Shell Evolution of exotic nuclei. Hiro IWASAKI

Transcription

1 Shell Evolution of exotic nuclei studied from level lifetime measurements Hiro IWASAKI NSCL/MSU

2 In beam gamma Spectroscopy vs In-beam decay Spectroscopy Knockout reactions Coulomb excitation Direct reactions Gamma Transition detection strengths as from a Tag lifetime of measurements reactions

3 Dawn of In-beam decay Spectroscopy with RIB - triaxiality in N=Z 64 Ge - Target K.Starosta, A.Dewald et al. Phys.Rev.Lett.99(2007) Degrader Differential Recoil distance method with the plunger device 64 Ge Köln- Plunger 65 Ge v1 v2 E1 E2 E2 properties (21,2 + ) 22 + New Shell Model (JUN45) M.Honma et al. PRC80(09) E1 E Wu. 40Wu. 0.1Wu. E 64 Ge 65 Ge v1 v2 triaxiality? 27º 23º Intermediate between -soft and -rigid

4 Lifetime group In-beam decay Spectroscopy bound states Doppler-shift effects of rays (E2/M1/E1) Recoil Distance Method (RDM) 64 Ge K.Starosta et al., PRL99 (07) Pd A.Dewald et al., PRC 78 (08) (R) 16,18,20 C talk by P.Fallon, neutron-rich Ni, Fe Doppler-shift Attenuation Method (DSAM) 13 B H.Iwasaki et al., PRL 102 (09) ReA3 Line-Shape Analysis NSCL Shadowing effects for ray detection (E2/M1) Recoil Shadow Method (RSM) unbound states Particle-plunger method Recoil Distance Method (RDM) - Lifetime of 2p emitter - Lifetime of p emitter of astrophysical interest Indirect lifetime measurement Resonance search and width 12 O D.Suzuki, H.Iwasaki et al., PRL103 (09) Spectroscopy of unbound nuclei 16,18 C H.Ong et al., PRC78 (08) C D.Suzuki et al., PLB 666 (08) 222

5 13 B

6 the evolution of the shell structure along the N=8 line The N = 8 isotones represent a drastic evolution of the shell structure from stable nuclei to exotic nuclei towards the neutron drip line. 16 O, 15 N, 14 C magic nuclei with the closed-shell structure for eg. Ex(2 + ) = 6-7 MeV N = 8 shell gap 12 Be (sd) 2 configuration in the ground state (A.Navin et al.prl85(00)266,s.d.pain et al.,prl96(06)032502) 11Li 11 Li dominance of the s-orbit -> formation of halo (H.Simon et al.,prl83(99)496) 9Li

7 13 B : transition from magic 14 C to exotic 12 Be Little is known for the odd-mass 13 B nucleus, which is expected to show transitional features from 14 C to 12 Be Ex [MeV] 6 [2] T1/2(fs) 4.83 MeV 43(35) < > J pi [1] 1/2 + (1,5,7/2) - (1,5,7/2) - (1,3,5/2) + (1,5,7/2) - (1,3,5/2) + H.Iwasaki et al.plb481(00)7, 491(00)8 S.Shimoura et al., PLB 654(07)87 N.Imai et al., PLB673(09)179 G.S. is dominated by the p-shell configuration 3/2 - from the magnetic moment (close to Schmit limit) [3] [1] 11 B(t,p) R.Middleton, Nucl. Phys. 51 (64) 50 [2] lifetime M.J.Throop Phys. Rev. 179 (69) 1011 [3] g-factor R.L. Williams Jr. Phys. Rev. C3 (71) 2149

8 Studies on 13 B with RI beams Gamma studies with RI beams are possible, but, still difficult, for odd-mass nuclei due to a small level spacing Gamma ray spectra measured by knock-out reaction 9 Be( 14 B, 13 B*) transfer reaction a( 12 Be, 13 B*) 12 Be(a,t) 13 B Identify 3.7,4.1,4.8-MeV states (FWHM 1% for v/c =0.3 ) NaI array Segmented Ge array V.Guimaraes et al., Phys. Rev. C 61 (2000) S.Ota et al., Phys. Lett. B 666 (2008) 311

