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Of the 3000 nuclei that have been produced in laboratories, only about 300 are stable. The other nuclei are short-lived and decay until the valley of stability is reached. The study of these rare isotopesthat often only exist for fractions of a second has proven crucial for the understanding of the nature of the nuclear force and provides crucial input for nuclear astrophysics in the quest to explain the isotopic abundances in the Universe.
A fundamental question in nuclear physics is how many neutrons a given number of protons can bind. The limit of existence for neutron-rich nuclei, the neutron drip-line beyond which an isotope cannot bind additional neutrons, is firmly established only up to oxygen ( Z=8). Experiments that are aimed at establishing the existence of the most neutron-rich nuclei are extremely challenging as these most weakly-bound nuclei are produced with very small cross section. From an experimental point of view, the rare isotopes have to be produced, detected and unambiguously identified. The fragmentation of a stable heavy-ion beam followed by the in-flight separation of the reaction residues and their detection in less than 1 second is at present the only approach. In the US, this possibility exists at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University. A beam of stable 48Ca (Z=20, N=28) accelerated by the laboratory’s coupled cyclotron facility to 140 MeV/nucleon was fragmented in the collision with a 970 mm thick tungsten target. The reaction products were separated by the A1900-S800 two-stage magnetic fragment separator and guided onto a detector setup that measures event-by-event the energy loss, total kinetic energy, time of flight and position of each reaction product transmitted by the ion optical system. The figure shows the unambiguous identification of the most neutron-rich species reached in the experiment.
Over a total period of 7.6 days and with an average rate of 5.0 x 1011 48Ca beam particles per second, three events of 40Mg (Z=12, N=28) were unambiguously detected and identified from their energy loss and flight time among the zoo of less exotic reaction residues – 40Mg had never been observed before. Also the existence of 42Al (Z=13, N=29) could be proven and one candidate event of the even heavier nucleus 43Al was identified. While most theories that attempt to predict the limits of nuclear existence calculate 40Mg to be bound, the existence of 42Al came as a surprise with respect to many nuclear models. The existence of 42Al indicates that the neutron drip-line is likely be much further out than previously thought, meaning that many more neutron-rich nuclei might exist. For 42Al, neutrons are expected to start to fill the p3/2 orbital which has a maximum occupancy of 4 neutrons. Calculations predict that the energy of this orbital will remain constant as neutrons are added. This may indicate that, with the observation of 42Al, one should expect aluminum isotopes out to 45Al (Z=13, N=32) to be bound, where the p3/2 orbit is fully occupied. Even heavier aluminum isotopes might also exist if the energy splitting between the neutron p3/2 and p1/2 orbitals becomes small as suggested by some calculations that include forces with tensor terms.
T. Baumann et al., Discovery of 40Mg and 42Al suggests neutron drip-line slant towards heavier isotopes, Nature 449, 1022 (2007).