The two-neutron halo nucleus Be has been investigated in a kinematically complete measurement of the fragments (Be and neutrons) produced in dissociation at 35 MeV/nucleon on C and Pb targets. Two-neutron removal cross-sections, neutron angular distributions and invariant mass spectra characteristic of a halo were observed and the electromagnetic (EMD) contributions deduced. Comparison with three-body model predictions indicate that the halo wavefunction contains a large ν(2s1/2) 2 admixture. The EMD invariant mass spectrum exhibited a relatively narrow structure near threshold (Edecay=1.8±0.1 MeV, Γ = 0.8±0.4 MeV) consistent with a soft-dipole excitation. PACS number(s): 27.20.+n6, 25.60.Dz, 25.60.Ge, 24.30.Gd Typeset using REVTEX 2 The size and distribution of matter in the nucleus have long played a central role in nuclear physics. Indeed, such gross properties reflect the combined effects of many fundamental aspects of nuclei. For the stable nuclei, measurements employing conventional probes, such as high energy electron and hadron scattering, have shown that the neutron and proton distributions exhibit essentially identical radii [1]. In contrast, for some light nuclei far from stability, which combine a large neutron excess with very weak binding, large differences have been found. Such “halo” systems are well described by a core, resembling a normal nucleus, surrounded by an extended valence neutron density distribution [2]. In general terms the halo may be regarded as a threshold phenomenon whereby the loosely bound valence neutrons tunnel with significant probability into the classically forbidden region outside the core potential. Within a simple quasideuteron description, the extent of the halo is governed by the separation energy and reduced mass of the system [3]. Under more realistic considerations the development of the halo is also influenced by the centrifugal barrier [4]. In the cases of He, Be and Li, which have been investigated experimentally in considerable detail, the valence neutrons occupy the 2s1/2 and/or 1p3/2,1/2 single-particle orbitals. In Be the configuration of the halo neutrons would, in a näive shell model prescription, be ν(d5/2) . Sophisticated models suggest, however, that a ν(s1/2) 2 admixture is also present [5–8]. Unfortunately, a paucity of experimental data [9–11] has precluded the elucidation of the structure of Be beyond the matter radius [12–15]. Compared to the other halo systems, the comparatively strong binding of the valence neutrons in Be (S2n=1.34±0.11MeV [16,17]) combined with the ν(d5/2) 2 component may provide a new window on continuum excitations, including the long sought-after Soft-Dipole Resonance (SDR) [18,19]. The goal of the present study was thus to explore the halo structure and continuum excitations of the two-neutron halo nucleus Be. The tool chosen was a kinematically complete measurement of the fragments (Be and two neutrons) from the dissociation of an intermediate energy beam of Be on C and Pb targets. Such a measurement allowed the two-neutron removal cross sections, neutron angular distributions and invariant mass 3 spectra to be extracted (the results of an analysis of the neutron-neutron correlations have been presented elsewhere [20]). The use of C and Pb targets permitted the electromagnetic component of the dissociation (EMD) to be deduced. The Be beam (∼130 pps) was prepared using the LISE3 spectrometer and a 63 MeV/nucleon O primary beam bombarding a thick Be production target. The mean energy of the beam at the mid-point of the secondary breakup targets was 35 MeV/nucleon. The energy spread in the beam was 10% and was compensated for by a time-of-flight (TOF) measurement over a 24 m flight-path between a parallel-plate avalanche counter (PPAC) located at the first focus of the spectrometer and the beam identification Si-detector. The beam particles were tracked onto the breakup targets (C 275 mg/cm, Pb 570 mg/cm) using two position sensitive PPAC’s (resolution FWHM ≈ 1-2 mm). Owing to the mixed nature of the secondary beam (50 % Be) the incoming ions were identified on a particle-by-particle basis using the TOF information combined with the energy loss derived from a Si-detector (300 μm) located just upstream of the target. The charged fragments from breakup were identified using a large area (5×5 cm) position sensitive (FWHM ≈ 0.5mm) Si-CsI telescope (Si 500 μm, CsI 2.5 cm) centred at zero degrees and located 11.4 cm downstream of the target. The energy response of the telescope (FWHM = 1.5%) was calibrated using various mixed secondary beams containing Be with energies straddling that expected for Be fragments arising from the dissociation of Be. In order to account for events arising from reactions in the telescope, data was also acquired without a reaction target with the beam energy reduced by the amount corresponding to the energy loss in the C and Pb targets. The neutrons emitted at forward angles were detected using the 99 