Varying Shapes of Selenium Isotopes

The chain of Selenium isotopes, with Z=34 protons, is thought to undergo a sudden shape change around N=36 from a disk-like to an elongated shape, which provides stringent test of nuclear structure models. The exact isotope for which this shape transition actually happens was uncertain until a recent experiment performed at NSCL’s reaccelerator facility ReA3. For this, radioactive 72Se was produced from 78Kr at 30% of the speed of light which was broken apart on a beryllium target and separated into a pristine-quality ion beam of 9% of the speed of light that was scattered off a 208Pb target. The energy of the 72Se beam was low enough so that the scattering process was only mediated by the electromagnetic field generated by the protons inside Se and Pb ions. This Coulomb excitation process excited 72Se from its ground state to the first excited state.  The probability of this excitation to happen – the cross section – as a function of the scattering angle changes minutely depending on the shape of 72Se. The measurement reported in Physical Review Letters was sensitive enough to determine the quadrupole moment due to shape deformation, and assign elongated shape to 72Se, while 70Se was shown earlier to have an oblate, or disk-like, shape. This measurement showed that the shape transition in the neutron-deficient Se isotopes does not occur until 70Se.   

J. Henderson et al., Phys. Rev. Lett. 121, 082502 (2018).

selenium isotopes