Heavy Target Nuclei Research Articles

A microscopic calculation of fragment formation in nucleus-nucleus collisions

A microscopic model using effective and free nucleon-nucleon scattering cross sections is used to calculate the yields of projectile-like fragments from nucleus-nucleus collisions from 20 MeV/ A to 2 GeV/ A. Good agreement with reaction cross-section and fragment cross-section measurements is obtained. The enhanced yields of neutron-rich fragments observed experimentally at low beam energies from collisions of projectiles with heavy targets are reproduced somewhat better by the inclusion of a neutron-rich surface on the heavy-target nuclei. Each fragment mass is produced in a strongly localized region of the distance of closest approach between the colliding nuclei; lighter fragments come from small distances and the heavier ones from more peripheral collisions.

Cryring - A Small Storage and Acceleration Ring for Heavy Ions

from a 400 kV accelerator * Injectors for negative (hydrogen) ions and electrons [intended for cooling as well as experiments) are included in the project. Regarding the extraction of accelerated heavy ions for nuclear and atomic sFectroscopy, it should be emphasized that the project aims at very heavy ions (Xe-L') which will be energetic enough 10 overcome the Coulomb barrier of heavy target nuclei. The different injectors can be separately used for experiments when not in operation for CRYRINS. It should also be emphasized that since CRYSIS is a pulsed ion source, beam-sharing will be simplified, making it possible to run two or perhaps even three different experiments simultaneously.

Search for anomalous fragments produced in collisions with heavy target nuclei and in delta Z = 1 peripheral interactions.

We exposed a stack of CR39 track detectors containing Ag foils to a 1.7-GeV/nucleon 56Fe beam and investigated the anomalous mean-free-path effect. Neither the whole set of 7517 nor a subset of 2542 interacting fragments produced probably in the Ag target show an effect. By combining the data of this and an earlier experiment we can also exclude an effect for 3219 interacting fragments produced in delta Z = 1 collisions.

Electromagnetic excitation of aligned 7Li nuclei.

Electric ground-state properties of $^{7}\mathrm{Li}$ have been measured in a consistent way by observation of the Coulomb interaction of aligned $^{7}\mathrm{Li}$ ions with heavy target nuclei. Besides the quadrupole moment (${Q}_{s}$=-3.70\ifmmode\pm\else\textpm\fi{}0.08 ${e}^{2}$ ${\mathrm{fm}}^{2}$) and the B(E2,${(3/2)}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${(1/2)}^{\mathrm{\ensuremath{-}}}$) value (8.3\ifmmode\pm\else\textpm\fi{}0.5 ${e}^{2}$ ${\mathrm{fm}}^{4}$), the tensor moments of the nuclear polarizability ${\ensuremath{\tau}}_{12}$ and ${\ensuremath{\tau}}_{11}$ were determined (${\ensuremath{\tau}}_{12}$=${\ensuremath{\tau}}_{11}$=0.23\ifmmode\pm\else\textpm\fi{}0.06 ${\mathrm{fm}}^{3}$). Present theoretical investigations on the structure of $^{7}\mathrm{Li}$ reveal difficulties in providing a unified description of all four properties.

Monte Carlo Calculations on Nuclear Spallation Reactions with High Energy Protons above Several GeV

Monte Carlo calculations on nuclear spallation reactions with high energy protons were performed by using a model for intranuclear cascade reactions in the energy range up to 1000 GeV and a nuclear model for the evaporation process. The scale factor of the characteristic particle production distance δ and the nuclear density reduction factor D were used in the cascade model in order to simulate the energy dependence of particle production from heavy target nuclei and mass-yield distributions for spallation products in the incident proton energy range above several GeV. The general characteristics of calculated mass-yield curves were determined primarily by the two parameters δ and D and they are in reasonable agreement with experimental data.

Multifragmentation of Big Nuclei

Heavy target nuclei split into many fragments when bombarded by protons or heavy ions at very high energies (Ep > 10 GeV). A statistical model is presented, which calculates the relative phase-space available for the various multifragment decay channels. It is shown that the Coulomb-interaction between the fragments is crucial. Including it by a self-consistent mean field method leads to a significant enhancement of the survival of the most heavy fragments and a strong suppression of the probability for splitting into many medium fragments. At a temperature of ∼ 5 MeV an U-shaped mass-yield curve results from the model calculations for most of the heavy targets, whereas they show a clear fission peak for U-targets. Over the whole fragment-mass region the experimental mass-yield distributions for limiting fragmentation can thus be reproduced. Further investigations and refinements of the model are needed before a final judgement about the importance of the multi-fragment phase-space relative to dynamical effects is possible.

Reactions between 12C and heavy target nuclei at 84 MeV/u

Using an experimental setup which allows the coincident investigation of the various projectile and target fragments the reactions between 84 MeV/u 12C projectiles and different targets were studied. Impact parameters have been deduced from the size and multiplicity of projectile fragments. Using a “minimum bias” triqqer relative cross sections have been determined for the different reaction channels. New results on the production of medium-heavy fragments are presented.