elements of the DéMoN array [21]. The array covered angles between +13 and -40 in the horizontal plane and ±14 in the vertical with the modules arranged in a staggered configuration at distances between 2.5 and 6.5 m from the target [21]. Such a geometry provided for a relatively high two-neutron detection efficiency (1.5%) whilst reducing the rate of cross-talk — both intrinsically and via the use of an off-line rejection algorithm — to negligible levels [21,22]. A threshold of 15 MeV on the neutron energy was applied in the off-line analysis 4 to eliminate contamination from the small number of evaporation neutrons arising from the target. The results obtained for the two-neutron removal cross sections, σ−2n ( Be identified in the telescope), the single-neutron angular distributions, dσ/dΩ (Be and neutron), and the associated angle integrated (0-40) cross sections, σn, are displayed in table I and figure 1a. In addition, the average neutron multiplicities have been derived (mn = σn/σ−2n) and are also listed. The single-neutron angular distributions are well characterised by a Lorentzian lineshape [9,22] and the corresponding momentum width parameters, Γn, have been tabulated. The large neutron removal cross sections and relatively narrow neutron distributions, while not as pronounced as for Li [9], clearly indicate the halo character of Be. The present results improve considerably on the earlier measurements of Riisager et al. [9] which suffered from poor statistics (no angular distribution could be constructed for a heavy target) and were restricted to a limited angular range. The multiplicities obtained for the two targets are instructive in terms of the reaction mechanisms leading to dissociation [23]. For a light target, unless the halo neutrons are highly spatially correlated, the reaction is expected to proceed via single-neutron removal (absorption or diffraction) followed by the in-flight decay of Be. As approximately equal contributions are expected for absorption and diffraction [23] the average neutron multiplicity should be 1.5, in accordance with that measured here (table I). This scenario is also supported by the single-neutron angular distribution for the C target which is well reproduced assuming passage via a low-lying resonance in Be [22,24]. In the case of a heavy target, nuclear and Coulomb dissociation are present. Given that Coulomb dissociation should be associated with a multiplicity of 2, the average multiplicity for dissociation on Pb should be between 1.5 and 2, as observed. The enhanced cross section for dissociation on the Pb target is indicative of a large EMD contribution. Assuming that the nuclear–Coulomb interference is small, the C target data (which arises essentially from nuclear induced reactions) may be scaled to estimate the nuclear contribution to breakup on Pb [19,22]. Assuming a root-mean-square radius of 3.2 5 fm for Be [14,15], σ −2n(Pb) = 0.85±0.07 b and, consequently, σ EMD −2n (Pb) = 1.45±0.40 b. The latter can be compared to the value of 0.47±0.15 b measured at 800 MeV/nucleon [19]. Importantly, for halo nuclei, the EMD cross section is dominated by the E1 component [25,26]. An enhancement with decreasing beam energy is thus expected, owing to the large amount of dipole strength near threshold (see below) coupled with the weighting of the virtual photon spectrum to low photon energies [27]. Assuming that the neutron angular distribution arising from nuclear dissociation on Pb is identical to that measured for the C target, the single-neutron angular distribution for EMD has been constructed (figure 1b) and the corresponding integrated cross section and average multiplicity derived (table I). Interestingly, the angular distribution remains narrow and forward peaked whilst the multiplicity is consistent with the value of 2 expected for EMD, confirming the validity of the methods used to estimate the contribution arising from nuclear breakup. The invariant mass spectra, reconstructed from the measured momenta of the beam and fragments (Be and two neutrons) from breakup, are displayed in figure 2a and b for the C and Pb targets. The EMD spectrum (figure 2c) has been deduced, as described above, following subtraction of the estimated nuclear contribution to reactions on Pb. As for the spectra obtained with the C and Pb targets, the EMD spectrum exhibits enhanced strength around 2 MeV decay energy (Edecay). Given the complex nature of the response function of the present setup, a detailed Monte Carlo simulation, including the influence of all nonactive materials, was developed based on the GEANT package [22]. The results shown in figure 2 were obtained following the descriptions for dissociation on C and Pb outlined earlier. In the case of the nuclear induced reactions a single low-lying state in Be (E0 = 0.5 MeV, Γ0 = 0.5± 0.4 MeV) was assumed to be populated following the diffraction of one of the halo neutrons [22,28]. The EMD was simulated under the assumpti

