First observation of low-energyγ-ray enhancement in the rare-earth region

Abstract

The γ-ray strength function and level density in the quasi-continuum of 151,153 Sm have been measured using BGO shielded Ge clover detectors of the STARLiTeR system. The Compton shields allow for an extraction of the γ strength down to unprecedentedly low γ energies of ≈ 500 keV. For the first time an enhanced lowenergy γ-ray strength has been observed in the rare-earth region. In addition, for the first time both the upbend and the well known scissors resonance have been observed simultaneously for the same nucleus. HauserFeshbach calculations show that this strength enhancement at low γ energies could have an impact of 2-3 orders of magnitude on the (n,γ) reaction rates for the r-process nucleosynthesis.

Statistical γ -decay properties of Ni64 and deduced ( n,γ ) cross section of the Campo1, Garrote2, Eriksen3 et al. 2016Phys. Rev. CSelf Cite

“…(19). For deformed nuclei, the GDR strength is defined as the sum of two components, each of them given by Eq.…”

“…For deformed nuclei, the GDR strength is defined as the sum of two components, each of them given by Eq. (19). Therefore, the GDR parameters that needed to be determined were T f , E E1,i , E1,i , and σ E1,i , where i = 1,2 indicates whether the oscillation is parallel or perpendicular to the main symmetry axis.…”

“…One of the most surprising features observed in the γ -strength function is the increased probability of γ decay with γ energies below 2-4 MeV, first seen in 56,57 Fe in 2004 [11]. Since then, this low-energy enhancement or upbend has been measured for several nuclei from Sc to Sm [11][12][13][14][15][16][17][18][19] and confirmed with an independent method for 95 Mo [20]. The physical mechanisms underlying this enhancement are still not clarified.…”

Low energy magnetic radiation enhancement in thef7/2shell

“…In the last decade things have become more complicated, since measurements of the γSF [10,[14][15][16][17][18][19][20][21][22][23][24][25][26] have revealed a newly observed minimum around E γ ≃ 2 − 4 MeV, so besides the high E γ enhancement, there is also a low E γ enhancement. The first attempts to understand the low-E γ enhancement [14,15,17] used the KMF model to describe the GDR; the contribution of the giant magnetic dipole resonance to the total γSF is fitted by a Lorentzian, similarly to the E2 resonance, while the low-E γ region is described by a separate term that has a power-law parametrization.…”

“…Recent studies in rare earth nuclei have shown [9,10] that bumps in the E γ ≈ 3 MeV region are of M1 character. Actually, the M1 transitions seem to play an active role in the γSF being described also by a Lorentz line [11] based on the existence of a resonance that originates from spin-flip excitations in the nucleus [12,13].…”