Heavy-ion Reaction Studies Research Articles

Mass identification by means of Energy-Time-of-Flight technique using large area silicon detector in a 4[formula omitted] array: The CHIMERA case

The study of heavy-ion reactions in the Fermi energy regime requires detection systems covering a high portion of the solid angle and using several identification techniques. Silicon detectors are widely used in this field. The so called E−ToF identification technique combines the energy and time of arrival of ions stopped in the detector to perform mass identification and calculate the velocity. However, when dealing with time measurements non-linear effects in detector and electronics responses have to be taken into account. We present an empirical method, as developed in the case of CHIMERA multi-detector, describing these non-linear effects and allowing for mass identification of ions stopped in the silicon detector by means of a semi-automatic procedure. Correlation between the non-linear effects affecting the ToF technique and the rise-time of the silicon detector signal will also be presented and discussed.

Unfolding reaction mechanisms in C12 fusion with Zr below 7 MeV/nucleon

Heavy-ion reaction studies in the incident energy regime $\ensuremath{\le}$ 20 MeV/nucleon have suggested multiple signatures of preequilibrium emission during the compound nucleus formation phenomenon and the presence of incomplete fusion, both of which vary with the incident energy and projectile-target combination. However, the available reaction data are insufficient to accurately decipher these contributions as the reports have a finite dependency on the predictions from theoretical models. Therefore, sound data interpretation with robust nuclear models is fundamental to unfolding the underlying processes. We report a new study of excitation functions of residues produced from the $^{12}\mathrm{C}+\mathrm{Zr}$ reactions up to 6.3 MeV/nucleon energy. Cross sections of 16 evaporation residues were measured through off-beam $\ensuremath{\gamma}$-ray spectroscopy. Excitation functions of the evaporation residues are reported at 14 energy points and compared with the reaction model code empire-3.2.2, with variations in nuclear level density. Additionally, the overall contribution of precompound and compound nuclear processes is discussed quantitatively within the realms of empire-3.2.2 model code.

Nuclear Structure and Decay Data for A=231 Isobars

The present evaluation of currently known nine nuclides of A=231: (231Rn, 233Fr, 231Ra, 231Ac, 231Th, 231Pa, 231U, 231Np and 231Pu) represents an update and revision of previous A=231 evaluation by 2013Br04. All the known decay and reaction data are evaluated, with recommended properties given in the Adopted Levels, Gammas datasets, for gamma-ray energies, photon branching ratios, level half-lives, spins, parities, transition probabilities, and configuration assignments. For 231Rn, only an identification of the nuclide has been made without an experimental determination of its half-life. For 231Fr, no excited states are known. About 30 levels in 231Ra up to 1774 keV excitation are known only from β− decay of 231Fr with level half-lives determined for five excited states. For 231Ac, excited states up to 3122 keV are known from β− decay, (t,α), and fragmentary information for high-spin levels from two heavy-ion reaction studies, with lifetimes of excited states measured for five levels. Extensive structure and rotational band information for 231Th is available for a large number of levels up to 1714 keV from β− and α decays, (n,γ) thermal and resonance, single-particle transfer reactions (d,p), (d,t) and (3He,α), with limited data for high-spin levels from one secondary reference, and half-lives available for five excited states. Very detailed structure data are available for 231Pa up to 2139 keV from β−, ε and α decays, (α,t), (d,d′) and high-spin data from heavy-ion Coulomb excitation and (p,2nγ) reaction, with level half-lives known for 34 excited states, mostly deduced by evaluators from transition probabilities determined in Coulomb excitation. Only eight excited states are known in 231U up to 1268 keV from ε and α decays, with no data for half-lives of excited states. For 231Np and 231Pu, only the ground-state is known from α decays, with a tentative level at 324 keV in 231Pu. Measurements of nuclear rms charge radius and isotope shifts for 231Ra have been made by 2018Ly01, and that for rms charge radius for 231Fr by 2014Bu06.235U is a well known nuclide for nuclear structure and reactor applications. There have been a large number of studies of its alpha decay to levels in 231Th, including some recent ones such as 2018Ma03 and 2017Le03, yet evaluators' analysis of the decay scheme suggests that, while α-particle transitions are well established, but several issues still remain about intricacies of gamma-ray and conversion electron spectroscopy which need to be resolved through dedicated experiments for a complete understanding of the decay scheme characteristics, although, this sort of research project would seem a challenging spectroscopic problem. As commented in individual datasets for other decays, important spectral information is lacking for most of the decay schemes, with almost no information available for some of these, resulting in many incomplete decay schemes in A=231 mass chain.

