Fig. 1 Side View of DALI from Up-stream 1 H( 17 C, 16 C → 15 C + n + γ)X Raw Doppler Effect Corrected Abstract DALI is a γ-ray detector, which is an array of NaI(Tl) crystal. By incorporating information from DALI signals, an excited state at 5.45 MeV in 16 C has newly been identified in a preliminary invariant mass spectrum of the 1 H( 17 C, 16 C → 15 C + n + γ)X reaction. In order to reproduce the previous result with better statistics and to gain further insights into the spectroscopic nature of this as well as other states in 16 C nuclei, another experiment is being planned using the invariant mass method in inverse kinematics. For the future experiment, suitable position of the DALI was determined by a GEANT simulation, and the results will be presented. DALI : γ-ray detector The Excited States of 16 C The Optimization of the DALI What is DALI? γ-ray Detector : An array of NaI(Tl) crystal 48 NaI Detectors (scintillator & PMT) Layer Structure : 4 layers  Ring Shape : Beam or target at the center Fig. 2 Top View and Side View of DALI DALI in R364n experiment RIPS in RIKEN : RI beam (In-flight separation) Inverse Kinematics : Unstable isotopes (ex. 17 C) Target : Cryogenic Hydrogen (LH 2 ) DALI (NaI) Detector  Surrounding the LH 2 target  To detect γ-rays from the target (assumption) Fig. 3 Experimental Setup of R364 Experiment Summary I.DALI is an γ-ray detector, and it was used in the RIPS experiment (R364n). II.The new state of 16 C (5.45 MeV) was found by using the signals from several detectors including DALI through the 1 H( 17 C, 16 C → 15 C + n + γ)X reaction III.The optimized position of DALI along the beam line is 40 cm far from the target toward downstream. IV.The distortion of the γ-ray peak is the problem to be solved. (ex. Assuming that the γ-ray source is located at the center of DALI) Invariant Mass : Excitation Energy : (n : neutron, f : fragment) Invariant Mass Spectrum Sharp peak at 0.46 MeV Other small peaks γ-ray Energy Spectrum Lower peak : Background Upper peak : 0.74 MeV (Corresponding to the first excited state of 15 C) Two peaks in Coincidence Newly found state of 16C (R364n at 5.45 MeV) First Excited State of 15 C : Isomer (T 1/2 =2.61 ns) → γ-ray can be emitted at the place far from the target → Low efficiency Adjustment of the Position (to downstream) → Increase efficiency for the future experiment Geant 3 Simulation : GDALI code Motivation & Method Note : The first excited state of 15 C is an isomer. (T 1/2 =2.61 ns) Distortion of the peak (Doppler effect correction) Result Best Efficiency at 40 cm (13.5%) to downstream along beam line from the original (R364n) setup. → Gain : 3.8 (R364n setup) Distortion of the peak due to the isomer state (the small plot) → DALI is farther from the target, more distortion of the γ-ray peak occurs. Collaborators (RIKEN, R364n) Seoul National University : Y.Satou, J.Hwang Tokyo Institute of Technology : T.Nakamura, T.Sugimoto, Y.Kondo, N.Matsui, Y.Hashimoto, T.Nakabayashi, T.Okumura, M.Shinohara RIKEN : N.Fukuda, T.Motobayashi, Y.Yanagisawa, N.Aoi, S.Takeuchi, T.Gomi, Y.Togano, H.Sakurai, H.Otsu, M.Ishihara RikkyoUniversity : S.Kawai Tokyo University : H.J.Ong, T.K.Onishi Center for Nuclear Study (CNS) Tokyo University : S.Shimoura, M.Tamaki Tohoku University : T.Kobayashi, Y.Matsuda, N.Endo, M.Kitayama