Nucleon charge exchange in the (π,πN) reaction at resonance energies

The semiclassical trajectory model of Benioff, which treats the nuclear structure of the target by means of the harmonic oscillator model, has been adapted to the (..pi..,..pi..N) reaction in the (3,3) resonance region. Excitation functions have been calculated for comparison with the experimental results on the following reactions: /sup 12/C(..pi../sup + -/,..pi../sup + -/n), /sup 25/Mg(..pi../sup + -/,..pi../sup + -/p), and /sup 197/Au(..pi../sup + -/,..pi../sup + -/n). The nucleon charge exchange probability P was treated as an adjustable parameter and found to decrease from approx.0.2 for light elements to < or approx. =0.05 for heavy elements. The value of P for /sup 12/C is supported by a parameter-free estimate on the basis of a modified free nucleon-nucleon scattering model. The decrease in P with increasing target A is attributed to nuclear size and structure effects. The calculated excitation functions are in good agreement with the experimental results for both light and heavy target nuclei. Experiments that would constitute a more stringent test of various aspects of the model are proposed.

Mass-yield distribution in proton and heavy ion induced target fragmentation processes

It is shown that nuclear target fragmentation in proton and heavy ion induced reactions, in particular the following experimental facts concerning the mass-yield distribution can be understood in terms of a semiclassical model:(i) its independence on the mass of the projectile at approximately the same incident energies,(ii) its trend of approaching a limit at higher bombarding energies,(iii) its “U-formed” shape at sufficiently high bombarding energies. Standard methods in statistical theory of chemical equilibrium are used to calculate the mass-yield distribution for medium and heavy target nuclei in high-energy nuclear reactions where the Coulomb interaction between the fragments is taken into account selfconsistently. The result shows: The fact that the decaying rest target nucleus and its fragments are bounded objects of finite size and finite charge have significant influences, especially on the form of the mass-yield distributions.

Propagation of a high energy heavy ion beam through matter

Abstract The propagation of high energy nuclei through water is calculated considering the energy loss and the fragmentation of the nuclei. The probabilities for the production of beam fragments in nuclear collisions are derived from proton target cross sections. For heavier target nuclei an empirical scaling factor is used. In the paper the production of fragments from monoenergetic beams of C, Ne and Ar for various absorber depths is investigated. From the energy spectra of the remaining beam particles and of the fragments depths dose curves are derived. These results are compared with experimental data.

Scattering of polarized tritons byBe9andC12

The differential cross section and the analyzing power ${A}_{y}$ for $^{9}\mathrm{Be}(\stackrel{\ensuremath{\rightarrow}}{t},t)^{9}\mathrm{Be}$ and $^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{t},t)^{12}\mathrm{C}$ elastic scattering have been measured at 15 and 17 MeV over the angular range ${\ensuremath{\theta}}_{\mathrm{lab}}=17.5\ensuremath{-}{165.0}^{\ensuremath{\circ}}$. The data in the forward hemisphere have been analyzed with a standard optical model. A large depth and conventional geometry parameters for the spin-orbit potential have been found. The results of the present analysis are in general agreement with previous work involving polarized tritons scattered from heavy target nuclei and show important differences from investigations of the scattering of polarized helions on light nuclei.[NUCLEAR REACTIONS $^{9}\mathrm{Be}(\stackrel{\ensuremath{\rightarrow}}{t},t)^{9}\mathrm{Be}$, $^{12}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{t},t)^{12}\mathrm{C}$, $E=15,17$ MeV; measured $\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}(\ensuremath{\theta})$, ${A}_{y}(\ensuremath{\theta})$; deduced optical model parameters.]

Elastic scattering of 9Be on heavy target nuclei

The elastic scattering of 9Be ions on Mg, Al, Si, Ca, Ni, Ag and Au target nuclei has been studied at 14, 20 and 26 MeV projectile energy. Excitation curves on Si were measured at four angles in the energy range of 14–20 MeV. A universal optical potential fitting all these angular distributions was found and ambiguities of the parameters are discussed. For each target the interaction distance, the strong absorption distance and the sensitive regions were calculated.

Interactions of 2 GeV/Nucleon 16O with Light and Heavy Emulsion Nuclei

Heavy ion interactions in nuclear emulsions induced by 2 GeV/nucleon 16O have been studied. The reaction cross sections determined are 337-85+56 mb, 1007-174+175 mb and 2180-231+304 mb for interactions with H, CNO and AgBr, respectively. Energy-, angular- and transverse momentum distributions of non-relativistic protons (E400 MeV) and all He nuclei have been analysed. Statistical models assuming small correlations among nucleon momenta can account for the main part of the He emission. The energy- and transverse momentum distributions of protons, emitted from light as well as heavy target nuclei indicate that intranuclear nucleon-nucleon scattering is the most important proton emission process. The angular distributions of both high energy protons and high energy He nuclei emitted from the target nucleus differ considerably in central and peripheral 16O-AgBr interactions. In order to explain the angular distributions of high energy protons and He nuclei in events with extremely small impact parameter, non-statistical phenomena, possibly hydrodynamic shock wave emission, must be considered.

