Relativistic Coulomb Excitation of Neutron-Rich 54,56,58Cr

Herbert Hübel

Helmholtz-Institut für Strahlen- und Kernphysik

Universität Bonn

Germany

ParticipantsParticipants

A. BürgerA. Bürger, H. Hübel, A. Al-Khatib, P. Bringel, A. Neußer, A.K. Singh, D. Mehta, T.S. Reddy, H. Hübel, A. Al-Khatib, P. Bringel, A. Neußer, A.K. Singh, D. Mehta, T.S. ReddyUniversity of Bonn, GermanyUniversity of Bonn, Germany

T. SaitoT. Saito, A. Banu, T. Beck, F. Becker, P. Bednarczyk, H. Geissel, J. Gerl, M. Gorska, H. Grawe, , A. Banu, T. Beck, F. Becker, P. Bednarczyk, H. Geissel, J. Gerl, M. Gorska, H. Grawe, J. Grebosz, M. Hellström, M. Kavatsyuk, O. Kavatsyuk, Kojouharov, N. Kurz, R. Lozeva, S. J. Grebosz, M. Hellström, M. Kavatsyuk, O. Kavatsyuk, Kojouharov, N. Kurz, R. Lozeva, S. Mandal, N. Saito, H. Schaffner, H. Weick, M. Winkler, H.J. WollersheimMandal, N. Saito, H. Schaffner, H. Weick, M. Winkler, H.J. WollersheimGSI Darmstadt, GermanyGSI Darmstadt, Germany

G. Benzoni, A. Bracco, F. Camera, B. Million, O. WielandG. Benzoni, A. Bracco, F. Camera, B. Million, O. WielandUniversity of Milano, ItalyUniversity of Milano, Italy

E. Clement, A. GörgenE. Clement, A. Görgen G. HammondG. HammondCEA Saclay, FranceCEA Saclay, France Keele University, UKKeele University, UK

P. Reiter, P. DoornenbalP. Reiter, P. Doornenbal M. Kmiecik, A. Maj, W. MeczynskiM. Kmiecik, A. Maj, W. MeczynskiUniversity of Köln, GermanyUniversity of Köln, Germany University of Krakow, PolandUniversity of Krakow, Poland

S. MuralitharS. Muralithar Z. PodolyakZ. PodolyakNSC New Delhi, IndiaNSC New Delhi, India University of Surrey, UKUniversity of Surrey, UK

C. WheldonC. WheldonHMI Berlin, GermanyHMI Berlin, Germany

Physics MotivationPhysics Motivation

Shell structure of nuclei far off stability may differ from that of Shell structure of nuclei far off stability may differ from that of nuclei near the valley of stabilitynuclei near the valley of stability

Shell structure is also important for astrophysics applications, Shell structure is also important for astrophysics applications, e.g. for nuclear synthesis r-process abundance calculationse.g. for nuclear synthesis r-process abundance calculations

Shell structure is related to the monopole part of the NN Shell structure is related to the monopole part of the NN interactioninteraction

e.g. S = 0 (spin flip), e.g. S = 0 (spin flip), l = 0 (spin-orbit partners), T = 0 (proton-l = 0 (spin-orbit partners), T = 0 (proton-neutron interaction): strongly binding in the two-body interactionneutron interaction): strongly binding in the two-body interaction

Causes large monopole shifts at large neutron or proton excess due to missing interaction partners

Effect on spin-orbit splitting

T. Otsuka et al., Eur. Phys. J. A 13, 69 (2002)

E. Caurier et al., Eur. Phys. J. A 15, 145 (2002)

M. Honma et al., Phys. Rev. C 69, 034335 (2004)

H. Grawe, Springer Lecture Notes Phys. 651, 33 (2004)

Neutron-rich nuclei with N = 28 to 40:Neutron-rich nuclei with N = 28 to 40:

p1/2

f5/2

p3/2

f7/2

T = 1 (2p1/2)2 monopole strongly binding in some interactions

Modification of the spin-orbit splitting

M. Honma et al., Phys. Rev. C 69, 034335 (2004)

E. Caurier et al., Eur. Phys. J. A 15, 145 (2002)

g9/2

50

28

Position of p1/2 uncertain

Prediction subshell at N = 32,34

Differences between effective potentials

Experimental data are needed to test the potentials used in calculations

Neutron-rich region around Z = 24, N = 32Neutron-rich region around Z = 24, N = 32

In the Ca isotopes E(2+) increases at N = 32, but not in the Ni isotopes

Ti and Cr isotopes also show the increase in E(2+), B(E2) for 54Ti32 low

Experimental quantities sensitive to shell closure:Experimental quantities sensitive to shell closure:Separation energiesSeparation energies22++ energies and B(E2) values energies and B(E2) values

Experiments with Experiments with FRSFRS--RISINGRISING setup at setup at GSIGSI

FRS = FRagment Separator

RISING = Rare ISotope INvestigation at GSI

GSI = Gesellschaft für SchwerIonenforschung Darmstadt, Germany

Layout of the FRS-RISING setup at GSILayout of the FRS-RISING setup at GSIRadioactive beams produced by fragmentation and separated by Radioactive beams produced by fragmentation and separated by

