Low-energy Heavy-ion Reactions Research Articles

Medically relevant radioisotope production through low energy heavy ion reactions

Medically relevant radioisotope production through low energy heavy ion reactions

Development of a low-pressure Multi-Mesh THGEM detector for fission experiments at FRIB

A small-area imaging detector prototype for position measurement of fission fragments produced in low energy heavy-ion reactions is presented in this study. The detector readout is equipped with a 2-dimensional, position-sensitive gaseous avalanche readout based on the novel Multi-Mesh Thick-GEM (MM-THGEM), and is coupled to a delay-line board for particle localization. The prototype has an effective area of 10 × 10 cm2 and is operated in isobutane at low pressure (7–10 Torr). We present and discuss a series of systematic evaluation tests performed by irradiating the detector with α-particles and fission fragments emitted by small-rate sources (241Am, 249Cf and 252Cf). Position and time resolutions of about 0.42 mm and 1 ns were achieved, respectively. This work serves as a benchmark for the development of a large-scale array of detectors for experiments with fission and fission-like reaction products at the Facility for Rare Isotope Beams (FRIB).

Investigation of sCVD diamond detectors for low energy heavy-ion reactions

The performance characteristics of single-crystalline diamond detectors at varying rates of heavy-ions below 50 MeV/u have been investigated. When tested with a 228Th source, the energy resolution of these detectors is better than 1.0% FWHM for 8.8 MeV α-particles. With a slight reduction in the energy resolution, simultaneous timing resolution of 314 ps has been achieved. Isotopic resolution has been achieved up to magnesium using a diamond telescope at low rates with a 20 MeV/u 20Ne + 12C reaction. At moderate rates of direct 7.5 MeV/u 78Kr, polarization effects are observed, though this effect can be diminished by increasing the applied voltage on the detector up to a limit. After an accumulated dose of 6.9 × 108 direct 78Kr particles transmitting through the detector, a degradation in the energy resolution of the 8.8 MeV peak from 1.6% to 5.3% is observed, indicating that permanent damage has been done to the detector.

Semi-classical approaches to heavy-ion reactions: fusion, rainbow, and glory

A semi-classical approximation has been a powerful tool in understanding the dynamics of low-energy heavy-ion reactions. Here we discuss two topics in this regard, for which Mahir Hussein was a world leading pioneer. The first topic is heavy-ion fusion reactions of neutron-rich nuclei, in which the breakup process of the projectile nucleus plays a crucial role. The second is rainbow and glory scattering, for which characteristic oscillatory patterns in differential cross sections can be well understood in terms of intereferences among several semi-classical trajectories.

Kinetic-energy dissipation and fluctuations in strongly damped heavy-ion collisions within the stochastic mean-field approach

Background: Microscopic mean-field approaches have been successful in describing the most probable reaction outcomes in low-energy heavy-ion reactions. However, those approaches are known to severely underestimate dispersions of observables around the average values that has limited their applicability. Recently it has been shown that a quantal transport approach based on the stochastic mean-field (SMF) theory significantly improves the description, while its application has been limited so far to fragment mass and charge dispersions. Purpose: In this work, we extend the quantal transport approach based on the SMF theory for relative kinetic energy dissipation and angular momentum transfer in low-energy heavy-ion reactions. Results: As the first application of the proposed formalism, we consider the radial linear momentum dispersion, neglecting the coupling between radial and angular momenta. We analyze the total kinetic energy (TKE) distribution of binary reaction products in the $^{136}$Xe+$^{208}$Pb reaction at $E_\mathrm{c.m.}=526$ MeV and compare with experimental data. From time evolution of single-particle orbitals in TDHF, the radial diffusion coefficient is computed on a microscopic basis, while a phenomenological treatment is introduced for the radial friction coefficient. By solving the quantal diffusion equation for the radial linear momentum, the dispersion of the radial linear momentum is obtained, from which one can construct the TKE distribution. We find that the calculations provide a good description of the TKE distribution for large values of energy losses, TKEL $\gtrsim$ 150 MeV. However, the calculations underestimate the TKE distribution for smaller energy losses. Further studies are needed to improve the technical details of calculations. (Shortened due to the word limit)

