Experimental level densities of atomic nuclei

Abstract

It is almost 80 years since Hans Bethe described the level density as a non-interacting gas of protons and neutrons. In all these years, experimental data were interpreted within this picture of a fermionic gas. However, the renewed interest of measuring level density using various techniques calls for a revision of this description. In particular, the wealth of nuclear level densities measured with the Oslo method favors the constant-temperature level density over the Fermi-gas picture. From the basis of experimental data, we demonstrate that nuclei exhibit a constant-temperature level density behavior for all mass regions and at least up to the neutron threshold.PACS. PACS-key 21.10. Ma, 25.20.Lj, 25.40.Hs

Verification of detailed balance for γ absorption and emission in Dy isotopes

“…The remarkable similarities between all seven nuclear level densities in panel (f) reveal the same gross properties, which confirm that these mid-shell dysprosium isotopes (N = 94 − 98) have their Fermi surfaces embedded between single particle orbits with similar density and nuclear deformation. We also observe that the nuclear level densities are close to a straight line in the log-plot, in accordance with previous findings using the Oslo method [38,39]. The parallel level densities are also evident in Fig.…”

“…When using data above ≈ 2∆, the χ 2 result of a fit to the Fermi gas model is significantly worse than a fit to the CT model, see Fig.9 in Ref. [39]. The CT parameters given in Table II are extracted from a fit to our experimental data above ≈ 2∆ for all the studied isotopes.…”

“…There is increasing evidence that the level density follows closer the constant temperature formula than the Fermi gas formula for excitations above the pairing gap 2∆ [38,39]. Assuming a constant temperature, T , the energy dependence in the expression σ 2 J = ΘT is given by the moment of inertia, Θ, which goes from ≈ 50% of the rigid body moment of inertia (Θ rigid ) at the ground state and approaches Θ rigid at S n .…”

Effect of broken axial symmetry on the electric dipole strength and the collective enhancement of level densities in heavy nuclei

“…in the pairing dominated phase below the phase transition point. This finds support in a recent analysis of level density data [77,80,83,84]; we adjust qp (0) as the state density at the lower end of the interpolation just above the ground state and it can be fixed there locally by regarding known spectral data (E x < ∆ 0 , J), similar as has been done previously [35,85,86]. We also tested a global approximation by setting qp (0)=0.3/∆ o for the state density at the lower end of the interpolation region, and this has a minor effect for E x ≳ E pt , as in most nuclei E pt is smaller than the neutron binding energy S n. This is shown in Fig.…”

Quasicontinuum γ decay of Zr91,92 : Benchmarking indirect ( n,γGuttormsen1, Goriely2, Larsen3 et al. 2017Phys. Rev. CSelf Cite260180View full textAdd to dashboardCite

“…The spin cutoff parameter σ is traditionally determined by a close-to rigid moment of inertia. Since σ 2 = ΘT /h 2 [32] and the nuclear temperature T is assumed to be approximately constant for 2∆ < E < S n [33,34], σ 2 follows the energy dependence of the moment of inertia Θ.…”