For the first time, physicists have observed excited states in the very neutron-deficient odd-odd nucleus, lanthanum-120. The study was published in Physics Letters B, and was conducted by researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences, and their collaborators from France, Finland, South Africa and other countries.  

As medium-heavy nuclei approach the N=Z line, it is predicted that the proton-neutron interaction will be enhanced, having a measurable impact on the structure of excited states. This may also be accompanied by shape changes of nuclei, which exhibit non spherical shapes, such as "rugby ball" (prolate), “pancake” (oblate), “pear” (octupolar), or “kiwi” (triaxial) shapes.  

Lanthanum-120 is a rare β-delayed proton emitter discovered in 1984. Due to its extremely low production cross-section, separating and identifying lanthanum-120 has posed a great challenge. Over the past 40 years, experimental physicists have not been able to successfully measure the excited states of lanthanum-120. 

"It is crucial to experimentally measure the excited states to explore the underlying mechanism of the structure evolution in the extremely proton-rich lanthanum nuclei,” said Assoc. Prof. LYV Bingfeng at IMP, one of the corresponding author of this study.  

The researchers utilized a state-of-the-art setup composed of a mass separator and high-efficiency gamma detector arrays to search for the excited states in lanthanum-120, at the accelerator laboratory of the University of Jyvaskyla, Finland. They then established the level structure of lanthanum-120.  

The researchers found that the spin at which the energies of odd- and even-spin cascades of gamma rays of lanthanum-120 are crossing follows the systematic trend. Moreover, they discovered that the transition probabilities exhibit a strikingly different pattern, with a significant staggering between odd and even spins, which is distinct from neighboring nuclei.  

"Our data, combined with theoretical models, suggest that lanthanum-120 exhibits a pronounced triaxial deformation. This study also indicates that the proton-neutron interaction plays an essential role in describing the structure of odd-odd nuclei close to the proton drip line in the A≈120 mass region,” said Costel Petrache from the University of Paris-Saclay, France, another corresponding author of this study. 

The deexcition and decay mode of lanthanum-120. (Image by Assoc. Prof. LYV)