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Nuclear structure of light exotic nuclei from break-up reactions
Nuclear Physics A 746 (2004) 479c–482c
Nuclear structure of light exotic nuclei from break-up reactions D. Cortinaa , J. Fernandez-Vazqueza , T. Aumannb , T. Baumannc , J. Benlliurea , M.J.G. Borged , L.V. Chulkovb , U. Datta Pramanikb , C. Forss´ene , L. M. Frailed , H. Geisselb , J. Gerlb , F. Hammacheb , K. Itahashif , R. Janikg , B. Jonsone , S. Mandalb , unzenbergb , T. Ohtsuboh , A. Ozawaf , K. Markenrothe , M. Meistere , M. Mockog , G. M¨ d i j Y. Prezado , V. Pribora , K. Riisager , H. Scheitk , R. Schneiderl , G. Schriederm , ummererb , I. Szarkag , H. Weickb H. Simonb , B. Sitarg , A. Stolzc , P. Strmeng , K. S¨ GSI-E212 collaboration a
Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain
b c
NSCL, Michigan State University, East Lansing, MI–48824, USA
d e f
GSI, Planckstrasse 1, 64291 Darmstadt, Germany
Instituto de Estructura de la Materia, CSIC, E–28006 Madrid, Spain
Chalmers Tekniska H¨ogskola och G¨oteborgs Universitet, SE–412 96 G¨oteborg, Sweden
RIKEN,2-1 Hirosawa Wako, Saitama 3051-01,Japan
g
Faculty of Mathematics and Physics, Comenius University, 84215 Bratislava, Slovakia
h
Department of Physics, Niigata University, 950-2181, Japan
i
Kurchatov Institute, RU–123182 Moscow, Russia
j
Institut for Fysik og Astronomi, Aarhus Universitet, DK-8000 Aarhus C, Denmark
k l
Max-Planck Institut f¨ ur Kernphysik, D-69117 Heidelberg, Germany
Physik-Deptartment E12, TU M¨ unchen, D-85748 Garching, Germany
m
Institut f¨ ur Kernphysik, TU Darmstadt, D-64289 Darmstadt, Germany
One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool to characterise the ground state properties of several neutron-rich isotopes in the sd-shell. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after one-nucleon removal were measured. The relative contributions of the remaining fragments in their ground and excited states have been determined from measurements of γ rays in coincidence with the longitudinal momentum distributions. In particular the breakup of 23 O has been investigated. The interpretation of our measurements, in the framework of a simple theoretical model, favours a spin and parity assignment of 1/2+ for the 23 O ground state in agreement with shell model predictions. 0375-9474/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2004.09.153
480c
D. Cortina et al. / Nuclear Physics A 746 (2004) 479c–482c
1. INTRODUCTION Neutron-rich oxygen isotopes near the neutron dripline present very exciting issues. It is well stablished today that the last bound oxygen isotope is 24 O [1–3]. This experimental fact reinforce the idea of the N = 16 magic number replacing the N = 20 gap for the heavy dripline nuclei [4]. 23 O is a key nucleus to understand the structure of light neutron-rich isotopes. Consequently, it has been subject of new interest and several experiments have been performed because the interpretation of different inclusive experimental results [5,6] lead to different spin and parity assignment for its ground state. A first experiment performed at GANIL [5] measuring the inclusive longitudinal momentum distributions of 22 O fragments emerging after breakup of 23 O in a C target interpreted the results in the framework of the p − sd shell-model. Almost at the same time, another experiment at GSI [7] revealed a significantly larger interaction cross-section for this nucleus. This fact was associated with a possible neutron halo structure of the 23 O ground state and with the existence of a sub-shell closure at N = 14 in the oxygen chain. A more recent experiment performed at RIKEN [6], where the inclusive one- and twoneutron breakup was investigated, suggested evidences of structural changes of the 22 O core in 23 O ground state and proposed I+ =5/2+ as ground state spin and parity for 23 O, in contradiction with shell model predictions. However, the interpretation of these results is controversial and was refuted in [8]. For a better understanding of this problem gamma coincidence data are crucial. We have therefore performed an experiment at GSI to disantangle the 22 O g.s contribution to the 23 O wave function from any other contribution of 22 O excited states. 