Weakly bound and unbound
nuclei
Takashi Nakamura
Department of Physics,
Tokyo Institute of Technology
The 9th Japan-China Joint Nuclear Physics Symposium (JCNP2015), 7-12/Nov/2015, Osaka Univ.
Contents
Introduction:
Drip Lines and Barely bound/unbound nuclei
Probes: Coulomb and Nuclear Breakup
at intermediate energies
SAMURAI Facility at RIBF at RIKEN
Coulomb/Nuclear Breakup of Borromean 2n-Halo 22C
Spectroscopy of 25O,26O -- @ SAMURAI at RIBF
Summary and Outlook
Characteristic Nuclear structure near neutron-drip line?
b-Stable
Neutron Drip-line
Beyond
1n separation energy
Sn=8MeV
p n
More
n-rich
Sn<1MeV
-1MeV<Sn<0MeV
continuum
p
n
continuum
p
n
Neutron Drip Line
Fermi levels between n and p—Very Different
Valence neutron – Loosely-Bound/Unbound
Neutron Halo/Skin (Neutron-matter surface)
Continuum
Strong cluster-like correlation
dineutron?
A.B.Migdal
Strongly correlated “dineutron”
on the surface of a nucleus
Dineutron?
Sov.J.Nucl.Phys.238(1973).
n
n
Unbound
a= -18 fm
n n
S=1
Dineutron:
@ Low-dense neutron skin/halo?
/surface of neutron star?
M.Matsuo
PRC73,044309(2006).
A.Gezerlis, J.Carlson,
PRC81,025803(2010)
Electric-Dipole (E1) Response of 11Li
11Li
n-star
12
n
n
core
Dineutron Correlation
Strongly Polarized
 Strong E1 Excitation
B( E1)  1.42  0.18 e 2 fm2 ( Erel  3MeV )
14
 12  4818
deg .
T.N. et al. PRL96,252502(2006).
Indirect Hint of Dineutron Correlation
Evolution Towards the Stability Limit
Where is the neutron drip line?
What are characteristic features of drip-line nuclei?
How does nuclear structure evolve towards the drip line?
Shell?
Ca
K
Deformation?
Ar
Cl
Halo?
S
Drip Line? Si P
N=28
Continuum? Al
He
H
Mg
Na
Ne
F
N=20
N=16
O
N
37Mg
C
B
Be
Li
21C
N=8
18B 19B
25O 26O28
22C
31Ne
O
1n halo known
2n halo known
4n halo/skin
Probes– Nuclear and Coulomb breakup
of drip-line nuclei at intermediate energies
Probe-1: Nuclear Breakup – Case of 1n Halo
e.g. 1n knockout reaction of 31Ne
31Ne
240MeV/u
(TN et al. , PRL112,142501 (2014).)
 30
30Ne P ( Ne)
31Ne* 30
Ne
n
C
Target
(g)
p
f
P// ( 30 Ne)
g ray in coincidence  30Ne(2+) / 30Ne(0+) Contribution
s-1n and P// distribution  of valence n, configuration
Theory: Eikonal Approximation
New 1n Halo Systems 31Ne and 37Mg:
31Ne
S n  0.1500..16
10 MeV
30Ne
core
p-wave halo neutron
Deformation b~0.5 (~ 3:2)
Strongly deformed although it is N=21 (close to 20)
37Mg:
N.Kobayashi, TN et al., PRL 112, 242501, 2014
Probe-1 Nuclear Breakup – Case of 2n Halo
e.g. 22C
22C*
20C
22C

 20
P(n), P(n), P( C)
12C
ds/dErel
Inelastic scattering
n
n
2+ (?)
