Michael Jones
LBNL
Understanding the Nature of the Low-energy Enhancement in the Photon Strength Function of 56Fe
The recent discovery of the low-energy enhancement in the photon strength function (PSF) of medium mass nuclei in the Fe and Mo region [1] has attracted great experimental and theoretical attention, as it may represent a new decay-mode [2]. The presence of an enhanced decay probability of low-energy gammas rays below the neutron threshold has the potential to greatly affect a broad range of applications including the astrophysical r-process and nuclear reactors [3-4]. Recent shell model calculations on 94Mo, 95Mo and 90Zr show that the enhancement could be due to a large B(M1) strength for low energy gamma-rays caused by orbital angular momentum recoupling of high-j orbits [5], while other mechanisms suggest an enhanced E1 strength [6]. A recent experiment designed to confirm the multipolarity and determine the electric or magnetic character of transitions in the region of the PSF enhancement in 56Fe was performed at ATLAS/ANL using GRETINA and the Phoswich Wall [7]. A 16 MeV proton beam was used to inelastically excite an 56Fe target to the quasi-continuum where it promptly decayed by gamma-ray emission. The PSF can be extracted using two-step cascades from the quasi-continuum to specifc low-lying levels by a model independent method first employed in 95Mo [8]. This method is being extended to take advantage of GRETINA as a polarimeter to obtain angular and polarization information in the region of the low-energy enhancement of the PSF. The results of this experiment as well as plans for future work will be discussed. [1] T. K. Eriksen et al. Phys. Rev. C 90, 044311 (2014) [2] B.A. Brown and A.C. Larsen, Phys. Rev. Lett, 252502 (2014) [3] Report of the Nuclear Physics and Related Computational Science R&D for Advanced Fuel Cycles Workshop, DOE Offices of Nuclear Physics and Advanced Scientific Computing Research. (August 2006). [4] A.C. Larsen and S. Goriely, Phys. Rev. C 82, 014318 (2010) [5] R. Schwengner, S. Frauendorf, and A.C. Larsen, Phys. Rev. Lett. 111, 232504 (2013) [6] E. Litvinova and N. Belov, Phys. Rev. C 88, 031302(R) [7] D. G. Sarantites et al. Nuclear Instruments and Methods A, 790, 42-55 (2015) [8] M. Wiedeking et al. Phys. Rev. Lett. 108, 162503 (2012)