Projectile-target Combinations Research Articles

Radiochemical studies on charge and mass distribution in medium energy fission

Present status of work on charge and mass distribution in medium energy fission of compound nuclei produced using various target projectile combinations is reviewed. Systematics of charge distribution parameters and their dependence on excitation energy of the fissioning nucleus is presented. Variation of mass distribution with excitation energy and mass and charge of the fissioning nucleus is discussed. Qualitative explanation of the data has been sought in terms of existing theories of fission. The importance of charge and mass distribution studies in identifying the different reaction mechanisms in heavy ion induced reaction is highlighted.

Collisions of deformed nuclei: A path to the far side of the superheavy island

A detailed understanding of complete fusion cross sections in heavy-ion collisions requires a consideration of the effects of the deformation of the projectile and target. Our aim here is to show that deformation and orientation of the colliding nuclei have a very significant effect on the fusion-barrier height and on the compactness of the touching configuration. To facilitate discussions of fusion configurations of deformed nuclei, we develop a classification scheme and introduce a notation convention for these configurations. We discuss particular deformations and orientations that lead to compact touching configurations and to fusion-barrier heights that correspond to fairly low excitation energies of the compound systems. Such configurations should be the most favorable for producing superheavy elements. We analyze a few projectile-target combinations whose deformations allow favorable entrance-channel configurations and whose proton and neutron numbers lead to compound systems in a part of the superheavy region where α half-lives are calculated to be observable, that is, longer than 1 μs.

Mass resolved low-energy ion backscattering spectrometry at target-to-projectile mass ratios near unity

The basic features of mass resolved ion (back-)scattering spectrometry (MARISS) at target-to-projectile mass ratios μ near unity have been explored using a quadrupole based secondary ion mass spectrometer with a simple (biased) energy analyzer. Sputter cleaned targets of Mg, Al, Si, Ti, Nb and Mo were bombarded at near-normal incidence with isotopically pure beams of Ne +, Ar + and Kr +, at impact energies E i between 1.5 and 10 keV. Positively charged scattered ions as well as secondary ions, with final energies E f and charge states n = 1 and 2, were energy analysed by ramping the target bias. The energy spectra showed narrow binary scattering surface peaks (full width at half maximum 15–28 eV) with shoulders or tails on the low and the high-energy side (due to subsurface scattering and double or multiple scattering, respectively). The peak-to-“background” ratio was significantly different for the projectile-target combinations studied (as high as 40–80 for Ne Al,Si and Kr Nb ; only 2–4 for Ne Mg and Ar Ti ). The fractional sca E f E i ranged from as low as 1.9 × 10 −3 to 3.7 × 10 −2 (1.08 ≤ μ ≤ 1.4). Mass analysis was indispensable for removing interference with secondary i All binary scattering events appear to be associated with inelastic energy losses Q. Assuming Q = 45 eV for scattering of Ne + from Mg, Al and Si, the true scattering angle θ has been determined as a function of the scattering energy (124 ≤ θ ≤ 139°). For scattering of 3–9 keV Ar + from Ti and 10 keV Kr + from Nb, Q was found to be ∼ 90 and ∼ 250 eV, respectively. Using four different Kr isotopes the characteristic velocity for neutralization of Kr + escaping from Nb (1.5 × 10 6 cm/s) could be determined without knowing the scattering cross section. Relatively high yields of Ne 2+ were recorded in scattering from Mg, Al and Si (up to 10% of the Ne + yield), with no shoulders on the low-energy side of the single scattering peaks ( Q ≈ 120 eV). Ionization of reemitted inert gases by two-particle gas-phase interaction has also been observed.

Electron promotion in low-energy rare-gas ion scattering from solid surfaces

In order to investigate the mechanism of electronic excitation during rare-gas ion scattering from surfaces, molecular orbital energies of various diatomic collision systems have been calculated on the basis of the discrete variational X α method. Doubly-excited autoionization is caused by the highly promoted 4fσ orbital of Ne colliding with the third-row elements of the Periodic System (NaAr), with decrease of the electron transition probability for heavier target atoms. By contrast, one-electron excitation leading to reionization is basically caused by the pseudo-crossing of the molecular orbitals. This is the reason why reionization is observed experimentally for a larger variety of projectile-target combinations compared to autoionization. Reionization may not be predicted sufficiently by the conventional treatment based on the diabatic correlation diagram. It is found that the probability of reionization is suppressed if the ionic level has a bonding character due to the interaction with the target d orbitals.

Ion-induced electron emission from solid surfaces: information content of the electron number statistics

Measured electron number statistics resulting from the impact of singly and multiply charged atomic, cluster and fullerene ions on atomically clean metal (polycrystalline gold) and insulator (lithium fluoride) surfaces depend in a characteristic manner on the given projectile-target combinations and projectile impact energies. Comparison with model statistics delivers, apart from the respective total electron yields, further information on the total numbers and single-electron emission probabilities of the complete electron ensemble taking part in the emission process. For potential emission induced by slow multicharged ions on gold, the present analysis provides new insights on hollow atom formation and decay in front of the surface, in excellent agreement with recently calculated hollow atom autoionization rates.

