Spectroscopy of 17C via one-neutron knockout reaction

A spectroscopic study of 17C was performed via the one-neutron knockout reaction of 18C on a carbon target at RIKEN-RIBF. Three unbound states at excitation energies of 2.66(2), 3.16(5), and 3.97(3) MeV (preliminary) were observed. The energies are compared with shell-model calculations and existing measurements to deduce their spin-parities. From the comparison, the states at 2.66(2) and 3.97(3) MeV are suggested to be 1/2 and 3/2, respectively. From its decay property, the state at 3.16(5) MeV is indicated to be 9/2.


Introduction
The unique features of neutron-rich nuclei, such as non-canonical shell structures differing from those near stability, have been a subject of intensive studies in both theory and experiment.Among the neutron-rich nuclei, neutron-rich carbon isotopes have attracted particular attention in recent years a e-mail: sjkim06@snu.ac.kr due to their anomalous level structures.For example, the ground state spin-parities (J π ) of 15,17,19 C nuclei, 1/2 + , 3/2 + , and 1/2 + , do not follow naive shell-model expectations.The 2 + excitation energy, E x (2 + 1 ) = 1588(20) keV, of 20 C 14 turned out to be not much different from that of neighboring eveneven 18 C 12 of 1585 (10) keV [1].This migration of the 2 + 1 energy is distinct from that of oxygen isotopes, where E x (2 + 1 ) exhibits a sudden jump at 22 O 14 , signifying the emergence of the shell gap at the neutron number N = 14 [2].The breakdown of the N = 14 gap in carbon isotopes has been ascribed to the near degeneracy of the s 1/2 and d 5/2 orbits and to the reduced neutron-neutron interaction when going from oxygen to carbon nuclei [1].Detailed mechanisms and interplay of these effects, however, remain to be answered, requiring further studies.Here we present a spectroscopic study on a neutronrich odd carbon isotope, 17 C, performed to furnish information on energy levels, especially above the neutron threshold.To populate neutron unbound states in 17 C, the one-neutron knockout reaction of 18 C on a carbon target was utilized.

Experiment
The experiment was performed at the Radioactive Isotope Beam Factory (RIBF) at RIKEN, operated by the RIKEN Nishina Center and the Center for Nuclear Study of the University of Tokyo, using the SAMURAI (Superconducting Analyser for MUlti particles from RAdio Isotope beams) spectrometer [3] during the first physics runs using the apparatus.For the spectroscopy of 17 C through the oneneutron knockout reaction from 18 C, beam particles were identified by the Bρ-TOF-ΔE method and the 18 C beam was selected.The 18 C beam with the mid-target energy of 245 MeV/nucleon and a momentum spread of ±3% impinged on a carbon reaction target having a thickness of 1.8 g/cm 2 .The beam intensity was about 2300 pps and the mass resolution was A/ΔA = 770 in σ.
The unbound states in 17 C produced by one-neutron knockout of 18 C immediately decayed into a 16 C fragment and a neutron.Downstream of the target, the charged particles were bent by a superconducting dipole magnet having a central magnetic field of 3 T.They were also identified by the Bρ-TOF-ΔE method and the 16 C fragment was identified in terms of both charge and mass.The fragment mass resolution was A/ΔA = 250 in σ.The momentum vector of the neutron was obtained by the position and timing information from the neutron hodoscope, NEBULA, consisting of 120 modules of plastic scintillators for neutron detection (NEUT) and 24 modules of plastic scintillators for charged-particle veto (VETO).For NEUT, TOF resolution was 270 ps for a flight length of around 11 m.A γ-ray detector array, DALI2 [4], having 140 blocks of NaI(Tl) scintillators surrounded the target to measure γ rays emitted from the excited fragments.

