Extraction of the giant monopole resonance strength with multipole decomposition analysis

Abstract

It has been established from previous studies that the inelastic scattering of α particles at Eα ≥ 200 MeV, particularly at very-forward scattering angles including 0◦ , is an effective experimental probe for studying the Isoscalar Giant Monopole Resonance (ISGMR) strength distribution (E0) in atomic nuclei. In recent studies at iThemba LABS, cross sections measured at 0◦ and 4◦ were used to extract the E0 strength in 40,42,44,48Ca via the corrected Difference-of-Spectra (DoS) method. This method is necessary to preserve any high energy-resolution information as obtained using the K = 600 spectrometer at iThemba LABS. However, it depends on the published contributions of multipoles with L > 0 from the same studies to which the DoS-derived E0 strengths are compared. Consequently, this raises concerns regarding the degree of independence of the iThemba LABS DoS results. The use of an alternative technique, the Multipole Decomposition Analysis (MDA), to extract the E0 strength distribution is, therefore, necessary. In general, for this type of analysis, it is crucial to collect data at more angles than just 0◦ and 4◦. This enhances the dataset and facilitates the inclusion of combinations of different multipolarities. However, based on previous inelastic 6Li scattering results at 100 MeV/u on 12C and 93Nb at the Research Center for Nuclear Physics (RCNP) in Japan, it is considered that while a restricted MDA procedure (i.e. using data from only 0◦ and 4◦) may not precisely determine the strengths of higher multipolarities, it does allow for an accurate extraction of the E0 component independently of other studies. Therefore, this work aims to conduct an MDA on previously acquired experimental datasets for the 40,42,44,48Ca isotopes and perform a comparison with the results from the previous DoS method. Furthermore, comparisons are made with the ISGMR strength results obtained from RCNP and the Texas A&M University (TAMU) Cyclotron Institute. It will be shown that the MDA-extracted ISGMR strength results from the present study and those from the DoS method show reasonable and consistent agreement with those from RCNP for all four isotopes.