Speaker
Francesca Giacoppo
(University of Oslo)
Description
F. Giacoppo1, L. Bernstein2, D. Bleuel2, P.A.Butler3, T. K. Eriksen1, A. Görgen1, M. Guttormsen1, T. W. Hagen1, A.C. Larsen1, H. T. Nyhus1, T. Renstrøm1, S. Rose1, S. Siem1, G.M. Tveten1, A.Voinov4, M. Wiedeking5, and J. Wilson6
1Department of Physics, University of Oslo, N-0316 Oslo, Norway
2Lawrence Livermore National Laboratory, USA
3Oliver Lodge Laboratory, University of Liverpool, United Kingdom
4Department of Physics and Astronomy, Ohio University, USA
5 iThemba LABS, South Africa
6 IPN Orsay, CNRS-IN2P3, Orsay, France
The density and width of nuclear excited states increase with excitation energy towards the particle separation energies creating a quasicontinuum of levels in heavier nuclei. Nuclear properties in this excitation-energy region are measured in term of statistical quantities such as the nuclear level density (NLD) and the radiative strength function (RSF) which represents the average γ-ray transition probability between states of a given energy difference.
The NLD and the RSF are fundamental input parameters for calculating nuclear reaction cross-sections and reaction rates relevant for astrophysical applications. Indeed recent studies clearly show that relatively small changes to the overall shape of the RSF, such as pygmy resonances, can have an order of magnitude effect on the rate of elemental formation in the r-process. Often measured RSF are completely lacking or are insufficient to carry out calculations and simplistic model parametrizations or extrapolations have to be made, which can lead to significant uncertainties in the expected results.
The Oslo nuclear physics group has developed a method to determine simultaneously the NLD and the RSF from particle-γ coincidence measurements [1, 2]. Several experiments have been carried out at the Oslo Cyclotron Laboratory (OCL) using light particle beams on various target materials.
Recently an unexpected low-energy increase in the radiative strength function of light and medium-mass nuclei, such as 56,57Fe [3] and 93-98Mo [4], has been measured at OCL. This enhancement has been confirmed in 95Mo with a different experimental technique and model-independent analysis [5]. At present there exists no theoretical model, which can reproduce these results. However, the question is whether this structure persists when approaching the neutron drip line: in this case an increase of the neutron-capture rates up to two orders of magnitude is expected [6].
An experimental program to study the gamma-ray strength function of neutron reach nuclei has been recently proposed at HIE-ISOLDE [7]. This contribution aims at presenting the features of the Oslo method and the technical requirements for its applicability to an inverse-kinematic experiment with radioactive beams. The present status of the low-energy increase and the possible impact on reaction rates relevant in astrophysics will be also discussed.
[1] A. Schiller et al., Nucl. Instrum. Methods Phys. A 447, 498 (2000).
[2] A. C. Larsen et al., Phys. Rev. C 83, 034 315 (2011).
[3] A.Voinov et al., Phys. Rev. Lett. 93, 142504 (2004).
[4] M. Guttormsen et al., Phys. Rev. C 71, 044307 (2005);
[5] M. Wiedeking et al., Phys. Rev. Lett. 108, 162503 (2012).
[6] A. C. Larsen and S. Goriely, Phys. Rev. C 82, 014318 (2010).
[7] S. Siem et al. “Statistical properties of warm nuclei: Investigating the low-energy enhancement in the gamma strength function of neutron rich nuclei”. Submitted proposal to INTC – October 2012
Primary author
Francesca Giacoppo
(University of Oslo)
