Inelastic $\alpha$-particle scattering at energies of a few hundred MeV and very-forward scattering angles including $0^\circ$ has been established as a tool for the study of the isoscalar giant monopole (IS0) strength distributions in nuclei. An independent investigation of the IS0 strength in nuclei across a wide mass range was performed using the $0^\circ$ facility at iThemba Laboratory for Accelerator Based Sciences (iThemba LABS), South Africa, to understand differences observed between IS0 strength distributions in previous experiments performed at the Texas A\&M University (TAMU) Cyclotron Institute, USA and the Research Center for Nuclear Physics (RCNP), Japan. The isoscalar giant monopole resonance (ISGMR) was excited in $^{ 58}$Ni, $^{ 90}$Zr, $^{ 120}$Sn and $^{ 208}$Pb using $\alpha$-particle inelastic scattering with $196$ MeV $\alpha$ beam and scattering angles $\theta_{ \text{ Lab}} = 0^\circ$ and $4^\circ$. The K$600$ magnetic spectrometer at iThemba LABS was used to detect and momentum analyze the inelastically scattered $\alpha$ particles. The IS0 strength distributions in the nuclei studied were deduced with the difference-of-spectra (DoS) technique including a correction factor for the $4^\circ$ data based on the decomposition of $L > 0$ cross sections in previous experiments. IS0 strength distributions for $^{ 58}$Ni, $^{ 90}$Zr, $^{ 120}$Sn and $^{ 208}$Pb are extracted in the excitation-energy region $E_{ \rm x} = 9 - 25$ MeV.Using correction factors extracted from the RCNP experiments, there is a fair agreement with their published IS0 results. Good agreement for IS0 strength in $^{ 58}$Ni is also obtained with correction factors deduced from the TAMU results, while marked differences are found for $^{ 90}$Zr and $^{ 208}$Pb.
Categories: nucl-ex