Elastic 3He scattering from even Ar isotopes and backward-angle anomalies in the systematics of elastic 3He scattering

Abstract Angular distributions have been measured for 3 He elastic scattering from 36, 38, 40 Ar at 26.5 MeV and from 36 Ar and 40 Ca at energies between 24.5 and 28 MeV. This scattering clearly shows features of “anomalous” backward-angle scattering, which is discussed in the systematics of 3 He scattering from heavier target nuclei. The data for “anomalous” scattering can be described by optical potentials which show features significantly different from those of “normal” scattering. These features are smaller radius parameters for the real optical potential and a strongly reduced volume integral for the imaginary potential.

Comparison ofHe3-,He4-, andC12-induced nuclear reactions in heavy-mass targets at medium excitation energies. II. Reaction model calculations

The excitation functions for neutron emission following fusion of $^{3}\mathrm{He}$, $^{4}\mathrm{He}$, and $^{12}\mathrm{C}$ ions with a variety of heavy target nuclei, as determined in the previous communication, are compared to the statistical, exciton, and hybrid reaction model predictions. An initial exciton number of 5 is determined for $\ensuremath{\alpha}$-particle-induced reactions by the simple exciton model. The initial exciton number of 7 is consistent with the $^{3}\mathrm{He}$-induced reactions by the exciton model. The hybrid-model predictions are found to be consistent with the $^{3}\mathrm{He}$- and $^{4}\mathrm{He}$-induced reactions using the initial exciton numbers of 3 and 4, respectively. The initial pre-equilibrium exciton states are assumed to be $2p\ensuremath{-}1n$ and $2p\ensuremath{-}2n$ pure particle states, respectively. The models permitting pre-equilibrium particle emission are not consistent with the data on $^{12}\mathrm{C}$-induced reactions; however, a statistical model calculation with level density parameter $a=\frac{A}{20}$ yields reasonable agreement with the cross sections. Angular momentum effects are discussed in terms of the model predictions and the isomer ratio for helion population of $^{186}\mathrm{Ir}^{g}$ and $^{186}\mathrm{Ir}^{m}$ are determined.[NUCLEAR REACTIONS $^{197}\mathrm{Au}(^{12}\mathrm{C},xn)$, $^{209}\mathrm{Bi}(^{3}\mathrm{He},xn)$, $^{209}\mathrm{Bi}(\ensuremath{\alpha},xn)$, $^{187}\mathrm{Re}(^{3}\mathrm{He},xn)$, $^{187}\mathrm{Re}(\ensuremath{\alpha},xn)$, $^{197}\mathrm{Au}(^{3}\mathrm{He},xn)$ statistical model, exciton model, hybrid model, optical model, angular momentum; predict $\ensuremath{\sigma}(E)$ for ($\mathrm{xn}$) reactions.]

Nucleus-nucleus collisions at relativistic energies

For relativistic beam nuclei up to 4He and medium heavy target nuclei, absorption cross sections and partial production cross sections are computed. Eclipse corrections are found to be essential. For C, N, and O beam nuclei, Coulomb dissociation is computed.

Energy Division inπ−-Nucleus Collisions at 1.5 GeV

The angular distributions and secondary spectra have been determined for protons and charged pions emergent from collisions of 1.5-GeV negative pions with emulsion nuclei. Expressed in terms of numbers of particles per collision, these results are compared to the predictions of the Monte Carlo intranuclear cascade, averaged over the light- and heavy-target nuclei. The charged-pion spectrum is found to be in large disagreement with that calculated. With included observations on neutral-pion emission, the fraction of primary energy carried away by pions is (16\ifmmode\pm\else\textpm\fi{}4)%, where the calculation predicts 48%. Possible reasons for this discrepancy are discussed, and a long-standing question of apparent energy imbalance in these collisions is revived.

Semiclassical treatment of heavy ion elastic scattering

A generalized semiclassical treatment for the elastic scattering of heavy ions is developed in the presence of a complex optical potential. The scattering phase shift and its derivatives with respect to the impact parameter are calculated after extending the JWKBL approximation. The results are compared with experimental data for the elastic scattering of16O from medium and heavy target nuclei and relation between the present treatment and that of the optical model and Regge pole analysis is established

Fission of U, Bi, Au, and Ag Induced by 29-GeVN14Ions

The binary fission cross sections measured with mica track detectors are threefold larger than with protons of 29 GeV. Ternary events are \ensuremath{\sim} 1% of binary. With U, Bi, and Au there is little preference for emission of fragments into the forward hemisphere. There is no indication of interactions which involve large momentum transfers to heavy target nuclei from 29-GeV $^{14}\mathrm{N}$ ions. However, with Ag targets there is evidence of significant momentum transfer.

Coulomb disintegration of 6Li

The Coulomb disintegration of 6Li in the strong Coulomb field of a heavy target nucleus into alpha and deuteron particles is studied. The total cross section is calculated asa function of the cluster overlap parameter and of the 6Li incident energy. Satisfactory results are obtained.