Stochastic Variational Method for Viscous Hydrodynamics

In this short review, we focus on some of the subjects, related to J. Cleymans’ pioneering contribution of statistical approaches to the particle production process in heavy-ion collisions. We discuss these perspectives from the effects of stochastic processes in collective variables of hydrodynamic description, which is described by a stochastic variational method. In this connection, we stress also the necessity of the inclusion of surface and quantum effects in the study of relativistic heavy-ion reactions.

A Compact Spectrometer for Heavy Ion Experiments in the Fermi energy regime

A Compact Spectrometer for Heavy IoN Experiments (CSHINE) has been recently built for the studies of heavy ion reactions at intermediate energies. CSHINE consists of a Si-Si-CsI telescope array and a parallel plate avalanche counter (PPAC) array. Each telescope consists of a thin single-sided silicon strip detector and a thick double-sided silicon strip detector backed by a 3 × 3 CsI(Tl) crystal hodoscope. The thicknesses of the silicon detectors are optimized in order to identify various types of light charged particles in a wide energy range. PPAC is a large-area two-dimensional position-sensitive gas detector, which records fission fragments with the intrinsic efficiency of approximately 100% but filters out light charged particles at properly selected working conditions. In the phase-II run, four Si-Si-CsI telescopes and three PPACs have been installed and operated in the beam experiment with 25 MeV/u Kr+Pb reactions. Particle identification for isotopes from Hydrogen to Boron has been achieved with the Si-Si-CsI telescopes, while excellent timing and position resolution have been achieved with the PPACs.

Performance results of National Array of Neutron Detectors (NAND) facility at IUAC

The 100 element neutron detector array facility, National Array of Neutron Detectors (NAND) has been installed at Inter University Accelerator Centre (IUAC), New Delhi for experiments in heavy-ion reaction studies. The detector array consists of 100 organic liquid scintillators, each of size 5”×5” coupled to 5 in. diameter photomultiplier. The liquid cells are mounted at 175 cm from the target with full array covering 3.3% of 4π solid angle. We present an overview of the design and implementation of various subsystems of the array. Description of the mechanical structure, development of detectors and nuclear electronics for signal processing, data acquisition, characterization of detectors and test results are presented. Results on the performance characteristics of the array are given based on the offline tests using radioactive sources and commissioning test by in-beam experiments. The system demonstrates good n−γ discrimination and time-of-flight performance.

Development of nuclear chemistry at Mainz and Darmstadt

Abstract This review describes some key accomplishments of Günter Herrmann such as the establishment of the TRIGA Mark II research reactor at Mainz University, the identification of a large number of very neutron-rich fission products by fast, automated chemical separations, the study of their nuclear structure by spectroscopy with modern detection techniques, and the measurement of fission yields. After getting the nuclear chemistry group, the target laboratory, and the mass separator group established at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, a number of large international collaborations were organized exploring the mechanism of deeply inelastic multi-nucleon transfer reactions in collisions of Xe and U ions with U targets, Ca and U ions with Cm targets, and the search for superheavy elements with chemical separations after these bombardments. After the Chernobyl accident, together with members of the Institute of Physics, a powerful laser technique, the resonance ionization mass spectometry (RIMS) was established for the ultra-trace detection of actinides and long-lived fission products in environmental samples. RIMS was also applied to determine with high precision the first ionization potentials of actinides all the way up to einsteinium. In the late 1980ies, high interest arose in results obtained in fusion-evaporation reactions between light projectiles and heavy actinide targets investigating the chemical properties of transactinide elements (Z≥104). Remarkable was the observation, that their chemical properties deviated from those of their lighter homologs in the Periodic Table because their valence electrons are increasingly influenced by relativistic effects. These chemical results could be reproduced with relativistic quantum-chemical calculations. The present review is selecting and describing examples for fast chemical separations that were successful at the TRIGA Mainz and heavy-ion reaction studies at GSI Darmstadt.