Preequilibrium strength in light heavy-ion induced reactions up to 7 MeV/nucleon

The crux of this article is to analyze the contribution of the preequilibrium reaction process to the production of residues in the low energy heavy-ion reactions. The production cross-sections of a few radionuclides produced through the 3n-emission channel have been measured from the 7Li-, 11B- and 12C-induced reactions in the 89Y and 93Nb targets within the energy range of ∼2.4–6.4 MeV/nucleon. The measured excitation functions have been analyzed by comparing them with the statistical nuclear reaction models, based on the equilibrium and preequilibrium reaction mechanisms, in the framework of ALICE91 and EMPIRE3.2.2. The result shows that the mixing of equilibrium and preequilibrium emission of light or cluster particles is essential for the better reproduction of the measured cross-section data. Variation of the preequilibrium fractions with derived energy parameters has also been demonstrated.

The Symmetry Energy of the Nuclear EoS: A Study of Collective Motion and Low-Energy Reaction Dynamics in Semiclassical Approaches

In the framework of mean-field based transport approaches, we discuss recent results concerning collective motion and low-energy heavy ion reactions involving neutron-rich systems. We focus on aspects which are particularly sensitive to the isovector terms of the nuclear effective interaction and the corresponding symmetry energy. As far as collective excitations are concerned, we discuss the mixed nature of dipole oscillations in neutron-rich systems. On the other hand, for reactions close to the Coulomb barrier, we investigate the structure of pre-equilibrium collective dipole oscillations, focusing on their sensitivity to the symmetry energy behavior below normal density. Nucleon emission is also considered within the same context. The possible impact of other relevant terms of the nuclear effective interaction on these mechanisms is also examined. From this analysis we expect to put further constraints on the nuclear Equation of State, of crucial importance also in the astrophysical context.

Signatures of higher-multipole deformations and non-coplanarity as essential, additional degrees-of-freedom in heavy-ion reactions

Eloquent display of signatures of including higher-multipole (octupole and hexadecupole) deformations βλi, i=1,2,λ=2–4, and non-coplanar degrees-of-freedom Φ are shown here for the first time as essential, additional variables in low-energy heavy-ion reactions (HIR), within the dynamical cluster-decay model (DCM) of the collective clusterization process. In other words, a description in terms of quadrupole-deformed, “optimally” oriented coplanar nuclei is not sufficient, which therefore calls for both higher-multipole deformations (β3i and β4i; i=1,2) and non-coplanarity (Φ≠0∘) as the additional degrees-of-freedom. Such a convincing, real result appears in terms of the non-compound-nucleus (nCN) effect, the nCN cross section σnCN-content in σfusion, which is different for different compound nuclei (CN), i.e., CN specific, varying from an almost complete impure to pure CN decay while going successively from (β2i, θ2iopt, Φ=0∘) to (β2i-β4i, “compact” oriented θci, Φ≠0∘), which is further independent of the radioactive or nonradioactive nature of CN formed in different entrance channels, i.e., also independent of entrance channel target-projectile nuclei. This study plus our earlier study of spontaneous C14-cluster radioactivity provide a new set of variables, in terms of higher-multipole deformations and non-coplanarity degrees-of-freedom, to explore the real outcome of any low-energy HIR (plus spontaneous decay), which could in fact be already close to Nature.

Enhanced nucleon transfer in tip collisions of U238+Sn124

Multinucleon transfer processes in low-energy heavy ion reactions have attracted increasing interest in recent years aiming at production of new neutron-rich isotopes. Clearly, it is an imperative task to further develop understanding of underlying reaction mechanisms to lead experiments to success. In this paper, from systematic time-dependent Hartree-Fock calculations for the $^{238}$U+$^{124}$Sn reaction, it is demonstrated that transfer dynamics depend strongly on the orientations of $^{238}$U, quantum shells, and collision energies. Two important conclusions are obtained: (i) Experimentally observed many-proton transfer from $^{238}$U to $^{124}$Sn can be explained by a multinucleon transfer mechanism governed by enhanced neck evolution in tip collisions; (ii) Novel reaction dynamics are observed in tip collisions at energies substantially above the Coulomb barrier, where a number of nucleons are transferred from $^{124}$Sn to $^{238}$U, producing transuranium nuclei as primary reaction products, that could be a means to synthesize superheavy nuclei. Both results indicate the importance of the neck (shape) evolution dynamics, which are sensitive to the orientations, shell effects and collision energies, for exploring possible pathways to produce new unstable nuclei.

