Neutron occupancy of the 0d5/2 orbital and the N = 16 shell closure in 24O

Open AccessInternational audienceOne-neutron knockout from 24O leading to the first excited state in 23O has been measured for a proton target at a beam energy of 62 MeV/nucleon. The decay energy spectrum of the neutron unbound state of 23O was reconstructed from the measured four momenta of the 22O fragment and emitted neutron. A sharp peak was found at Edecay=50±3 keV, corresponding to an excited state in 23O at 2.78±0.11 MeV, as observed in previous measurements. The longitudinal momentum distribution for this state was consistent with d  -wave neutron knockout, providing support for a Jπ assignment of 5/2+. The associated spectroscopic factor was deduced to be C2S(0d5/2)=4.1±0.4 by comparing the measured cross section (View the MathML source) with a distorted wave impulse approximation calculation. Such a large occupancy for the neutron 0d5/2 orbital is in line with the N=16 shell closure in 24O.The neutron drip-line nucleus 24O, which has been studied extensively in recent years [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] and [14], is now considered a doubly-closed-shell nucleus. In particular, the high excitation energy of the first 2+ state [7] and small quadrupole transition parameter [14] are strong indicators of an N=16 spherical shell closure. In addition, Kanungo et al. [8] found a large spectroscopic factor – C2S(1s1/2)=1.74±0.19 – for one-neutron knockout from 24O reflecting an almost complete occupancy of the 1s1/2 orbital.In this Letter we report on the spectroscopic factor for d5/2 neutron removal from 24O. The first excited state of 23O, which is neutron unbound [15], [13] and [16], was populated by one-neutron knockout from 24O with a proton target. The excitation energy, cross section, and longitudinal momentum distribution were determined and allowed the 0d5/2 neutron-hole nature of this state to be identified. The large spectroscopic factor deduced for this state is in line with the N=16 shell closure in 24O.The experiment was performed at the RIPS facility [17] at RIKEN. The experimental setup has been described in Refs. [14] and [18], and is depicted in Fig. 1. The secondary 24O beam was produced using a 1.5 mm-thick Be production target and a 95 MeV/nucleon 40Ar primary beam of ∼40 pnA. The average intensity of the 24O beam was ∼4 particles/sec. The momentum of the secondary beam was determined particle-by-particle by measuring the position at the dispersive focus F1 of RIPS with a parallel plate avalanche counter. The energy loss (ΔE  ) and time-of-flight (TOF) were measured using 350 μm-thick silicon and 500 μm-thick plastic scintillator detectors, respectively, at the achromatic focus F2. The liquid-hydrogen (LH2) target [19] was installed at the achromatic focus F3. The effective target thickness and the mid-target energy of 24O were 159±3 mg/cm2 and 62 MeV/nucleon, respectively. The 24O beam incident on the target was tracked particle-by-particle by using two multi-wire drift chambers installed just upstream of the target. The target was surrounded by an array consisting of 48 NaI(Tl) crystals (DALI) to detect γ rays from de-excitation of the fragments