GDR Built in Rn Nuclei – High Spin Selection
M. Kmiecik, A. Maj, W. Królas, W. Męczyński, J. Styczeń, M. Ziębliński, M. Brekiesz,
B. Million1, A. Bracco1, F. Camera1, S. Leoni1, O. Wieland1,S. Brambilla1, B. Herskind2,
M. Kicińska-Habior3
1
INFN Milano, Italy; 2NBI Copenhagen, Denmark; 3Warsaw University, Poland
Y [a. u.]
The shape of nuclei at large angular momenta, close to the fission barrier, was studied using the GDR decay measurement. For this purpose we have chosen the reaction of 96 MeV 18O on 198Pt forming
the compound nucleus 216Rn at angular momentum distribution with lmax=42 , and an excitation energy
E*=56 MeV. This reaction feeds the high-spin isomers in 211Rn (T1/2=201 ns, I=63/2-) and 212Rn (T1/2=154
ns, I=30+) through the intense channels 4n and 5n.
The experiment was performed at LNL in Legnaro using the recoil-catcher geometry technique [1]. The
residual nuclei, ejected from the target, were stopped in a 6 m thin Mylar catcher having central 6 mm hole
for the beam. The delayed radiation emitted by the stopped residues was detected in a cylindrical eightsegmented BGO shield. A Ge-detector was installed at 146o for discrete -radiation measurement. To measure high-energy -rays, the target was surrounded by the HECTOR array and the efficient multiplicity filter
HELENA.
The signals from the prompt high-energy -rays from the target were recorded on the tape only when followed by the delayed radiation detected in the catcher set-up. Additionally, we measured the time between
the reaction and the isomeric decay, and the sum energy of the delayed -transitions. To select the GDR decay feeding the high-spin isomer we set the gate on the time spectrum corresponding to the isomer lifetime.
The obtained GDR spectra were analysed with
the Monte Carlo version of the CASCADE code
total
based on the statistical model. The calculated spectra
f > 12
7
10
isomer gated
were fitted to the experimental data (total and for the
cascade total
multiplicity fold >12). The best fit was obtained for
cascade f > 12 (I ~> 30 )
6
the GDR energy EGDR≈13.5 MeV and the GDR
10
cascade I >= 40 
width GDR≈6.1 MeV. Fig. 1 shows the analysed
experimental data and the results of the performed
5
10
calculations. The total (not gated) data are presented
in comparison to the spectra gated both with high
4
fold (>12) and with the isomeric decays. The exper10
imental GDR spectrum corresponding to high folds
(>12) was fitted with calculated one assuming spin
3
10
higher than 30. The data corresponding to the isomer gating are well described by the calculations
with the same GDR parameters, but only if one se2
10
lects angular momenta of compound nucleus higher
than 40. This shows that the isomeric decays gates
1
imply the choice of nuclei possessing the highest
10
4
6
8
10
12
14
16
18
20
angular momenta, very close to the fission limit.
E [MeV]
Since the GDR parameters for all analysed data are
the same, the preliminary results of our analysis
show that the chosen subset of Rn nuclei close to the
fission barrier have small deformations. This is con- Fig. 1. The experimental GDR spectra (points)
sistent with the results of the thermal shape fluctua- corresponding to total, high fold gated, and gattion model which predicts for heavy nuclei basically ed with isomer data, and the results of Monte
no spin dependence of the effective nuclear shapes Carlo CASCADE code fit (lines) with
EGDR=13.5 MeV and GDR≈6.1 MeV
probed by the GDR [2].
[1]. A. Maj et al., Proceedings of Camerino Conference “Exotic Nuclei at the Drip Line”, 2002, in print;
[2]. M. Mattiuzzi et al., Nucl. Phys. A612 (1997) 262; M. Kmiecik et al., Nucl. Phys. A674 (2000) 29.