Elsevier

Nuclear Physics A

Volume 669, Issues 1–2, 10 April 2000, Pages 51-64
Nuclear Physics A

Halo excitations in fragmentation of 6He at 240 MeV/u on carbon and lead targets

https://doi.org/10.1016/S0375-9474(99)00825-8Get rights and content

Abstract

Dissociation of a 240 MeV/u beam of 6He, incident on carbon and lead targets, has been studied in kinematically complete experiments to investigate low-lying excitation modes in the halo nucleus 6He. It is shown that alignment effects characterize the inelastic scattering and allow an unambiguous assignment of the spin of a narrow resonance observed in the excitation energy spectrum. The differential cross sections for the 6He inelastic scattering on carbon and lead targets were deduced from the measured momenta of the two neutrons and the α-particle. An analysis of these distributions shows that quadrupole and, possibly, monopole excitations characterize the hadronic interaction, while the dipole mode is dominating in Coulomb dissociation. Neither theoretically predicted new resonance states in 6He nor nuclear excitation of a dipole mode were found. Direct evidence has been obtained for strong suppression of Coulomb post-acceleration in direct Coulomb breakup in a lead target.

Introduction

The investigations of nuclei in the dripline regions have over the past decade yielded many interesting new both theoretical and experimental results. Special interest has been devoted to nuclei where the separation energy of the last bound nucleon or nucleon pair is so low that a nuclear halo may be developed (see the review papers [1], [2], [3]). Halo states are characterized by a dilute tail of nuclear matter extending far into the classically forbidden region1. The existence of soft modes of excitation in such nuclei has been discussed in many theoretical papers [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] and low-lying dipole modes have been observed in Coulomb dissociation experiments using beams of halo nuclei [16], [17], [18], [19]. Investigations of dipole excitations in hadronic interactions [20], [21] and excitation of higher multipolarities [19], [22], [23] have also been performed.

There exists a vast amount of experimental data about 6He [24], and there is a general consensus that the only excited state below the 3H+3H threshold is found at 1797±25 keV (Iπ=2+, Γ=113±20 keV) [24]. However, several types of low energy resonances have been predicted to be present in 6He [8], [9], [14], [11], [25], [26]. One expects 0+ monopole resonances (breathing mode) in the excitation energy (E) region 5.0–6.5 MeV, 1+ resonances around E=3.5-4.5 MeV and, in addition to the 1797 keV state, a second quadrupole resonance at about E=3.5-4.0 MeV. According to some calculations [14], these resonances are narrow and situated at lower excitation energies. The low-energy dipole resonance in 6He has not been found experimentally. It is, however, interesting to note that the non-resonant modes in the inelastic scattering reaction 6He(p,p′) are predicted to be important with the main contribution coming from the dipole and quadrupole modes [25], [26]. It should also be mentioned that non-resonant fragmentation is established to be a direct break-up channel for light, stable nuclei with well-developed cluster structure, e.g., 6Li and 7Li [27], [28], [29], [30].

This paper describes a continuation of the analysis of data from dissociation of 240 MeV/u 6He in carbon and lead targets, obtained in a kinematically complete experiment performed at GSI. The special emphasis of the present analysis is to search for excitation modes in 6He of the types mentioned above. We have recently [19] presented cross sections for different reaction channels of 6He, quantitative results of its E1 continuum strength distribution and the first evidence for low-lying strength of different multipolarities in the spectra. In this paper we address some additional peculiarities in the data.

The analysis is made using traditional methods applicable for reactions with beams of stable nuclei, without taking the few-body nature of 6He into account. The inelastic scattering cross section is strongly sensitive to the transferred angular momentum (Δl) which allows to distinguish between the excitation modes of different multipolarity. It will also be shown that spin-alignment effects are pronounced and that they give valuable information about the continuum.

Section snippets

Experimental method

The radioactive beam of 6He was produced in an 8 g/cm2 Be production target by fragmentation of a primary 18O (340 MeV/u) beam from the heavy-ion synchrotron SIS at GSI and subsequently separated in the FRS by analysis. The secondary beam of 240 MeV/u 6He was then brought to a carbon target (thickness 1870 mg/cm2) or to a lead target (thickness 870 mg/cm2) placed in front of the large-gap dipole magnetic spectrometer (ALADIN). The neutrons, recorded in coincidence with an α-particle, were

Spin alignment

Let us first consider the interactions of 6He in the carbon target. The 6He excitation energy spectrum extracted from the measured momenta, shown in Fig. 1, reveals a narrow peak at the low energy part of the spectrum. The total cross section is 30±5 mb for excitation energies below 10 MeV, and the peak contributes with 4.0±0.8 mb [19]. The low-energy peak can be interpreted as the first excited state in 6He at 1797 keV based on its energy. However, a shakeoff mechanism as proposed in [32] may

Excitations in the Coulomb field

The 6He excitation-energy spectrum obtained with the lead target is shown in Fig. 6a. The distribution reveals a wide peak centered at about 2.5 MeV excitation energy. The narrow peak, which can be seen on the low-energy slope, corresponds to the nuclear excitation of the 6He first 2+ state with a cross section equal to 14±4 mb [19]. The dominant part of it is due to Coulomb dissociation (520±110 mb [19] ) and can be associated with E1-dipole oscillations. A detailed discussion of this mode was

Summary

Experimental data on diffractive dissociation of 6He on carbon and lead targets at 240 MeV/u have been discussed. Excitation-energy spectra, inelastic differential cross sections and angular correlations of the decay products were presented and analyzed.

Nuclear excitation of the 6He first excited state has been confirmed by the observation of the angular correlations connected with spin alignment effects. Analysis of these correlations gave evidence that the main component of the 6He(2+, E=1.8

Acknowledgements

This work was supported by the BMBF under Contracts 06 DA 820, 06 OF 112 and 06 MZ 864 and by GSI via Hochschulzusammenarbeitsvereinbarungen under Contracts DA RICK, OF ELZ, MZ KRK and partly supported by the Polish Committee of Scientific Research under Contract PBZ/PB03/113/09, EC under contract ERBCHGE-CT92-0003, CICYT under contract AEN92-0788-C02-02 (MJGB) and WTZ under contract RUS 609-96. One of us (B.J.) acknowledges the support through an Alexander von Humboldt Research Award.

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