Invariant-mass spectroscopy of neutron-rich Be isotopes

ContentsBreakup reactions of 14Be on a proton target

Inelastic scattering (14Be)One-neutron removal reaction (13Be)

Y. KondoRIKEN Nishina Center

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CollaboratorsY. Kondo, T. Nakamura, Y. Satou, T. Matsumoto, N. Aoi, N. Endo, N. Fukuda, T. Gomi,

Y. Hashimoto, M. Ishihara, S. Kawai, M. Kitayama, T. Kobayashi, Y. Matsuda, N. Matsui,

T. Motobayashi, T. Nakabayashi, K. Ogata, T. Okumura, H. J. Ong, T. K. Onishi, H. Otsu,

H. Sakurai, S. Shimoura, M. Shinohara, T. Sugimoto, S. Takeuchi, M. Tamaki, Y. Togano,

Y. Yanagisawa

Tokyo Institute of Technology RIKEN Nishina Center Tohoku University Rikkyo University Kyushu University University of Tokyo Center for Nuclear Study (CNS), University of Tokyo

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Nuclear Chart

Exotic structures▷Neutron halo▷Magicity loss (12Be, 32Mg)▷Di-neutron correlation? 　 (6He, 11Li)▷Different deformation of proton/neutron density(16C)

Neutron halo

Magicity loss

Different deformations of Protons and neutronsDi-neutron?

13Be, 14Be

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14Be▷Drip-line nucleus▷Two neutron halo▷Borromean (12Be+n, n+n systems are unbound)▷No bound excited states

excited states locate above the neutron separation energy (S2n=1.26MeV)

13Be▷Unbound nucleus▷Low-lying levels are not clarified

Several experimental results are not consistent

Breakup of 14Be on proton

14Be and 13Be

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Inelastic scattering

One-neutron removal reaction

14Be▷Angular distribution Jp assignment▷cross section collectivity

13Be▷Momentum distribution

of 13Be system Jp assignment

Breakup Reactions on a low-Z target

p

12Ben

n

14Be

1np

12Be

n

n2np

Be12p

q 12Ben

n

14Be*

~ 70 MeV/u

1np

p

12Ben

n

14Be12Be

n

Be12p

q 12Ben

13Be

n~ 70 MeV/u• Coulomb breakup cross section is small

Invariant mass 22 ii pEM

Momentum Distribution of 13Be (transverse)

2/513

22/113

12/113

014 05/2Be01/2Be11/2Be0Be dps

l=0 l=1 l=2

Momentum distribution spin-parity assignment of 13Be

l=2

l=1

l=0

Example of momentum distribution width of P distribution▷depends on the orbital

angular momentum of a knocked-out neutron

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Experiment

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Experimental Setup

Primary beam18O 100 MeV/u

Production targetBe 　 6 mm

Plastic scintillator1 mm

14BeEnergy : ~ 70MeV/uIntensity : ~8,000 counts/sPurity : 90%

RIPS （ RIKEN 　 Projectile-fragment Separator)

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Experimental Setup

14Be

PPACAngle of 14Be

Dipole magnet

Drift chamber (FDC)Particle Identification

Drift chamber (MDC)Angle of 12Be

NaI(Tl) scintillator g ray from 12Be

Reaction TargetLiquid H2 (227 mg/cm2)

charged particle Hodoscope(plastic scintillator)

Velocity of 12Be

Neutron counter(plastic scintillator)

Veto counter

12Be

n

Detect 12Be and (a) neutron(s) in coincidence

~ 70 MeV/u

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Experimental Setup (photo)

Dipole MagnetTarget

Drift Chamber

He bag

Hodoscope

Neutron Detector

RIPSBeam

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Neutron counter

Charged particle VETO(thin plastic scintillators)

Neutron counter

Beam direction

~2m%5.21

Efficiency

For 1n detection

Neutron Counter54bars6x6x214cm3

Relative energy spectrum (12Be+n+n)Angular distribution

Results (Inelastic scattering)

p

12Ben

n

14Be

1np

12Be

n

n2np

Be12p

q12Be

n

n

14Be(2+)

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Inelastic scattering14Be+p 14Be* 12Be+n+n

▷Select Mn=2 (detection multiplicity) crosstalk rejection (position, timing)

Neutron Crosstalk Analysis

1

2

Two neutron event

CrosstalkOne neutron is detected by two (or more) detectors

Crosstalk events

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1

NEUT-BNEUT-A

Same Wall event

Different Wall event

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Comparison with 14Be+C data (previous experiment)

p(14Be,12Be+n+n)69 MeV/nucleon

neutron crosstalk events are eliminatedefficiency and acceptance are corrected

Similar peak at around 0.3MeV was observed

C(14Be,12Be+n+n)68 MeV/nucleon(previous exp.)

T. Sugimoto, T. Nakamura, Y. Kondo et alPLB 654,160 (2007)

14Be(2+)

Er=0.28(1)MeV

DL=2

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14Be+p experiment

DWBA analysisTwo optical potentials

(A) A.A. Korsheninnikov et al. PLB343, 53 (1995)(B) R.L. Varner et al.