9 Level lifetime measurements at IKP, Cologne experiment 7 Li( 7 Li,p) 13 B* at 5.4 MeV low-energy fusion reaction Doppler-shift Attenuation method (DSAM) target 7 LiF +Au backing Ge 1 Cluster detector from Euroball + 5 co-axial + particle coincidence measurement

10 Observed gamma spectrum for 13 B Previos work (with 1 Ge(Li) ) M.J.Throop Physica l Review 179, 1011 (1969) MeV 4.83 ( 12 C/ 11 B) Present work

11 Doppler-shift analysis Observed (Doppler-shifted) energies are written by Eobs E0(1 ini F( ) cos ) Attenuation factor F ( ) 1 (Full Doppler-shifted) F( ) 0 (stopped) < 30 fs 50(20) fs 20

12 Line-shape analysis for the 3.53-MeV state Gamma-ray peak consists of three components: 1 decay in flight during the target or degrader 3 2 decay in flight after the degrader 3 stopped component target degrader 1 2 Mean lifetime 1.3 (3) ps 3 2 This value corresponds to very small transition probabilities 1 B(M1)< 7.2x10-4 W.u. B(E2)< 0.81 W.u.

13 A comparison with shell model calculations Since the 3.53-MeV state is populated via L=2 transfer momentum in the 11 B(p,t) 13 B reaction (J pi ( 11 B gs )=3/2 - ), this is the (lowest) negative-parity state with J=(1,3,5,7) - /2 A comparison with recent shell model calculations using the SFO interaction involving the improved p-n monopole interaction (T.Suzuki et al., PRC67(03)044302).

14 Level Scheme along the N=8 line the 3.53-MeV state is most likely 3/2 - state with the dominant neutron 2p2h intruder configuration 3/2 - gamma transition 3/2 - ground state H.Iwasaki, A.Dewald et al. Phys.Rev.Lett.102 (2009)

15 Shape coexistence in 13 B? A variety of rotational bands are predicted by the AMD (antisymmetrized molecular dynamics) calculation (Y.Kanada-En yo et al., Prog.Theo.Phys.120 No.5 (08) p917 ). ground state second 3/2 - state (present work) proton intruder 1/2 + state (S.Ota et al., PLB666(08)311)

16 17 C

17 Structure of 17 C Sn=760keV Three weakly-bound states (nearly degenerate) connected with M1/E2 transitions Inversion between 3/2+, 5/2+ states deformation? lowering 1/2+ state halo?

18 Modified Recoil Shadow Method Increased detectors improved statistics Various combinations increased sensitivity towards lifetime Measurement with/without lead shield R wpb /R wopb NO uncertainty due to angular distribution of -ray

19 Level lifetime measurements of 17 C 1 st (1/2 + )->gs. 18.9(5.3)ps (5/2 + ) 333 kev 583(56)ps (1/2 + ) 212 kev 3/2 + 1 st ; B(M1; (1/2 + ) 3/2 + ) = (1.0±0.1) x 10-2 [μ N2 ] 2 nd ; B(M1; (5/2 + ) 3/2 + ) = +3.2 ( ) x 10-2 [μ N2 ] 2 nd (5/2 + )->gs.

20 Level Schemes and B(M1) among N=11 isotones

21 Comparison with shell model calculations The large imbalance in B(M1) strengths between the first and second excited states in 17 C provides a sensitive test for shell model interactions. SFO-tls = SFO + tensor ( + ) T. Suzuki and T.Otsuka Phys. Rev. C78 (2008) (R) Further, by reducing <1s1/20d5/2;J=2,T=1 V 0d5/20d3/2;J=2,T=1> <1s1/20d5/2;J=2,T=1 V 0d5/20d3/2;J=2,T=1> by 20%, SFO-tls interaction gives the B(M1) values Of 0.01 N 2 and N 2 (exp: 0.01 and N 2 ) halo effects?