Erel ( 22 C ) ( E x  S 2 n )
21C*
20C
22C
n
C
P//(20C)
s
d
ds/dErel
Knockout reaction 12
12C

 20
P(n), P( C)
Erel ( 21 C )
N.Kobayashi,TN,PRC2012
Probe-2: Coulomb Breakup
Photon absorption of a fast projectile
22C
22C*
g(virtual photon)
b>0.3
High-Z Target
(Pb)
Equivalent Photon Method
dsCB 16p3
dB(E1)
=
N
(E
)
dEx 9hc E1 x dEx
Cross section = (Photon Number)x(Transition Probability)
C.A. Bertulani, G. Baur, Phys. Rep. 163,299(1988).
Halo  Soft E1 Excitation
(E1 Concentration at Ex<1MeV)


 20
P(n), P(n), P( C)
Invariant Mass
E x , Erel
20C
n
n
Di-neutron Correlation
Single particle state (Halo)
SAMURAI Facility at RIBF at RIKEN
RIKEN RI Beam Factory (RIBF)
Completed in 2007
New-Generation RI-beam facility
SCRIT
2013
SLOWRI
2014
ZDS
2008
SAMURAI
2012
SRC
BigRIPS
2007
Rare RI
RING
2014
SHARAQ
2009
SRC: World Largest Cyclotron (K=2500 MeV)
Heavy Ion Beams up to 238U at 345MeV/u (Light Ions up to 440MeV/u)
eg.
Increasing Year by Year!
48Ca beam (345 MeV/nucleon) ~200pnA (250pnA max.) 48
Ca:~500pnA 2014
238U beam (345 MeV/nucleon) ~12pnA (15pnA max.)
238U: ~ 30 pnA 2015
SAMURAI
Superconducting Analyzer for MUlti-particle from RAdio Isotope Beam
Kinematically Complete measurements by detecting multiple particles
in coincidence
RI beam
from BigRIPS
Superconducting
Magnet
(3T, 2m dia. Pole
80cm gap)
Large momentum acceptance
Brmax / Brmin ~ 2 – 3
Good Momentum Resolution
Dp/p~ 1/1500
Large Bending Angle (~60deg)
+4 Tracking Detectors
T.Kobayashi NIMB317, 294 (2013)
Target
rotatable
Proton
Large angular acceptance for n
Neutron(s)
Heavy Ion
+-8.8 deg (H) x +-4.4 deg(V)
(~50% coverage < Erel ~ 5MeV)
Moderate Erel Resolution
DE = 200 keV (s) at Erel=1MeV
Stage: Rotatable (-5 -- 95 degrees)
Variety of Physics Opportunities
SAMURAI
Superconducting Analyzer for MUlti-particle from RAdio Isotope Beam
March 2012
Breakup Experiments at SAMURAI
-- Results from Day-One campaign
experiments at SAMURAI
Coulomb Breakup of 19B and 22C, Nakamura et al.
Study of 18B,21C, and excited states of 19B,22C, Orr et al.
Structure of Unbound Oxygen Isotopes 25O,26O, Kondo et al.
F
O
N
C
B
Be
Li
He
H
21C
N=8
18B 19B
25O 26O28O
22C
22C
(Z=6,N=16)
 Prominent 2n-Halo?
K.Tanaka et al., PRL 104, 062701(2010).
S2n= 0.14(46) MeV
L.Gaudefroy et al. PRL109,202503(2012).
sR(mb)
Reaction cross section measurements
(<rm2>)1/2=5.4(9) fm c.f. ~3.5 fm11Li
22C
Z=8
19C
20C
 N=16 Magicity?
20
16
A
1d3/2
2S1/2
1d5/2
A.Ozawa et al., PRL 84, 5493 (2000).
