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 densities and radiative strength functions (RSFs) in 160,161 Dy have been extracted using the ( 3 He,αγ) and ( 3 He, 3 He ′ γ) reactions, respectively. The data are compared to previous measurements on 161,162 Dy. The energy distribution in the canonical ensemble is discussed with respect to the nucleon Cooper pair breaking process. The gross properties of the RSF are described by the giant electric dipole resonance. The RSF at low γ-ray energies is discussed with respect to temperature dependency. Resonance parameters of a soft dipole resonance at Eγ ∼ 3 MeV are deduced.

The level densities and radiative strength functions (RSFs) of 50,51 V have been extracted using the ( 3 He,αγ) and ( 3 He, 3 He ′ γ) reactions, respectively. From the level densities, microcanonical entropies are deduced. The high γ-energy part of the measured RSF fits well with the tail of the giant electric dipole resonance. A significant enhancement over the predicted strength in the region of Eγ < ∼ 3 MeV is seen, which at present has no theoretical explanation.

The nuclear level densities of 116,117 Sn below the neutron separation energy have been determined experimentally from the ( 3 He,αγ ) and ( 3 He, 3 He γ ) reactions, respectively. The level densities show a characteristic exponential increase and a difference in magnitude due to the odd-even effect of the nuclear systems. In addition, the level densities display pronounced step-like structures that are interpreted as signatures of subsequent breaking of nucleon pairs.

The radiative strength function of 117Sn has been measured up to the neutron separation energy using the (3He, 3He' gamma) reaction. An increase in the slope of the strength function around E gamma=4.5 MeV indicates the onset of a resonancelike structure, giving a significant enhancement of the radiative strength function compared to standard models in the energy region 4.5

Level densities for93−98 Mo have been extracted using the ( 3 He,αγ) and ( 3 He, 3 He'γ) reactions. From the level densities thermodynamical quantities such as temperature and heat capacity can be deduced. Data have been analyzed by utilizing both the microcanonical and the canonical ensemble. Structures in the microcanonical temperature are consistent with the breaking of nucleon Cooper pairs. The S-shape of the heat capacity curves found within the canonical ensemble is interpreted as consistent with a pairing phase transition with a critical temperature for the quenching of pairing correlations at Tc ∼ 0.7 − 1.0 MeV.

The scandium isotopes 44,45 Sc have been studied with the 45 Sc( 3 He,αγ) 44 Sc and 45 Sc( 3 He, 3 He ′ γ) 45 Sc reactions, respectively. The nuclear level densities and γ-ray strength functions have been extracted using the Oslo method. The experimental level densities are compared to calculated level densities obtained from a microscopic model based on BCS quasiparticles within the Nilsson level scheme. This model also gives information about the parity distribution and the number of broken Cooper pairs as a function of excitation energy. The experimental γ-ray strength functions are compared to theoretical models of the E1, M1, and E2 strength, and to data from (γ,n) and (γ,p) experiments. The strength functions show an enhancement at low γ energies that cannot be explained by the present, standard models.