Quadrupole Strength in the Neutron-rich nickel-72 Isotope: the Order is Restored
A new measurement of excited-state lifetimes performed at NSCL provides new insights into the evolution of nuclear structure along the nickel proton-magic isotopic chain. Isotopic chains of proton-magic nuclei, like calcium (20 protons) and nickel (28 protons), offer an unprecedented view into the changes of nuclear structure as neutrons are added on top of an inert core of protons away from the valley of beta stability. They are also most often the first benchmarks for developing nuclear models that aim to capture the changes to the nuclear structure in the neutron-rich regime. The lifetimes of low-lying excited states are directly related to nuclear collective modes (meaning all the particles are participating in quadrupole deformation or vibration). Hence, they provide an important link to nuclear structure models.
Researchers from University of Tennessee and 11 other universities and institutions, including a team from NSCL, determined the lifetimes of the lowest 2+ and 4+ states in nickel-72 using the recoil-distance Doppler-shift method, a model-independent probe to determine excited-state lifetimes in nuclei and hence the transition probabilities connecting the nuclear states. The approach exploits the well-known Doppler effect encountered when radiation is emitted by nuclei in flight. This effect is similar to the change in sound frequency you hear when a police car with sirens passes by. The measurement was enabled by combining state-of-the-art equipment available at NSCL, namely the GRETINA gamma-ray detection system, the S800 spectrograph, and the TRIPLEX plunger device.
The results show that nickel-72 exhibits enhanced collectivity relative to nickel-68, a nickel isotope that displays some characteristics of a doubly-magic nucleus, but the new data are at odds with the higher collectivity earlier reported for neighboring nickel-70. The NSCL result indicates that the evolution of collectivity in the neutron-rich nickel isotopes toward the N=50 doubly-magic nucleus nickel-78 is less dramatic than previously implied, supporting theoretical predictions and earlier conclusions from an NSCL measurements on the collectivity of nickel-74.
A paper on this research was published in Physical Review Letters in March 2016: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.122502