 Heaviest s-wave dominant 2n Halo within reach?
c.f. 3s1/2 Halo: Far away for the current and near-future RIB facilities
N~60-70
SAMURAI Experiment May/2012
First Full Exclusive Coulomb/Nuclear Breakup Measurement of 22C and
22N
DE
23N
~10cps RIBF
(~10cph RIPS)
20C
22C
Pb3.26g/cm2
C191.79g/cm2
C
19B
17B
19B
n
22C+CAZ+X
n
2.8
3
3.2
3.4
3.6
3.8
7
4
A/Z
240MeV/nucleon
22C
6
Z
22C
19C20C
5
20C
4
17B
19B
14Be
3
2.4 2.6 2.8 3
3.2 3.4 3.6 3.8 4
A/Z
Heavy Ion + 1 neutron :
22C(22N)+C21C*20C+n
22F 23F 24F 25F 26F 27F 28F 29F 30F
21O 22O 23O 24
O
22N21C*20C+n
20N 21N 22N 23N
S.Leblond
Preliminary
19C 20C 21C 22C
18B
19B
Spokesperson:
N.A.Orr, J.Gibelin
31F
22N21C*20C+n
S.Mosby et al.(MSU)
NPA909,69(2013).
N=16
s
20
16
22C21C*20C+n
1d3/2
Preliminary
2S1/2
1d5/2
Erel(MeV)
8
2
2
1s1/2
22C
s d
1p1/2
1p3/2
0
0
1
2
3
Edecay(MeV)
4
5 Spectra
1
2
Edecay(MeV)
from S.Leblond/N.A. Orr
3
Coulomb Breakup of 22C (20C+n+n Spectrum)
Spokesperson: T.Nakamura
22C+Pb22C* 20C+n+N
counts
R.Minakata (Tokyo Tech)
0
1
2
3
4
5
Erel(MeV)
6
7
8
9
10
Study of unbound nuclei 25O and 26O at SAMURAI
Spokesperson Yosuke Kondo
Experimental study of unbound oxygen isotopes
towards the possible double magic nucleus 28O
T. Otsuka et al., PRL105, 032501 (2010).
24Ne 25Ne 26Ne 27Ne 28Ne 29Ne 30Ne 31Ne 32Ne
23F 24F 25F 26F 27F 28F 29F 30F
22O 23O 24
O 25O 26O 27O 28O
21N 22N 23N
20C 21C 22C
19B
N=16?
Drip line
Drip line
31F
Z=8
N=20
Oxygen Anomaly
3N force: significant at N>16
G. Hagen et al., PRL108, 242501(2012).
H. Hergert et al., PRL110, 242501(2013).
S.K.Bogner et al., PRL113, 142501(2014).
Continuum Effect:
A.Volya, V.Zelevinski, PRL94,052501(2005).
nn correlations:
L.V. Grigorenko et al., PRL111,042501(2013).
K. Hagino, H. Sagawa PRC89,014331(2014).
2n radioactivity of
E. Lunderberg et al.
PRL108, 142503 (2012)
~770keV
26O?
25O+n
⊗ (nd3/2)2
26O: 24O(0+)
24O+2n
?
x
26O
L.V. Grigorenko et al. PRC 84, 021303 (2011)
C. Caesar et al.PRC88, 034313 (2013)
Er < 200keV
Z. Kohley et al,PRL110,152501 (2013)
T1/2=4.5+1.1-1.5ps
(3ps systematic error)
2n radioactivity?
Usual 1n decay
G~MeV or keV
Er < 120keV (95% CL)
t < 5.7ns
Large uncertainty of experimental Excite
study state at 4.2MeV?
• Only upper limit is given for the ground state energy
• Large systematic error in the lifetime measurement
• Excited State of 26O?
Experimental Setup at SAMURAI at RIBF
C target
Ionization (1.8g/cm2)
Chamber
2Plastics
2 MWDCs
MWDC
Superconducting
Dipole Magnet
(B=3.0T)
NEBULA
n
n
27F
~210MeV/u
(from BigRIPS)
DALI2
MWDC
Hodoscope
24O
Decay energy spectrum of 25O
Edecay
Edecay=750keV
25O
gs
26F+C 25O*24O+n
24O+n
Kondo et al.