The measurement of the two-particle correlation function for different particle species allows to obtain information about the development of the particle emission process: the space-time properties of emitting sources and the emission time sequence of different particles. The single-particle characteristics and two-particle correlation functions for neutral and charged particles registered in forward direction are used to determine that the heavy fragments (deuterons and tritons) are emitted in the first stage of the reaction (pre-equilibrium source) while the majority of neutrons and protons originates from the long-lived quasi-projectile. The emission time sequence of protons, neutrons and deuterons has been obtained from the analysis of non-identical particle correlation functions.

The measurement of the two-particle correlation function for different particle species allows to obtain information about the development of the particle emission process: the space-time properties of emitting sources and the emission time sequence of different particles. The single-particle characteristics and two-particle correlation functions for neutral and charged particles registered in forward direction are used to determine that the heavy fragments (deuterons and tritons) are emitted in the first stage of the reaction (pre-equilibrium source) while the majority of neutrons and protons originates from the long-lived quasi-projectile. The emission time sequence of protons, neutrons and deuterons has been obtained from the analysis of non-identical particle correlation functions.

The first exclusive breakup measurements for the nucleus Be-9 are presented. Breakup via several discrete states is observed following scattering off C-12 and Pb-208. The results support the prediction of a recent microscopic cluster calculation for a strong n+Be-8(2(+)) state component in the second excited state.

Ar+Ni collisions at 77 MeV/u were studied in the experiment E286 performed at GANIL. An important advantage of this experiment was an application of the neutron detector DEMON for registration of both neutral and charged particles. This feature allows to compare characteristics of neutrons and protons detected by the same detector and gives a possibility to determine the influence of the Coulomb field on the proton emission. Estimation of a charge of the emitting source was performed by comparing energy spectra of neutrons and protons detected under identical experimental conditions. The experimental results were compared with the prediction of the SIMON model.

Mass-energy distributions (MEDs) and capture-fission cross sections have been measured in the reaction 48Ca + 208Pb → 256No at the energies Elab=206–242 MeV using a double-arm time-of-flight spectrometer CORSET. It has been observed that MED of the fragments consists of two parts, namely, the classical fusion–fission process corresponding to the symmetric fission of 256No and quasi-fission "shoulders" corresponding to the light fragment masses ∼60–90 u and complimentary heavy fragment masses. The quasi-fission "shoulders" have a higher total kinetic energy (TKE) as compared with that expected for the classical fission. A mathematical formalism was employed for the MEDs fragment decomposition into fusion–fission and quasi-fission components. In the fusion–fission process a high-energy Super-Short mode has been discovered for the masses MH=130–135 u and the TKE of ≈233 MeV.

Results of the experimental study of , , dT, pT, and, for the first time, nT, , correlations in the region of small relative momenta are presented. The data analysis provides an estimate of the effective source size or emission time.

Nuclear dissipation in capture reactions is investigated using backtracing, a new analysis protocol. Combining analysis procedure with dynamical models, the difficult and long-standing problem of competition and mixing between quasifission and fusion-fission is solved for the first time. The nature of the relevant dissipation is determined as one-body dissipation. At low excitation energy where shell effects are strongly effective, the shape of the mass distribution could be a powerful check of the nature and the magnitude of the dissipation.

The cluster properties of neutron-rich light nuclei are considered. Resonantparticlespectroscopytechniqueandexperimentalstudiesof clustering usingposition-sensitivechargeparticledetectorsis discussed. Someexperimental evidence of possible existence of molecular, exotic cluster structures is reviewed.

. Results of the experimental study of p 4 He , d 3 He , dT, pT, p 3 He and, for the first time, nT, n 3 He , n 4 He correlations in the region of small relative momenta are presented. The data analysis provides an estimate of the effective source size or emission time.

By comparing theoretical and experimental excitation functions of evaporation residues resulting from the same compound nucleus or heavy and superheavy nuclei, it is possible to understand the effect of the entrance channel and the shell structure of reacting nuclei on the fusion mechanism. The competition of complete fusion with the quasifission process is strongly related to the intrinsic fusion barrier B fus * and the quasifission barrier B qf as well as the size of the well in the nucleus-nucleus potential. In our calculations of the excitation functions for capture, fusion, and evaporation residues, we use the relevant variables such as mass asymmetry of nuclei in the entrance channel, potential energy surface, driving potential, spin distribution, and surviving probability of compound nucleus that are responsible for the mechanism of the fusion-fission process. As a result, we obtain a beam energy window for the capture of the nuclei before the system fuses and the Γ n /Γ f ratio at each step along the deexcitation cascade of the compound nucleus. Calculations performed in the framework of the model taking into account the nuclear shell effect and shape of colliding nuclei allow us to reach useful conclusions about the mechanism of the fusion-fission process and the production of the evaporation residues. We analyze the 40 Ar + 176 Hf, 86 Kr + 130 Xe, and 124 Sn + 92 Zr reactions leading to 216 Th*; the 32 S + 182 W and 60 Ni + 154 Sm reactions leading to 214 Th*; the 48 Ca + 248 Cm reaction leading to the 296 116 compound nucleus; and the 48 Ca + 249 Cf reaction leading to the 297 118 compound nucleus.

Two-neutron correlation functions as well as single neutron energy spectra were used to determine the parameters of the emitting sources in Ar-Ni reaction at 77 MeV/u. The neutrons were registered in angular ranges 4degrees-22degrees and 55degrees-66degrees. By a choice of the neutron energy range and measurement angles we are able to determine the space-time parameters of the preequilibrium and the quasi-projectile separately. The source velocities and temperatures were determined by fitting the multisource model parameters to the single-neutron inclusive energy spectra. The space-time parameters were extracted from the fit of correlation functions with a Gaussian (space) and exponential (time) distributions.

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 intermediate energy (30-50 MeV/nucleon) Li-11, Be-14, and B-15 beams. Some six events were observed that exhibit the characteristics of a multineutron cluster liberated in the breakup of Be-14, most probably in the channel Be-10+(4)n. The various backgrounds that may mimic such a signal are discussed in detail.