With the advent of radioactive isotope beam facilities, it was found that whilst some magic numbers lost their magicity, i.e, the shell gaps became diminished, new ones emerged. A prominent case of the latter is N=34 in the 54Ca isotope, the spectroscopy of which revealed the nucleus as being of doubly-magic character [1], confirming a prediction made some ten years prior [2]. More recently, this notion has been reinforced by mass measurements, showing a marked decrease in S2n values between 54Ca and 56Ca [3]. It is desirable to get a more detailed understanding of this prominent nucleus and conduct first spectroscopy of the more exotic Ca isotopes.
Experimental investigations around the neutron-rich Ca region were conducted at the Radioactive Isotope Beam Factory, operated by the RIKEN Nishina Center and the Center for Nuclear Study, University of Tokyo. A 240 pnA beam of 70Zn30+ was accelerated to 345 MeV/nucleon and secondary beams of isotopes were produced from its fragmentation on a 10-mm-thick 9Be target. The cocktail beam was transported through the BigRIPS fragment separator, to the MINOS device, which housed a 151-mm-thick liquid hydrogen target surrounded by a time-projection chamber to track ejected protons following knockout reactions. Excited states of the reagents decayed via γ-ray, or neutron emission. In the latter case, the DALI2+ array of 226 NaI(Tl) crystals was arranged around the LH2 target for high-efficiency γ-ray detection. For neutron detection, the NeuLAND and NEBULA plastic scintillator arrays were positioned ~15 m upstream of MINOS. Reaction products were identified and their momenta determined with the SAMURAI spectrometer and its associated detectors.
During the course of the talk, the physics interests of the neutron-rich Ca isotopes shall be detailed within the context of the relevant nuclear forces that drive shell evolution in the region. Following an explanation of the measurement objectives, the experiment will be described followed by a presentation of the results from both proton and neutron knockout reactions.
[1] D. Steppenbeck et al., Nature (London) 502, 207 (2013).
[2] T. Otsuka, R. Fujimoto, Y. Utsuno, B. A. Brown, M. Honma,and T. Mizusaki, Phys. Rev. Lett. 87, 082502 (2001).
[3] S. Michimasa et al., Phys. Rev. Lett.121, 022506 (2018)