EURISOL User Group Workshop Jan 14-18, Florence, Italy STRUCTURE NEAR THE NEUTRON DRIP LINE AT N=24 Zdeněk Dlouhý Nuclear Physics Institute ASCR, CZ Řež, Czech Republic for the GANIL-Orsay-Dubna- Řež -Bucharest collaboration

Halo nuclei Island of inversion- Disappearence of magicity Shells at N = 40, 50 Properties of Neutron-rich nuclei at the drip line Magic numbers:2,8,20,28,40,50,82….

The most well known one- and two-neutron halo nuclei are 11Be and 11Li nuclei 11 Be Z=4, N=7 11 Li Z=3, N=8 10 Be 2n 9 Li 1n

Disappearance of standard doubly-magic nucleus near the neutron drip line 10 He Z=2, N=8 A.N. Ostrowski et al., Phys. Lett. B338, 13 (1994). A. Korsheninnikov et al., Phys. Lett. B326, 31 (1994).

Island of inversion Island of Inversion N Z=8

Search for existence of neutron-rich isotopes 26,28O

GANIL Experimental Areas GANIL Experimental Areas In-Flight- Fragmentation In-Flight- Fragmentation Method for Radioactive Method for Radioactive ions beams ions beams  Fragment separator LISE3  Spectrometer SPEG

Search for the bound 26,28O nuclei at LISE3 N=16 Fragmentation of the 36 S 16+ (78.1AMeV) beam with a mean intensity 800 enA has been used for the search for the existence of bound 26,28 O nuclei. A dashed and solid lines show N=20 and N=16 isotones. The heaviest known 29 F isotope is clearly visible (519 events). Fragmentation of the 36 S 16+ (78.1AMeV) beam with a mean intensity 800 enA has been used for the search for the existence of bound 26,28 O nuclei. A dashed and solid lines show N=20 and N=16 isotones. The heaviest known 29 F isotope is clearly visible (519 events). No events corresponding to 26 O, 28 O and to heavier isotopes than N=16 for C and N have been observed. No events corresponding to 26 O, 28 O and to heavier isotopes than N=16 for C and N have been observed.

H.Sakurai et al., Phys.Lett. 448B, 180 (1999)

Disappearance of standard doubly-magic nuclei near the neutron drip line 10 He Z=2, N=8 A.N. Ostrowski et al., Phys. Lett. B338, 13 (1994). A. Korsheninnikov et al., Phys. Lett. B326, 31 (1994). 28 O Z=8, N=20 O. Tarasov et al., Phys. Lett. B (1997). H. Sakurai et al., Phys. Lett. B (1999).

ѵ Intruder states neutrons 18 Z=8 28O

dEZ TOFA/Z

The evolution of the shell closures at large N/Z ratios is one of the most fascinating quest in nuclear structure. The confirmation of shell closure and magic numbers is evidenced usually using one of following experimental approaches : 1) Study of masses and separation energies 2) Determination of energies of the first excited state (E2) of even-even nuclei, 3) The reduced transition probability B(E2; 0+ → 2+) value along an isotopic chain of proton-magic nuclei, provides a sensitive signature of shell evolution.

Study of masses and separation energies

The shell structure which is plainly visible when inspecting a graph of the two-neutron separation energy, defined by S 2n (N,Z) = M(N-2,Z) -- M(N,Z) + 2 M n versus the number of neutrons, N. For one-neutron separation energy we get S n (N,Z) = M(N-1,Z) -- M(N,Z) + 1 M n but the pairing effect must be avoided. For a given Z, the general tendency for S2n is to fall steadily as an N increases.

Nuclear mass measurement at SPEG The masses of 31 neutron-rich nuclei in the range A = have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S, and P isotopes are seen to exhibit a change in shell structure around N = 28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. Evidence for shape coexistence is provided by the observation of an isomer in 43 S. F.Sarazin et al., Phys.Rev.Lett. 84, 5062 (2000)

S2n versus N

S2n versus Z

In–beam gamma ray spectroscopy

Experimental Setup BaF 2 array In-beam gamma ray spectroscopy g.s. Exc. De-exc.

In beam gamma ray spectroscopy HPGe clovers BaF 2 – upper array Beam HPGe clovers SPEG

N=14 and 16 shell gaps in neutron-rich oxygen isotopes M.Stanoiu et al., Physical Review C 69, (2004) The nonobservation of any γ- decay branch in 23O and 24O suggests that their excited states lie above the neutron decay thresholds. From this, as well as from the level schemes proposed for 21O and 22O, the size of the N=14 and 16 shell gaps in oxygen isotopes was discussed in the light of shell-model calculations. 24 O

Isotopes near N=16 measured at GANIL 20O, 22O, 24O 26Ne, 28Ne, 24Mg, 28Mg, 30Mg, 32Mg 30Si, 32Si, 34Si Energies of the first 2 + states vs neutron number Isotopes near N=20 32S, 34S, 36S, 36Ar, 38Ar, 36Ca, 38Ca, 40Ca O O Ne Mg Si ? S Ar Ca ?

Changes in neutron magic numbers for neutron-rich nuclei Instability of 10 He (Z = 2, N = 8) and 28 O (Z = 8, N = 20) Disappearance of neutron standard magic numbers N = 8 and N = 20 Appearance of new neutron magic numbers N = 6 and N = 16 New doubly magic nuclei 8 He (Z = 2, N = 6) and 24 O (Z = 8, N = 16) (below neutron decay threshold no bound excited states) Adding +1 proton we obtain: Cores of halo nucleus 9 Li (Z = 3, N = 6) and 25 F (Z = 9, N = 16) halo nucleus 11 Li (Z = 3, N = 8) and F (Z = 9, N = 18 – 22)

Doubly magic Nuclei – bases for Nuclear Halo’s or very Neutron-rich Nuclei?

Change of effective single particle energies in Si and O

Search for 33 F

Conclusions Conclusions  Disappearance of doubly magic nuclei 10 He and 28 O  Changes in magic numbers near driplines - N=6,16  New doubly magic nuclei near driplines - 8 He and 24 O  Triton separation energy of odd Z nuclei shows the posibility of existence N=24 subshell in neutron-rich nuclei

Collaboration GANIL, Caen, France R.Anne, M.Lewitowicz, W.Mittig, F.de Oliveira, P.Roussel-Chomaz, H.Savajols, M.G.Saint-Laurent IPN, Orsay, France F.Azaiez, M.Belleguic, C.Donzaud, J.Duprat, D.Guillemaud-Mueller, S.Leenhardt, A.C.Mueller, F.Pougheon, J.E.Sauvestre, O.Sorlin, M. Stanoiu FLNR, JINR, Dubna, Russia Yu.Penionzhkevich, S.Lukyanov, Yu.Sobolev LPC, Caen, France N.L.Achouri, J.C.Angelique, S.Grevy, N.Orr NIPNE, Bucharest-Magurele, Romania C.Borcea, A.Buta, I. Stefan, F. Negoita, Atomki, Debrecen, Hungary Zs.Dombradi, D.Soher, J.Timar, NPI, ASCR, Řež, Czech Republic D.Baiborodin, J.Mrázek, G.Thiamová, J.Vincour, Z.D.

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