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Register a new userEvidence for dipole nature of the low-energy $gamma$ enhancement in $^{56}$Fe
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The $gamma$-ray strength function of $^{56}$Fe has been measured from proton-$gamma$ coincidences for excitation energies up to $approx 11$ MeV. The low-energy enhancement in the $gamma$-ray strength function, which was first discovered in the ($^3$He,$alphagamma$)$^{56}$Fe reaction, is confirmed with the ($p,p^primegamma$)$^{56}$Fe experiment reported here. Angular distributions of the $gamma$ rays give for the first time evidence that the enhancement is dominated by dipole transitions.
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A model-independent technique was used to determine the $gamma$-ray Strength Function ($gamma$SF) of $^{56}$Fe down to $gamma$-ray energies less than 1 MeV for the first time with GRETINA using the $(p,p)$ reaction at 16 MeV. No difference was observed in the energy dependence of the $gamma$SF built on $2^{+}$ and $4^{+}$ final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.
The {gamma}-ray strength function and level density in the quasi-continuum of 151,153Sm have been measured using BGO shielded Ge clover detectors of the STARLiTeR system. The Compton shields allow for an extraction of the {gamma} strength down to unprecedentedly low {gamma} energies of about 500 keV. For the first time an enhanced low- energy {gamma}-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. Hauser-Feshbach calculations show that this strength enhancement at low {gamma} energies could have an impact of 2-3 orders of magnitude on the (n,{gamma}) reaction rates for the r-process nucleosynthesis.
Isospin properties of dipole excitations in 74 Ge are investigated using the ({alpha},{alpha}{gamma}) reaction and compared to ({gamma},{gamma}) data. The results indicate that the dipole excitations in the energy region of 6 to 9 MeV adhere to the scenario of the recently found splitting of the region of dipole excitations into two separated parts: one at low energy, being populated by both isoscalar and isovector probes, and the other at high energy, excited only by the electromagnetic probe. Relativistic quasiparticle time blocking approximation (RQTBA) calculations show a reduction in the isoscalar E1 strength with an increase in excitation energy, which is consistent with the measurement.
The electric dipole strength in 120Sn has been extracted from proton inelastic scattering experiments at E_p = 295 MeV and at forward angles including 0 degree. Below neutron threshoild it differs from the results of a 120Sn(gamma,gamma) experiment and peaks at an excitation energy of 8.3 MeV. The total strength corresponds to 2.3(2)% of the energy-weighted sum rule and is more than three times larger than what is observed with the (gamma,gamma) reaction. This implies a strong fragmentation of the E1 strength and/or small ground state branching ratios of the excited 1- states.
The electromagnetic character of the $Delta I=1$ transitions connecting the one- to zero-phonon and the two- to one-phonon wobbling bands should be dominated by an $E2$ component, due to the collective motion of the entire nuclear charge. In the present work it is shown, based on combined angular correlation and linear polarization measurements, that the mixing ratios of all analyzed connecting transitions between low-lying bands in $^{135}$Pr interpreted as zero-, one-, and two-phonon wobbling bands, have absolute values smaller than one. This indicates predominant $M1$ magnetic character, which is incompatible with the proposed wobbling nature. All experimental observables are instead in good agreement with quasiparticle-plus-triaxial-rotor model calculations, which describe the bands as resulting from a rapid re-alignment of the total angular momentum from the short to the intermediate nuclear axis.
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