s): Nelson, Sarah; Gregorich, Kenneth; Dragojevic, Irena; Ellison, Paul; Garcia, Mitch Andre; Gates, Jacklyn; Stavsetra, Liv; Ali, Mazhar; Nitsche, Heino | Abstract: The new reaction 208Pb(59Co,n)266Mt was studied using the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. A cross section of 7.7+5.2-3.3 pb was measured at a compound nucleus excitation energy of 14.9 MeV. The measured decay properties of 266Mt and its daughters correspond well with existing data. We compare this experimental result to transactinide compound nucleus formation model predictions, and the previously studied 209Bi(58Fe,n)266Mt reaction.

Until recent years most "cold fusion" type reactions for production of odd-Z TANs used targets of 209 Bi instead of 208 Pb. The more asymmetric 209 Bi-based reactions were chosen because they were expected to have a larger cross section for the 1n exit channel product as a result of the lower effective fissility of the reaction [2]. This preference of a slightly more asymmetric system determined which reactions were used in the experimental discoveries of bohrium (Bh, Z = 107) [3,4], meitnerium (Mt, Z = 109) [5,6], roentgenium (Rg, Z = 111) [7], and a recent report on the production of Z = 113 [8].
In addition to the idea that the slightly more asymmetric reactions would give rise to higher cross sections, theoretical predictions have been made about these reactions. Świątecki, Siwek-Wilcyńska, and Wilczyński's "Fusion By Diffusion" (FBD) model [9][10][11] employs a three step description of heavy element formation by cold fusion reactions. The first step is the sticking, or capture step, where the projectile and target nuclei come into contact and are captured in a mutual nuclear and Coulomb potential minimum. The second step is the "diffusion" along the elongation coordinate, coalescing the target and projectile into a single body. The "survival" of the nucleus by emission of one neutron instead of undergoing fission or other competing de-excitation methods is the final step. Experimentally determined cross sections are typically reproduced within a factor of two by these FBD model predictions [12][13][14][15].
In an effort to investigate the role the entrance channel plays in TAN compound nucleus formation, we have undertaken a series of paired reactions which produce the same CN. These paired reactions differ by changing only the location of one proton between the target and projectile nuclei. Recently we have reported on the reaction pairs producing 258 Db via the 209 Bi( 50 Ti,n) and 208 Pb( 51 V,n) reactions [15], and 262 Bh via the 209 Bi( 54 Cr,n) and 208 Pb( 55 Mn,n) reactions [13,16]. Additionally, the pair of reactions producing 272 [22]. The decay chains observed passed through known nuclides, allowing confident assignment of Z and A. The most recent work on 266 Mt led by Hofmann et al. in 1997 [21] resulted in a three-point excitation function comprised of twelve decay chains. They fit these three data points with a Gaussian function, obtaining a peak cross section of 7.5 ± 2.7 pb. The observed alpha particle energies of 266 Mt vary between 10.48 -11.74 MeV, which is not unexpected because of its two unpaired particles. GSI reports a half-life of ms for the decay of 266 Mt. 6 .
The 208 Pb( 59 Co,n) 266 Mt experiment was conducted at the LBNL 88-Inch Cyclotron using the BGS. The BGS separates out evaporation residues (EVRs) from unreacted beam and undesirable reaction products by their differing magnetic rigidities in dilute He gas, and has been described previously in [23,24]. The beam of 59 Co 13+ passed through a 45 g/cm 2 -nat C carbon window used to separate the vacuum of the beamline from the 67 Pa of He fill gas of the BGS and its target chamber. The target wheel consisted of nine arc-shaped targets with a nominal areal density of ~460 g/cm 2 208 Pb metal on 35 g/cm 2 nat C. A thin <10 g/cm 2 layer of nat C was applied to the downstream side of the targets to improve infrared cooling and prevent target material loss. The target wheel rotation speed was 5-10 Hz. Calculations of the energy loss through the vacuum window, target, and backing were performed with SRIM-2003 [25,26].
