Statistical-model description of γ decay from compound-nucleus resonances

Abstract

The statistical model of compound-nucleus reactions predicts that the fluctuations of the partial gamma decay widths for a compound-nucleus resonance are governed by the Porter-Thomas distribution (PTD), and that consequently the distribution of total gamma-decay widths is very narrow. However, a recent experiment [1] reported large fluctuations of the total gamma-decay widths in the 95 Mo(n, γ) 96 Mo* reaction, contrary to this expectation. Furthermore, in recent theoretical works it was argued that sufficiently strong channel couplings can cause deviations of the partial width distributions from PTD. Here, we investigate whether the combined influence of a large number of nonequivalent gamma-decay channels, each of which couples weakly to the compound-nucleus resonances, can modify the statistics of the partial widths. We study this effect in neutron scattering off 95 Mo within a random-matrix model that includes coupling to the entrance neutron channel and to the large number of gamma channels. Using realistic coupling parameters obtained from empirical models for the level density and the gamma strength function, we find that the PTD describes well the distribution of partial widths for all decay channels, in agreement with the statistical-model expectation. Furthermore, we find that the width of the distribution of the total gamma-decay widths is insensitive to wide variations in the parameters of the gamma strength function, as well as to deviations of the partial-width distributions from the PTD. Our results rule out an explanation of the recent experimental data within a statistical-model description of the compound nucleus. arXiv:1909.06250v1 [nucl-th]

Electric and magnetic dipole strength in 112,114,116,118,120,124Sn

“…The inelastic proton scattering experiments were performed in 2015 and 2017 at RCNP. In 2015, 112,116,124 Sn and with lesser statistics 118,120 Sn were measured. In the second experimental campaign in 2017, 112,116,124 Sn were measured again to improve statistics.…”

“…In the second experimental campaign in 2017, 112,116,124 Sn were measured again to improve statistics. Additionally, data on 114,118,120,122 Sn were taken. The measurements used the Grand Raiden spectrometer [71].…”

“…Dispersion matching between the beams and the magnetic spectrometers used to detect the scattered particles allows for high-resolution measurements of the order ∆E/E = (1 − 2) × 10 −4 . Here, we report the results of a study of the stable tin isotopes 112,114,116,118,120,124 Sn performed at RCNP. A decomposition of the dominant E1 and spin-M 1 modes can be achieved either by an MDA of the cross sections [6] or independently by the measurement of a combination of polarization transfer observables [1].…”

Tests of the Porter-Thomas Distribution for Reduced Partial Neutron Widths

“…The results of Refs. [1,2] have, therefore, caused quite a stir in the theoretical nuclear-physics community [5][6][7][8][9][10][11]. The Porter-Thomas distribution follows from orthogonal invariance, one of the pillars of random-matrix theory for time-reversal invariant systems.…”

“…The terms that couple an orthogonally invariant Hamiltonian to the neutron channel and the numerous gamma channels break orthogonal invariance and may cause deviations of the distribution of reduced partial neutron widths from the Porter-Thomas distribution. That possibility has been investigated in a number of papers [6][7][8][9][10][11]. It is now firmly established, however, that breaking of orthogonal invariance due to coupling to the channels is by far too weak to account for the discrepancy found in Refs.…”