2. EXPERIMENT AND RESULTS The experiment was performed at the FRagment Separator(FRS) at GSI. The secondary beams were produced by nuclear fragmentation of relativistic 40 Ar at 1 A.GeV, on a carbon target. A complete description of the experimental technique used can be found in [9–11]. The results we obtained for the inclusive breakup fragment longitudinal momentum distribution widths after correction of the intrinsic momentum resolution are shown in Figure 1 for all the secondary beams studied in the experiment. The longitudinal momentum distribution have been obtained following the procedure described in [9–11]. They show a quite constant value (∼ 200 MeV/c) and a sudden decrease for 22 O and 23 F fragments corresponding with the filling of the 2s1/2 shell. The intrinsic momentum resolution was experimentally evaluated for each secondary beam and amounts to 19 ± 1 MeV/c (FWHM) for 23 O. The corresponding one-neutron removal cross sections have been extracted counting the incident projectiles and the fragments in front and behind the breakup target. This ratio was corrected by the experimental transmission evaluated with the code MOCADI after adjustment of the simulated fragment longitudinal momentum width to the measured ones. We present in Table 2 the measured cross sections for oxygen isotopes. The error includes statistical and systematics uncertainties. The observed experimental trend shows a smooth increase of the one-neutron removal cross-section. We can also observe, in
481c
D. Cortina et al. / Nuclear Physics A 746 (2004) 479c–482c
Fragment 18 O 19 O 20 O
Figure 1. FWHM’s of longitudinal momentum distribution for different isotopes of Nitrogen (triangles), Oxygen (squares) and Fluorine (circles) after one-neutron removal on a carbon target.
σ−1n (mb) 56 ± 7 56 ± 6 72 ± 7
Fragment 21 O 22 O
σ−1n (mb) 70 ± 7 85 ± 10
Figure 2. Results for the inclusive oneneutron removal cross section after oneneutron removal of 19−23 O at 939 MeV/u on a carbon target.
contrast to the results obtained in [6,7], that the one-neutron removal cross section of 23 O is not significantly big in order to assign a neutron halo character to this nucleus. The inclusive longitudinal momentum distribution (plong ), of 22 O residual fragments after one-neutron removal from 23 O obtained in this experiment is shown on the left side of Figure 3. In this figure the solid line corresponds to the Gaussian fit of the experimental data used to determine the width of the momentum distribution. The FWHM for this inclusive measurement is found to be 133 ± 10 MeV/c considering the correction due to the intrinsic momentum resolution of our spectrometer. The γ-rays emitted during de-excitation of 22 O were recorded with NaI detectors. The analysis of the γ-ray spectrum reveals three γ energies at 1.3, 2.6, and 3.2 MeV. This spectrum has been interpreted using previous experimental data and shell model calculations [8,12,13]. Assuming that all the 22 O excited levels decay through the first excited state at 3.2 MeV, we can use this peak to gate on the longitudinal momentum distribution in order to obtain the exclusive distribution that are shown on the right side of Figure 3. The FWHM for these exclusive measurements are found to be 126 ± 20 MeV/c and 236 ± 20 MeV/c for 22 O in its ground and any excited state, respectively. The procedure followed to analyse the coincidence spectrum can be found in Ref. [9–11]. The cross-sections corresponding to the exclusive momentum distribution amounts to 50±10 mb and 35±8 for the 22 O ground state and excited states, respectively. The experimental momentum distribution for the one-neutron removal channel leaving the 22 O core in its ground state is compared in Figure 4 to theoretical model calculations based in the Eikonal approximation [14]. Two calculations are shown for angular momenta l = 0 and l = 2. Clearly, the distribution assuming a 2s1/2 neutron coupled to the 22 O(0+ ) core is in much better agreement with the data. We can thus conclude that the ground state spin of 23 O is I π = 1/2+ . The experimental distribution is, however, slightly wider than the prediction for l = 0 The calculated neutron-knockout cross section for the configuration (d5/6 )6 (s1/2 )1 from the 1s-shell amounts to 51 mb and thus in agreement with the experimental value of
482c
D. Cortina et al. / Nuclear Physics A 746 (2004) 479c–482c
0.5
0.6
mb
0.2
0.4 0 0.3 FWHM = 237 ± 20 MeV/c
22O
exc.
0.2
0.2
/MeV/c )
incl.
dσ-1n /dp(
mb
dσ-1n/dp( /MeV/c )
0.4
mb
gs.
