EURISOL User Group Workshop

Jan 14-18, 2008. Florence, Italy

STRUCTURE NEAR THE NEUTRON DRIP LINE AT N=24

Zdeněk Dlouhý

Nuclear Physics Institute ASCR, CZ-25068 Řež, Czech Republic

for the GANIL-Orsay-Dubna- Řež -Bucharest collaboration

Halo nucleiHalo 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 inversionIsland of inversion

Island of Inversion

N

Z=8

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

GANIL Experimental AreasGANIL Experimental Areas

In-Flight- FragmentationIn-Flight- Fragmentation Method for RadioactiveMethod for Radioactive ions beamsions beams

Fragment separator LISE3

Spectrometer SPEG

Search for the bound Search for the bound 26,28O nuclei at LISE326,28O nuclei at LISE3

N=16

Fragmentation of Fragmentation of thethe3636SS16+16+(78.1AMeV) beam with a (78.1AMeV) beam with a mean intensity 800 enA has been mean intensity 800 enA has been used for the search for the used for the search for the existence of bound existence of bound 26,2826,28O nuclei. O nuclei. A dashed and solid lines show A dashed and solid lines show N=20 and N=16 isotones. The N=20 and N=16 isotones. The heaviest known heaviest known 2929F isotope is F isotope is clearly visible (519 events).clearly visible (519 events).

No events corresponding to No events corresponding to 2626O, O, 2828O and to heavier isotopes than O and to heavier isotopes than N=16 for C and N have been N=16 for C and N have been observed.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. B409 64 (1997).H. Sakurai et al., Phys. Lett. B448 180 (1999).

ѵ

Intruder

states

16

18 2220

24

neutrons

18

Z=8 28O

dE Z

TOF A/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

S2n(N,Z) = M(N-2,Z) -- M(N,Z) + 2 Mn

versus the number of neutrons, N.

For one-neutron separation energy we get

Sn(N,Z) = M(N-1,Z) -- M(N,Z) + 1 Mn

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 = 29 - 47 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 43S.

F.Sarazin et al., Phys.Rev.Lett. 84, 5062 (2000)

S2n versus N

S2n versus N

S2n versus Z

In–beam gamma ray spectroscopyIn–beam gamma ray spectroscopy

Experimental Setup

BaF2 array

In-beam gamma ray spectroscopy

g.s.

Exc. De-exc.

In beam gamma ray spectroscopy

HPGeclovers

BaF2 – upper array

Beam

HPGe

clovers

SPEG

NN=14 and 16 shell gaps in neutron-rich =14 and 16 shell gaps in neutron-rich oxygen isotopesoxygen isotopes

M.Stanoiu et al., Physical Review C 69, 034312 (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.

24O

Isotopes near N=16 measured at GANIL

20O, 22O, 24O26Ne, 28Ne, 24Mg, 28Mg, 30Mg, 32Mg30Si, 32Si, 34Si

Energies of the first 2+ states vs neutron number

Isotopes near N=20

32S, 34S, 36S,36Ar, 38Ar, 36Ca, 38Ca, 40Ca

OO

Ne

Mg

Si

? S

ArC

a

?

Changes in neutron magic numbers for neutron-rich nuclei

•Instability of 10He (Z = 2, N = 8) and 28O (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 8He (Z = 2, N = 6) and 24O (Z = 8, N = 16)(below neutron decay threshold no bound excited states)

Adding +1 proton we obtain:

Cores of halo nucleus 9Li (Z = 3, N = 6) and 25F (Z = 9, N = 16)

halo nucleus 11Li (Z = 3, N = 8) and 27-31F (Z = 9, N = 18 – 22)

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

Change of effective single particle energies in Si and O

Search for Search for 3333FF

20168 2424

Conclusions Conclusions

Disappearance of doubly magic nuclei 10He and 28O

Changes in magic numbers near driplines - N=6,16

New doubly magic nuclei near driplines - 8He and 24O

Triton separation energy of odd Z nuclei shows the posibility of existence N=24 subshell in neutron-rich nuclei

Collaboration

GANIL, Caen, FranceR.Anne, M.Lewitowicz, W.Mittig, F.de Oliveira, P.Roussel-Chomaz, H.Savajols,

M.G.Saint-Laurent

IPN, Orsay, FranceF.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, HungaryZs.Dombradi, D.Soher, J.Timar,

NPI, ASCR, Řež, Czech Republic D.Baiborodin, J.Mrázek, G.Thiamová, J.Vincour, Z.D.

Thanks for your attention Thanks for your attention