Edecay=749(10) keV
G= 88(6) keV
C.R.Hoffman et al.,(MSU)
PRL100, 152502 (2008)
Previous Works
(GSI) C.Caesar, PRC 2013
(MSU) Hoffman, PRL 2008
Decay energy spectrum of 26O (24O+2n, 24O+n)
26O
gs
26O
gs
24O+n+X
Counts /20keV
Counts /50keV
27F+C26O24O+2n
26O
1st
New!
27F+C26O,25O
26O
1st+
25O
gs
Edecay(24O+n)
Edecay(24O+n+n)
=Edecay(24O+n)+Edecay(25O+n)
Edecay(24O+n+n)
Finite Value!
Edecay ( 26 O gs )  18  3( stat )  4( syst ) keV
11
Edecay ( 26 O1st )  1.2800..08
MeV Most likely 2+
No peak at ~4.2MeV
n
24O
n
Barely unbound nucleus
Present Results
on 25O and 26O
26O
1.27(11) MeV
(2+)
749(10) keV
25O+n
24O+n+n
18(5) keV
25O+n
26O
Centrifugal
Barrier
Extremely weakly
UNBOUND state!
E
0+
• N=16 shell closure is confirmed
• USDB fail to describe E(2+) of 26O
 Effect of pf shell?
3N-forces, pf shell, continuum
Submitted to PRL
Next step: Study of 27O and 28O in Nov.-Dec
Summary and Outlook
Coulomb and Nuclear Breakup
Useful tool to probe weakly-bound states (HALO)
near/beyond the drip-line 31Ne TN, N.Kobayashi, PRL2014,
37Mg N.Kobayashi, TN, PRL2014.
SAMURAI Facility at RIBF (since 2012)
Powerful equipment for various experiments using RI beams
Coulomb/Nuclear Breakup of 22C at SAMURAI
S.Leblond, J.Gebelin, M.Marques, N.Orr, R.Minakata, S.Ogoshi, TN, et al.
21C spectrum  pin down s and d 1hole state of 22C
Large Coulomb breakup cross sections
Spectroscopy of 25O,26O at SAMURAI
Y.Kondo, J.Tsubota, TN et al.
26O(0+gs): Very weakly unbound 2n states  Correlation? Continuum?
26O(2+): Found for the first time at E =1.28(11) MeV  Shell Evolution?
rel
 Keys to understand Oxygen drip-line anomaly, Shell Evolution
Near Future: Variety of unbound states along n-drip line
 28O (Possible unbound doubly magic nucleus, 24O+4n)
Day-one Collaboration
Tokyo Institute of Technology: Y.Kondo, T.Nakamura, N.Kobayashi, R.Tanaka,
R.Minakata, S.Ogoshi, S.Nishi, D.Kanno, T.Nakashima, J. Tsubota, A. Saito
LPC CAEN: N.A.Orr, J.Gibelin, F.Delaunay, F.M.Marques, N.L.Achouri, S.Leblond,
Q. Deshayes
Tohoku University : T.Koabayshi, K.Takahashi, K.Muto
RIKEN: K.Yoneda, T.Motobayashi ,H.Otsu, T.Isobe, H.Baba,H.Sato, Y.Shimizu, J.Lee,
P.Doornenbal, S.Takeuchi, N.Inabe, N.Fukuda, D.Kameda, H.Suzuki, H.Takeda,
T.Kubo
Seoul National University: Y.Satou, S.Kim, J.W.Hwang
Kyoto University : T.Murakami, N.Nakatsuka
GSI : Y.Togano
Univ. of York: A.G.Tuff
GANIL: A.Navin
Technische Universit¨at Darmstadt: T.Aumann
Rikkyo Univeristy: D.Murai
Universit´e Paris-Sud, IN2P3-CNRS: M.Vandebrouck
Backup
Mass identification@SAMURAI
22O
27F+CAO
Counts
@SAMURAI
21O
~13s
separation!
23O
@GSI
22O
C. Caesar et al.
PRC(2013).
23O 24
O
24O
A
Clear Particle identification! A/DA=310 (1s)
 High resolving power of the SAMURAI spectrometer
Eg=1.6MeV
gated