22O
dσ-1n/dp (
22O
0.6
/MeV/c )
FWHM = 134 ±10 MeV/c
FWHM = 127 ± 20 MeV/c
0.4
l=2
0.3
0.2
0.1
l=0
0.1 0 -200
0
-200
0
200 -200
0
200
0
-120
-40
40
120
200
plong (MeV/c)
plong (MeV/c)
Figure 3. Left: Inclusive longitudinal momentum distributions for 22 O fragments after one-neutron removal from 23 O. Right top: Exclusive longitudinal momentum for 22 O in its ground state. Right bottom: Exclusive longitudinal momentum distribution for 22 O in any excited state.
Figure 4. Ground state exclusive momentum distribution for 22 O fragments after one-neutron knock-out reaction from 23 O compared with calculations assuming l = 0 and l = 2
50 ± 10 mb. This result confirms the large spectroscopic factor for the s-neutron (C 2 S = 0.8) obtained by Brown et al [8]. However, a quite significant discrepancy has been found when comparing the neutron-knockout cross section contributions involving excited states with the experimentally deduced cross-section. 3. CONCLUSION We have measured for the first time the 22 O distribution after one-neutron knock-out of O in coincidence with the 22 O γ de-excitation thereby demonstrating that the groundstate spin of 23 O is I π = 1/2+ . This result provides a clear solution to the discrepancy of the ground-state spin and parity assignment of 23 O.
23
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
O.Tarasov et al., Phys. Lett B409 (1997) 64. A. Ozawa et al., Nucl. Phys. A673 (2000) 411. H. Sakurai et al., Phys. Lett. B448 (1999) 180. R. Kanungo et al., Phys. Lett. B 528, 58 (2002) E. Sauvan et al., Phys. Lett. B 491, 1 (2000) R. Kanungo et al., Phys. Rev. Lett. 88, 142502 (2002) A. Ozawa et al, Nucl. Phys. A693 (2001) 32. A. Brown, G. Hansen and J. Tostevin., Phys. Rev. Lett 90 (2003) 159201-1. D. Cortina-Gil et al., Phys. Lett. B 529, 36 (2002) D. Cortina-Gil et al., Nucl. Phys. A 720, 3 (2003) D. Cortina-Gil et al., submitted to Phys. Rev. Lett. P.G. Thirolf et al., Phys. Lett. B 485 (2000) 16. M. Stanoiu et al., Phys. Rev. C 69 (2004) 034312. P.G. Hansen, Phys. Rev. Lett. 77 (1996) 1017.
Nuclear structure of light exotic nuclei from break-up reactions D. Cortinaa , J. Fernandez-Vazqueza , T. Aumannb , T. Baumannc , J. Benlliurea , M.J.G. Borged , L.V. Chulkovb , U. Datta Pramanikb , C. Forss´ene , L. M. Frailed , H. Geisselb , J. Gerlb , F. Hammacheb , K. Itahashif , R. Janikg , B. Jonsone , S. Mandalb , unzenbergb , T. Ohtsuboh , A. Ozawaf , K. Markenrothe , M. Meistere , M. Mockog , G. M¨ d i j Y. Prezado , V. Pribora , K. Riisager , H. Scheitk , R. Schneiderl , G. Schriederm , ummererb , I. Szarkag , H. Weickb H. Simonb , B. Sitarg , A. Stolzc , P. Strmeng , K. S¨ GSI-E212 collaboration a
Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain
b c
NSCL, Michigan State University, East Lansing, MI–48824, USA
d e f
GSI, Planckstrasse 1, 64291 Darmstadt, Germany
Instituto de Estructura de la Materia, CSIC, E–28006 Madrid, Spain
Chalmers Tekniska H¨ogskola och G¨oteborgs Universitet, SE–412 96 G¨oteborg, Sweden
RIKEN,2-1 Hirosawa Wako, Saitama 3051-01,Japan
g
Faculty of Mathematics and Physics, Comenius University, 84215 Bratislava, Slovakia
h
Department of Physics, Niigata University, 950-2181, Japan
i
Kurchatov Institute, RU–123182 Moscow, Russia
j
Institut for Fysik og Astronomi, Aarhus Universitet, DK-8000 Aarhus C, Denmark
k l
Max-Planck Institut f¨ ur Kernphysik, D-69117 Heidelberg, Germany
Physik-Deptartment E12, TU M¨ unchen, D-85748 Garching, Germany
m
Institut f¨ ur Kernphysik, TU Darmstadt, D-64289 Darmstadt, Germany
One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool to characterise the ground state properties of several neutron-rich isotopes in the sd-shell. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after one-nucleon removal were measured. The relative contributions of the remaining fragments in their ground and excited states have been determined from measurements of γ rays in coincidence with the longitudinal momentum distributions. In particular the breakup of 23 O has been investigated. The interpretation of our measurements, in the framework of a simple theoretical model, favours a spin and parity assignment of 1/2+ for the 23 O ground state in agreement with shell model predictions. 0375-9474/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2004.09.153
