Low energy dipole strength from large scale shell model calculations

Abstract

Abstract. Low energy enhancement of radiative strength functions has been deduced from experiments in several mass regions of nuclei. Such an enhancement is believed to impact the calculated neutron capture rates which are crucial input for reaction rates of astrophysical interest. Recently, shell model calculations have been performed to explain the upbend of the γ-strength as due to the M1 transitions between closelying states in the quasi-continuum in Fe and Mo nuclei. Beyond mean-field calculations in Mo suggested, however, a non-negligible role of electric dipole in the low energy enhancement. So far, no calculations of both dipole components within the same theoretical framework have been presented in this context. In this work we present newly developed large scale shell model appraoch that allows to treat on the same footing natural and non-natural parity states. The calculations are performed in a large sd − p f − gds model space, allowing for 1p-1h excitations on the top of the full p f -shell configuration mixing. We restrict the discussion to the magnetic part of the dipole strength, however, we calculate for the first time the magnetic dipole strength between states built of excitations going beyond the classical shell model spaces. Our results corroborate previous findings for the M1 enhancement for the natural parity states while we observe no enhancement for the 1p-1h contributions. We also discuss in more detail the effects of configuration mixing limitations on the enhancement coming out from shell model calculations.

Constraints on the dipole photon strength functions from experimental multistep cascade spectra

“…The SM calculations on light A 80 nuclei [26][27][28][29] suggest that the amplitude of the zero-ε γ M1 limit decreases with increasing nuclear deformation β 20 , part of this strength being transferred to higher energies in the ε γ 2-3 MeV range where the scissors mode appears for deformed nuclei.…”

“…Usually, it is assumed that the electromagnetic response of a nucleus depends only on the γ -ray energy ε γ ; this assumption is known as the Brink hypothesis [20]. In reality, a nonzero limit of the dipole strength for transitions between excited states has been reported from experiment [21,22] and later proposed also from the shell model (SM) calculations [23][24][25][26][27][28][29]. For this reason, the D1M + QRPA calculations were complemented with a phenomenological low-ε γ contribution both for the E 1 and M1 strengths; this extended model is referred to as D1M + QRPA + 0lim.…”

“…Such a low-energy contribution of the dipole PSFs, inspired partly by the SM calculations, remains, however, not fully understood in many aspects. For instance, systematic SM calculations for light A 80 nuclei [26][27][28][29] suggest that the overall strength of the low-ε γ M1 contribution and its energy dependence may vary with deformation, but little experimental constraints exist. Also a clear experimental evidence for the E 1 or M1 character of the strength at ε γ → 0 is missing.…”

Reference database for photon strength functions

“…When considering the deexcitation PSF, deviations from the photoabsorption strength can be expected, especially for γ-ray energies approaching the zero limit. In particular, shellmodel calculations [129,[182][183][184][185][186][187][188] predict an exponential increase of the M1 deexcitation PSF at decreasing γ-ray energies approaching zero. Such an upbend of the PSF observed experimentally (see e.g.…”

How do we infer shell effects at high-excitation energies? A new spectroscopic probe to search for magic numbers

“…To date, the EM character of the LEE remains undetermined experimentally, although polarization asymmetry measurements of γ rays in 56 Fe show an admixture of M1 and E1 radiation [30]. Various interpretations of the LEE have been proposed, explaining its dipole origin as M1 [31][32][33][34][35][36] and E1 [37] dipole radiation [38]. Shell-model (SM) calculations consistently support the M1 nature of the LEE [31][32][33][34].…”