Professor Sun Baohua's team from the School of Physics at Beihang University has made a significant breakthrough in nuclear physics, systematically revealing the universal physical behavior of empirical scaling factors in charge-changing cross-section measurements and demonstrating for the first time the unique advantages of heavy reaction targets such as Pb for precisely extracting proton distribution radii of unstable rare isotopes. The findings were published in Physical Review Letters under the title "Best Reaction Target to Determine Proton Distribution Radii of Atomic Nuclei."

Protons and neutrons are tightly bound to form atomic nuclei, and rare isotopes with low production yields and short lifetimes serve as key probes for exploring the structure of microscopic matter. The proton distribution radius characterizes the spatial distribution of protons within a nucleus and is a crucial physical quantity for studying nuclear structure and testing theoretical models. Precise measurements of this quantity in unstable nuclei not only deepen our understanding of how nucleons bind and where the limits of matter structure lie, but also help astrophysicists constrain the nuclear equation of state and model the evolution of massive neutron stars.

In the article, the authors found that a heavy target such as Pb is most suitable for determining the proton distribution radii of unstable nuclei through charge-changing cross-section (σ_cc) measurements. Experimentally, low-Z targets are routinely used to determine nucleon distribution radii of unstable isotopes. This approach has recently been extended to study proton distribution radii from σ_cc measurements. However, empirical scaling factors have to be introduced to apply the Glauber models.

The researchers systematically investigated the scaling factor using 39 new σ_cc data of 18  p-shell nuclei on hydrogen, carbon, silver, and lead targets at around 240  MeV/nucleon. Together with the existing data, they revealed a universal dependence of the scaling factor on both the masses of target nuclei and the separation energies of projectile nuclei. The scaling factors decrease with increasing target-nucleus mass and converge to 1 for the highest-Z target, making the scaling unnecessary. They concluded that instead of a low-Z target, employing a heavy target such as Pb in σ_cc measurements is the best option to determine the proton distribution radii of unstable nuclei.

This study overturns the previous experimental paradigm favoring light targets and reveals the advantages of using heavy targets such as Pb for charge-changing reaction measurements to extract proton distribution radii of unstable rare isotopes, providing a new and reliable experimental pathway for studying proton distributions in nuclei far from stability.

Xu Junyao, a doctoral student at the School of Physics, is the first author of the paper. Professor Sun Baohua, Professor Isao Tanihata, and Associate Professor Zhao Jianwei are co-corresponding authors. Beihang University is the primary affiliation for the research. Collaborating institutions include the Institute of Modern Physics of the Chinese Academy of Sciences, the Shanghai Institute of Applied Physics of the Chinese Academy of Sciences, Sun Yat-sen University, Sichuan University, the University of Chinese Academy of Sciences, and other domestic and international universities and research institutions. The research was supported by the National Natural Science Foundation of China and "the Innovative Talent Introduction Base for Radiation Physics and Advanced Nuclear Energy Materials Discipline."

Article link: https://doi.org/10.1103/l85h-wzy5

Editor: Lyu Xingyun