Eighty years of research on super-heavy nuclei

Professor Walter Greiner, our mentor, colleague, and friend, passed away in the age of eighty. During his lifetime, the search for elements beyond uranium started and elements up to the so far heaviest one with atomic number 118 were discovered. In this talk I will present a short history from early searches for ‘trans-uraniums’ up to the production and safe identification of shell-stabilized ‘Super-Heavy Nuclei’ (SHN). The nuclear shell model reveals that these nuclei should be located in a region with closed shells for the protons at Z = 114, 120 or 126 and for the neutrons at N = 184. The outstanding aim of experimental investigations is the exploration of this region of spherical SHN. Systematic studies of heavy ion reactions for the synthesis of SHN revealed production cross-sections which reached values down to one picobarn and even below for the heaviest species. The systematics of measured cross-sections can be understood only on the basis of relatively high fission barriers as predicted for nuclei in and around the island of SHN. A key role in answering some of the open questions plays the synthesis of isotopes of element 120. Attempts aiming for synthesizing this element at the velocity filter SHIP will be reported.

The evaporation residue in the fission state of barium nuclei within relativistic mean-field theory

The evaporation residue of barium isotopes is investigated in a microscopic study using relativistic mean field theory. The investigation includes the isotopes of barium from the valley of stability to the exotic proton-rich region. The ground as well as neck configurations for these nuclei are generated from their total nucleonic density distributions of the corresponding state. We have estimated the constituents (number of nucleons) in the elongated neck region of the fission state. We found the α-particle as the constituent of the neck of Ba-isotopes, referred to as the evaporated residue in heavy-ion reaction studies. A strong correlation between the neutron and proton is observed throughout the isotopic chain.

Determination of neutron-induced fission cross-sections of unstable nuclei via surrogate reaction method

Heavy ion reaction studies around Coulomb barrier energies have been generally used to investigate the effect of the structure of projectile/target on reaction dynamics. Other than providing an understanding of basic physics of the reaction dynamics, some of these reactions have been used as tools to serve as surrogates of neutron-induced compound nuclear fission cross-sections involving unstable targets. In this paper, we report some of the recent results on the determination of neutron-induced fission cross-sections of unstable actinides present in Th–U and U–Pu fuel cycles by surrogate reaction method by employing transfer-induced fission studies with 6,7Li beams.

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.

Studying Heavy Ion Collisions Using Methods From Cosmic Microwave Background (CMB) Analysis

We present and discuss a framework for studying the morphology of high- multiplicity events from relativistic heavy ion collisions using methods commonly em- ployed in the analysis of the photons from the Cosmic Microwave Background (CMB). The analysis is based on the decomposition of the distribution of the number density of (charged) particles expressed in polar and azimuthal coordinates into a sum of spherical harmonic functions. We present an application of the method exploting relevant symme- tries to the study of azimuthal correlations arizing from collective flow among charged particles produced in relativistic heavy ion collisions. We discuss perspectives for event- by-event analyses, which with increasing collision energy will eventually open entirely new dimensions in the study of ultrarelaticistic heavy ion reactions.

Near-barrier reactions with radioactive ion beams at the ReA3 facility

The new ReA3 re-accelerated beam facility at the National Superconducting Cyclotron Laboratory (NSCL) will provide high quality radioactive ion beams (RIBs), produced from fast fragmentation reactions, at energies around the Coulomb barrier. These radioactive isotopes can have exotic properties such as neutron/proton skins, halos, or unexpected changes in their shell structure. ReA3 will allow us to explore how these exotic properties manifest themselves in low-energy reactions. In particular, heavy-ion fusion induced with medium mass RIBs remains almost completely unexplored as only three fusion reactions using RIBs, between fluorine (Z = 9) and tin (Z = 50), have ever been measured. A new research program focused on the study of heavy-ion fusion reactions is being developed to take advantage of the world-unique RIBs offered by the ReA3 facility. Along with an overview of the ReA3 facility, details about three devices being developed for exploration of fusion reactions induced by RIBs will be presented.

LOW DENSITY NUCLEAR MATTER IN FERMI ENERGY COLLISIONS

Recent heavy ion reaction studies aimed at elucidating the properties of low density matter produced in near Fermi-energies are discussed. At the lowest densities and temperatures large alpha mass fractions are observed. The symmetry free energies derived from isoscaling analyses are far above those obtained in common effective interaction calculations, reflecting cluster formation, primarily of alpha particles, not included in such calculations. Theoretical modeling of these results requires the consideration of cluster formation and medium modifications of cluster properties.

Charged particle identification by pulse shape discrimination with single-sided segmented silicon-pad detectors

A charged particle detector array consisting of 50 Si–CsI detector telescopes for study of heavy-ion reactions is under construction in BARC-TIFR Pelletron-LINAC facility. Single-sided segmented silicon-pad detectors of 300 μm thickness and 400 mm 2 total active area have been fabricated for this purpose and their design parameters along with static and dynamic characteristics are presented. A low-cost solution for implementing the pulse-shape-based particle discrimination technique with the silicon-pad detectors has been demonstrated through in-beam measurements and the results obtained are presented.

