Nonstatistical interpretation of delayed neutron emission — simple shell model approach

Beta-delayed neutron emission from93?100Rb to excited states in the residual Sr isotopes

“…Furthermore, the Pj-values are decisive in determining the shape and absolute strength of the upper part of S~ of nuclei far from F-stability. As already mentioned introductory, over and above yielding information on possible selectivity in highenergy F-decay via 'doorway states' [6, 9-13, 16-], the partial neutron branching ratios may also reflect nuclear structures of initial and final states connected in neutron emission [4,10,14,23]. Therefore, in the discussion about the validity of a purely statistical description of delayed neutron decay [8], a possible test may be to examine whether simultaneous agreement between experiment and statistical model calculations for neutron spectrum shapes as well as for the population of individual final states can be obtained when assuming a smooth, structureless F-strength function and when describing partial level widths for neutron emission by optical model transmission coefficients [10].…”

“…These deviations from optical model predictions for p-wave neutron emission suggest that structure effects in neutron decay from highly excited states in 94Sr to the lowest energy levels in 93Sr have to be considered. As has first been pointed out by Shihab-Eldin [4], pre-equilibrium neutron emission to the ground state and the first excited levels in the final nucleus which may have significant differences in the general makeup of their wave functions, will show up in a blocking or favouring of certain decay channels. On the basis of the nuclear shell model [53,54], and supported by the shape of the experimental /3-strength function [14], the ground state configuration of 9'*Rb (J~=3-) may be 4 5…”

“…Following Shihab-Eldin [4], neutron emission from these configurations can only occur through admixtures in their wave functions obtained via scattering events that lead to a neutron in an unbound state. Considering p-wave neutron emission to the ground and first excited state at 213 keV in 93Sr, the blocking of the decay channel to the latter level might be explained as due to its specific configuration containing a neutron single particle excitation which is not or only to a minor extent formed by allowed ,qdecay to emitter states just above B, in 948r.…”

“…In this case, a statistical model is expected to fulfill the postulate [10] of a simultaneous reproduction of the experimental neutron spectrum envelopes and the P Lvalues. If, however, the neutron emitting states possess an intermediate structure originating from the preceding /3--decay to favourable shell model configurations [2,4,10,23], 'preequilibrium' neutron emission to levels in the residual nucleus should reflect the differences in the wave functions of initial and final states connected in neutron decay.…”

“…During the last years, experimental results on highresolution spectroscopy of /3-delayed neutrons and their interpretation have raised extensive discussions about the existence and identification of discrete nuParts of this work wilI be presented in the Doctoral Dissertation of A.Schr6der, to be submitted to Johannes Gutenberg Universit~it Mainz 2 Present address: IAEA, A-1400 Wien, Austria 3 Present address: TUV Bayern, D-8046 Garching bei Mfinchen, FRG clear structures beyond gross statistical properties in high-energy /3-decay, and about their consequences in nuclear physics and astrophysics [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Up to recently, most attempts to describe/3-delayed neutron emission were based on the comparison of predictions from statistical model calculations with integral experimental data like total and partial neutron emission probabilities and singles neutron spectra.…”