Phys. Rep. 201, 57 (1991) (CH89)

　　 d =1.40(19) fm (14Be+p)

p(14Be,12Be+n+n)69 MeV/nucleon

14Be(2+)

Erel(12Be+n+n) (MeV)

Erel=0.25(1) MeVs =12.5±0.2±1.6 mb

(A)

(B)p(14Be,14Be(2+) )69 MeV/nucleon

Y. Kondo, T. Nakamura, Y. Satou et al.: to be submitted

Width is dominated by the experimental resolution

(~100keV (1 )s @ 0.25MeV)

relrel 19.0~ EE (1s)

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2+ energy & deformation length

2+ energyLower than 12Be

Deformation lengthSmaller than 12Be

Proton/neutron collectivities can be deduced (now in progress)

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Phase space decay▷ 14Be(21

+) 12Be+n+nSequential Decay▷ 14Be(21

+)　　 13Be+n

(Erel=0.1MeV)　　 12Be+n+n

(Erel=0.15MeV)

Decay of 14Be(21+)

sequential

phase space

12Ben

n

Ec-n1

En-n

Ec-n2

Ec-(nn)

Relative energy spectrum (12Be+n)Transverse momentum distributions

Results(One-neutron removal)

1np

p

12Ben

n

14Be12Be

n

Be12p

q 12Ben

13Be

n

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Mn=1 events

Subtraction of Inelastic Component

Mn=1 events

Inelastic channelEstimated from Mn=2 events

One-neutron removal channel

Corresponds to 14Be(2+)

One-neutron removal channel(one neutron is emitted)

knocked out

Inelastic channel(two neutrons are emitted)

not detected

two cases in Mn=1 events▷ inelastic component should be subtracted

p(14Be,12Be+n+n)69 MeV/nucleon

14Be(2+)

Erel(12Be+n+n) (MeV)

Erel=0.25(1) MeVs =12.5±0.2±1.6 mb

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13Be12Be+n+g

13Be12Be(1-)+n(Eg=2.7MeV)

13Be12Be(2+)+n(Eg=2.1MeV)

s=11(2)mb(Erel=0~4MeV)

s=5.3(7)mb(Erel=0~4MeV)

12Be+n s=89(6)mbs for 12Be+n+g is small

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Two peaks at 0.5MeV, 2MeV

Transverse momentum distribution

(not longitudinal)▷Width of momentum distributions

are different between peak regions

Relative Energy Spectrum

Erel(12Be+n) (MeV)

p(14Be,12Be+n)

s=89(6)mb(Erel=0-4MeV)

0.25-0.75MeV 2.0-2.5MeV

Px resolution~30MeV/c

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Relative energy spectrum▷ p- and d-wave components Breit-Wigner shape

▷ s-wave component G.F. Bertsch et al: PRC 57, 1366 (1998)

Momentum distribution CDCC calculation (by T. Matsumoto)13Be is assumed to be a core in 14Be

▷ 13Be-p interaction JLM interaction J. Jeukenne et al.: PRC16, 80 (1977)

▷ n-p interaction R.A. Malfliet and J.A.Tjon NPA127, 161 (1969)

▷ 13Be-n potential Wood-Saxon form

Depth is adjusted to reproduce the separation energy

Fitting of Erel spectrum and momentum distributions

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22)sin()cos(

1

relrelrel

relrel

akk

akk

kdE

d

/2 BE /2 relrel Ek

a : Scattering length

4/22rrelrel

EEdE

d

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0.5 MeV peak p-wave resonance

2 MeV peak d-wave resonance

Relative Energy Spectrum

p

s

d

ps

d

Erel(12Be+n) (MeV)

p(14Be,12Be+n)

s=89(6)mb(Erel=0-4MeV)

0.25-0.75MeV 2.0-2.5MeV

p

ds

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p-wave component▷ Er=0.50(1) MeV▷ G=0.36(2) MeV

consistent with Gsp (l=1)▷ Jp=1/2-

d-wave component▷ Er=2.48(7) MeV▷ G=2.4(2) MeV

larger than Gsp (l=2)other state @ 2MeV?

Relative energy spectrum

s component as~ -3fm

d state Er=2.48(7) MeV Γ=2.4(2)MeV

p state Er=0.50(1) MeV Γ=0.36(2)MeV

single particle width

p-wave @0.50MeV Gsp~0.5MeV

d-wave @2.48MeV Gsp~1.4MeV

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Summary of the observed levels

The 2+ state in 14Be locates lower than the g.s. of 13BeSequential decay process is energetically forbidden

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The low-lying negative parity state Intruder state

Low-lying state of 13Be

Thiswork

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Shell model calculation

▷ PSDMK D.J. Millener et al.: NPA255, 315 (1975)

Provides the shell closure at 12Be

▷ SFO (spin-flip p-n monopole interaction) T. Suzuki et al.: PRC67, 044302 (2003)

resonably reproduce the magicity loss at 12Be

Energy Levels of 12Be and 13Be

13Be

PSDMKHigher excitation energy of 1/2-

SFOGround state of 1/2- good!several states at ~2MeV

Intruder 1/2- statedisappearance of N=8 magicity

explained by spin-flip p-n monopole interaction

12Be

13Be

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▷energy gap between [220 ½] and [101 ½] orbitals disappears with large prolate deformation

▷Large quadrupole deformation (b~0.6) of 12Be H. Iwasaki et al. PLB481, 7

(2000)

intruder 1/2- state of 13Be indicate large deformation?

Deformation?Ref) A. Bohr and B.R. Mottelson Nuclear structure Vol.1

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Breakup reactions (14Be+p)

Inelastic scattering▷2+ state of 14Be▷Phase space decay of 2+ state

One-neutron removal reaction▷Low-lying p-wave (intruder) resonance of 13Be

Summary