Results and Discussion
The energies of the unbound states in 17 C were reconstructed by the momentum vectors of the 16 C fragment and neutron utilizing the invariant mass method.As preliminary results, three unbound states at the relative energies of 0.66(5), 1.92(1), and 3.23(3) MeV corresponding to the excitation energies (E X ) of 3.16(5), 2.66(2), and 3.97(3) MeV, respectively, were observed.The 3.16-MeV state was observed to decay into the 2 + state in 16 C at 1.760(3) MeV [5].The states at 2.66 and 3.97 MeV are associated with p-wave neutron knockout, as evidenced by their momentum distributions (both longitudinal and transverse ones) which are well described by the Glauber model calculations for L = 1 [6], and correspond well to the reported 1/2 − [2.71(2) MeV] and 3/2 − [3.93(2) MeV] states [7], respectively.definitely known and this state is plotted in both panels of Fig. 1 in black."WBT" and "WBT 0.7V 01 " refer to the calculations using the WBT interaction [10] within the 1 w basis and the p-sd model space and those with reduced two-body matrix elements (by multiplying a factor 0.7) for the J = 0 and T = 1 channels [7], respectively."WBT no w restriction" provides energies from the NuShellX code [11] which can perform w unconstrained calculations.Calculations utilizing the coupled-cluster effective interaction [12] and the monopole based universal interaction in the p-sd model space [13] are denoted as "CCEI" and "V MU ", respectively.
Ueno et al. [7] reported that the 1/2 − energy is underpredicted by about 1 MeV with "WBT", while such an underprediction is significantly remedied with "WBT 0.7V 01 ".The present "WBT no w restriction" reproduces the energy of the 1/2 − state well together with that of 3/2 − .Concerning the "WBT no w restriction" predictions for negative parity states, there is no candidate for the 3.16-MeV state.This state, instead, has two candidate states, 9/2 + and 3/2 + , in the positive parity state diagram, Fig. 1 (b).In an attempt to clarify the nature of this state, its decay property was examined within the shell model.This state has been observed to populate predominantly the 2 + state of 16 C [5].For the 3/2 + assumption, there are two routes for its decay, namely via s-wave and d-wave neutron emissions.From a simple estimate for the partial width (Γ), assuming that it is proportional to the product of the spectroscopic factor (C 2 S ) for decay, the wave number (k = 2μE decay / c), the radius of the decay daughter (R), and the transmission through the centrifugal barrier (v l ) [14] as we obtain that 97.5% of the decay is induced by s-wave.Here E decay is the decay energy and μ is the reduced mass of the decay products.For this s-wave decay, the single-particle resonance width is predicted as 4.84 MeV [14].A resonance having a width of this order will be hardly visible as a distinct peak structure.For d-wave decay, the single-particle width is calculated as 0.03 MeV [14].The 9/2 + assignment for the 3.16-MeV state, thus, remains as a plausible solution (a 9/2 + state can decay to 16 C(2 + ) only through the d-wave neutron emission).In one-neutron knockout from 18 C, the 9/2 + state is not populated via one-step processes; 9/2 + orbit occupancy of neutrons is expected to be zero.Multiple step processes, such as (a) inelastic excitation of 18 C followed by sequential decay into the 9/2 + state in 17 C and/or (b) initial population of 18 C(2 + ) followed by d 5/2 neutron knockout, might be responsible for the population of the 9/2 + state.

Summary
Three unbound states at E x = 2.66(2), 3.16(5), and 3.97(3) MeV in 17 C were observed by the oneneutron knockout reaction of 18 C using the SAMURAI spectrometer in RIBF at RIKEN.While the reported shell-model calculation with the WBT interaction had to be carried out using the two-body matrix elements of the particles within the sd orbits to obtain a proper excitaion energy spectrum for the observed negative parity states in 17 C, the calculation utilizing the WBT interaction without w restriction in the model space provided an appropriate description of them.The state at 3.16 MeV is suggested to be a 9/2 + state from an examination of its decay property.

Fig. 1 2 ExcitationFigure 1 .
Figure 1.Negative (a) and positive (b) parity states in 17 C. "Exp."refers to the states observed in this study.For others see the text.