Our group has been involved in planning and constructing the Large Area Neutron Detector LAND which is one of the big spectrometers used at the SIS heavy-ion synchrotron accelerator facility of Gesellschaft für Schwerionenforschung, GSI, Darmstadt.
Sketch of LAND (without veto detector)
LAND has a front face of 2m x 2m and 1m depth. It features a multilayer structure of passive converter (Fe) and active scintillator material.The detector is subdivided into 200 independently operating modules and a 40-element charged-particle veto detector.
Sketch of a single neutron detector paddle
main function concerns time-of-flight measurements with very good position as well as time resolutionwhich gives excellent momentum resolution pn/pn = 1.5 x 10 -2 . The neutron detection efficiency is >90% for >200 MeV neutrons. The LAND Collaboration has been in several research projects mentioned below.
Cluster model for 11Li
A novel structural feature called the neutron halo has been found in a number of light, extremely neutron-rich nuclei. The combination of the low neutron separation energy and the short range of the nuclear force allows the neutron or a cluster of neutrons to tunnel into the space surrounding the nuclear core so that neutrons are present with appreciable probality at distances much larger than the normal nuclear radii. In this weakly bound structure, simple few-body or cluster models largely account for the most general properties of halo nuclei.
We investigate light halo nuclei such as 14Be, 11Be, 11Li, 8He and 6He with a combination of detectors (see figure), including the LAND detector.
The halo nuclei are produced by projectile fragmentation of a ca. 400 MeV/u 18O-beam from SIS, separated by the fragment recoil seperator FRS and delivered as radioactive beams to the detector system shown in the figure. The direction of the incoming beam, here 280 MeV/u is determined with multiwire drift chambers (MWDC) (1,2). The charged fragments from the target (3) are analyzed using the ALADIN magnet (4), MWDC's (5), and a plastic wall (6). The position and flight time of the coincident neutrons are measured with LAND (7). The left insert shows the momentum distribution of the 9Li fragment in the direction perpendicular to the pole plane of the magnet, and the right insert depicts the radial momentum distribution of neutrons in coincidence with the 9Li fragment.
Present emphasis of the experiments is on the information on the wave functions of the halo neutrons contained in measured momentum distributions, on the reconstructed Lorentz invariant mass (excitation energy) spectrum of the halo nuclei themselves and of sequential decay products (e.g. 10Li in the fragmentation of 11Li), on spin alignment of the fragments and neutrons.
Coulomb excitation using relativistic heavy ions is a promising tool to investigate giant resonances. The intense electromagnetic field acting in a close collision of heavy ions contains Fourier components of several tens of MeV and the adiabaticity cut-off lies well beyond the region of giant resonances. Thus, in relativistic heavy-ion collisions of typically 1 GeV/u, high cross sections for giant resonance excitation are observed. If a projectile is excited in a Pb-target all decay products are kinematically focused in a narrow forward cone where they can be detected with a detection system similar to the one used for the study of the fragmentation of halo nuclei. ALADIN and peripheral detectors are used to identify the heavy fragment and the LAND detector registers the neutrons evaporated in the decay of the giant resonance state. By reconstructing the Lorentz invariant mass, the excitation energy spectrum can be reconstructed. In this spectrum, the LAND collaboration could identify the two-phonon giant dipole resonance in 136Xe, 208Pb and 238U. Position and widths of these highly exotic states are consistent with a harmonic oscillator. In 238U, also Coulomb fission has been investigated and the possibility to study giant resonances built on the second potential minimum is considered.
While these experiments were performed with stableprojectiles we have also started a program where Coulomb excitation of radioactive projectiles from the FRS is studied. This is done for broad ranges of mass numbers of a given projectile Z to study the E1 strenght as a fucnction of isospin. For increasingly neutron-rich isotopes, a fragmentation of the E1 strength is expected with considerable strength shifted to unusually low excitation energies.
The LAND detector has also been involved to study collective phenomena in central collisionsof two Au nuclei at 400-800 MeV/u in collaboration with the FOPI detector. The forward wall of the latter was used to reconstruct the reaction plane from the momentum distribution of light charged particles, and LAND was used for the first time to observe the "squeezeout" of neutrons perpendicular to this reaction plane. The squeezeout signal is found to rise strongly with the transverse momentum of the neutrons. This dependence of the azimuthal anisotropy follows a universal curve independent of beam energy if the neutron momenta are measured in fractions of the projectil momentum per mass unit. Also the kinetic-energy spectra of mid-rapidity neutrons scale with the kinetic energy of the projectile. This is in agreement with predictions from non-viscous fluid dynamics and provides a very general constraint to microscopic models of nucleus-nucleus collisions at relativistic energies.