Experimental nuclear level densities at excitation energies below the neutron threshold follow closely a constant-temperature shape. This dependence is unexpected and poorly understood. In this work, a fundamental explanation of the observed constant
-temperature behavior in atomic nuclei is presented for the first time. It is shown that the experimental data portray a first-order phase transition from a superfluid to an ideal gas of non-interacting quasiparticles. Even-even, odd-$A$, and odd-odd level densities show in detail the behavior of gap- and gapless superconductors also observed in solid-state physics. These results and analysis should find a direct application to mesoscopic systems such as superconducting clusters.
The level densities and $gamma$-ray strength functions of $^{105,106,111,112}$Cd have been extracted from particle-$gamma$ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation ener
gy. The $gamma$-ray strength functions display no strong enhancement for low $gamma$ energies. However, more low-energy strength is apparent for $^{105,106}$Cd than for $^{111,112}$Cd. For $gamma$ energies above $approx$ 4 MeV, there is evidence for some extra strength, similar to what has been previously observed for the Sn isotopes. The origin of this extra strength is unclear; it might be due to $E1$ and $M1$ transitions originating from neutron skin oscillations or the spin-flip resonance, respectively.
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 ($^3$H
e,$alphagamma$)$^{56}$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.
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.
The nuclear level density and the gamma-ray strength function have been determined for 43Sc in the energy range up to 2 MeV below the neutron separation energy using the Oslo method with the 46Ti(p,alpha)43Sc reaction. A comparison to 45Sc shows that
the level density of 43Sc is smaller by an approximately constant factor of two. This behaviour is well reproduced in a microscopical/combinatorial model calculation. The gamma-ray strength function is showing an increase at low gamma-ray energies, a feature which has been observed in several nuclei but which still awaits theoretical explanation.
Radiative strength functions of 117Sn has been measured below the neutron separation energy using the (3He,3Hegamma) reactions. An increase in the slope of the strength functions around E_gamma= 4.5 MeV indicates the onset of a resonance-like structu
re, giving a significant enhancement of the radiative strength function compared to standard models in the energy region 4.5 <= E_gamma <= 8.0 MeV. For the first time, the functional form of this resonance-like structure has been measured in an odd tin nucleus below neutron threshold in the quasi-continuum region.
The nuclear level densities of 116,117Sn below the neutron separation energy have been determined experimentally from the (3He,alpha gamma) and (3He,3He gamma) reactions, respectively. The level densities show a characteristic exponential increase an
d 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 scandium isotopes 44,45Sc have been studied with the 45Sc(3He,alpha gamma)44Sc and 45Sc(3He,3He gamma)45Sc reactions, respectively. The nuclear level densities and gamma-ray strength functions have been extracted using the Oslo method. The experi
mental 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 gamma-ray strength functions are compared to theoretical models of the E1, M1, and E2 strength, and to data from (gamma,n) and (gamma,p) experiments. The strength functions show an enhancement at low gamma energies that cannot be explained by the present, standard models.
