Nuclear structure of 8B studied by proton resonance scatterings on 7Be Intense RI beam production at CRIB using the cryogenic gas target system Measurement results of proton elastic/inelastic resonance scatterings on 7Be H. Yamaguchi, Y. Wakabayashi, S. Hayakawa, G. Amadio, S. Kubono, H. Fujikawa, A. Saito, J.J. He, 　　T. Teranishi, Y.K. Kwon, Y. Togano, N. Iwasa, 　K. Inafuku, A. Kim, M. Niikura, D.N. Binh, and L.H. Khiem Center for Nuclear Study, Univ. of Tokyo, Kyushu University, RIKEN, Chung-Ang Univ. Rikkyo Univ., Tohoku Univ., Ewha women’s Univ., IPE

CRIB Introduction CNS Radio-Isotope Beam separator Low energy(<10MeV/u) RI beams, in-flight method Primary beam from K=70 AVF cyclotron Momentum (Magnetic rigidity) separation by “double achromatic” system, and velocity separation by a Wien filter. Orbit radius: 90 cm, solid angle: 5.6 msr, momentum resolution: 1/850. 2007 Dec 5 OMEG 07

Low-Energy RI beam Productions at CRIB Reactions such as (p,n), (d,p) and (3He,n) in inverse kinematics are used. Other RI beams have been produced at CRIB: 7Be, 8Li, 13N, 17N, 18F, 21Na, 22Mg , 23Mg, 25Al, 26Si, 39Ar,… typically 104-106 pps RI beam Primary beam Reaction Cross section Target Intensity 10C 6.1 A MeV 10B(4+) 7.8 A MeV (200 pnA) p(10B,10C)n 2 mb CH4 gas 1.3 mg/cm2 (1.6105 pps) 14O 6.7 A MeV 14N(6+) 8.4 A MeV (500 pnA) p(14N,14O)n 8 mb (1.7106 pps) 12N 3.9 A MeV 200 pnA 3He(10B,12N)n 5 mb 3He gas 0.25 mg/cm2 2.5103 pps 11C 3.4 A MeV 3He(10B,12N*)n 12N*  11C+p 20 mb 1.6104 pps 2007 Dec 5 OMEG 07

Cryogenic target: design Features: Lq. N2 cooling (automatic refill) for the better cooling power (~100 W) and thicker target. Forced target gas flow (>30 l/min) to have a better cooling, and to avoid target thickness reduction by the high-current beam. Oxygen density monitoring 2007 Dec 5 OMEG 07

Cryogenic target: results H2 gas target of 760 Torr and 80 mm-long worked at 85K stably for a 7Li2+ beam of 1.3 pμA. (Heat deposit…7.4W). Secondary beam: 7Be4+ at 4.0 MeV/u, purity 75% (without degrader/ WF). 2x108 pps was achieved. 2007 Dec 5 OMEG 07

Target thickness reduction by heat It is known that a high heat deposit (>5 mW/mm) of the beam can reduce the effective target thickness (J. Görres et al., 1980). In our measurement, the effective target thickness was reduced by ~30% due to the high heat deposit of 7.4W (65 mW/mm in the gas). We succeeded in minimizing the reduction to ~5% by making a forced circulation of the target gas. Poster P-21 (Wakabayashi) H. Yamaguchi et al., submitted to NIM A. 2007 Dec 5 OMEG 07

7Be(p,g)8B Astrophysically important reaction: 7Be(p,g)8B 8B neutrino … important for the solar model. Nucleosynthesis in some enviroments (hot p-p chain etc.) Astrophysical S-factor S17(0)…determined by 7Be(p,g)8B cross section. For a precise determination of S17, resonances in 8B may affect to the result ⇒ structure must be studied Now we know S17(0) with the uncertainty of 6-8%. Junghans et al, PRC (2003), Cyburt et al., PRC (2004), Schümann et al., PRC (2006). 2007 Dec 5 OMEG 07

7Be+p elastic resonance scattering Study the structure of 8B States at 0.77 MeV and 2.32 MeV are well known, but no clear knowledge above 3.5 MeV. A broad (>4 MeV) state at 3.5 MeV …2-　was reported; low-lying 2s state? The resonance may contribute to the 7Be(p,g) cross section even around the solar energy. V.Z. Gol’dberg et al, JETP Lett. (1998), another measurement in G.V. Rogachev et al, Phys. Rev. C (2001). 2007 Dec 5 OMEG 07

Method (Experimental setup) Thick target method: all the 7Be were stopped in the CH2 target, and recoil protons (Emax=23 MeV) were detected by silicon detectors (60 msr x 4 sets, covering up to 45 degree). Eproton⇒Ecm⇒Eex 2007 Dec 5 OMEG 07

Experimental Setup (picture) 2007 Dec 5 OMEG 07

Advantages over past measurements Wide energy range Ecm up to 6.7 MeV (< 3.5MeV in the past measurements). Wide angle 0-45 degree in qLAB (fixed angle in the past). Inelastic scattering 7Be(p,p)7Be* Inelastic events to the excited state at 429 keV have to be separated from inelastic events. NaI detectors were used to detect 429 keV gamma rays. 2007 Dec 5 OMEG 07

R-matrix fit for the 2- level The peak around Ecm=2.3 MeV is assumed to be from the interference of the known 3+ level (Eex=2.3 MeV) and the wide 2- state. (Same assumption with Rogachev et al). R-matrix fit…a sensitive tool for the determination of the parameters of the 2- state. 2007 Dec 5 OMEG 07

Determination of the 2- state parameters 1s-limits for the energy and the width are shown. We obtained consistent results between the data of 3 different angle ranges. 2007 Dec 5 OMEG 07

Fit for the whole energy range An example of the R-matrix fit for the whole energy region, using the determined parameters of the 2- state. The overall shape was roughly reproduced by introducing two new states.(1- state is expected to exist to make a doublet with the 2- state.) 2007 Dec 5 OMEG 07

How much the 2- state affects to S17(0)? Junghans et al. (2003)…precise direct measurement of 7Be(p,g), S17(0) = 21.4±0.6(expt.)±0.6(theo.). Fitted by a sum of a Micro-cluster model calculation (Descouvemont and Baye; DB) normalized to the experiment and resonant contribution (Breit-Wigner) of the 1+ and 3+. What if we introduce the broad 2- resonance? Both of DB and tail of the 2- resonance vary slowly with energy. Is it possible that some part of DB is actually from the contributions from the 2- resonance ? DB 3+ 1+ 2007 Dec 5 OMEG 07

S17(0) with 2- resonance Contribution from 2- resonance was evaluated by Breit-Wigner formula using our results. 1+ Results for 3 different Gg are shown. Orange curve…Gp was twiced from our best value. 3+ Direct+Resonance 2007 Dec 5 OMEG 07

Summary A low-energy (4.0 MeV/u) and intense (max. 2 x 108 pps) 7Be beam was successfully produced at CRIB, using a cryogenic gas target. We have measured proton elastic scatterings on 7Be at CRIB, and the result supports (or does not deny) the existence of the broad 2- state of 8B. Gol’dberg et al. E=3 MeV, G=1-4 MeV. Rogachev et al. E=(3.5 +/- 0.5) MeV, G=(8 +/- 4) MeV. Our best (preliminary) value: E=(3.1 +/- 0.4) MeV, G=(2.8 +/- 1.7) MeV. We made a simple evaluation of the resonant contribution of the 2- state for the S17 using the Breit-Wigner formulation. The result shows that the 2- state does not affect to the determination of S17(0) significantly. 2007 Dec 5 OMEG 07