Studio della forza di pairing nel meccanismo di trasferimento di neutroni con lo spettrometro MAGNEX

Francesco CappuzzelloUniversity of Catania & INFN-LNS

Particle-Particle correlations in nuclei

Pairing Vibrations(p,t) and (t,p) reactions on Sn and Pb targets

Strong excitation of the 0+ ground states

Harmonic behaviour of the excitations

Eigenstates of a general harmonic oscillator in a gauge space with ∆N = ±2 and ∆L = 0

Analogy between shape and pairing vibrationsD. Bes and R.A. Broglia Nucl. Phys. 80 (1965) 289-313.

A. Bohr and B. Mottelson, Nuclear Structure, Vol. II (Benjamin, New York, 1975).

R. Middleton and D. J. Pullen, Nuclear Phys. 51 (1964) 77.

J.H. Bjerregaard et al., Phys. Lett. B 24 (1967) 568.

(t,p) ∆N=+2

(p,t) ∆N=-2

Analogy between p-h, p-p and h-h excitations

Particle-Particle correlations

Giant Resonances

Collective p-p or h-h excitations

R.A. Broglia and D. Bes PLB 69 (1977) 129-133

Giant Pairing Vibrations (GPV)

Predicted properties of the GPV (heavy nuclei)

• Excitation Energy ~ 15 – 20 MeV

• FWHM ~ 7.8 A-1/3

• Collective nature

• L = 0 angular momentum transfer

Recent theoretical studies on light nuclei

E.Khan et al., PRC 69, 014314 (2004)

B.Avez et al., PRC 78, 044318 (2008)

R. Middleton et al., Nucl. Phys. 51 (1964) 77

J.H. Bjerregaard et al., PLB 24 (1967) 568

Collective p-h excitations

Pairing vibrations

NEVER OBSERVED

GPV requires L = 0 transfer!

In transfer reactions typically large amount of linear and angular momentum is transferred

The role of the reaction mechanism

The role of the incident energy

Near the Coulomb barrier the angular momentum transfer is minimized

Drawback:

1. The angular distributions are peaked at the grazing angle and are not sensitive to the structure of the populated states

2. Q-value matching rules typically suppress the cross section at high excitation energy where the GPV is expected

At high incident energy the mechanism is characterized by a large amount of angular momentum transfer and by deep inelastic collisions

At energies between 5 and 10 times the Coulomb barrier the angular distributions are sensitive to the final populated states

N. Anyas-Weiss et al., Phys. Rep. 12 (1974) 201-272

The role of the involved nuclei

• Brink’s matching conditions D.M. Brink, Phys. Lett. B 40 (1972) 37-40

)(

/

0/)(21

0//

2121

0

22

11

21012

22110

iiffeff

eff

ZZZZQQmvk

evenlevenl

vRQRRkL

RRkk

−−=

=

=+=+

≈+−+−=∆

≈−−=∆

λλ

λλ

λλ

• The survival of a preformed pair in a transfer process is favored when the initial and final orbitals are the same

The 13C(18O,16O)15C reaction at 84MeV

18O6+ Tandem beam at Einc = 84 MeV (7.5 times above the Coulomb barrier)

13C self-supporting target 99% enriched, thickness 50 μg/cm2

16O ejectiles momentum analysed by the MAGNEX spectrometer (LNS – Catania)

Three angular settings (Ω ∼ 50 msr) θ = 3° ÷ 13°

θ = 7° ÷ 19°

θ = 13° ÷ 25°

Main features Values Maximum magnetic rigidity 1.8 T m Solid angle 50 msr Momentum acceptance ± 13% Momentum dispersion for k = -0.104 (cm/%) 3.68

First order momentum resolution

xMDRx

p ∆= 5400

ScatteringChamber Quadrupole Dipole

Focal PlaneDetector

Measured resolution

Energy ∆E/E ∼ 1/1000Angle Δθ ∼ 0.3°Mass Δm/m ∼ 1/160

Algebraic trajectory-reconstruction

MAGNEX

Particle Identification Technique

F.Cappuzzello et al. NIMA 621 (2010) 419-423

18O + 13C reaction at 84 MeV and scattering angle of θlab = 7°-19°

qpB ∝ρ

ΔE –E histogram

coun

ts

Mass (a.u.)14 15 16 17 18 19 20

inelastic

stripping pick-up

7° < θlab< 13°

Comparison of different reaction channels for18O + 13C at 84 MeV

Efficiency for the production of different chargestates estimated using the program INTENSITYJ.A.Winger et al., NIM B70(1992) 380-392

Enhancement of the two-neutron stripping channel

Relevant contribution of the direct transfer of the neutron pair

Reconstructed angle%energy

),,,,,( iiiiiii lyxX δφθ= One gets labθ Scattering angle

eccE Excitation energy

16O angle % energy spectrum

Enhancement of the known L = 0 transition (state at 3.103 MeV) and of the 11 MeV bump

13Cg.s. (1/2-) → 15Cgs (1/2+) L = 113Cg.s. (1/2-) → 15C0.74 (5/2+) L = 313Cg.s. (1/2-) → 15C3.103 (1/2-) L = 013Cg.s. (1/2-) → 15C4.22 (5/2-) L = 213Cg.s. (1/2-) → 15C4.66 (3/2-) L = 213Cg.s. (1/2-) → 15C6.84 (7/2-, 9/2-) L = 413Cg.s. (1/2-) → 15C7.35 (7/2-, 9/2-) L = 4

S.Truong and H.T.Fortune, PRC 28,977(1983)

g.s.