22 12 O

23 Shell evolution along the Z=8 line Level schemes for low-lying states in 12,14 O with Z=8 and 12 Be, 14 C with N=8

24 Missing-mass spectroscopy on the proton-rich nucleus 12 O via (p,t) 12 O unbound => missing-mass spectroscopy p( 14 O, 12 O)t at 50 MeV/u Characteristic features -High energy recoil energy of tritons (> 60MeV) use of a thick target (1mm t )

25 p( 14 O, 12 O)t experiment at GANIL (collaboration between IPN-Orsay, Ganil, CEA-Saclay (France) and Univ.of Tokyo, RIKEN, RCNP (Japan) p( 14 O, 12 O)t at 50 MeV/u triton MUST II Spectrometer 14 O beam Cryogenic target Solid Hydrogen 10 C(+2p) Si telescope triton beam Cryogenic Solid Hydrogen Target at GANIL P.Dolegieviez et al. NIM564(06)32

26 New Results on 12 O the Results the mirror symmetry in the breakdown of magic number 8 12 O N = 8 Magic number 8 The 12 O spectrum indicates the first evidence for the excited state at 1.8 MeV, of which J is suggested to 0 + or 2 + from angular distribution data. 12 O 14 O 16 O 14 C 12 Be proton-rich stable neutron-rich Z = 8 Exp shell model

27 Proposed mechanisms for the shell evolution in exotic nuclei [ Isospin symmetric ] proton-neutron monopole interaction - tensor force - the spin-isospin part of the central force for eg. proton J=j+1/2 (1p3/2) neutron J=j-1/2 (1p1/2) [ Isospin asymmetric ] Effects due to weak binding - neutron halos : 2S 1/2 orbital - ls-splitting : d Vls dr 14 C 12 Be [ Isospin?? ] proton neutron Clustering and/or deformation - Nilsson orbit - Molecular orbit (alpha + alpha + Xn) Sigma orbit Isospin symmetric mechanism should be responsible?

28 Open Questions : Resonance Widths (lifetimes) gs = 0.6(5) MeV ex = 1.2(6) MeV Other data for the ground-state width gs = 0.40(25) MeV (G.J.KeKelis et al. PRC17(78)1929) gs = 0.578(205) MeV (R.A.Kryger et al. PRL74(95)860) Theoretical Predictions R-matrix formula for one-proton sequential decay (F.C.Barker PRC59(99)535) gs < 0.1 MeV 3-body model (core+n+n) calculation (L.V.Griorenko et al. PRL88(02)042502) gs 0.06 MeV

29 In Summary, I presented in-beam decay spectroscopic studies on exotic nuclei, with an emphasis on the shell evolution in light neutron-rich / neutron-deficient nuclei. By applying new and conventional methods in nuclear spectroscopy, a variety of low-lying properties of interest have been investigated. Possible mechanisms, which are responsible for the shell evolution, have been discussed in comparison with recent experimental data.

30 Collaborators, (U.of Tokyo/RIKEN/CNS/RCNP Japan) H.J.Ong, N.Imai, H.Sakurai, T.Nakao, N.Aoi, H.Baba, S.Bishop, Y.Ichikawa, M.Ishihara, Y.Kondo, T.Kubo, K.Kurita, T.Motobayashi, T.Nakamura, T.Okumura, T.K.Onishi, S.Ota, M.K.Suzuki, S.Takeuchi, Y.Togano, Y.Yanagisawa D.Suzuki, H.Otsu, S.Michimasa, M.Takechi, H.Okamura, H.Baba (Koeln Germany) A.Dewald, A.Gelberg, J.Jolie, C.Fransen, M.Hackstein, W.Rother, T.Pissulla, K.O.Zell, P.Petkov (GANIL/IPN/CEA-Saclay France) D.Beaumel, P.Roussel-Chomaz, Y.Blumenfeld, E.Pollacco, A.Gillibert, L.Nalpas, J.Guillot, J.A.Scarpaci, M.Assie, A.Remis., V.Lapoux, X.Mougeot, I.Mukha, F.Hammache, O.Sorlin, S.Franchoo, N.deSereville, A.Drouart, F.Marechal, I.Stefan