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The Giant Pairing Vibration in 14C and 15C nuclei

Manuela Cavallaro

INFN-Laboratori Nazionali del Sud Catania (Italy)

Nature

Comm.

6 (2015) 6743 Nature Comm. 6 (2015) 6743

INFN – LNS, INFN – CT,

University

of

C

atania, Catania,

Italy

IPN – Orsay, France

INFN and

University

of Padova,

Italy

UFF,

Niteroi

,

Brasil

The pairing force R.A. Broglia and D. Bes PLB 69 (1977) 129

Giant

Pairing Vibration (GPV)Collective excitation of a pair of particles (or holes) across major shells

Giant

Resonances

Collective

p-p or h-h

excitations

Giant

Pairing

Vibrations

(

GPV

)

Collective p-h excitations

Analogy with

Giant

Resonances (GDR, GQR)

Investigation of effects of the

pairing interaction

in the structure of atomic nuclei

GPV propertiesExcitation Energy ~ 72 A-1/3 FWHM

~ 1-2 MeV

L = 0 multipolarityCollectivity: B(GPV) ~ B(PV)Excitation Energy ~ 20 MeVMost

of

theoretical

prediction

limited

to

heavy

nuclei

(Pb and Sn

isotopes

)

R.A

.

Broglia

and D. Bes PLB 69 (1977)

129-133

L.Fortunato

et al. EPJ A14, 37-42(2002)

H.Shimoyama

and

M.Matsuo

, PRC 88, 054308 (2013)

Some work on

light nuclei

(Oxygen isotopes) E.Khan

et al. PRC 69 (2004) 014314

B.Avez

et al. PRC 78 (2008) 044318

GPV should be populated by two-nucleon transfer reactions

What are the hypotheses?Giant Pairing Vibrations

(

GPV)must exist!Particle-hole symmetry (basic symmetry for systems of interacting fermions)Mean field description of the system ground state (true for nuclei)

Why so difficult to observe?Previous (p,t) and (t,p) experiments on Pb and Sn isotopes never observed GPV

Criticisms in the population of GPV GPV requires

L

= 0 In transfer reactions typically large amount of angular momentum is transferred, especially at high excitation energyIncident energy

Near the Coulomb barrier, weak sensitivity to angular momentum transfer

At high incident energy, Q-value matching

N.

Anyas

-Weiss et

al.

Phys. Rep. 12 (1974)

201

S.

Kahana

and A. J.

Baltz

Advances in Nuclear Physics Vol. 9

Projectile/target

Brink’s matching conditions

S

urvival

of a

preformed pair

in a transfer

process favored if the

initial and final orbitals are the

same

D.M.

Brink, Phys. Lett. B

40 (1972)

37

Our study

12,13C(18O,16O)14,15C reactions at 84 MeV

(18O,16O) reaction is a good probe to study pairingPopulation of L=0 transitions is favorable (Brink’s matching conditions)Preformed neutron pair in 18

O

The

energy

of 3 times the Coulomb barrier is a good compromise between selectivity and sensitivity to low angular momentum transfer

Experimental setup18O beam from Tandem at 84 MeV and Superconducting Cyclotron at 270 MeV

12

C and 13C thin targets (50 µg/cm2)Ejectiles detected by the MAGNEX spectrometerAngular settings

6°, 12°, 18°

3° <

24°

INFN Laboratori Nazionali del Sud

Catania

F. Cappuzzello et al Eur. Phys. J. A (2016) 52:167 (review paper)Measured resolution:Energy E/E  1/1000Angle θ 

0.3°

Mass m/m  1/160 The MAGNEX magnetic spectrometerLarge acceptance:Energy

-28%, +20%

Angle

50

msr

Is the reaction mechanism for (

18

O,16O) under control?

1. Transfer yieldsEnhancement of the

two-neutron

transfer channelcountsEjectile

Mass

(

a.m.u

.)

14

15

16

17

18

19

20

inelastic

stripping

pick-up

16

O

17

O

18

O

19

O

18

O+

13

C

7

° <

θ

lab

< 13

°

15

O

Ejectile Mass (a.m.u.)

σ

(mb)

The 2n transfer is not a 2

nd

order process

TRANSFER OF A CORRELATED PAIR

We

compared

the transfer

yields

for

inelastic

scattering

,

one

-,

two

-,

three-neutron

transfer in the

same

conditions

13

C(

18O,17O)14C

12

C(

18

O,

16

O)

14

C

6.73

(3

-

)

7.34

(2

-

)

7.01

(2

+

)

8.32

(2

+

)

10.74

(4

+

)

In the (

18

O,

16

O), the

suppression of

s.p

. states

, which would require an uncorrelated transfer of 2n and the breaking of the initial pair in the

18

O

g.s.

