Inclusive projectile fragmentation in the spectator model

Grazing-angle singles spectra for projectile fragments from nuclear collisions exhibit a broad peak centered near the beam velocity, suggesting that these observed fragments play only a “spectator” role in the reaction. Using only this spectator assumption (but not DWBA), we find that a “prior form” formulation of the reaction leads, via closure, to a 〈ψ∥W∥ψ〉-type estimate of the inclusive spectator spectrum, thus relating it to the reaction cross section for the “participant” with the target. We show explicitly that this expression includes an improved multi-channel version of the Udagawa-Tamura formula for the “breakup-fusion” or incomplete-fusion cross section, and identifies it as the fluctuation part of the participant-target reaction cross section. A Glauber-type estimate of the distorted wave functions which enter clearly shows how the width of the peak in the spectator spectrum arises from the “Fermi motion” within the projectile, as in the simpler Serber model, but is modified by the “overlap geometry” of the collision.     

Isospin effects of projectile fragmentation in a Boltzmann-Langevin approach

The iso spin effects of projectile fragmentation at intermediate energies are investigated using an iso spindependent Boltzmann-Langevin model.The collisions of mass-symmetric reactions including ~(58)Fe,~(58)Ni+~(58)Fe,and~(58)Ni at intermediate energies,in the 30 to 100 MeV/A range,are studied for different symmetry energies.Yield ratios of the isotopic,isobaric,and isotonic pairs of fragments from the intermediate-mass region using three symmetry energies are extracted as functions of the N/Z ratio of the composite systems in the entrance channel and the incident energies.It is found that the yield ratios are sensitive to symmetry energies,especially for neutron-rich systems,and the calculations using soft symmetry energy are closer to the experimental data.The iso spin effect is stronger for the soft symmetry energy,owing to the competition of the repulsive Coulomb force and the symmetry energy attractive force on the proton.For the first time,the splits are presented,revealing a transition from the iso spin equilibrium at lower energies to translucency at intermediate energies.The results show a degree of transparency in that intermediate mass fragments undergo a transition from dependence on the composite systems in the entrance channel to reliance on the projectile and target nuclei.     

Manipulating atomic fragmentation processes by controlling the projectile coherence

We have measured the scattering angle dependence of cross sections for ionization in p+H2 collisions for a fixed projectile energy loss. Depending on the projectile coherence, interference due to indistinguishable diffraction of the projectile from the two atomic centers was either present or absent in the data. This shows that, due to the fundamentals of quantum mechanics, the preparation of the beam must be included in theoretical calculations. The results have far-reaching implications on formal atomic scattering theory because this critical aspect has been overlooked for several decades.     

Polarization in projectile fragmentation andg-factor measurements for neutron-rich nuclei

A method to produce beams of polarized unstable nuclei was developed, which fully exploits the advantageous features of the projectile fragmentation reaction. The method was applied tog-factor measurements of several neutron-rich nuclei. We present experimental results so far obtained and discuss capabilities and limitations the present polarization method has.     

Isospin dependence of projectile fragmentation and neutron-skin thickness of neutron-rich nuclei

By investigating the cross section distributions of fragments produced in the 140 A MeV ^40,48Ca+⁹Be and 1 A GeV ^124,136Xe+Pb reactions, the isospin dependence of projectile fragmentation in fragment production is studied. In the framework of the statistical abrasion-ablation model, the 1 A GeV ¹³⁶Xe+²⁰⁸Pb reaction is calculated. By adjusting the diffuseness parameter in neutron density distribution of ¹³⁶Xe, we find the isospin dependence of projectile fragmentation in fragment production is sensitive to the neutron-skin thickness of the projectile nucleus.     

Disappearance of isospin effect in projectile fragmentation at intermediate energy

The 140~MeV/u ^40,48Ca+^9Be and ^58,64Ni+^9Be reactions are simulated by the statistical abrasion ablation model, and the simulation results are compared to the National Superconducting Cyclotron Laboratory (NSCL) experimental data. By comparing the fragment isotopic distributions of ^40,48Ca and ^58,64Ni, we study the isospin effect in the projectile fragmentation induced by the neutron-rich nuclei at intermediate energy experimentally and theoretically. It is found that the isospin effect in projectile fragmentation decreases and even disappears as the violence of the collision increases.     

Nuclear fragmentation degree in the production of light projectile fragments

The projectile-target multiplicity correlations at the lowest (3.7 A GeV) and highest (200 A GeV) available energies in Sulfur-emulsion interactions are investigated in the framework of the participant-spectator model. We examined the light projectile spectator (singly, N_z=1 and doubly, N_z=2 charged projectile fragments). The measured average multiplicities of N_z=1 and N_z=2 at both incident energies showed a decrease with increasing the fragment charge, as well as the average values within experimental errors showed independence on the incident energy. The multiplicity measurements of different emitted secondary charged particles (produced particles, shower tracks, and target fragments, gray and black tracks) in the interactions of ³²S-Em at 3.7 A and 200 A GeV were carried out. However, the correlation between the mean multiplicities of secondary charged particles and both of the singly and doubly charged projectile fragments were investigated to shed some light on the interface of spectator and participant regions, and on the possible influences of the phenomena from the two regions on the collision dynamics.     

