PHYSICAL REVIEW C 72, 064327 (2005) β -decay of the neutron-rich nucleus 18 N Z. H. Li, Y. L. Ye, H. Hua, ∗ D. X. Jiang, Y. M. Zhang, F. R. Xu, Q. Y. Hu, G.L. Zhang, Z. Q. Chen, T. Zheng, C. E. Wu, J. L. Lou, X. Q. Li, D. Y. Pang, S. Wang, and C. Li School of Physics and MOE Key Laboratory of Heavy Ion Physics, Peking University, Beijing 100871, China H. S. Xu, Z. Y. Sun, L. M. Duan, Z. G. Hu, R. J. Hu, H. G. Xu, R. S. Mao, Y. Wang, X. H. Yuan, H. Gao, L. J. Wu, H. R. Qi, T. H. Huang, F. Fu, F. Jia, Q. Gao, X. L. Ding, J. L. Han, and X. Y. Zhang Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China (Received 2 September 2005; published 30 December 2005) The β -decay of 18 N has been studied using β -n and β -γ coincidence methods. The 18 N ions were produced by the fragmentation of the E/A = 68.8 MeV 22 Ne beam on a thick beryllium target. A newly constructed neutron detector system with wide energy detection range and low-energy detection threshold was used. The 619 ± 2 ms half-life of the 18 N β -decay was found to be in very good agreement with the previous measurements. Transitions to 11 β -delayed neutron emitting states in 18 O have been observed with a total branching ratio of 6.98 ± 1.46%. The Gamow-Teller β -decay strengths of 18 N to these levels were deduced and compared with the shell model calculations. The intensities of the strong γ -ray transitions in 18 O were found to be consistent with recent work. DOI: 10.1103/PhysRevC.72.064327 PACS number(s): 21.10.Pc, 23.40.−s, 23.90.+w, 27.20.+n I. INTRODUCTION For light neutron-rich nuclei, their β -decays are often characterized by the large decay energies (10∼20 MeV), which will lead to the population of excited states with a wide excitation range, in particular particle unbound states, in the daughter nuclei. As a result, delayed neutrons or other particles may be emitted following the emission of β -rays. Such a complex decay scheme yields a great deal of information on β -decay properties of the neutron-rich mother nucleus and nuclear structures of the daughter nucleus, which provides a stringent test of the validity of structure models, such as the shell model. To get an unambiguous and precise determination of the complex decay scheme, as well as the information about the quantum state of the mother nucleus and states in the daughter nucleus, the coincidence measurements of the delayed neutrons or other particles with the β -rays are usually necessary. We present here the spectroscopy of β -decay of the neutron- rich nucleus 18 N using β -n and β -γ coincidence methods. The β -decay of nucleus 18 N has been studied extensively [1–8] since it was first observed by Chase et al. [1] in 1964, and the spectroscopy of the daughter nucleus 18 O has been established quite well via numerous studies [2–5,9]. Since the β -decay energy of 18 N is 13.899 MeV, which lies above the alpha threshold at S α =6.227 MeV and the neutron threshold at S n = 8.044 MeV, delayed emissions of neutrons or alpha particles can occur. The total branching ratio to α-emitting states has been found to be 12.2 ± 0.6% by Zhao et al. [4] and a β -delayed neutron emission probability (P n ) of 14.3 ± 2.0% has been measured by Reeder et al. [6]. Among these measurements, one interesting feature is that a broad alpha group was observed at an excitation energy of 9.00 MeV with a width of about 500 keV [4,5]. In a recent work by Scheller ∗ Corresponding author. Email address: Hhua@hep.pku.edu.cn et al. [2], an attempt was made to measure the energy spectrum of delayed neutrons, and nine neutron peaks were observed at an energy from 0.99 to 3.26 MeV with a total branching ratio of 2.2 ± 0.4%, which only accounted for a small part of total neutron-emission probability (P n ) of 14.3 ± 2.0% by Reeder [6]. In Ref. [2], the neutron detection threshold prohibited detection of the delayed neutrons of 800 keV resulting in a lesser value for the total branching ratio of 18 N to neutron emitting states in 18 O. Thus the missing ∼12% branching ratio is likely to proceed to states in 18 O between 8.044 MeV (neutron threshold) and ∼9.0 MeV (low-energy neutron detection threshold in the 18 N experiment of Ref. [2]). Based on the above considerations, the present work focused on the low energy spectra of delayed neutrons from the β -decay of 18 N. One recently constructed neutron detector system, which is designed for the study of β -delayed neutron emitting nuclei with wide energy range and low-energy detection threshold, was used and two missing branches to the neutron emitting states in 18 O were observed. II. EXPERIMENT The present experiment was performed at the Institute of Modern Physics (IMP), Lanzhou, China. The primary beam for the experiment was 68.8 MeV per nucleon 22 Ne ions produced by HIRFL, which impinged on a 9 Be target with a thickness of 292.5 mg/cm 2 . The 18 N fragments were collected, separated, and purified using the Radioactive Ion Beam Line in Lanzhou (RIBLL) [10]. The present experimental setup is shown in Fig. 1. The 18 N beam passed through a thin aluminum window (10 µm thick) which separated the vacuum of the beam line from the air. Before the 18 N beam was finally stopped in a thin plastic scintillation detector, referred to as the implantation detector (NE102, 4.0 cm × 5.0 cm × 3.0 mm thick), it also passed an energy degrader (whose thickness was adjustable) and a silicon surface-barrier E detector (325 µm thick, 0556-2813/2005/72(6)/064327(6)/$23.00 064327-1 ©2005 The American Physical Society