Nuclear level density and $\gamma$-ray strength functions of $^{121,122}$Sn below the neutron separation energy are extracted with the Oslo method using the ($^3$He,$^3$He$^\prime\gamma$) and ($^3$He,$\alpha \gamma$) reactions. The level densities of $^{121,122}$Sn display step-like structures, interpreted as signatures of neutron pair breaking. An enhancement in both strength functions, compared to standard models for radiative strength, is observed in our measurements for $E_\gamma \gtrsim 5.2 $ MeV. This enhancement is compatible with pygmy resonances centered at $\approx 8.4(1)$ and $\approx 8.6(2)$ MeV, respectively, and with integrated strengths corresponding to $\approx1.8^{+1}_{-5}%$ of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in $^{116-119}$Sn. Experimental neutron-capture cross reactions are well reproduced by our pygmy resonance predictions, while standard strength models are less successful. The evolution as a function of neutron number of the pygmy resonance in $^{116-122}$Sn is described as a clear increase of centroid energy from 8.0(1) to 8.6(2) MeV, but with no observable difference in integrated strengths

In this work, we have reviewed the Oslo method, which enables the simultaneous extraction of level density and gamma-ray transmission coefficient from a set of particle-gamma coincidence data. Possible errors and uncertainties have been investigated. Typical data sets from various mass regions as well as simulated data have been tested against the assumptions behind the data analysis.Comment: 23 pages, 33 figures. As published in Phys. Rev.

The nuclear level densities of 118,119 Sn and the γ-ray strength functions of 116,118,119 Sn below the neutron separation energy are extracted with the Oslo method using the ( 3 He, αγ) and ( 3 He, 3 He γ) reactions. The level density function of 119 Sn displays step-like structures. The microcanonical entropies are deduced from the level densities, and the single neutron entropy of 119 Sn is determined to be (1.7±0.2) k B . Results from a combinatorial model support the interpretation that some of the low-energy steps in the level density function are caused by neutron pair-breaking. An enhancement in all the γ-ray strength functions of 116−119 Sn, compared to standard models for radiative strength, is observed for the γ-ray energy region of (4 − 11) MeV. These small resonances all have a centroid energy of 8.0(1) MeV and an integrated strength corresponding to 1.7(9)% of the classical Thomas-Reiche-Kuhn sum rule. The Sn resonances may be due to electric dipole neutron skin oscillations or to an enhancement of the giant magnetic dipole resonance.

Using the REX-ISOLDE facility at CERN the Coulomb excitation cross sections for the 0 + gs → 2 + 1 transition in the β-unstable isotopes 100,102,104 Cd have been measured for the first time. Two different targets were used, which allows for the first extraction of the static electric quadrupole moments Q(2 + 1 ) in 102,104 Cd. In addition to the B(E2) values in 102,104 Cd, a first experimental limit for the B(E2) value in 100 Cd is presented. The data was analyzed using the maximum likelihood method. The provided probability distributions impose a test for theoretical predictions of the static and dynamic moments. The data are interpreted within the shell-model using realistic matrix elements obtained from a G-matrix renormalized CD-Bonn interaction. In view of recent results for the light Sn isotopes the data are discussed in the context of a renormalization of the neutron effective charge. This study is the first to use the reorientation effect for post-accelerated short-lived radioactive isotopes to simultaneously determine the B(E2) and the Q(2 + 1 ) values.

163,164 Dy nuclei have been measured by use of the Oslo method on data from pick-up ( 3 He, α) and inelastic scattering ( 3 He, 3 He ) reactions, respectively. The level densities for these dysprosium isotopes together with previously measured [160][161][162] Dy are extracted in the region below the neutron binding energy. Thermodynamic properties are deduced within both micro-canonical and canonical ensemble theories. A phase transition from the pair-correlated state at low energies to a less correlated or uncorrelated state is studied in both ensembles. It is investigated whether the temperature of the nucleus is constant or a varying function of excitation energy. It is found that above an excitation energy of 3 MeV the temperature of all five dysprosium nuclei have a constant value within the experimental uncertainties. The impact of a constant-temperature level density versus a Fermi gas level density is discussed with respect to the canonical heat capacity.