Study of¹⁹C by One-Neutron Knockout

Abstract

Abstract. The spectroscopic structure of 19 C, a prominent one-neutron halo nucleus, has been studied with a 20 C secondary beam at 290 MeV/nucleon and a carbon target. Neutron-unbound states populated by the one-neutron knockout reaction were investigated by means of the invariant mass method. The preliminary relative energy spectrum and parallel momentum distribution of the knockout residue, 19 C * , were reconstructed from the measured four momenta of the 18 C fragment, neutron, and beam. Three resonances were observed in the spectrum, which correspond to the states at E x = 0.62(9), 1.42(10), and 2.89(10) MeV. The parallel momentum distributions for the 0.62-MeV and 2.89-MeV states suggest spin-parity assignments of 5/2 + and 1/2 − , respectively. The 1.42-MeV state is in line with the reported 5/2 + 2 state. a

Evidence of strong dynamic core excitation inC19resonant break-up

“…Although this is supported by sd shellmodel calculations (middle right column of Fig. 1), the existence of a bound 5/2 + state seems to be excluded according to knock-out experiments [30][31][32]. In addition, an unbound 5/2 + state was found at RIKEN in the break-up of 19 C on protons at 70 MeV/nucleon [33] and more recently in a one-neutron knockout reaction at 290 MeV/nucleon [32].…”

“…1), the existence of a bound 5/2 + state seems to be excluded according to knock-out experiments [30][31][32]. In addition, an unbound 5/2 + state was found at RIKEN in the break-up of 19 C on protons at 70 MeV/nucleon [33] and more recently in a one-neutron knockout reaction at 290 MeV/nucleon [32]. Semi-microscopic predictions and shell-model calculations suggest a strong overlap of this state with the 2 + core excited state [34].…”

“…(Color online) Experimental (far left column)[29,33] and calculated spectrum of19 C in a shell-model calculation with OXBASH using the WBP interaction[40,41] (middle right column), and within a semi-microscopic core-plus-valenceparticle calculation (P-AMD, far right column) which takes into account core excitations as described in[34]. We include a second experimental spectrum (middle left column) due to the discrepancy raised by the latest knock-out experiments[30][31][32].…”