Speaker
Description
Can alpha particles be the basic building blocks of atomic nuclei? The conventional mean-field picture with nucleons as basic degrees of freedom is considered to dominate, particularly in the ground state of the doubly magic nucleus $^{16}\mathrm{O}$. On the other hand, alpha cluster theories have predicted their existence in the ground state of $^{16}\mathrm{O}$ [1, 2, 3]. Recently, proton-induced alpha-knockout reactions have been established as an effective probe for studying alpha clusters in the nuclei [4]. The reaction cross section of $^{16}\mathrm{O}(\mathrm{p,p\alpha})$ is a good measure for the number of alpha clusters.
We performed an experiment at RCNP using a 400 MeV proton beam incident on an oxygen-containing target. A double-arm spectrometer analyzed the energies and momenta of recoil protons and alpha particles emitted by the $^{16}\mathrm{O}(\mathrm{p,p\alpha})^{12}\mathrm{C}$ reaction.
The alpha separation energy spectrum (Fig.1) and its yield provides direct evidence of an alpha clusters in the ground state of $^{16}\mathrm{O}$. Furthermore, three distinct peaks revealed in the spectrum, corresponding to different motions of the alpha clusters in atomic nuclei. The momentum distribution of alpha clusters in $^{16}\mathrm{O}$, which analysis is ongoing, will clarify their motion.
In this presentation, we will discuss the experimental results of the $^{16}\mathrm{O}(\mathrm{p,p\alpha})^{12}\mathrm{C}$ reaction and their interpretation.
[1] D.M. Brink et. al, $\it{Phys.Lett.B.}\ \textbf{33}$, 143-146 (1970)
[2] R. Bujker and F. Iachello, $\it{Phys.Rev.Lett.}\ \textbf{112}$, 152501 (2014).
[3] E. Epelbaum et. al, $\it{Phys.Rev.Lett.}\ \textbf{112}$, 102501 (2014).
[4] J. Tanaka, Z.H. Yang et. al, $\it{Science}\ \textbf{371}$, 260-264 (2021).