FRSFRS

Primary beam: 86Kr 480 MeV/A

Production target: 8Be 2.5 g/cm2

Reaction target: Au 1.0 g/cm2

54,56,58Cr ions: 100 MeV/A

SCI1 and SCI2 give TOF: v/c, MW1,2: multiwire detectors

MUSIC ionization chamber gives energy loss: Z

HECTOR: BaF2 scintillation detectors, not used here

15 Ge-Cluster detectors, 7 encapsulated Ge crystals each

CATE: Si-CsJ CAlorimeter TElescope for E, E

RISING -ray detectors around the Au reaction target

Ge-Cluster detectorsGe-Cluster detectorsSeven encapsulated Ge crystals in common vacuumSeven encapsulated Ge crystals in common vacuum

Efficiency ~60 % each, hexagonal taperedEfficiency ~60 % each, hexagonal tapered

Ge Cluster detectorsGe Cluster detectors

15 Clusters arranged in two rings at 1515 Clusters arranged in two rings at 1500 and 36 and 3600

Absolute efficiency determined with Absolute efficiency determined with 6060Co source:Co source:1.15% at 1.332 MeV, with Lorentz boost 2.31%1.15% at 1.332 MeV, with Lorentz boost 2.31%

Energy dependence determined with Energy dependence determined with 152152Eu sourceEu source

Good timing of BaFGood timing of BaF22 detectors of HECTOR array used to detectors of HECTOR array used to identify and suppress background identify and suppress background

Multiwire detectorsMultiwire detectors MW1 and MW2 used for incoming beam tracking:Extrapolation to interaction point on the targetTogether with CATE ➔ determine scattering angle and angle of emission

20 x 20 cm2,Resolution: 1mm ⇒ 5mm @ target

tracking:

po

MW1 MW2 CATEAu target

γ

pi

θs

θγ

Multiwire extrapolation to target

Fragment IdentificationFragment Identification

Fragment identification before Au target

Z: 0.8%

56Cr

Z

A/Q

A/Q:1.1%(with Z gate)

CACAlorimeter lorimeter TETElescope lescope CATECATE

∆E• 0.3 mm thick Si detectors • Z identification• Position sensitive

E• CsI detectors • Mass identification

56Cr(Coulomb excitation)

56Cr + 197Au

∆E

E

Ion identification after the target

CATE events

Event-by-event Doppler correction of Event-by-event Doppler correction of -ray energies-ray energies

Determine v/c from TOF

Tracking of incoming and outgoing Cr ions and angle of Ge crystal with respect to ion gives actual -ray emission angle

tracking:

po

MW1 MW2 CATEAu target

γ

pi

θs

θγ

-Ray Energy (keV)

30 keV

16 keV

834

Counts

Scattering angle of Cr ionsScattering angle of Cr ionsSelection of Coulomb-excitation eventsSelection of Coulomb-excitation events

Scattering angle (deg)

200

C

o

u

n

t

s

0 Limit in scattering angles 0.6o to 2.8o corresponds to impact parameters of 40 to 10 fm, respectively

Details of the three experimentsDetails of the three experiments

5454Cr: ~4 x 10Cr: ~4 x 1033 particles/s, 22 h, 45% particles/s, 22 h, 45% 5454CrCr

5656Cr: ~1 x 10Cr: ~1 x 1033 particles/s, 20 h, 35% particles/s, 20 h, 35% 5656CrCr

5858Cr: ~3 x 10Cr: ~3 x 102 2 particles/s, 55 h, 25% particles/s, 55 h, 25% 5858CrCr

Trigger condition: SCI2 and one CATE CsI

Time gate on prompt peak, Doppler-shift correction, gate on scattering angle, gate on incoming and outgoing Cr ions

500 600 700 800 900 1000 1100 1200

200

400

600

800

1000

1200

1400

1600

1800

2000

Cr56

500 600 700 800 900 1000 1100 1200

200

400

600

800

1000

1200

1400

Cr58

Gamma-ray spectra of Gamma-ray spectra of 54,56,5854,56,58CrCr

1006 keV

58Cr 880 keV

500 600 700 800 900 1000 1100 1200

500

1000

1500

2000

2500

3000

3500

4000

4500

Cr54

54Cr 835 keV 56Cr

Comparison to theoryComparison to theory

Calculations:

T. Otsuka et al., Phys. Rev. Lett. 87, 082502 (2001)

T. Otsuka et al., Eur. Phys. J. A 13,69 (2002)

M. Honma et al., Phys. Rev. C 69, 034335 (2004)

E. Caurier et al., Eur. Phys. J. A 15, 145 (2002)

Experimental B(E2) value lower for 56Cr32 than for 54Cr and 58Cr

Experimental 2+ energy high for 56Cr32

Theory does not reproduce the 56Cr B(E2) value

Similar results for 52,54,56Ti (MSU)D.-C. Dinca et al., preprint

PRELIMINARY

SummarySummary

54,56,5854,56,58Cr ions produced by spallation of high-energy Cr ions produced by spallation of high-energy 8686Kr on Be and separated by FRSKr on Be and separated by FRS

54,56,5854,56,58Cr Coulomb excited on Au target at 100 MeV/ACr Coulomb excited on Au target at 100 MeV/A

B(E2,0B(E2,0++ - 2 - 2++) determined) determined

E(2E(2++) higher and B(E2) smaller for ) higher and B(E2) smaller for 5656CrCr3232 than for than for neighbors neighbors (preliminary)(preliminary)

Evidence for subshell closure at N = 32Evidence for subshell closure at N = 32

Discrepancy to large-scale shell model calculationsDiscrepancy to large-scale shell model calculations