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Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes


A fundamental question in nuclear physics is what combinations of neutrons and protons can make up a nucleus. Many hundreds of exotic neutron-rich isotopes have never been observed; the limit of how many neutrons a given number of protons can bind is unknown for all but the lightest elements1, owing to the delicate interplay between single particle and collective quantum effects in the nucleus. This limit, known as the neutron drip line, provides a benchmark for models of the atomic nucleus. Here we report a significant advance in the determination of this limit: the discovery of two new neutron-rich isotopes—40Mg and 42Al—that are predicted to be drip-line nuclei2. In the past, several attempts to observe 40Mg were unsuccessful3,4; moreover, the observation of 42Al provides an experimental indication that the neutron drip line may be located further towards heavier isotopes in this mass region than is currently believed. In stable nuclei, attractive pairing forces enhance the stability of isotopes with even numbers of protons and neutrons. In contrast, the present work shows that nuclei at the drip line gain stability from an unpaired proton, which narrows the shell gaps and provides the opportunity to bind many more neutrons5,6.

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Figure 1: Section of the chart of nuclides for light, neutron-rich nuclei.
Figure 2: Schematic diagram of the coupled cyclotron facility16.
Figure 3: Particle identification plots of observed isotopes.


  1. 1

    Thoennessen, M. Reaching the limits of nuclear stability. Rep. Prog. Phys. 67, 1187–1232 (2004)

    ADS  CAS  Article  Google Scholar 

  2. 2

    Audi, G., Wapstra, A. H. & Thibault, C. The Ame2003 atomic mass evaluation. Nucl. Phys. A 729, 337–676 (2003)

    ADS  Article  Google Scholar 

  3. 3

    Notani, M. et al. New neutron-rich isotopes, 34Ne, 37Na and 43Si, produced by fragmentation of a 64 A MeV 48Ca beam. Phys. Lett. B 542, 49–54 (2002)

    ADS  CAS  Article  Google Scholar 

  4. 4

    Lukyanov, S. M. et al. Experimental evidence for the particle stability of 34Ne and 37Na. J. Phys. G Nucl. Part. Phys. 28, L41–L45 (2002)

    CAS  Article  Google Scholar 

  5. 5

    Otsuka, T., Matsuo, T. & Abe, D. Mean field with tensor force and shell structure of exotic nuclei. Phys. Rev. Lett. 97, 162501 (2006)

    ADS  Article  Google Scholar 

  6. 6

    Brown, B. A., Duguet, T., Otsuka, T., Abe, D. & Suzuki, T. Tensor interaction contributions to single-particle energies. Phys. Rev. C 74, 061303(R) (2006)

    ADS  Article  Google Scholar 

  7. 7

    Möller, P., Nix, J. R., Myers, W. D. & Swiatecki, W. J. Nuclear Ground-State Masses and Deformations. At. Data Nucl. Data Tables 59, 185–381 (1995)

    ADS  Article  Google Scholar 

  8. 8

    Samyn, M., Goriely, S., Bender, M. & Pearson, J. M. Further explorations of Skyrme-Hartree-Fock-Bogoliubov mass formulas. III. Role of particle-number projection. Phys. Rev. C 70, 044309 (2004)

    ADS  Article  Google Scholar 

  9. 9

    de Gennes, P. G. Superconductivity of Metals and Alloys (Addison-Wesley, Reading, Massachusetts, 1989)

    MATH  Google Scholar 

  10. 10

    Goriely, S., Samyn, M., Pearson, J. M. & Onsi, M. Further explorations of Skyrme–Hartree–Fock–Bogoliubov mass formulas. IV: Neutron-matter constraint. Nucl. Phys. A 750, 425–443 (2005)

    ADS  Article  Google Scholar 

  11. 11

    Artukh, A. G. et al. New isotopes 21N, 23O, 24O and 25F, produced in nuclear reactions with heavy ions. Phys. Lett. 32B, 43–44 (1970)

    ADS  Article  Google Scholar 

  12. 12

    Langevin, M. et al. Production of neutron-rich nuclei at the limits of particles stability by fragmentation of 44 MeV/u 40Ar projectiles. Phys. Lett. 150B, 71–74 (1985)

    ADS  CAS  Article  Google Scholar 

  13. 13

    Guillemaud-Mueller, D. et al. Particle stability of the isotopes 26O and 32Ne in the reaction 44 MeV/nucleon 48Ca + Ta. Phys. Rev. C 41, 937–941 (1990)

    ADS  CAS  Article  Google Scholar 

  14. 14

    Tarasov, O. et al. Search for 28O and study of neutron-rich nuclei near the N = 20 shell closure. Phys. Lett. B 409, 64–70 (1997)

    ADS  CAS  Article  Google Scholar 

  15. 15

    Lutostansky, Yu. S., Lukyanov, S. M., Penionzhkevich, Yu. E. & Zverev, M. V. Neutron drip line in the region of O–Mg isotopes. Particles Nuclei Lett. 115, 86–93 (2002)

    Google Scholar 

  16. 16

    Gelbke, C. K. Rare isotope research capabilities at the NSCL today and at RIA in the future. Prog. Particle Nucl. Phys. 53, 363–372 (2004)

    ADS  CAS  Article  Google Scholar 

  17. 17

    Morrissey, D. J., Sherrill, B. M., Steiner, M., Stolz, A. & Wiedenhoever, I. Commissioning the A1900 projectile fragment separator. Nucl. Instrum. Meth. B 204, 90–96 (2003)

    ADS  CAS  Article  Google Scholar 

  18. 18

    Bazin, D., Caggiano, J. A., Sherrill, B. M., Yurkon, J. & Zeller, A. The S800 spectrograph. Nucl. Instrum. Methods Phys. Res. B 204, 629–633 (2003)

    ADS  CAS  Article  Google Scholar 

  19. 19

    Tarasov, O. B. et al. New isotope 44Si and systematics of the production cross sections of the most neutron-rich nuclei. Phys. Rev. C 75, 064613 (2007)

    ADS  Article  Google Scholar 

  20. 20

    Retamosa, J., Caurier, E., Nowacki, F. & Poves, A. Shell model study of the neutron-rich nuclei around N = 28. Phys. Rev. C 55, 1266–1274 (1997)

    ADS  CAS  Article  Google Scholar 

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This work was supported by the US National Science Foundation.

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Correspondence to T. Baumann.

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Baumann, T., Amthor, A., Bazin, D. et al. Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes. Nature 449, 1022–1024 (2007).

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