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A new approach to the production and detection of bound neutron clusters is presented. The technique is based on the breakup of beams of very neutron-rich nuclei and the subsequent detection of the recoiling proton in a liquid scintillator. The method has been tested in the breakup of 11 Li, 14 Be and 15 B beams by a C target. Some 6 events were observed that exhibit the characteristics of a multineutron cluster liberated in the breakup of 14 Be, most probably in the channel 10 Be+ 4 n. The various backgrounds that may mimic such a signal are discussed in detail.

Mean square radii of light radioactive projectiles, including exotic isotopes close to the neutron drip line ("Li, 14Be, and 17B), have been deduced from total reaction cross-section measurements, using a simple microscopic model. The isospin dependence of nuclear radii is discussed and compared to other experimental results.

Resonances in the unbound nucleus 11 N have been studied, using the resonance scattering reaction 10 Cϩp. The data give evidence for three states above the 10 Cϩp threshold with energies 1.30, 2.04, and 3.72 MeV. These states can be interpreted, in a potential-model analysis, as the ground state and the first two excited states with spin-parity 1/2 ϩ , 1/2 Ϫ , and 5/2 ϩ arising from the shell-model orbitals 1s 1/2 , 0p 1/2 , and 0d 5/2. A narrow state superposed on a broad structure found at higher energy could be interpreted as the mirror state of the 3/2 Ϫ in 11 Be shifted down in energy. This shift would suggest a large radius of the potential. ͓S0556-2813͑96͒50110-X͔

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Reactions of the halo systems lIBe and 11Li (at 460 and 280 MeV/nucleon) with a carbon target demonstrate that (n + 9Li) has an (unbound) l = O ground state very close to the threshold. The neutron halo of 11Li has appreciable (1S 1 / 2 )2 and (OP1/2)2 components. PACS numbers: 25.60.+v, 21.10.Pc, 25.70.Mn, 27.20.+n There is currently a surge of interest in the IOLi problem caused by the work of Kryger et al. [1], who reported a narrow central peak in the relative velocity spectrum of the fragments (n + 9Li) produced in fragmentation of an 18 0 beam at 80 MeV/nucleon. The peak was interpreted as evidence either for the ground state of IOLi, in this case probably the missing s state, or for an l = 0,1 resonance near 2.5 MeV. Weak evidence for a structure close to threshold [2] favors the first interpretation. A low-Iying P state is also present near 0.5 MeV according to very similar data presented by Young et al. [2] and by Bohlen el al. [3]. These two papers give a detailed summary of previous work on lOLi.The properties of IOLi are essential for the understanding of the neutron halo associated with the 11 Li ground state [4]. Thompson and Zhukov [5] found that a neutron s state at low energy would lead to an 11 Li halo with about egual admixtures of (lS 1 / 2 )2 and (OP1/2)2. This gave much better agreement with the narrow momentum distributions observed in fragmentation experiments and was in line with the suggestion by Barranco, Vigezzi, and Broglia [6], who attributed the forward-peaked angular distributions of neutrons from 11 Li incident on light targets to the decay in ftight of lOLi. (The 5He intermediate state also dominates the breakup of 6He [7]. ) We have produced the intermediate state lOLi in singlenucleon stripping reactions of the halo states 11 Be and 11 Li. The word "stripping" is employed here in the sense introduced in 1947 by Serber to describe reactions of 190 MeV deuterons; see his recent remarks on the subject [8]. At high bombarding energies, the sudden approximation is valid, and the halo neutron will, after the collision, to a good approximation be in a state characteristic of the projectile and not in an eigenstate of (9Li .+ n). We shall in the following develop a line of arguments that first infer the properties of IOLi from the known structure of 11 Be and then use the results to draw conclusions about the 11 Li halo.The radioactive beams were generated in a 8 g/cm 2 Be target by fragmentation of a 18 0 beam from the heavy-ion synchrotron SIS at GSI aod separated io the fragment separator [9] FRS by magnetic analysis. The secondary beams of 280 MeV / nucleon for 11 Li and 460 MeV / nucleon for 11 Be were transported via the storage ring ESR to a carbon target (thickness 1.29 g/cm 2 ) placed directIy in front of the large-gap magnetic spectrometer [10,11] ALADIN. Neutrons coincident with the nuclear fragment were detected in the calorimeter LAND [12] placed 11 m behind the carbon target and having an active detector area of 2 X 2 m 2 and a total thickness of 1 m. The neutron ...