MoNA Physics

Nuclear Physics Near the Neutron Drip-line

The study of nuclei close to the neutron drip-line and the investigation of nuclear systems even beyond the drip-line has greatly expanded in recent years. This increased interest is based on new phenomena that already have been observed or that are predicted to occur in these nuclei. At the neutron drip-line, the binding energy of a single neutron vanishes, leaving a system that is unbound with respect to prompt neutron emission. So far, the study of neutron-rich nuclei has been limited to the lightest elements, and the neutron drip-line has been experimentally verified only up to oxygen (Z = 8). With the coupled cyclotron facility, it should be possible to determine the actual position of the neutron drip-line up to sulfur (Z = 16). This will offer a large number of neutron-rich systems to be studied for the first time. Near the neutron drip-line, sequences of isotopes with odd neutron numbers are encountered that are unbound, while the next heavier isotope with even neutron number is bound. The investigation of these neutron-unbound systems can provide important insight into the interaction between nucleon and nucleus far from stability.

New phenomena that have been observed so far in nuclei close to the neutron drip-line include nuclear systems with very diffuse surfaces caused by loosely bound states of low angular momentum, which are referred to as halo states due to their large spatial extension. For light nuclei like 6He, 11Li, and 11Be, theoretical few-body calculations lead to quite accurate descriptions of their experimental properties. Only recently could these studies be extended to heavier systems, revealing more complex structures. Furthermore, nuclear closed shells, which manifest themselves in especially stable systems with magic numbers of nucleons, are found to wash out towards the neutron drip-line. Collective excitations are also affected significantly by the large N/Z ratio of neutron-rich nuclei. The shift of giant dipole resonance strength to lower energies has been predicted, and soft modes of giant resonances are studied in light halo nuclei.

An already well-established technique to study neutron-rich systems is to measure the products of a breakup reaction. In order to be able to make a full reconstruction of the nucleus before the breakup, the detection of the neutron (or neutrons) in coincidence with the charged breakup fragment is necessary. Although breakup cross sections for loosely bound systems are relatively high, the beam intensities for these extreme nuclei are typically low, and a highly efficient detector is especially needed if one wants to study the correlation of multiple neutrons. Breakup reactions also have been applied to form neutron-unbound states, where the effects of final-state interaction are revealed by measuring the constituents of the unbound state in coincidence.

Sequential particle decay spectroscopy

Studies of neutron-unbound ground states of nuclei far from stability have been performed only for light elements. The Z-dependence of the s1/2-spin states for all N = 7 nuclei has been established. The N = 7 isotones attract theoretical interest due to the appearance of intruder states from the 1s0d shell, which was first observed in the one-neutron halo 11Be. Extending the study of these intruder states to nuclei with larger N/Z ratios requires the observation of unbound neutron single-particle states, which have been observed applying the method of sequential particle decay spectroscopy [1,2]. This technique is based on the coincident measurement of the decay product of the unstable fragment and a neutron in a collinear geometry. The measured relative velocity spectrum is directly related to the decay energy and thus the mass of the decaying system. Excited states can also be observed with this method. Due to the strong forward focusing of the reaction products, both decay products, the charged fragment as well as the neutron, have to be detected at zero degrees.

Study of multi-neutron halos in breakup reactions

Studies that require the use of a large-area neutron detector with multi-hit capability are those investigating the nuclear structure of Borromean nuclei, which have a multi-neutron halo. Borromean nuclei are three-body systems with unbound binary subsystems. A common technique to examine the wave function of halo nucleons is to measure the momentum distribution of the core fragment after removal of the halo nucleon. Due to the conservation of momentum, this distribution is directly related to the momentum of the removed nucleon. Theoretical calculations that take the reaction mechanism into account are able to reconstruct the wave function. A common feature of halo nuclei are narrow momentum distributions of the core fragment, which are connected to a large spatial extension of the halo nucleon's wave function by Heisenberg's uncertainty principle. This elegant method fails, however, in the examination of Borromean halo nuclei, since the removal of one loosely bound neutron leaves an unbound subsystem. In order to study these nuclei, it is essential to also measure the emitted neutron, so that a reconstruction of the momentum can be achieved.

A number of multi-nucleon halo nuclei may be produced and studied at the NSCL coupled cyclotron facility, as for example 19B, which may have a large number of loosely bound neutrons that could form a halo of four or even six neutrons. Already 15B has shown some indication of having a halo structure, and although the binding energy of 19B is unknown, it could be very low (16B and 18B are known to be unbound). At the coupled cyclotron facility, the predicted production rate for 19B is sufficient for detailed studies using the methods of sequential particle decay or breakup reactions. It may therefore be possible to investigate the interaction of many neutrons in a diffuse, nearly proton-free environment. The heaviest two-neutron halo that is within reach of the coupled cyclotron facility is probably 32Ne. The MoNA neutron detector will be an essential instrument for these future studies as the capability of detecting multiple neutrons with a high efficiency is indispensable for these experiments.

Collective excitations in neutron-rich systems

Collective excitations of nuclei can probe both global and specific nuclear properties. The giant dipole resonance (GDR) is a sensitive tool for studying non-uniform charge distributions in nuclei. For example, the halo structure in light neutron-rich nuclei could result in a completely different electro-magnetic response as compared to stable nuclei [3]. Measuring the response of extremely neutron-rich systems will help us to understand the collective motion of these nuclei and to extrapolate to the collective motion of neutron matter.

The study of the GDR in exotic nuclei relies on its large excitation cross section and a clean separation and identification from other giant resonances. Two important factors are necessary to excite the GDR strongly in the projectiles: high beam energies and high-Z targets. At high beam energies, GDR formation is dominated by Coulomb excitation, which is proportional to Z2 of the target. The GDR is located above the particle evaporation threshold, so the excited projectiles will break up in flight. One possible experimental technique that can be applied to reconstruct the excitation function of the projectile has recently been used for the first time in the study of the GDR in 20O. The excitation energy was reconstructed from the breakup fragment and the neutrons after Coulomb dissociation [4]. The projectile residues were measured in coincidence with all emitted particles (charged breakup fragments, neutrons, and gamma rays). Due to the high beam energy of the projectiles, all decay products were strongly forward focused and could be detected in a forward wall of particle detectors. A resolution of about 1 MeV has been achieved with this method [4], wich requires beam intensities of 104 particles per second. The proposed highly efficient neutron detector will be essential for measurements of this kind at the NSCL, where the most neutron-rich oxygen isotope 24O and neutron-rich nuclei in even heavier mass regions like 48Ar are within reach of the coupled cyclotron facility.

References

[1] R.A. Kryger et al., Phys. Rev. C 53 (1996) 1971

[2] M. Thoennessen et al., Phys. Rev. C 59 (1999) 111

[3] Proceedings of the Topical Conference on Giant Resonances, edited by A. Bracco and P.F. Bortignon, Nucl. Phys. A 649 (1999)

[4] T. Aumann et al., Nucl. Phys. A649 (1999) 297c