480c
D. Cortina et al. / Nuclear Physics A 746 (2004) 479c–482c
1. INTRODUCTION Neutron-rich oxygen isotopes near the neutron dripline present very exciting issues. It is well stablished today that the last bound oxygen isotope is 24 O [1–3]. This experimental fact reinforce the idea of the N = 16 magic number replacing the N = 20 gap for the heavy dripline nuclei [4]. 23 O is a key nucleus to understand the structure of light neutron-rich isotopes. Consequently, it has been subject of new interest and several experiments have been performed because the interpretation of different inclusive experimental results [5,6] lead to different spin and parity assignment for its ground state. A first experiment performed at GANIL [5] measuring the inclusive longitudinal momentum distributions of 22 O fragments emerging after breakup of 23 O in a C target interpreted the results in the framework of the p − sd shell-model. Almost at the same time, another experiment at GSI [7] revealed a significantly larger interaction cross-section for this nucleus. This fact was associated with a possible neutron halo structure of the 23 O ground state and with the existence of a sub-shell closure at N = 14 in the oxygen chain. A more recent experiment performed at RIKEN [6], where the inclusive one- and twoneutron breakup was investigated, suggested evidences of structural changes of the 22 O core in 23 O ground state and proposed I+ =5/2+ as ground state spin and parity for 23 O, in contradiction with shell model predictions. However, the interpretation of these results is controversial and was refuted in [8]. For a better understanding of this problem gamma coincidence data are crucial. We have therefore performed an experiment at GSI to disantangle the 22 O g.s contribution to the 23 O wave function from any other contribution of 22 O excited states. 2. EXPERIMENT AND RESULTS The experiment was performed at the FRagment Separator(FRS) at GSI. The secondary beams were produced by nuclear fragmentation of relativistic 40 Ar at 1 A.GeV, on a carbon target. A complete description of the experimental technique used can be found in [9–11]. The results we obtained for the inclusive breakup fragment longitudinal momentum distribution widths after correction of the intrinsic momentum resolution are shown in Figure 1 for all the secondary beams studied in the experiment. The longitudinal momentum distribution have been obtained following the procedure described in [9–11]. They show a quite constant value (∼ 200 MeV/c) and a sudden decrease for 22 O and 23 F fragments corresponding with the filling of the 2s1/2 shell. The intrinsic momentum resolution was experimentally evaluated for each secondary beam and amounts to 19 ± 1 MeV/c (FWHM) for 23 O. The corresponding one-neutron removal cross sections have been extracted counting the incident projectiles and the fragments in front and behind the breakup target. This ratio was corrected by the experimental transmission evaluated with the code MOCADI after adjustment of the simulated fragment longitudinal momentum width to the measured ones. We present in Table 2 the measured cross sections for oxygen isotopes. The error includes statistical and systematics uncertainties. The observed experimental trend shows a smooth increase of the one-neutron removal cross-section. We can also observe, in
481c
D. Cortina et al. / Nuclear Physics A 746 (2004) 479c–482c
Fragment 18 O 19 O 20 O
Figure 1. FWHM’s of longitudinal momentum distribution for different isotopes of Nitrogen (triangles), Oxygen (squares) and Fluorine (circles) after one-neutron removal on a carbon target.
σ−1n (mb) 56 ± 7 56 ± 6 72 ± 7
Fragment 21 O 22 O
σ−1n (mb) 70 ± 7 85 ± 10
Figure 2. Results for the inclusive oneneutron removal cross section after oneneutron removal of 19−23 O at 939 MeV/u on a carbon target.