Charged hadron distributions in pA and AA collisions at the CERN SPS

Single particle distributions of π ±, K ±, p, p and d near mid-rapidity from 450 GeV/c pA and 200 GeV/c per nucleon SA collisions are presented. Inverse slope parameters are extracted from the transverse mass spectra, and examined for indications of collective phenomena. Proton and antiproton yields are determined for different projectile-target combinations. First results from 160 GeV/c per nucleon PbPb collisions are presented.

Hadron structure and left-right asymmetry in inclusive production in single-spin hadron-hadron collisions.

Theoretical arguments and experimental facts are presented which show the following. Left-right asymmetries are expected to exist in a number of single-spin inclusive production processes. Measuring such asymmetries by using different types of projectile-target combinations, one can obtain useful information on the spin structure of hadrons in general, and determine the spin-dependent quark distribution functions in particular. Quantitative predictions for these asymmetries in various reactions are presented.

Distribution of the number of emitted electrons for MeV H+- and He2+-ion impact on metals.

The statistical distribution of the number of emitted electrons induced by MeV ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ impact on aluminum, copper, and gold targets was measured. The obtained results are very well represented by a P\'olya distribution. Based on a simplified theoretical picture the appearance of a P\'olya distribution in this context can be explained by cascade processes, which permits a quantitative estimation of the distributions in good agreement with the experiments for all investigated projectile-target combinations. The deviation from a Poisson distribution is given by the relative importance of cascade processes compared to projectile-induced processes.

Effective source sizes of low rapidity soft particle emission

Correlations of positive pions and protons measured with the Plastic Ball detector in ultrarelativistic nucleus-nucleus collisions are studied. Source parameters are extracted for various projectile-target combinations. While the proton source can be explained by geometry, the pion source shows more subtle effects, which may be related to a very large source component. A comparison with the RQMD model for 200 A GeV 32S + Au reactions reveals discrepancies for the pion source.

Cross Sections for Electron Capture in Relativistic Atomic Collisions

Cross sections for the capture of electrons from single-electron targets into bare projectiles moving at relativistic velocities are calculated using the relativistic eikonal approximation. The calculations are performed for projectile energies between 0.2 and l04 GeV/u and for projectile-target combinations that presumably are of interest for future experiments, namely Au and U colliding with C, Al, Cu, Ag, Au, and U and vice versa. State-to-state cross sections are given for initial and final 1s1/2, 2s1/2, 2p1/2 and 2p3/2 states.

Total and 2n-removal cross sections of the neutron-rich isotopes 8,9,11Li

Total interaction cross sections have been measured for 8Li on C and Pb targets, for 9Li on C, Al, Cu, Sn and Pb targets, as well as for 11Li on C, Sn and Pb targets at about 80 MeV/nucleon. In addition, 2n-removal cross sections for 11Li have been extracted. These measurements are used to determine the nuclear and the electromagnetic part of the cross sections for the different projectile-target combinations. The experimental results are compared to different model calculations. These comparisons allow one to draw conclusions on the matter density distribution of the neutron-rich lithium isotope 11Li. By comparing our data on the electromagnetic dissociation of 11Li with all the other data available in the literature, we are able to put constraints on the dipole-strength distribution in 11Li.

Complex fragment production and multifragmentation in 139La-induced reactions at 35, 40, 45 and 55 MeV/u

Complex fragment emission ( Z>3) has been studied in the reactions of 35, 40, 45 and 55 MeV/u 139La+X. Charge, angular, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sectins. Multifragment events increase with both bombarding energy and entrance-channel mass asymmetry. The excitation functions for multifragment events rise strongly with excitation energy. These excitation functions are independent of the target-projectile combination and bombarding energy suggesting, the formation of an intermediate nuclear system, whose decay properties depend mainly on its excitation energy and angular momentum.

Effects of prefission neutron emission on the fission fragment angular distributions in heavy-ion-induced fission.

Earlier calculations of fission fragment anisotropies made with the use of the standard saddle point statistical model have been corrected to include the effects of prefission neutron emission. These calculations are compared with the data reported earlier for the fissioning systems formed by projectile-target combinations of $^{10}\mathrm{B}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, and $^{19}\mathrm{F}$ on $^{232}\mathrm{Th}$ and $^{237}\mathrm{Np}$ and $^{9}\mathrm{Be}$ on $^{232}\mathrm{Th}$. It is seen that for these systems the conclusions reached earlier regarding the entrance channel dependence of the fission fragment anisotropy as a function of mass (charge) asymmetry are not affected by the inclusion of corrections due to prefission neutron emission.

Double-λ hypernuclear production in high-energy heavy-ion collisions

The production of double-λ hypernuclei was studied regarding three different mechanisms: (1) coalescence of a nuclear fragment from peripheral collisions and two A particles, (2) coalescence of two A particles, neutrons and protons due to central collisions, and (3) conversion of a ξ hypernucleus. The production probability is estimated for several projectile-target combinations at 5 and 14.5 GeV/nucleon.

