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NUSTAR-SHE-03 GSI SCIENTIFIC REPORT 2009
172
GSITemplate2007
σ / nbarn
Cross-sections for production of neutron deficient Sg - isotopes in reactions
54 Cr + 206,207,208 Pb
F.P. Heßberger 1,2 , S. Antalic 3 , B. Streicher 1,4 , B. Sulignano 5,1 , D. Ackermann 1 , M. Block 1 , S. Heinz 1 ,
S. Hofmann 1,6 , J. Khuyagbaatar 1 , B. Kindler 1 , I. Kojouharov 1 , B. Lommel 1 , R. Mann 1
1 GSI-Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany; 2 Helmholtz - Institut Mainz, Mainz, Germany,
3 Comenius University Bratislava, Bratislava, Slovakia , 4 University of Groningen, The Netherlands, 5 CEA Saclay,
Gif-sur-Yvette, France, 6 Goethe-Universität Frankfurt, Frankfurt, Germany
Due to low cross-sections for the production of heaviest
nuclei the choice of the optimum reaction for specific
nuclides is crucial for nuclear structure investigations
based on α-γ-decay spectroscopy, which demand maximum
numbers of observed events. Following previous
decay studies of 255 No [1] and 257 Rf [2] an experiment to
investigate 259 Sg, the next heavier N=153 isotone, for
which only rough decay properties were known so far [3],
was performed at SHIP in March 2009. The most efficient
way to produce neutron deficient isotopes of seaborgium
is employing reactions of 54 Cr with Pb - isotopes. Excitation
functions for 54 Cr + 208 Pb [2] and 207 Pb [4] had been
measured previously. The results are displayed in fig. 1.
1
0,1
0,01
10 12 14 16 18 20 22 24 26 28 30 32
E* / MeV
Figure 1: Excitation functions for the reactions
208 Pb( 54 Cr,xn) 262-x Sg (blue symbols), 207 Pb( 54 Cr,xn) 261-x Sg
(red symbols),
206 Pb( 54 Cr,xn) 260-x Sg (wine symbols)
(x=1,2). The full symbols represent the 1n - channels, the
open symbols the 2n - channels. The lines represent the
results of HIVAP calculations (full line = 1n channel,
dashed line = 2n channel) for the corresponding systems.
The arrows denote the positions of the Bass model fusion
barriers [5] for the respective target - projectile combinations.
HIVAP [6] calculations using parameterisations to reproduce
the 1n and 2n cross-sections for the reaction 54 Cr +
208 Pb were performed. Evidently this parameterisation
satisfactorily reproduces also the 2n cross-sections for
54 Cr + 207 Pb, while the 1n cross-section is underpredicted
by roughly a factor of two. Going from 207 Pb to 206 Pb
HIVAP predicts a decrease by roughly factors of 2.5 and
4 for the 1n and 2n cross-sections, respectively, and in
addition a shift of the excitation energy for the 1n cross-
section maximum by ≈1.5 MeV towards higher values.
Since the thus expected cross-section of ≈150 pb for
259 Sg is only about a factor of two lower than the measured
value for the reaction 207 Pb( 54 Cr,2n) 259 Sg and an astonishing
high cross-section of ≈320 pb was obtained for
the reaction 208 Pb( 54 Cr,1n) 259 Sg [7] it seemed valuable to
test the reaction 206 Pb( 54 Cr,1n) 259 Sg as an alternative way
of production. The result is shown in fig. 1 (wine symbols).
A cross-section of ≈1 nb was measured for 259 Sg,
which is even about 50% higher than the 1n cross-section
using the more neutron rich target isotope 207 Pb. Differences
of the 2n cross-section are less dramatic. One data
point, close to the expected [6] maximum was taken. We
obtained σ(2n) = 0.14±0.04 nb at E * = 22.5 MeV, which
is roughly a factor of two higher than the predicted value
and a factor of two lower than the 2n cross-section for the
reaction 54 Cr + 207 Pb. This more 'regular' behavior does
not suggest an enhanced stability of 259 Sg against prompt
fission during the deexcitation process. The production
ratios σ( 260 Sg,2n) / σ( 259 Sg,2n) ≈ 1.7 and σ( 260 Sg,1n) /
σ( 259 Sg,1n) ≈ 0.6 do also not suggest a reduction of the
SHIP transmission for 260 Sg due to a short-lived isomeric
state decaying during the separation process. It rather
hints at a strong nuclear structure influence of the target
nucleus on the fusion probability at energies far below the
fusion barrier, established at E * = 23.2±0.5 MeV for the
three systems. More detailed measurements of excitation
functions will show if this is just a local effect connected
with the specific projectile or a more general phenomenon.
In the latter case, only slightly decreasing 1n crosssections
going from 208 Pb to the lighter isotopes 206 Pb or
even 204 Pb, would not only give new insight into the process
of 'cold' fusion, but also enhance our chances of continuing
our program on detailed nuclear structure investigations
of nuclei in the vicinity of the N = 152 subshell
towards isotopes of elements above seaborgium (Z=106).
References
[1] F.P. Heßberger et al., EPJ A 29, 165 (2006)
[2] B. Streicher, PHD, Comenius University Bratislava,
2006, and to be published
[3] F.P. Heßberger et al. EPJ A 41, 145 (2009)
[4] B. Sulignano, PHD, Johannes-Gutenberg Universität
Mainz, 2007
[5] R. Bass, Nucl. Phys. A 231, 45 (1974)
[6] W. Reisdorf, Z. Phys. A 300, 227 (1981)
[7] C.M. Folden III et al. Phys. Rev. C 79, 027602 (2009)