Adding neutrons changes how nuclei emit low‑energy gamma rays: a magnetic spike and a scissors‑like bump appear
This paper studies how the probability for a nucleus to emit gamma rays changes when you add neutrons. The authors calculated two kinds of gamma strength — magnetic dipole (M1) and electric quadrupole (E2) — across long chains of isotopes of molybdenum, iron, tin, germanium and gadolinium. They used the familiar spherical shell model and a newer triaxial projected shell model to see patterns as the neutron number changes.
To make the strength functions they computed many excited states and the transition probabilities between them. For example, in some cases they calculated the lowest ~40 states with spins J=0–6 and tens of thousands of M1 transition probabilities, then grouped those transitions into small energy bins to build the average strength. This lets them predict how the nucleus will de‑excite by gamma emission from different compound energies.
A clear finding is a strong low‑energy enhancement of magnetic (M1) strength, which the authors call Low Energy Magnetic Radiation (LEMAR). In nearly spherical nuclei LEMAR appears as a single spike near zero gamma energy and falls off roughly exponentially with energy. In mid‑shell nuclei — those with more deformation — a second feature shows up around 3 MeV. This bump is identified with the “scissors resonance,” an excitation that can be pictured as a swinging motion of the deformed neutron cloud against the deformed proton cloud.
The paper explains these features in simple physical terms. In a spherical nucleus, magnetic substates of a given single‑particle level are degenerate, and residual interactions spread their strength over many mixed states; this produces many low‑energy transitions and the LEMAR spike. When the nucleus is deformed, those substates split and produce two effects: fragmented transitions that form the scissors‑region bump and a remaining cluster of low‑energy transitions that keep the LEMAR spike. Pairing between nucleons and the interaction that mixes configurations reduce the strength of individual two‑quasiparticle lines and spread that strength among many states.