On the projectile fragmentation in heavy-ion reactions at intermediate energies

The two-body fragmentation of the projectile in the target field is described within a three-body dynamic model. Four types of reaction emerge: fusion at low energies and low impact parameters b, elastic and inelastic reactions at high b and two types of fragmentation in peripheral collisions. For high b the two projectile parts are emitted while for low b one fragment fuses with the target and only the other one can be detected. In the first case the part which has grazed the target has been strongly slackened and has a velocity of 0.7 V beam for small emission angles to 0.5 V beam for large angles. It might be a contribution to the relaxed fragment events detected at intermediate angles. In agreement with experimental data, the b² window for this kind of fragmentation is very narrow for heavy quasi-projectiles and widens with decreasing masses. This model allows to reproduce semi-quantitatively the mass distribution of the quasi-projectiles, the position of the maximum in the one- and two-peak energy spectra and the relative importance of the two types of fragmentation.     

Direct and non-direct contributions to projectile fragmentation at 20 MeV/nucleon

The spectrum of projectile-like fragments in the scattering of ¹⁶O on ²⁰⁸Pb at 315 MeV is analyzed in terms of direct and non-direct contributions. The direct contribution comes from a one-step breakup process whereas the non-direct contributions consist of two terms: inelastic scattering following breakup and complex multi-step processes.     

Precision Penning trap mass measurements of rare isotopes produced by projectile fragmentation

The low-energy beam and ion trap facility LEBIT at NSCL/MSU is at present the only facility where precision experiments are performed with stopped rare isotope beams produced by fast-beam fragmentation. LEBIT combines high-pressure-gas stopping with advanced ion manipulation techniques to provide brilliant low-energy beams. So far these beams have mainly been used for mass measurements on short-lived rare isotopes with a 9.4T Penning trap mass spectrometer. Recent examples include ^70m Br , located at the proton dripline, ³²Si and the iron isotopes ^63-65Fe . While the measurement of ³²Si helps to solve a long-standing dispute over the validity of the isobaric multiplet mass equation (IMME) for the A = 32 , T = 2 multiplet, the mass measurements of ^65m,g Fe marked the first time a nuclear isomeric state has been discovered by Penning trap mass spectrometry.     

Strange baryons production and projectile fragmentation in p−A collisions at the AGS

Experiment 910 (E910) has studied proton-nucleus collisions at 12.3 and 17.5 GeV incident energies using a variety of targets. Centrality selection is provided via “grey” track multiplicity which can be statistically related to ν, the number of scatterings of the proton in the target nucleus. Previously published semi-inclusive measurements of Ν production are discussed and preliminary measurements of “leading” Ν production in 17.5 GeV p-Au collisions are presented as a function of ν. The data indicate that the probability for the projectile to fragment into a strange baryon increases significantly with increasing ν for ν≤3; an extrapolation of the E910 data to nucleus-nucleus collisions can reproduce most of the enhancement in Ν production observed in Pb-Pb collisions at 40, 80 and 160 A. GeV. Preliminary measurements of Ξ^? production in 17.5 GeV p-Au collisions show a rapid increase in yield with increasing ν. The data are discussed in the context of the Van Hove fragmentation model. Preliminary proton and pion spectra are presented for 12.3 GeV p-Be collisions. The data show a rapid degradation of outgoing proton momenta with increasing N _grey while only modest changes are observed in the pion fragmentation spectra.     

Hyperon decays and quark-quark correlations

The nonleptonic weak Hamiltonian strongly favours the annihilation and creation of correlated quark pairs in spin zero and colour antitriplet states. It is demonstrated that bothS- andP-wave nonleptonic hyperon decays are well described by this mechanism if supplemented by factorization contributions. Apart from the use of mass values and the -nucleon coupling constant, the results are obtained in a parameter free way. Since the diquark properties applied are identical to the ones used in the calculation ofK2 decays and the long range contribution to theK _L –K _S mass difference, a coherent physical picture of dominant nonperturbative effects in the low energy domain is reached.Supported in part by the Bundersministerium für Forschung und Technologie, Bonn, FRG     

Hyperon production at COSY-TOF

The strangeness production program at the COSY-TOF experiment is discussed. The apparatus is shown emphasizing the technique to measure delayed decays. Results obtained for the reactions pp→K ⁺ Λp and pp→K ⁰ Σ ⁺ p are discussed.     

Hyperon magnetic moments in QCD

Hyperon magnetic moments are calculated in a model-free way using the QCD sum rules. The results are in agreement with experiment.