Low-energy Heavy-ion Reactions Research Articles

Noncollective excitations in low-energy heavy-ion reactions: Applicability of the random-matrix model

We investigate the applicability of a random-matrix model to the description of noncollective excitations in heavy-ion reactions around the Coulomb barrier. To this end, we study fusion in the reaction ${}^{16}$O + ${}^{208}$Pb, taking account of the known noncollective excitations in the ${}^{208}$Pb nucleus. We show that the random-matrix model for the corresponding couplings reproduces reasonably well the exact calculations, obtained using empirical deformation parameters. This implies that the model may provide a powerful method for systems in which the noncollective couplings are not so well known.

Zero Sound Propagation in Femto-Scale Quantum Liquids

Charge equilibration has been recognized as a dominant process at the early stage of low-energy heavy-ion reactions. The production of exotic nuclei is suppressed under the appearance of charge equilibration, in which the proton-neutron ratios of the final reaction products are inevitably averaged. Therefore charge equilibration plays one of the most crucial roles in the synthesis of chemical elements. Focusing on how and when the charge equilibration takes place, zero sound propagation in femto-scale quantum liquids is explained.

Role of noncollective excitations in low-energy heavy-ion reactions

We investigate the effect of single-particle excitations on heavy-ion reactions at energies near the Coulomb barrier. To this end, we describe single-particle degrees of freedom with the random matrix theory and solve the coupled-channels equations for one-dimensional systems. We find that the single-particle excitations hinder the penetrability at energies above the barrier, leading to a smeared barrier distribution. This indicates that the single-particle excitations provide a promising way to explain the difference in a quasi-elastic barrier distribution recently observed in $^{20}$Ne + $^{90,92}$Zr systems.

Unusual features of proton and α-spectra from low-energy heavy-ion reactions

Proton and α-particle spectra have been measured in low-energy 12C and 16O-induced reactions on Nb and Y targets with the primary aim of measuring the excitation energy and angular momentum dependence of nuclear level density. In the γ-multiplicity gated spectra, an unusual feature of a broad structure at high particle energies is observed in all the cases. In the case of proton spectra, the structures have compound nuclear origin and point towards an excitation energy and angular momentum-dependent enhancement which is beyond the conventional level density prescription. The broad structures in the α-spectra cannot be fully explained within the statistical model even with the enhanced level density. In this case, other reaction mechanisms like the transfer of α or 8Be to the target could also be important.

ESTIMATING NEUTRON DOSE EQUIVALENT RATES FROM HEAVY ION REACTIONS AROUND 10 MeV amu−1 USING THE PHITS CODE

It has been sometimes necessary for personnel to work in areas where low-energy heavy ions interact with targets or with beam transport equipment and thereby produce significant levels of radiation. Methods to predict doses and to assist shielding design are desirable. The Particle and Heavy Ion Transport code System (PHITS) has been typically used to predict radiation levels around high-energy (above 100 MeV amu(-1)) heavy ion accelerator facilities. However, predictions by PHITS of radiation levels around low-energy (around 10 MeV amu(-1)) heavy ion facilities to our knowledge have not yet been investigated. The influence of the "switching time" in PHITS calculations of low-energy heavy ion reactions, defined as the time when the JAERI Quantum Molecular Dynamics model (JQMD) calculation stops and the Generalized Evaporation Model (GEM) calculation begins, was studied using neutron energy spectra from 6.25 MeV amu(-1) and 10 MeV amu(-1) (12)C ions and 10 MeV amu(-1) (16)O ions incident on a copper target. Using a value of 100 fm c(-1) for the switching time, calculated neutron energy spectra obtained agree well with the experimental data. PHITS was then used with the switching time of 100 fm c(-1) to simulate an experimental study by Ohnesorge et al. by calculating neutron dose equivalent rates produced by 3 MeV amu(-1) to 16 MeV amu(-1) (12)C, (14)N, (16)O, and (20)Ne beams incident on iron, nickel and copper targets. The calculated neutron dose equivalent rates agree very well with the data and follow a general pattern which appears to be insensitive to the heavy ion species but is sensitive to the target material.

Fission dynamics with the 4π detector 8πLP

A 4π light charged particle spectrometer, named 8πLP, is in operation at the Laboratori Nazionali di Legnaro (Italy) for the study of the reaction mechanisms produced in low-energy heavy-ion reactions. The spectrometer has recently been used in a study of fission dynamics that involves the detection of light charged particles in the fission and evaporation residue channel in a system of intermediate fissility, as well as in a study of multinucleon transfer to heavy target. Data on the system 240 MeV 32S + 100Mo are presented. Dynamical effects extracted as a consequence of the comparison of the data to the statistical model calculations are discussed.

