Study of a fundamental nearest level spacing distribution of neutron resonances

Regular spacings of s-wave neutron resonances for the target nuclides 40Ca, 54Fe, 58Fe, 96Zr and 238U are discussed. An attempt is made to interpret these spacings with the phonon representation. The maximum and the width of the obtained Gaussian-like distribution for the nearest level spacings determine the phonon energy and their lifetime respectively. There is good agreement between the lifetime of phonons and that of neutron resonances based on the average neutron width. The long lifetimes of phonons for 96Zr and 238U allow to distinguish subharmonic resonances. The finite lifetime of phonons and the observation of subharmonics characterize the neutron resonance spectrum as a spectrum of an anharmonic oscillator. These spectra are observable at neutron separation energy for neutron closed shell and neutron excess nuclei, which have a small neutron separation energy. Therefore the study of neutron resonances provides us with an experimental signature that the nucleus has properties of a non-linear system. (orig.)