Program - Brookhaven National Laboratory

Using linearly-polarized laser-Compton scattering γ-rays, partial E1 and M1 photoneutron cross sections
along with total cross sections were determined for 207,208 Pb at four energies near threshold by measuring
anisotropies in photoneutron emission. Separately , total photoneutron cross sections were measured for
206,207,208 Pb with a high-efficiency 4π neutron detector. The E1 strength dominates over the M1 strength
in the neutron channel where E1 photoneutron cross sections clearly show extra strength of the pygmy
dipole resonance in 207,208 Pb near the neutron threshold corresponding to 0.32% - 0.42% of the Thomas-
Reiche-Kuhn sum rule. Several µ 2 N units of B(M1) ↑ strength were observed in 207,208 Pb just above neutron
threshold, which correspond to an M1 cross section less than 10% of the total photoneutron cross section.
This contribution is based on our recent publication [1].
Corresponding author: H. Utsunomiya
[1] T. Kondo, H. Utsunomiya, S. Goriely et al., Phys. Rev. C 86, 014316 (2012).
OF 4 2:40 PM
Experimental Level Densities and Gamma Strength Functions with the Oslo Method
S. Siem, A. Bürger, F. Giacoppo, A. Görgen, M. Guttormsen, T.W. Hagen, A.C. Larsen, H.T. Nyhus, J.
Rekstad, T. Renstrom, S.J. Rose, G. M Tveten., University of Oslo, Dept. of Physics, P.OBox 1048
Blindern, 0316 Oslo, Norway. L. A. Bernstein, D. Bleuel,, Lawrence Livermore National Laboratory 7000
East Avenue, Livermore, CA 94550, USA. F. Gunsing, CEA Saclay, DSM/Irfu/SPhN, F-91191
Gif-sur-Yvette Cedex, France. A. Schiller, A. Voinov, Department of Physics and Astronomy, Ohio
University, Athens, Ohio 45701, USA. M. Wiedeking, iThemba LABS, P.O. Box 722, 7129 Somerset
West, South Africa. J.N. Wilson, Institut de Physique Nucleaire d’Orsay, 15 rue G. Glemenceau, 91406
Orsay Cedex, France.
The nuclear physics group at the Oslo Cyclotron Laboratory has developed a unique technique to extract
simultaneously the level density and γ-strength function from primary γ-ray spectra [1]. These are
fundamental properties of the atomic nucleus and important input parameters in reaction cross-section
calculations, used in reactor physics simulations and astrophysics models of formation of heavy elements
in explosive stellar environments. I will give a short description of our facility and the Oslo method. Then
the latest result from experiments done in Oslo will be presented, including resent data on actinide nuclei,
relevant for the Thorium fuel cycle, where we observe a large orbital scissors strength for several actinide
nuclei [2]. We have also observed another resonance around 7-9 MeV in Sn isotopes which might be due
to neutron skin oscillations [3]. Both these resonances increase the gamma decay probability and should
be included cross-sections calculations. Finally an unexpected enhancement of the γ-strength function at
low gamma energy has been observed in several nuclei [4]. This low energy enhancement has the potential
of increasing neutron-capture rates with up to two orders of magnitude if also present in very neutron-rich
nuclei [5]. The present status of the low-energy enhancement, including new data on Cd and Zr, will also
be presented.
[1] A. Schiller et al., Nucl. Instrum. Methods Phys. Res. A 447, 494 (2000). [2] M.Guttormsen et al.
submitted to Phys. Rev. Lett [3] H.K. Toft et al., Phys. Rev. C 81, 064311 (2010). Phys. Rev. C 83,
044320 (2011) [4] A. Voinov et al., Phys. Rev. Lett 93, 142504 (2004); M. Guttormsen et al., Phys.Rev.
C 71, 044307 (2005); [5] A.C. Larsen et al., Phys. Rev. C82, 014318 (2010).
Session PA Neutron Cross Section Measurements
Thursday March 7, 2013
Room: Met East at 3:30 PM
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