Can the nuclear equation of state be measured in heavy-ion reactions at SIS energies?

The quantum molecular dynamics (QMD) approach is so far the only theory which can predict the transverse momentum transfer in high-energy heavy-ion collisions quantitatively. Boltzmann-Uehling-Uhlenbeck (BUU) type calculations overpredict the transverse momentum transfer by at least 50% for heavy projectile-target combinations at energies around 800 MeV/nucleon. With an improved version of the QMD, which conserves energy better than 1%, we investigate the consequences of this small momentum transfer especially the time evolution of the high-density zone created in central reactions Au(800 MeV/nucleon) + Au. We find that on the average the potential energy is lowered and only the most central nucleons feel a compressional energy of about 15 MeV/nucleon. The “bounce-off”, i.e. the transverse momentum transfer is not due to the release of compressional energy but caused to avoid compression and hence determined by the density gradient. This explains why BUU calculations are bound to overpredict the magnitude. Thus the transverse momentum transfer is only a quite indirect measure for the nuclear equation of state. The small compressional energy raises the question of whether the present accelerators are too low in energy to determine the nuclear equation of state.

Bethe-Bloch stopping power parameters for He, Ne, Ar, Kr, and Xe

Recent accurate measurements of the stopping powers of He, Ne, Ar, Kr, and Xe for protons and alpha particles in the respective energy intervals of 0.7–2.1 MeV and 1.0–3.0 MeV have been analyzed in the context of Bethe-Bloch theory modified by inclusion of Barkas-effect and Bloch terms. Values of various parameters of the theory were extracted through two- and three-parameter fits to the data. Results were compared with those of an earlier review for all such experiments with these projectile-target combinations.

Pion correlations in 1.8A GeV Ar on KCl and La and 1.2A GeV Xe on La.

Results are presented for pion interferometry measurements of 1.8A GeV Ar+KCl and Ar+La, and 1.2A GeV Xe + La at the Lawrence Berkeley Laboratory Heavy Ion Spectrometer System. The parameters R, \ensuremath{\tau}, \ensuremath{\lambda}, ${\mathit{R}}_{\mathrm{\ensuremath{\perp}}}$, and ${\mathit{R}}_{\mathrm{\ensuremath{\parallel}}}$ are presented for all three projectile-target combinations. The correlation between the extracted size of the pion source and the centrality of the collision is investigated as well as the freeze-out densities and the dependence of the source size on the mean momentum of the pion pairs. The experimental setup and analysis are discussed and comparisons made with the results of others. The phase space covered is at forward angles in the center-of-mass system.

Mass and charge release by the evaporation of particles from compound nuclei around mass 60

Reaction product cross sections have been measured for reactions of4He projectile with54,56Fe,58Ni and59Co targets in the bombarding energy range 14–27 MeV. The cross-section-weighted average charge\(\overline {\Delta Z}\) and mass\(\overline {\Delta A}\) removed from the compound system were deduced and compared with results reported for heavy-ion induced reactions. Variation of\(\overline {\Delta A}\) with E CN * is independent of the initial angular momentum of compound nucleus in theA=60 mass region in contrary to the rare-earth region. This would suggest a strong angular momentum removal from the compound nucleus by evaporation of particles forA≈60 nuclei. The average evaporated charge\(\overline {\Delta Z}\) depends on both the CN excitation energy E CN * and the compound nucleusN andZ numbers. The\(\overline {\Delta A}\) and\(\overline {\Delta Z}\) dependence on excitation energy and the projectile-target combination can be understood in terms of the statistical model calculations.

Dynamics of fission and quasi-fission revealed by pre-scission neutron evaporation

The dependence of pre-scission neutron multiplicities (ν pre) on the mass-split and total kinetic energy in fusion-fission and quasi-fission has been measured for a wide range of projectile-target combinations. The data indicate that the fusion-fission time scale is shorter for asymmetric splits than for symmetric splits, whilst there is no dependence on TKE. For quasi-fission reactions induced using 64Ni projectiles, ν pre falls rapidly with increasing TKE, indicating that these neutrons are emitted near to or after scission. A new interpretation of both neutron multiplicities and mean energies (the neutron clock-thermometer) allows the extraction of time scales with much less uncertainty than previously, and also gives information about the deformation from which the neutrons are emitted.

Pion production in nuclear collisions

The production of pions in nuclear collisions is analyzed in terms of a microscopic reaction model, where the free energy of the entrance channel is transferred to the final channel through the coupling of the relative motion to the internal excitation of N*-resonances. Such a model allows quite naturally for a consistent analysis of the exclusive and the inclusive production of pions. It turns out that the pionic fusion cross section is determined predominantly by the spectroscopic parentage between the initial target projectile combination and the final nucleus, whereas the inclusive part is dominated by the energy dependence of the phase space factor, i.e. by the number of the degrees of freedom which are relevant in a given kinematical situation. This model is applied to the analysis of available experimental data from threshold to the Δ(1232)-region. A consistent analysis of both the inclusive and the exclusive part of the pion spectrum is presented.