Experimental level densities of atomic nuclei Guttormsen, M.; Aiche, M.; Bello Garrote, F. L. ...
European physical journal. A, Hadrons and nuclei,
12/2015, Letnik:
51, Številka:
12
Journal Article
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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.
We analyze primary gamma -ray spectra of the odd-odd Np super(238) nucleus extracted from Np super(237)(d,p gamma )Np super(238) coincidence data measured at the Oslo Cyclotron Laboratory. The ...primary gamma spectra cover an excitation-energy region of 0< or =E sub(i)< or =5.4 MeV, and allow us to perform a detailed study of the gamma -ray strength as a function of excitation energy. Hence, we can test the validity of the generalized Brink-Axel hypothesis, which, in its strictest form, claims no excitation-energy dependence on the gamma strength. In this work, using the available high-quality Np super(238) data, we show that the gamma -ray strength function is to a very large extent independent of the initial and final states. Thus, for the first time, the generalized Brink-Axel hypothesis is experimentally verified for gamma transitions between states in the quasicontinuum region, not only for specific collective resonances, but also for the full strength below the neutron separation energy. Based on our findings, the necessary criteria for the generalized Brink-Axel hypothesis to be fulfilled are outlined.
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