Speaker
Dr
Jones Michael
(LBNL)
Description
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 quasicontinuum where it promptly decayed by gamma-ray emission. The PSF can be extracted using two-step cascades from the quasicontinuum to specific 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. Preliminary results will be discussed.
References:
[1] T. K. Eriksen, et al. Phys. Rev. C 90, 044311 (2014)
[2] B.A. Brown and A.C. Larsen, Phys. Rev. Lett. 113, 252502 (2014)
[3] M.B. Chadwick, et al. Nucl. Data Sheets 112 2887 (2011)
[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)
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Contracts No. DE-AC02-05CH11231, DE-SC0014442, and No. DE-AC02-06CH11357. This research used resources of ANL's ATLAS facility, which is a DOE Office of Science User Facility.
Primary author
Dr
Jones Michael
(LBNL)
Co-author
ANL 1564 Collaboration
(ATLAS/ANL)