Nuclear Data Sheets for A = 169

Experimental data pertaining to all nuclei with mass number A = 169 (Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt) have been evaluated. Level schemes from both radioactive decay and reaction studies are presented, along with associated tables of experimental data and adopted properties for levels and γ rays. These include data from several recent high-precision measurements of intensities for γ rays emitted in 169Yb ε decay; these are of special interest because of their use as calibration standards. Excited-state information is newly available for Re, Os, Ir and Pt, and heavy-ion reaction studies have significantly expanded our knowledge of excited states in Lu through Ta. More extensive work on ε decay from W is needed and new experimental work will be required to resolve a discrepancy between the Jπ values deduced for a 180-keV level in 169Ta based on extensive band structure from (HI,xnγ) work (Jπ = 1/2–) and TDPAD measurements (J = 5/2). The present evaluation for A = 169, which considers data received by 15 June 2008, supersedes the 1991 evaluation (with a literature cutoff date of March 1990) by V. Shirley, published in Nuclear Data Sheets 64, 505 (1991).

The application of neutron activation analysis, scanning electron microscope, and radiographic imaging for the characterization of electrochemically deposited layers of lanthanide and actinide elements

Lanthanide and actinide targets are prepared at the University of Mainz by molecular plating, an electrochemical deposition from an organic solvent, for heavy-ion reaction studies at GSI. To acquire information about deposition yield, target thickness, and target homogeneity, the following analysis methods are applied. With neutron activation analysis (NAA) the deposition yield and the average thickness of the deposited material is determined. We report on the analytical procedure of NAA performed subsequent to the molecular plating process. Scanning electron microscope (SEM) is used to determine the morphology of the target surfaces. In combination with energy dispersive X-ray spectrometer (EDS), we also could obtain qualitative information about the chemical form of the deposited material. So far, gadolinium, samarium, holmium, and uranium targets have been investigated with SEM/EDS. With radiographic imaging (RI), targets of uranium prepared by molecular plating and by vacuum vaporization are investigated. This method is suitable to obtain information about the spatial distribution, the homogeneity, and the thickness of the target layer deposition.

The equation of state of dense matter: from nuclear collisions to neutron stars

The equation of state (EoS) of dense matter represents a central issue in the study of compact astrophysical objects and heavy ion reactions at intermediate and relativistic energies. We have derived a nuclear EoS with nucleons and hyperons within the Brueckner–Hartree–Fock approach, and joined it with quark matter EoS. For that, we have employed the MIT bag model, as well as the Nambu–Jona-Lasinio and the color dielectric models, and found that the NS maximum masses are not larger than 1.7 solar masses. A comparison with available data supports the idea that dense matter EoS should be soft at low density and quite stiff at high density.

Applications of Skyrme energy-density functional to fusion reactions spanning the fusion barriers

The Skyrme energy density functional has been applied to the study of heavy-ion fusion reactions. The barriers for fusion reactions are calculated by the Skyrme energy density functional with proton and neutron density distributions determined by using restricted density variational (RDV) method within the same energy density functional together with semi-classical approach known as the extended semi-classical Thomas–Fermi method. Based on the fusion barrier obtained, we propose a parametrization of the empirical barrier distribution to take into account the multi-dimensional character of real barrier and then apply it to calculate the fusion excitation functions in terms of barrier penetration concept. A large number of measured fusion excitation functions spanning the fusion barriers can be reproduced well. The competition between suppression and enhancement effects on sub-barrier fusion caused by neutron-shell-closure and excess neutron effects is studied.

The Ring Counter (RCo): A high resolution IC–Si–CsI(Tl) device for heavy ion reaction studies at 10–30 MeV/A

An annular detector (Ring Counter, RCo) is presented, which has been designed and built to detect and identify in mass and charge light charged particles and fragments with very low energy thresholds and high energy resolution. It complements the GARFIELD apparatus, operating at INFN Laboratori Nazionali di Legnaro, to detect the forward emitted products of nuclear heavy ion reactions. It consists of eight sectors of a three-stage telescope, each one formed by an ionization chamber followed by eight strips of a silicon detector and by two CsI(Tl) scintillators. Construction features and performances are described and discussed in details.