Experimental First Order Pairing Phase Transition in Atomic Nuclei

Abstract

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 constanttemperature 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.

Experimentally constrained(p,γ)Y89and(n,γ)Larsen1, Guttormsen2, Schwengner3 et al. 2016Phys. Rev. C334241View full textAdd to dashboardCite

“…[27] within the constant-temperature (CT) approximation of the level-density function. This approach gives a very good description of the functional form of experimental level densities above ≈ 2∆ [28], where ∆ ≈ 1 MeV is the pair-gap parameter. However, it yields rather low (and constant) values for the spincutoff parameter.…”

“…Then, a 3D scan of the area around the full-energy peaks was performed to get a correct estimate of the underlying spectrum, and finally removing the carbon and oxygen lines. Experimental response functions have been recorded in-beam for γ transitions of 13 C, 16 O, 28 Si, and 56 Fe. The unfolding procedure is described in detail in Ref.…”

La137,138,139(n,γ) cross sections constrained with statistical decay properties of Kheswa1, Wiedeking2, Brown3 et al. 2017Phys. Rev. C

“…This behavior is due to odd-odd 138,140 La nuclei having one extra degree of freedom that generates an increase in ρ(E x ) compared to odd-even 139 La. The horizontal difference between NLDs of oddodd and odd-even nuclei has been related to the pair gap parameter, while the vertical difference is a measure of entropy excess for the quasiparticle [22]. The constant temperature behavior of the NLDs (above the pair-breaking energy) is a consistently observed feature [20], that is also confirmed by the HFB + Comb predictions, and has been interpreted as a first-order phase transition [22].…”

“…The γSF has the potential to significantly impact reaction cross sections and therefore astrophysical element formation [17,18] and advanced nuclear fuel cycles [19]. Measurements of the NLD provides insight into the evolution of the density of states for different nuclei [20] and can be used to determine nuclear thermodynamic properties such as entropy, nuclear temperature, and heat capacity as a function of E x [21,22].…”

Nuclear level density: Shell-model approach

“…The philosophy behind this approach is usually explained [59] in terms of the first-order phase transition that goes through the latent heat at fixed temperature. Although typically this assumes the melting of the Cooper pairs but in fact one can also talk about other types of correlated structures which are undergoing something similar to the liquid-gas phase transition or even the first stage on the road to multifragmentation.…”