0.74

6.84

7.35

3.10

34.

22

4.66

15C excitation energy (MeV)

13C(18O,16O)15C9 < θlab < 12

15C excitation energy (MeV)

Cou

nts

Cou

nts

g.s.

0.74

6.84

7.35

3.10

34.

22

4.66

13C(18O,16O)15C4.5 < θlab < 7

?

?

Good candidate for GPV?

Preliminary spectra of 15C

1E+0

1E+1

1E+2

1E+3

1E+4

10 20 30 40 50

dσ/dω

[µb/

sr]

θCM [°]

13C(18O,16O)15C at 84 MeV, θsc = 7° ÷ 18°

6.84 MeV L = 4

12.9 MeV

x10

Comparison of the angular distributions

State at Ex = 11 MeV13CGS (1/2-) (18O ,16O)15C3.103 (1/2-) L = 0

13CGS (1/2-) (18O ,16O)15Cgs (1/2+) L = 1

13CGS (1/2-) (18O ,16O)15C6.84 (7/2-÷9/2-) L = 4

1E+0

1E+1

1E+2

1E+3

1E+4

10 20 30 40 50

dσ/dω

[µb/

sr]

θCM [°]

13C(18O,16O)15C at 84 MeV, θsc = 7° ÷ 18°

g.s. MeV L = 1

12.9 MeV

1E+0

1E+1

1E+2

1E+3

1E+4

10 20 30 40 50

dσ/dω

[µb/

sr]

θCM [°]

13C(18O,16O)15C at 84 MeV, θsc = 7° ÷ 18°

3.1 MeV L = 0

12.9 MeV

14

Transfer experiments on different targets

9Be, 11B, 12,13C, 28Si, 58,64Ni, 128Sn, 208Pb

Absolute horizontal position calibration of the FPD

Relative calibration of the pads for the horizontal position measurement

Absolute vertical calibration (drift time)

Relative calibration of the silicon detectors energy

Identification of 16O8+ ions at the FPD

Representation in the focal plane parameters (Xfoc, θfoc, Yfoc, φfoc)

Construction of the spectrometer transport matrix

Preliminary spectrum of 11Be

18O beam 84 MeV

9Be target 114µg/cm2 + collodion 6 µg/cm2

g.s

+ 0.

32 +

14C

.

1.78

9Be(18O,16O)11Be12C(18O,16O)14CΘlab = 8°

g.s.

+ 0

.32

1.78

2.69

3.89

+ 3

.96

5.24

6.72

11Be excitation energy (MeV)

Cou

nts ?

12C target

9Be target

Preliminary spectrum of 13B

11B(18O,16O)13BΘlab = 8.5°

g.s.

18O

4.13

5.39

6.17

+ 6

.43

13B excitation energy (MeV)

Cou

nts

?

18O beam 84 MeV

11B target 33µg/cm2 + Formvar 4 µg/cm2

12C target 50 µg/cm2

Contribution due to carbon impurities subtracted

17

64Ni(18O,16O)66Ni

Xfoc (m)

12° < Θlab < 24°

Θfo

c(r

ad)

Xfoc (m)

coun

ts

64Ni(18O,16O)66NiEbeam = 84 MeVΘlab ≈ 18°Max E* 28 MeV

g.s.

1.42

14C

14C

PRELIMINARY

18

MAGNEX + EDEN MAGNEX to measure high resolution energy spectra for well identified reaction products

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

Unique facility to study the resonant states of neutron rich nuclei (low separation

energy)

19

MAGNEX + EDEN

Memorandum Of Understanding

INFN -LNS

IN2P3 – IPN Orsay

Proposal at the LNS PACfor the commissioning

12C(18O,17O)13C

16O(18O,17O)17O

To setup electronics and study efficiency and resolution

13C(18O,16O)15C 2 neutron coincidences

Conclusions and outlooks

A broad resonance observed in 15C spectrum populated via (18O,16O)

at 84 MeV

Compatible with GPV?

Analysis of the angular distributions in progress

Break-Up calculations will be done to study the continuum

background (TDSE)

Different targets (9Be, 11B, 28Si, 58Ni, 66Ni, 120Sn, 208Pb) explored via

(18O,16O), data analysis in progress

Working group

F.Cappuzzello1,2, D.Carbone1,2, M.Cavallaro2, A.Cunsolo1,2,A.Foti1,3, M.Bondì1,2, G.Santagati1,2, G.Taranto1,2, R.Linares4,R.Chen1,5

1. Dipartimento di Fisica e Astronomia, Università degli Studi di Catania, Italy2. Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali del Sud, Italy3. Istituto Nazionale di Fisica Nucleare – Sezione Catania, Italy4. University of Sao Paulo, Institute of Nuclear Physics, Sao Paulo, Brazil5. Institute of Modern Physics, CAS, PRC

F. Azaiez, S.Franchoo, M.Niikura, J.A.Scarpaci

CNRS - IN2P3 – Institut de Physique Nucléaire d’Orsay, France