Heavy-ion induced electromagnetic interactions

Electromagnetic processes in heavy-ion interactions at energies far above the Coulomb barrier give access to a wealth of nuclear-structure information on exotic nuclei [1]. At energies of the order of 1 GeV/nucleon, collective nuclear states at low and at high excitation energies are excited in peripheral heavy-ion collisions with large cross sections. Due to Lorentz contraction, the mutual electromagnetic field contains high frequencies up to several tens of MeV/ħ. Surface vibrations and particular giant resonances can be studied even with moderate beam intensities. The large cross sections allow experiments with minimum beam intensities of 1 to 1000 ions/s, provided efficient devices for γ-ray and particle detection are implemented.

Electromagnetic excitation of the giant dipole resonance induced by high-energy beams on targets of high nuclear charge was pioneered at GSI in exploring the multi-phonon states of the dipole resonance [2]. This method was recently extended to secondary beams of exotic nuclei [3]. It was shown that, e.g., for neutron-rich oxygen isotopes, low-lying strength appears and that the usual pattern of the dipole resonance strength distribution dissolves [3]. With the proposed new facility, the measurement of the dipole strength of neutron-rich nuclei relevant for the astrophysical r-process will be feasible. In the region of the N=82 closed shell, for instance, the giant dipole strength can be deduced even beyond ¹³²Sn. For these heavier neutron-rich nuclei, the appearance of a new collective mode is predicted, the collective oscillation of valence neutrons (neutron skin) against the core, the so called soft dipole mode. The higher beam intensities also allow the study of giant quadrupole strength. Compared to dipole excitations, the required beam intensities are an order of magnitude larger. Giant resonance studies, in particular monopole and quadrupole excitations, will also be investigated at the NESR using the internal target and at the e-A collider (see the LoI's by the EXL and ELISe collaborations).

Besides resonant excitations, direct non-resonant transitions to the continuum occur for weakly bound nuclei. This ‘threshold strength’ is characteristic for the single-particle structure, being extremely sensitive to the spatial distribution of the valence nucleons. Similar to knockout, the l-value of the removed nucleon and spectroscopic factors can be deduced [4]. For a halo-like structure, cross sections become very large, and spectroscopic information can be obtained with beam intensities down to 0.1 ions/s. The continuum structure of drip-line nuclei was studied so far only for very light nuclei.  From a kinematically complete measurement of the decay not only the excitation spectrum, but also correlations can be studied as, e.g., in the three-body decay of Borromean halo nuclei like ¹¹Li or ⁶He [5,6]. The experimental technique discussed here also allows the extraction of (p, γ) and (n, γ) reaction rates, which essentially determine the astrophysical r-, and rp-reaction paths. While the direct measurement of these rates is very difficult, the (γ,p) and (γ,n) reaction can be measured by electromagnetic excitation using high-energy secondary beams [7].

[1] C.A. Bertulani and G. Baur, Phys. Rep. 163 (1988) 299.

[2] T. Aumann, P.F. Bortignon, H. Emling, Ann. Rev. Nuc. Part. Sci. 48 (1998) 351.

[3] A. Leistenschneider et al., Phys. Rev. Lett 86 (2001) 5442.

[4] U. Datta Pramanik et al., Phys. Lett. B 551 (2003) 63.

[5] T. Aumann et al., Phys. Rev. C 59 (1999) 1252.

[6] S.N. Ershov, B.V. Danilin and J.S. Vaagen, Phys. Rev. C 64 (2001) 064609.

[7] F. Schümann et al., Phys. Rev. Lett. 90 (2003) 232501.