Dynamical deformation in heavy ion reactions and the characteristics of quasifission products

The investigation of the characteristics of low-energy heavy ion reactions covering both fusion and quasifission is carried out within the dinuclear system (DNS) concept, which is developed to include the deformation variables of fragments in addition to the mass numbers of the fragments, so that the energy dissipation, nucleon exchange, and deformation evolutions of the colliding nuclei as well as their correlations are treated simultaneously, and the potential energy surface of the system is thus reaction-time dependent. The direct consequence of introducing the deformation of fragments as dynamical variables is that one must treat the orientation between the two deformed nuclei. This is solved by introducing a barrier function. It is found that the model can reproduce data about the mass, as well as the total kinetic energy and its dispersion, of the reaction products very well, revealing that the DNS model has a reasonable theoretical foundation and thus can reliably describe the reaction mechanism.

Microscopic description of production cross sections including deexcitation effects

Background: At the forefront of the nuclear science, production of new neutron-rich isotopes is continuously pursued at accelerator laboratories all over the world. To explore the currently-unknown territories in the nuclear chart far away from the stability, reliable theoretical predictions are inevitable. Purpose: To provide a reliable prediction of production cross sections taking into account secondary deexcitation processes, both particle evaporation and fission, a new method called TDHF+GEMINI is proposed, which combines the microscopic time-dependent Hartree-Fock (TDHF) theory with a sophisticated statistical compound-nucleus deexcitation model, GEMINI++. Results: The method is applied for multinucleon transfer processes in low-energy heavy ion reactions. It is shown that the inclusion of secondary deexcitation processes, which are dominated by neutron evaporation in the present systems, substantially improves agreement with the experimental data. The magnitude of the evaporation effects is very similar to the one observed in GRAZING calculations. TDHF+GEMINI provides better description of the absolute value of the cross sections for channels involving transfer of more than one protons, compared with the GRAZING results. However, there remain discrepancies between the measurements and the calculated cross sections, indicating a limit of the theoretical framework that works with a single mean-field potential. Possible causes of the discrepancies are discussed. Conclusions: In order to perfectly reproduce experimental cross sections for multinucleon transfer processes, one should go beyond the standard self-consistent mean-field description. Nevertheless, the proposed method will provide valuable information to optimize production mechanisms of new neutron-rich nuclei through its microscopic, non-empirical predictions. (Shortened due to the word limit)

Solitonic excitations in collisions of superfluid nuclei a qualitatively new phenomenon distinct from the Josephson effect

Recently, we have reported a novel role of pairing in low-energy heavy ion reactions at energies above the Coulomb barrier, which may have a detectable impact on reaction outcomes, such as the kinetic energy of fragments and the fusion cross section [arXiv:1611.10261, arXiv:1702.00069]. The phenomenon mimics the one studied experimentally with ultracold atomic gases, where two clouds of fermionic superfluids with different phases of the pairing fields are forced to merge, inducing various excitation modes of the pairing field. Although it originates from the phase difference of the pairing fields, the physics behind it is markedly different from the so-called Josephson effect. In this short contribution, we will briefly outline the results discussed in our recent papers and explain relations with the field of ultracold atomic gases.