,

reveals the minor role of the

two-step dynamics

2. Energy

spectra

Extreme Cluster Model (CRC) Relative

motion of the 2n system

frozen and separated by the c.m. Only the term with the 2n coupled to S = 0 participates to the transfer

Sequential

transfer

(DWBA)

Introducing

the

17

O +

13

C

intermediate

partition

Indication of the presence of two-neutron pairing correlations in the

14

C ground state

Coherent

sum

No

arbitrary

scaling

M. Cavallaro,

et

al., PRC 88 (2013) 054601

12

C(

18

O,

16

O)

14

Cg.s. @ 84 MeV

3. Angular distribution

The measured spectra

Energy spectra

S

n

S

2n

Unknown

Unknown

12

C(

18

O,

16

O)

14

C

13

C(

18

O,

16

O)

15

C

9° <

θ

lab

< 10°

Strong

population

of

states

with large 2n

core

overlap

Almost

complete

s

uppression

of the 0

+

2

, 0

+

3

states

Nature

Comm.

6

(2015) 6743

Gaussian function + linear background

E

x

=

13.7 ± 0.1

MeV

FWHM

=

1.9

± 0.3

MeV

16.43

16.72

Energy and width of the bumps

E

x

=

16.9 ± 0.1

MeV

FWHM

=

1.2 ± 0.3

MeV

New experiment @ 270 MeV

Using the

Superconducting Cyclotron at LNS

Same

centroid

and

width

T

he observed bumps have all the GPV signatures

Etx (MeV)

12

C

0

3.02

E

t

x

(

MeV

)

16.9

MeV

g.s.

g.s.

14

C

Δ

14

C -

Δ

12

C = 3.02

MeV

13

C

Δ

15

C -

Δ

13

C = 6.7

MeV

0

6.7

13.7

MeV

g.s.

g.s.

15

C

The excitation energy of the two resonances referred to the target ground state is the same (~ 20 MeV)

Pairing

energy

scale

E

t

x

=

E

x

+

M

r

-

M

t

14

C

GPV

15

C

GPV

Consistent with

cQRPA

predictions

E.Khan

et al. PRC 69 (2004) 014314

1.Excitation

energy

20.4

19.9

L

= 0

L

=

3

L

= 4

2.Multipolarity

Equal population of the

M

-states

in heavy-ion reactions near the Coulomb barrier

L ≠ 0

transitions:

featureless

shape

L

= 0

transitions: oscillating shape

S.

Kahana

and A. J.

Baltz

Advances in Nuclear Physics Vol. 9

GPV

Model independent indication of

L

=

0

Nature

Comm.

6

(2015) 6743

3.Collectivity

Transfer probability

(

semi-classical description of the relative motion)

Quantal

corrections

Elastic

scattering

Transfer probability

T

he

GPV strength i

s

predicted to be similar to that of the

L

= 0 transition to the ground state in

Pb

and

Sn

even-even

isotopes

W. von

Oertzen

and A.

Vitturi

,

Rep.

Prog

. Phys

.

64

(2001) 1247

R.A.

Broglia

and A.

Winther

,

Heavy Ion

Reactions,

(Addison-Wesley, 1991)

Neutron decay

Neutron decayMAGNEX + EDEN couplingMAGNEX to measure high resolution energy spectra for

charged reaction productsEDEN

to study the decaying neutrons emitted by the observed resonances with good efficiency and energy resolution

The EDEN neutron multidetector 40 liquid scintillator detectors (NE213)The cells are cylindrical, 5 cm thick with 20 cm diameter Possibility of

n -  discrimination

by pulse shape analysis Energy measurement by TOF with time resolution of 1 ns Typical energy resolution at a 1.7 m distance from the target: 60 keV for 850 keV neutrons and 500 keV for 6 MeV neutrons Intrinsic efficiency  50% for 1 MeV and 30% for 6 MeV neutrons Low discrimination threshold

H. Laurent et al., NIM

A 326,

417 (1993)

M. Cavallaro et al., NIM A

700

, 65 (2013

)

26

12

C

impurities

GPV

0.73±0.32

0.88±0.16

0.99±0.21

1.15±0.21

1.16±0.25

Neutron decay

MAGNEX

EDEN

M. Cavallaro, et al.,

Phys

. Rev. C 93, 064323 (2016)

MAGNEX

EDEN

The GPV mainly decays emitting two neutrons sharing the available energy, while the single neutron emission to the 14Cg.s.

is

ruled out

Not

enough

statistics

for

neutron-neutron-ion

coincidence

0.63±0.13

0.34±0.09

0.09

Neutron decay

M. Cavallaro, et al.,

Phys

. Rev. C 93, 064323 (2016)

GPV

The (18O,16O) reaction is a powerful tool to explore the effects of the pairing force The reaction mechanism is under controlFirst experimental observation of the GPVNext step: characterization of the GPV (experiments on different targets and measurements of neutron-neutron decay)

Transfer

yieldsEnergy spectraAngular distributionExcitation energyWidthMutipolarityCollectivity

Conclusions

Thank you