# Presentation: 284407

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

- Relativistic Coulomb Excitation of Neutron-Rich 54,56,58Cr
- Herbert Hübel
- Helmholtz-Institut für Strahlen- und Kernphysik
- Universität Bonn
- Germany

## Participants

- A. Bürger, H. Hübel, A. Al-Khatib, P. Bringel, A. Neußer, A.K. Singh, D. Mehta, T.S. Reddy
- University of Bonn, Germany
- T. Saito, 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.
- Mandal, N. Saito, H. Schaffner, H. Weick, M. Winkler, H.J. Wollersheim
- GSI Darmstadt, Germany
- G. Benzoni, A. Bracco, F. Camera, B. Million, O. Wieland
- University of Milano, Italy
- E. Clement, A. Görgen			G. Hammond
- CEA Saclay, France				Keele University, UK
- P. Reiter, P. Doornenbal			M. Kmiecik, A. Maj, W. Meczynski
- University of Köln, Germany			University of Krakow, Poland
- S. Muralithar				Z. Podolyak
- NSC New Delhi, India			University of Surrey, UK
- C. Wheldon
- HMI Berlin, Germany

## Physics Motivation

- Shell structure of nuclei far off stability may differ from that of nuclei near the valley of stability
- Shell structure is also important for astrophysics applications, e.g. for nuclear synthesis r-process abundance calculations
- Shell structure is related to the monopole part of the NN interaction
- 	e.g. S = 0 (spin flip), l = 0 (spin-orbit partners), T = 0 (proton-neutron 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:

- 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 = 32

## Experimental quantities sensitive to shell closure:Separation energies2+ energies and B(E2) values

-   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:Separation energies2+ energies and B(E2) values

## Experiments with FRS-RISING setup at GSI

- FRS = FRagment Separator
- RISING = Rare ISotope INvestigation at GSI
- GSI = Gesellschaft für SchwerIonenforschung 	Darmstadt, Germany

## Layout of the FRS-RISING setup at GSIRadioactive beams produced by fragmentation and separated by FRS

- 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

- RISING -ray detectors around the Au reaction target

**Notes:**
tatsächliche beamtime nachschlagen: ausgewertete und vorhandene Strahlzeit

power failure 24.9., 15:00
start of acq	26.9. 7:00
end of acq		29.9. 9:00
end of everything 29.9. 18:00

## Ge-Cluster detectorsSeven encapsulated Ge crystals in common vacuumEfficiency ~60 % each, hexagonal tapered

## Ge Cluster detectors

- 15 Clusters arranged in two rings at 150 and 360
- Absolute efficiency determined with 60Co source:
- 	1.15% at 1.332 MeV, with Lorentz boost 2.31%
- Energy dependence determined with 152Eu source
- Good timing of BaF2 detectors of HECTOR array used to identify and suppress background

## Multiwire detectors 

- MW1 and MW2 used for incoming beam tracking:
- Extrapolation to interaction point on the target
- Together with CATE ➔ determine scattering angle and angle of  emission

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

**Multiwire extrapolation to target**

**Notes:**
Strahlfleckabmessung: Durchmesser in cm ➔ tracking (Bild)
Detektorabmessung, Auflösung, Auflösung am target

strahlfleck: 56Cr nie ohne s4 slits
resolution: typical 0.5mm
1mm/(2067.1-591.5)*(4291-591.5) = 2.5mm

## Fragment Identification

**Fragment identification before Au target**

**Z: 0.8%**

**56****Cr**

**Z**

**A/Q**

## CAlorimeter TElescope CATE

- CAlorimeter TElescope CATE

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

**E**
** ****CsI detectors **
** ****Mass identification **

**56****Cr**
**(Coulomb excitation)**

**56****Cr**** + ****197****Au**

**∆****E**

**E**

- Ion identification after the target

## CATE events

## Event-by-event Doppler correction of -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

******-Ray Energy (keV)**

**30 keV**

**16 keV**

**                         ****834**

**Counts**

## Scattering angle of Cr ionsSelection 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 experiments

- 54Cr: ~4 x 103 particles/s, 22 h, 45% 54Cr
- 56Cr: ~1 x 103 particles/s, 20 h, 35% 56Cr
- 58Cr: ~3 x 102 particles/s, 55 h, 25% 58Cr

- 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

## Gamma-ray spectra of 54,56,58Cr

**1006 keV**

**58****Cr**

**880 keV**

**54****Cr**

**835 keV**

**56****Cr**

## Comparison 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**

## Summary

- 54,56,58Cr ions produced by spallation of high-energy 86Kr on Be and separated by FRS
- 54,56,58Cr Coulomb excited on Au target at 100 MeV/A
- B(E2,0+ - 2+) determined
- E(2+) higher and B(E2) smaller for 56Cr32 than for neighbors (preliminary)
- Evidence for subshell closure at N = 32
- Discrepancy to large-scale shell model calculations