Coupled Channel and Optical Model Approaches to Low-Energy Heavy-Ion Reactions

Coupled Channel and Optical Model Approaches to Low-Energy Heavy-Ion Reactions

Observables and open problems for NICA

The restoration of chiral symmetry in hot dense nuclear systems in competition with a transition to deconfined matter in central nucleus-nucleus collisions at NICA energies is a central problem of nuclear physics. To explore these transitions we study the production of hadrons in nucleus-nucleus collisions from 4 to 160A GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach that is extended to incorporate essentials aspects of chiral-symmetry restoration (CSR) in the hadronic sector (via the Schwinger mechanism) on top of the deconfinement phase transition as implemented in PHSD. The modeling of chiral-symmetry restoration in PHSD is driven by the pion-nucleon $\Sigma$ -term in the computation of the quark scalar condensate $\langle q \bar{q} \rangle$ that serves as an order parameter for CSR and is assumed to scale with the effective quark masses $ m_{s}$ and $ m_{q}$ . Furthermore, the nucleon scalar density $\rho_{s}$ , which also enters the computation of $\langle q \bar{q} \rangle$ , is evaluated within the nonlinear $\sigma$ - $ \omega$ model which is constrained by Dirac-Brueckner calculations and low-energy heavy-ion reactions. The essential impact of CSR is found in the Schwinger mechanism (for string decay) which fixes the ratio of strange to light quark production in the hadronic medium. We find that above $\sim 80$ A GeV the reaction dynamics of heavy nuclei is dominantly driven by partonic degrees-of-freedom such that traces of the chiral-symmetry restoration are hard to identify. Our studies support the conjecture of “quarkyonic matter” in heavy-ion collisions from about 5 to 40A GeV and suggest a microscopic explanation for the maximum in the $ K^{+}/\pi^{+}$ ratio at about 30A GeV which only shows up if in addition to CSR a deconfinement transition to partonic degrees-of-freedom is incorporated in the reaction dynamics.

Chiral symmetry restoration versus deconfinement in heavy-ion collisions at high baryon density

We study the production of strange hadrons in nucleus-nucleus collisions from 4 to 160 A GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach that is extended to incorporate essentials aspects of chiral symmetry restoration (CSR) in the hadronic sector (via the Schwinger mechanism) on top of the deconfinement phase transition as implemented in PHSD. Especially the $K^+/\pi^+$ and the $(\Lambda+\Sigma^0)/\pi^-$ ratios in central Au+Au collisions are found to provide information on the relative importance of both transitions. The modelling of chiral symmetry restoration is driven by the pion-nucleon $\Sigma$-term in the computation of the quark scalar condensate $<q {\bar q}>$ that serves as an order parameter for CSR and also scales approximately with the effective quark masses $m_s$ and $m_q$. Furthermore, the nucleon scalar density $\rho_s$, which also enters the computation of $<q {\bar q}>$, is evaluated within the nonlinear $\sigma-\omega$ model which is constraint by Dirac-Brueckner calculations and low energy heavy-ion reactions. The Schwinger mechanism (for string decay) fixes the ratio of strange to light quark production in the hadronic medium. We find that above $\sim$80 A GeV the reaction dynamics of heavy nuclei is dominantly driven by partonic degrees-of-freedom such that traces of the chiral symmetry restoration are hard to identify. Our studies support the conjecture of 'quarkyonic matter' in heavy-ion collisions from about 5 to 40 A GeV and provide a microscopic explanation for the maximum in the $K^+/\pi^+$ ratio at about 30 A GeV which only shows up if a transition to partonic degrees-of-freedom is incorporated in the reaction dynamics and is discarded in the traditional hadron-string models.

Fission dynamics with systems of intermediate fissility

A 4π light charged particle spectrometer, called 8π LP, is in operation at the Laboratori Nazionali di Legnaro, Italy, for studying reaction mechanisms in low-energy heavy-ion reactions. Besides about 300 telescopes to detect light charged particles, the spectrometer is also equipped with an anular PPAC system to detect evaporation residues and a two-arm time-of-flight spectrometer to detect fission fragments. The spectrometer has been used in several fission dynamics studies using as a probe light charged particles in the fission and evaporation residues (ER) channels. This paper proposes a journey within some open questions about the fission dynamics and a review of the main results concerning nuclear dissipation and fission time-scale obtained from several of these studies. In particular, the advantages of using systems of intermediate fissility will be discussed.