A BaF2 detector array for γ rays up to ∼30 MeV

A compact array of seven hexagonal BaF2 detectors, with cosmic ray background suppression, has been set up for the measurement of high energy photons up to ∼30MeV. The array has been used in the measurement of giant dipole photons in low energy heavy-ion reactions.

Analytical calculation of fusion cross-sections

We analyse the fusion cross-sections, calculated by using two different analytical parameterisations and compare them with the experimental data. Both the parameterisations are based on ion-ion potentials calculated within the framework of Skyrme energy density formalism. In the first case, the ion-ion potential (including the spin-density term) was parameterised and then, by adding the Coulomb potential, one could compute the fusion barrier analytically. In the second case, the calculated fusion barrier heights and positions were parameterised directly. Both of these (previously) reported parameterisations are used here to calculate the fusion barriers and fusion excitation functions for more than 50 reactions belonging to the s-d and f-shell nuclei. A detailed comparison of these parametrisations with the experimental and several other theoretical results shows that both of these parameterisations are able to reproduce the experimental data equally well. As the (second) direct parameterisation depends only on the charges and masses of colliding nuclei, it is very useful for predicting/ understanding the fusion process in low energy heavy-ion reactions.

Low-energy heavy-ion reactions: a link between nuclear structure and reaction dynamics

High precision data recently obtained in the study of multinucleon transfer and subbarrier fusion reactions at LNL are presented. The studies of transfer channels in the systems 40,48 Ca+ 124 Sn and 64 Ni+ 238 U revealed important effects not identified in the past, and demonstrated the possibility of a quantitative understanding of the role played by the various degrees of freedom in the reaction mechanism. Evidence of their influence on the fusion enhancements seem to show-up in the systems 40 Ca+ 124,116 Sn and 40 Ca+ 90,96 Zr, but, in general, the data still escape a consistent treatment.

On the origin of the radial flow in low energy heavy-ion reactions

The average transverse energy of nucleons and intermediate mass fragments observed in the heavy-ion reaction Xe(50 A MeV) + Sn shows the same linear increase as a function of their mass as observed in heavy-ion collisions up to the highest energies available today and fits well into the systematics. At higher energies this observation has been interpreted as a sign of a strong radial flow in an otherwise thermalized system. Inverstigating the reaction with quantum molecular dynamics simulations we find in between 50A MeV and 200A MeV a change in the reaction mechanism. At 50A MeV the apparent radial flow is merely caused by in-plane flow and Coulomb repulsion. The average transverse fragment energy does not change in the course of the reaction and is equal to the initial fragment energy due to the Fermi motion. At 200A MeV, there are two kinds of fragments: those formed from spectator matter and those from the center of the reaction. There the transverse energy is caused by the pressure from the compressed nuclear matter. In both cases we observe a binary event structure, even in central collisions. This demonstrates as well the non-thermal character of the reaction. The actual process which leads to multifragmentation is rather complex and is discussed in detail.

Extension of quantum molecular dynamics and its application to heavy-ion collisions.

In order to treat low-energy heavy-ion reactions, we make an extension of quantum molecular dynamics method. A phenomenological Pauli potential is introduced into effective interactions to approximate the nature of the Fermion many-body system. We treat the widths of nucleon wave-packets as time-dependent dynamical variables. With these modifications, our model can well describe the ground-state properties in wide mass range. Improvements due to the extension are also obtained in the nucleus-nucleus collision calculations.

Neutron-proton bremsstrahlung from low-energy heavy-ion reactions.

High-energy photon yields from the reactions $^{12}\mathrm{C}$${+}^{112,124}$Sn at 10 MeV/nucleon have been measured. The ratio of cross sections (${\mathit{E}}_{\ensuremath{\gamma}}$\ensuremath{\ge}30 MeV)\ensuremath{\sigma}${(}^{124}$Sn)/\ensuremath{\sigma}${(}^{112}$Sn) is 1.6\ifmmode\pm\else\textpm\fi{}0.2, which is larger than expected within the equal-participant model. The elementary n-p-\ensuremath{\gamma} cross section is evaluated within the neutral scalar \ensuremath{\sigma} meson exchange model, and implemented into a BUU code. The results of the BUU calculation suggest that the high-energy \ensuremath{\gamma}-ray yield differences for the two Sn isotopes arise from the differences in nucelon phase-space distributions. The sensitivity of the high-energy photon production to the initial phase-space conditions is explored.

