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Researchers measured how the nucleus of zinc-64 emits gamma rays and how many internal energy levels it has. Using two complementary experiments, they mapped the average gamma‑decay strength and the density of nuclear levels over a wide energy range. The data show a generally smooth, statistical pattern, but with a surprising reduction in transitions that go directly to the ground state. This could point to non‑statistical structure effects in the nucleus.
To get a broad picture the team combined two techniques. Below the neutron separation energy they used the Oslo method on a charged‑particle reaction (64Zn(p,p′γ)) at the Oslo Cyclotron Laboratory to extract the nuclear level density (NLD) and the gamma‑ray strength function (γSF). Above that energy they measured photoneutron (γ,n) cross sections at the NewSUBARU facility in Japan. The NewSUBARU beams were quasi‑monochromatic, produced by laser Compton scattering, covering beam maxima from about 12 to 21 MeV. The 64Zn target was 1.00 g of metal enriched to 99.4%, and detectors included a LaBr3(Ce) gamma detector and a high‑efficiency 4π neutron counter made from 203 He‑3 tubes.
What they observed is twofold. The level density in the quasi‑continuum region behaves like a constant‑temperature type trend, meaning the number of accessible states grows with excitation energy in a steady way. The γSF measured with the Oslo method is fairly smooth and shows a clear low‑energy enhancement for gamma energies below about 4 MeV. From the NewSUBARU data the team also probed a significant portion of the giant dipole resonance, a broad, collective excitation seen in many nuclei. Most unexpectedly, gamma decays from the quasi‑continuum that go directly to the 0+ ground state appear to be strongly hindered, with a hindrance factor of about κ ≈ 0.5.
Why this matters: zinc isotopes are structurally complex. They have only two protons outside a closed shell and display low‑lying excited 0+ states and shapes that can be triaxial (not simply spherical or axially deformed). A suppression of direct ground‑state transitions could signal non‑statistical effects in the decay pattern, for example differences in shape between the states involved. The combined Oslo and (γ,n) measurements provide data that can test and constrain nuclear theory, including large‑scale shell‑model calculations and models of collective modes.
Important caveats and uncertainties are noted by the authors. The interpretation of the hindered ground‑state transitions as a non‑statistical effect is tentative: other explanations may exist and further study is needed. The (γ,n) cross sections must be unfolded from the measured, energy‑spread beams, and the neutron efficiency calibration (done with a 252Cf source) carries its own uncertainty (quoted as about 2.2% in the experimental description). The Oslo method and the (γ,n) data cover complementary—but not identical—energy regions (Oslo below the neutron threshold, NewSUBARU above it), so a complete picture still depends on how the two datasets are combined and on future measurements.