We present a new iterative procedure to extract the level density and the γ strength function from primary γ spectra for energies close up to the neutron binding energy. The procedure is tested on simulated spectra and on data from the 173 Yb( 3 He,α) 172 Yb reaction.

Radiative strength functions (RSFs) for the 56,57 Fe nuclei below the separation energy are obtained from the 57 Fe( 3 He, αγ) 56 Fe and 57 Fe( 3 He, 3 He ′ γ) 57 Fe reactions, respectively. An enhancement of more than a factor of ten over common theoretical models of the soft (Eγ < ∼ 2 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) is observed. Two-step cascade intensities with soft primary transitions from the 56 Fe(n, 2γ) 57 Fe reaction confirm the enhancement.PACS numbers: 25.40. Lw, 25.55.Hp, 25.20.Lj, 27.40.+z Unresolved transitions in the nuclear γ-ray cascade produced in the decay of excited nuclei are best described by statistical concepts: a radiative strength function (RSF) f XL (E γ ) for a transition with multipolarity XL and energy E γ , and a level density ρ(E i , J π i ) for initial states i at energy E i with equal spin and parity J π i yield the mean value of the partial decay width to a given final state f [1]Most information about the RSF has been obtained from photon-absorption experiments in the energy interval 8-20 MeV, i.e., for excitations above the neutron separation energy S n . There, the giant electric dipole resonance (GEDR) is dominant. Data on the soft (E γ < 3-4 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) remain elusive. Corresponding data from discrete transitions show large fluctuations and are biased toward high transition strengths due to experimental thresholds. First data in the statistical regime have been obtained from the 147 Sm(n, γα) 144 Nd reaction [2]. They indicate a moderate enhancement of the soft E1 RSF compared to a Lorentzian extrapolation of the GEDR. For spherical nuclei, in the framework of Fermiliquid theory, this enhancement is explained by a temperature dependence of the GEDR width [3], the Kadmenskiȋ-Markushev-Furman (KMF) model. However, the experimental technique requires the presence of sufficiently large α widths and depends on estimates of both α and total radiative widths in the quasicontinuum below S n . The sequential extraction method developed at the Oslo Cyclotron Laboratory (OCL) [4] has enabled further investigations of the soft RSF by providing unique data for transitions in the quasicontinuum with sufficient averaging. For deformed rare-earth nuclei, it has been shown that the RSF can be described in terms of a KMF GEDR model, a spin-flip giant magnetic dipole resonance (GMDR), and a soft M 1 resonance [5,6]. In this work, we report on the first observation of a strong enhancement of the soft RSF in 56,57 Fe over the sum of the GEDR and GMDR models. This enhancement has been found in Oslo-type experiments and is confirmed independently by two-step cascade (TSC) measurements. To our knowledge, there exists at present no theoretical model which can explain an enhancement of this magnitude.The first experiment, the 57 Fe( 3 He, 3 He ′ γ) 57 Fe and 57 Fe( 3 He,αγ) 56 Fe reactions, was carried out with 45-MeV 3 He ions at the OCL. Particle-γ coincidences were measured ...

Radiative strength functions (RSFs) in 93−98 Mo have been extracted using the ( 3 He,αγ) and ( 3 He, 3 He ′ γ) reactions. The RSFs are U-shaped as function of γ energy with a minimum at around Eγ = 3 MeV. The minimum values increase with neutron number due to the increase in the lowenergy tail of the giant electric dipole resonance with nuclear deformation. The unexpected strong increase in strength below Eγ = 3 MeV, here called soft pole, is established for all 93−98 Mo isotopes. The soft pole is present at all initial excitation energies in the 5 − 8 MeV region.

The level density at low spin in the 161,162-Dy and 171,172-Yb nuclei has been extracted from primary gamma rays. The nuclear heat capacity is deduced within the framework of the canonical ensemble. The heat capacity exhibits an S-formed shape as a function of temperature, which is interpreted as a fingerprint of the phase transition from a strongly correlated to an uncorrelated phase. The critical temperature for the quenching of pair correlations is found at Tc=0.50(4) MeV.Comment: 8 pages including 4 figures, different method to extract Tc, different figures, text partly rewritte

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