Giant resonances in low-energy heavy-ion reactions

The low-energy light heavy-ion fusion cross sections have been examined in view of the presence of the giant-dipole resonance (GDR) and giant-quadrupole resonance (GQR) excitations. A correlation between the resonances observed in the heavy-ion reactions and the known GDR and GQR excitations has been established for both the gross and fine structures. The ∼3 MeV wide enhancement observed in the 12C + 13C → 25Mg fusion cross section overlaps with the lower part of the 25Mg GDR, and the ∼3 MeV wide resonance observed in the 9Be + 9Be → 18O → nn+ 16O and 7Li + 11B → 18O → nn + 16O reactions overlaps with the higher part of the T < = 1 18 O GDR observed via photonuclear excitation. The fine-structure resonances observed in the 12C + 12C → 24Mg reaction are strongly correlated with those of the 24Mg GQR observed via 24Mg(α, α') inelastic scattering.

Rare heavy-ion reactions close to the Coulomb barrier

A few aspects of low-energy heavy-ion reactions are considered, namely subbarrier fusion cross section enhancements, nucleon transfer at large internuclear distances and elastic scattering. The mechanism of channel coupling links together all these processes, and is responsible for the often surprising experimental evidences of recent years.

A kinematic coincidence technique for the study of low-energy heavy-ion reactions

A setup for kinematic coincidences has been developed for the study of low-energy binary heavy-ion reactions. It consists of a position-sensitive silicon detector, and a counter telescope with microchannel plates detectors and a conventional ΔE- E ionization chamber. The solid angle is around 0.6 msr. By measuring the time of flight of one of the ions, and both correlated scattering angles, we achieve mass and Q-value resolutions of ΔA/A⋍ 1 120 FWHM and ΔQ ⋍ 900 keV , respectively, for the elastic scatteri 58Ni + 64Ni near the barrier.

Transfer energy loss probability distributions in heavy ion collisions

Transfer energy loss probability distributions in low energy heavy ion reactions have been calculated in a realistic case for the first time. They are rather large in general and so they play a crucial role in the evolution of the dissipation mechanisms. It is proved that transfer processes, in peripheral collisions, can be treated as independent channels, generating energy loss spectra which can be described with a product of poissonian distributions.

Emission times in low-energy heavy-ion reactions by particle-particle correlations.

Particle-particle correlations (p-p and ..cap alpha..-..cap alpha..) vs relative momentum (P/sub rel/) are reported (theta/sub lab/approx. =19.5/sup 0/) for the reaction 140 MeV /sup 16/O+/sup 27/Al. A plot of the Galilean invariant cross section indicates that evaporative emission dominates the gross proton production even at this forward angle. The observed anticorrelation between two protons for small values of P/sub rel/ is analyzed to give an evaporation lifetime in the range of 1--5 x 10/sup -21/ s. The ..cap alpha..-..cap alpha.. correlation is completely different; it seems to arise from preformed /sup 8/Be.

Numerical method for the calculation of continuum excitation amplitudes in time-dependent external field problems.

We introduce a new numerical method for calculating continuum excitation probabilities of complex physical systems under the influence of external, time-dependent, and nonperturbative fields. The method utilizes a discretized form of the Hamiltonian on a space lattice and is particularly suited for large scale computations involving many-body systems. We perform a comparative study for a model problem by solving the same time-dependent Schr\"odinger equation in spherical and cylindrical coordinates. As a realistic example, we apply the method to the problem of prompt nucleon emission in low energy heavy-ion reactions.

Ultradipole photons in low-energy heavy-ion reactions.

We examine the theoretical mechanisms for producing high-energy photons in the reaction ..cap alpha..+ /sup 154/Sm and /sup 3/He+/sup 148/Sm at low bombarding energies. We find that neither potential field nor nucleon-nucleon collisions can account for the measured high-energy tail, leaving the source of these photons a mystery. Further experiments are suggested to help determine the origin of the photons.

Exciton-model approach to fast-particle emission in heavy-ion collisions.

Emission of fast particles in low-energy heavy-ion reactions is calculated using the exciton model. The initial state of the excitons is fixed by pure momentum-space considerations. By defining the projectile source and the target source as independent emitters of the fast particles, we calculate the angle-energy double-differential cross section. Angular dependence is obtained by fixing the total linear momenta of the projectile source and target source separately in addition to fixing their excitation energies. A linear-momentum-dependent level density is used for this purpose. This model is applied to several heavy-ion reactions whose energy is 10--20 MeV per particle. Our calculations reproduce the data for the high-energy part of the energy spectra well. Our model is one way of understanding the mechanism of the moving-source model.