contrast to the results obtained in [6,7], that the one-neutron removal cross section of 23 O is not significantly big in order to assign a neutron halo character to this nucleus. The inclusive longitudinal momentum distribution (plong ), of 22 O residual fragments after one-neutron removal from 23 O obtained in this experiment is shown on the left side of Figure 3. In this figure the solid line corresponds to the Gaussian fit of the experimental data used to determine the width of the momentum distribution. The FWHM for this inclusive measurement is found to be 133 ± 10 MeV/c considering the correction due to the intrinsic momentum resolution of our spectrometer. The γ-rays emitted during de-excitation of 22 O were recorded with NaI detectors. The analysis of the γ-ray spectrum reveals three γ energies at 1.3, 2.6, and 3.2 MeV. This spectrum has been interpreted using previous experimental data and shell model calculations [8,12,13]. Assuming that all the 22 O excited levels decay through the first excited state at 3.2 MeV, we can use this peak to gate on the longitudinal momentum distribution in order to obtain the exclusive distribution that are shown on the right side of Figure 3. The FWHM for these exclusive measurements are found to be 126 ± 20 MeV/c and 236 ± 20 MeV/c for 22 O in its ground and any excited state, respectively. The procedure followed to analyse the coincidence spectrum can be found in Ref. [9–11]. The cross-sections corresponding to the exclusive momentum distribution amounts to 50±10 mb and 35±8 for the 22 O ground state and excited states, respectively. The experimental momentum distribution for the one-neutron removal channel leaving the 22 O core in its ground state is compared in Figure 4 to theoretical model calculations based in the Eikonal approximation [14]. Two calculations are shown for angular momenta l = 0 and l = 2. Clearly, the distribution assuming a 2s1/2 neutron coupled to the 22 O(0+ ) core is in much better agreement with the data. We can thus conclude that the ground state spin of 23 O is I π = 1/2+ . The experimental distribution is, however, slightly wider than the prediction for l = 0 The calculated neutron-knockout cross section for the configuration (d5/6 )6 (s1/2 )1 from the 1s-shell amounts to 51 mb and thus in agreement with the experimental value of
482c
D. Cortina et al. / Nuclear Physics A 746 (2004) 479c–482c
0.5
0.6
mb
0.2
0.4 0 0.3 FWHM = 237 ± 20 MeV/c
22O
exc.
0.2
0.2
/MeV/c )
incl.
dσ-1n /dp(
mb
dσ-1n/dp( /MeV/c )
0.4
mb
gs.
22O
dσ-1n/dp (
22O
0.6
/MeV/c )
FWHM = 134 ±10 MeV/c
FWHM = 127 ± 20 MeV/c
0.4
l=2
0.3
0.2
0.1
l=0
0.1 0 -200
0
-200
0
200 -200
0
200
0
-120
-40
40
120
200
plong (MeV/c)
plong (MeV/c)
Figure 3. Left: Inclusive longitudinal momentum distributions for 22 O fragments after one-neutron removal from 23 O. Right top: Exclusive longitudinal momentum for 22 O in its ground state. Right bottom: Exclusive longitudinal momentum distribution for 22 O in any excited state.
Figure 4. Ground state exclusive momentum distribution for 22 O fragments after one-neutron knock-out reaction from 23 O compared with calculations assuming l = 0 and l = 2
50 ± 10 mb. This result confirms the large spectroscopic factor for the s-neutron (C 2 S = 0.8) obtained by Brown et al [8]. However, a quite significant discrepancy has been found when comparing the neutron-knockout cross section contributions involving excited states with the experimentally deduced cross-section. 3. CONCLUSION We have measured for the first time the 22 O distribution after one-neutron knock-out of O in coincidence with the 22 O γ de-excitation thereby demonstrating that the groundstate spin of 23 O is I π = 1/2+ . This result provides a clear solution to the discrepancy of the ground-state spin and parity assignment of 23 O.
23
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
O.Tarasov et al., Phys. Lett B409 (1997) 64. A. Ozawa et al., Nucl. Phys. A673 (2000) 411. H. Sakurai et al., Phys. Lett. B448 (1999) 180. R. Kanungo et al., Phys. Lett. B 528, 58 (2002) E. Sauvan et al., Phys. Lett. B 491, 1 (2000) R. Kanungo et al., Phys. Rev. Lett. 88, 142502 (2002) A. Ozawa et al, Nucl. Phys. A693 (2001) 32. A. Brown, G. Hansen and J. Tostevin., Phys. Rev. Lett 90 (2003) 159201-1. D. Cortina-Gil et al., Phys. Lett. B 529, 36 (2002) D. Cortina-Gil et al., Nucl. Phys. A 720, 3 (2003) D. Cortina-Gil et al., submitted to Phys. Rev. Lett. P.G. Thirolf et al., Phys. Lett. B 485 (2000) 16. M. Stanoiu et al., Phys. Rev. C 69 (2004) 034312. P.G. Hansen, Phys. Rev. Lett. 77 (1996) 1017.