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Register a new userFirst observation of low-energy {gamma}-ray enhancement in the rare-earth region
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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.
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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 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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Excited states in the well-deformed rare earth isotopes $^{154}$Sm and $^{166}$Er were populated via ``safe Coulomb excitation at the Munich MLL Tandem accelerator. Conversion electrons were registered in a cooled Si(Li) detector in conjunction with a magnetic transport and filter system, the Mini-Orange spectrometer. For the first excited $0^+$ state in $^{154}$Sm at 1099 keV a large value of the monopole strength for the transition to the ground state of $rho^2(text{E0}; 0^+_2 to 0^+_text{g}) = 96(42)cdot 10^{-3}$ could be extracted. This confirms the interpretation of the lowest excited $0^+$ state in $^{154}$Sm as the collective $beta$-vibrational excitation of the ground state. In $^{166}$Er the measured large electric monopole strength of $rho^2(text{E0}; 0^+_4 to 0^+_1) = 127(60)cdot 10^{-3}$ clearly identifies the $0_4^+$ state at 1934 keV to be the $beta$-vibrational excitation of the ground state.
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