Shape transitions and new interactions in neutron-rich nuclei

A new research result from the Michigan State University National Superconducting Cyclotron Laboratory (NSCL) suggests that elongated and squashed atomic nuclei may be more common than previously expected, even in isotopes with so-called magic numbers of protons and/or neutrons. The special, spherical nuclei we find in nature may be the exception rather than the rule. The finding challenges the simple conjecture that the quantum mechanical magic numbers in nuclei, that parallel the magic numbers of electrons in noble gas atoms, remain robust across the entire nuclear chart.

 

The NSCL segmented germanium array (SeGA), which allows for high-resolution in-beam gamma-ray spectroscopy of intermediate-energy beams from the laboratory's coupled cyclotrons, was used in a 2008 experiment that suggests that deformed atomic nuclei may be more common in nature than previously expected. 

 

The most common perception of the atomic nucleus is that it has a spherical shape.Indeed, for stable nuclei found in nature that possess a magic number (2, 8, 20, 28, 82, and 126) of protons and/or neutrons, this is the case. This picture began to change when evidence began to suggest that a magic number of protons or neutrons may not always yield a spherical nucleus. An example is at neutron number 20, where calcium-40, with equal (and magic) numbers of protons (20) and neutrons (20), shows pronounced stability and spherical shape. Magnesium-32, however, with the same magic neutron number (20) but only 12 protons, has a very deformed, elongated shape.

 

A 2008 NSCL result, published in Physical Review C, found evidence for a similar shape transition along neutron number 40. The researchers started witha beam of nickel-68, an isotope that contains 28 protons and 40 neutrons. Although neutron number 40 is not officially a "magic" number, the nucleus nickel-68 displays the same attributes as calcium-40, and is believed to be aspherical nucleus. The team performed successive two-proton knockout reactions, first knocking out two protons to create iron-66 and then knocking out two more to create chromium-64. 

 

NSCL researchers discovered there was a very small probability of removing protons to produce chromium-64, a result consistent with a non-spherical shape for this nucleus. The presence of nuclei with deformed shapes along a presumed magic number is evidence for the importance of new interactions in a nucleus that are not strongly manifested in stable nuclei, such as the tensor force.