Evolution of the Isoscalar Giant Monopole Resonance in the Ca Isotope Chain

Background: Two recent studies of the evolution of the isoscalar giant monopole resonance (ISGMR) within the calcium isotope chain report conflicting results. One study suggests that the monopole resonance energy, and thus the incompressibility of the nucleus, K-A, increase with mass, which implies that K-tau, the asymmetry term in the nuclear incompressibility, has a positive value. The other study reports a weak decreasing trend of the energy moments, resulting in a generally accepted negative value for K-tau. Differences in the observed trends have been attributed to the use of different techniques to account for instrumental background and the physical continuum. Purpose: An independent measurement of the ISGMR in the Ca isotope chain is provided to gain a better understanding of the origin of possible systematic trends. Methods: Inelastically scattering of 196 MeV alpha particles were observed at small scattering angles, including 0 degrees, for a range of calcium targets (Ca-40,Ca-42,Ca-44,Ca-48). The scattered alpha particles were momentum analyzed in the K600 magnetic spectrometer at iThemba LABS, South Africa. Monopole strengths spanning an excitation-energy range between 9.5 and 25.5 MeV were obtained using the difference-of-spectra (DoS) technique, adjusted to allow corrections for the variation of the angular shape of the sum of the L > 0 multipoles as a function of excitation energy, and compared with previous results that employed the multipole-decomposition analysis (MDA) techniques. Results: The structure of the E0 strength distributions of Ca-40,Ca-42,Ca-44 agrees well with the results from the previous measurement that support a weak decreasing trend of the energy moments, while no two datasets agree in the case of Ca-48. Despite the variation in the structural character of E0 strength distribution from different studies, we find for all datasets that the moment ratios, calculated from the ISGMR strength in the excitation-energy range that covers the resonance peak, display at best only a weak systematic sensitivity to a mass increase. Conclusion: Different trends observed in the nuclear incompressibility are caused by contributions to the E0 strength outside of the region covering the resonance peak, and in particular for high excitation energies. While procedures exist to identify and subtract instrumental background, more work is required to characterize and subtract continuum background contributions at high excitation energies.