The projectile energy expected to be optimal for production of 266 [16]. The sides of the FPD have three cards each on the top and bottom and one card on each side, and are referred to as "upstream" detectors. In addition, another set of three detector cards is placed immediately behind the main focal plane detector cards to detect light ionizing particles such as protons passing through the FPD, and is called the "punchthrough" detector. Additional details about the detector system can be found in previous publications [19]. conditions. The data files were analyzed offline, searching for EVR-and alpha-like events within the same energy gates as listed above, and >80 MeV spontaneous fission (SF) -like events (80 < E fission < 300 MeV, no MWPC signal). Once potential decay chains were identified through the offline searches, more specific searches were carried out to lifetimes of 10 4 seconds to try to identify Z = 99-100 decays with long half-lives.
The known decay properties of 266 Mt and its associated daughter products are presented in Figure 1A. The 291.5 MeV beam used in this study was 0.5 MeV below the threshold for production of the 2n product, 265 Mt. Accepted decay chains were restricted to an EVR correlated either to a minimum of two full-energy or reconstructed alpha decays, or to an alpha decay followed by an SF. The chain detection efficiency for conclusive identification of 266 Mt was calculated to be 0.92, using the method described in Chapter 2 of [29]. Table 1 contains a summary of the beam energy, integrated beam dose, and resulting cross section for this work as well as the most recent study by the GSI. Five decay chains attributed to the decay of 266 Mt were observed in this work, and these decay chains are depicted in Figure 1B. Half-life and cross section errors were treated as a special case of the Poisson distribution [30] and our reported error values are at the 68% confidence interval.
Of the five alpha decays of 266 Mt observed in these experiments, only two registered a full-energy signal in the FPD. These decays in chains 1 and 4 registered alpha particle energies of 11.26 and 10.67 MeV, respectively, which match the range of alpha energies observed previously [21]. No electron capture (EC) or SF decays attributable to 266 Mt were observed in this work, and we report a half-life of ms, consistent with the previously reported value of ms [21]. We assign an upper limit of <0.25 at the 84% confidence limit for SF decay, with a corresponding partial SF half-life upper limit of <0.013 s.  [31][32][33][34][35][36][37]. Figure 1B contains more specific information about individual nuclides.
A calculation of the expected number of randomly correlated decays was performed, using a method similar to the one described in [16]. EVRs followed by greater than two alpha-like events. Thus, it is statistically likely that the five alpha decay chains observed in this work are true events and not random correlations from unrelated signals.
A cross section of pb was measured at an excitation energy of 14.9 MeV in the 59 Co + 208 Pb reaction. Figure 2 illustrates the cross section data from the 58 Fe + 209 Bi excitation function reported by Hofmann et al. [21] and the 59 Co + 208 Pb reaction in this work. The two peak cross section values are the same within statistical uncertainty. Overall, the decay properties of 266 Mt and its daughters fit well with the previously reported values by GSI.
Świątecki has predicted 1n cross sections using a re-parameterized FBD model [9][10][11]. This re-parametrization was obtained by fitting to 18 1n cross section measurements, rather than the 12 that were available earlier, and has a second adjustable parameter reflecting drift in the asymmetry during the diffusion stage. For the 58 Fe + 209 Bi reaction, the prediction of 12.  To determine if the cross section measured in the 59 Co + 208 Pb reaction is truly at or very near the peak of the 1n excitation function, a full excitation function would need to be measured.
An additional 3-4 bombarding energies should be adequate to acquire the data needed for a more complete picture.
In  Black triangles in the upper right corner indicate the beam was turned off. "Esc" denotes alpha particles that exited the focal plane detector and missed the upstream detectors. Lifetimes following EC decay are the sum of the two decays. Dotted border indicates the decay was not directly observed. Colors signify decay the following decay modes: yellow = alpha, red = EC, green = SF.  Figure 2: Experimental results on 266 Mt. Filled squares represent GSI data from the 209 Bi( 58 Fe,n) reaction [21], the open square represents LBNL results with the 208 Pb( 59 Co,n) reaction. Horizontal error bars represent the energy width of the targets.