Extracting integrated and differential cross sections in low-energy heavy-ion reactions from backscattering measurements

We suggest new methods to extract elastic (quasi-elastic) scattering angular distribution and reaction (capture) cross sections from the experimental elastic (quasi-elastic) backscattering excitation function taken at a single angle. A novel Coulomb scattering relation between angular momentum and centrifugal energy is used. The methodology is developed for addressing complementary reaction observables, improving the description of elastic differential cross section.

Analysis of fragment distribution and associated effects in 12,13C induced reactions

The decay of 220 Ra * nucleus formed in two different entrance channels 12 C +208 Pb and 13 C +207 Pb is investigated over a wide range of incident energies using the dynamical cluster decay model (DCM). The DCM is a non-statistical model used to account for the decay of hot and rotating nuclei formed in low energy heavy ion reactions. The excitation functions are calculated by considering quadrupole (β2) deformations with optimum orientations [Formula: see text] of decaying fragments. The DCM-based cross-sections for evaporation residue (ER), fusion–fission, αxn and neutron decay processes find nice agreement with the reported experimental data over wide range of incident energies. The cross-sections corresponding to different decay mechanism are worked out within DCM by fitting neck length parameter (ΔR). The entrance channel and angular momentum effects are investigated in reference to the above-mentioned reaction channels. In addition to this, the fragment mass distribution is worked out by colliding 13 C weakly bound stable projectile with a variety of target nuclei resulting in 13 C +159 Tb , 13 C +181 Ta and 13 C +207 Pb reactions. At comparable projectile energies, the increase in target mass is shown to favor asymmetric fragmentation in the fissioning region. Besides this, the incomplete fusion (ICF) contribution is worked out for 12 C and 13 C channels by applying necessary energy corrections in the framework of DCM.

Dynamics of light, intermediate, heavy and superheavy nuclear systems formed in heavy-ion collisions

The dynamical description of light, intermediate, heavy and superheavy nuclei formed in heavy-ion collisions is worked out using the dynamical cluster decay model (DCM), with reference to various effects such as deformation and orientation, temperature, angular momentum etc. Based on the quantum mechanical fragmentation theory (QMFT), DCM has been applied to understand the decay mechanism of a large number of nuclei formed in low-energy heavy-ion reactions. Various features related to the dynamics of competing decay modes of nuclear systems are explored by addressing the experimental data of a number of reactions in light, intermediate, heavy and superheavy mass regions. The DCM, being a non-statistical description for the decay of a compound nucleus, treats light particles (LPs) or equivalently evaporation residues (ERs), intermediate mass fragments (IMFs) and fission fragments on equal footing and hence, provides an alternative to the available statistical model approaches to address fusion–fission and related phenomena.

Detector system for the study of low energy heavy ion reactions using kinematic coincidence technique

The characteristics and performance of a new detector system developed for the study of low energy heavy ion binary reactions using the kinematic coincidence technique are presented. The detector system has been developed to carry out experiments such as multi-nucleon transfer reactions using the General Purpose Scattering Chamber (GPSC) facility at IUAC [1,2]. The detector system consists of a pair of two-dimensional position sensitive multi wire proportional counter (MWPC) and a ΔE−E gas ionization chamber. Both MWPC have an active area of 5×5cm2, and provide position signals in horizontal (X) and vertical (Y) plane, and timing signal for time of flight measurements. The main design feature of MWPC is the reduced wire pitch of 0.025in. (0.635mm) in all electrodes, giving uniform field and faster charge collection, and usage of 10μm diameter in anode frame which gives higher gains. The position resolution of the detectors was determined to be 0.45mm FWHM and time resolution was estimated to be 400ps FWHM. The detector could handle heavy ion count rates exceeding 100kHz without any break down. The timing and position signals of the detectors are used for kinematic coincidence measurements and subsequent extraction of their mass and angular distributions. The ionization chamber has a conventional transverse field geometry with segmented anode providing multiple ΔE signals for nuclear charge (Z) identification. This article describes systematic study of these detectors in terms of efficiency, count rate handling capability, time, position and energy resolution.