Role Of Isospin Research Articles

Editorial: Biographical Memoirs , Volume 68

Editorial: <i>Biographical Memoirs</i> , Volume 68

Anthony Milner Lane. 27 July 1928—9 February 2011

Tony Lane came from humble beginnings to become one of the world's leading theoretical nuclear physicists. His career in the Theoretical Physics Division at the Atomic Energy Research Establishment (AERE) at Harwell was characterized by his outstanding successes in explaining experimental nuclear data. He pioneered the understanding of the important nucleon capture reactions by introducing new mechanisms of direct and semi-direct capture and, together with colleagues, he greatly advanced knowledge of nuclear analogue states, and the role of isospin in nuclear physics. With R. G. Thomas, he wrote a comprehensive review of R-matrix theory, applied to analyse resonances in nuclear reactions, which became one of the most cited papers in physics. His book Nuclear theory gave a good account of the use of pairing force theory in nuclear physics, and its application to nuclear collective motion.

Role of isospin and its conservation in neutron-rich fission fragments

Following on from our earlier paper Garg and Jain (2017 Phys. Scr. 92 094001) containing some initial results, we present a detailed discussion and complete results in this paper, which provide the first direct evidence for the validity of isospin as a nearly good quantum number in neutron-rich systems. The evidence comes from the reproduction of the general features of the partition-wise relative yields of neutron-rich fission fragments produced in two heavy-ion induced fusion fission reactions, namely 208Pb (18O, f) and 238U (18O, f), by using the concept of isospin conservation. To fix the isospin values and use the isospin algebra, we invoke what we term Kelson’s conjectures. We present a consistent scheme for isospin assignments based on these considerations. Our calculated results confirm that isospin behaves as an approximately good quantum number in neutron-rich systems, in this case, the fission fragments.

Antiferromagnet-Based Spintronic Functionality by Controlling Isospin Domains in a Layered Perovskite Iridate.

The novel electronic state of the canted antiferromagnetic (AFM) insulator strontium iridate (Sr2 IrO4 ) is well described by the spin-orbit-entangled isospin Jeff = 1/2, but the role of isospin in transport phenomena remains poorly understood. In this study, antiferromagnet-based spintronic functionality is demonstrated by combining the unique characteristics of the isospin state in Sr2 IrO4 . Based on magnetic and transport measurements, a large and highly anisotropic magnetoresistance (AMR) is obtained by manipulating the AFM isospin domains. First-principles calculations suggest that electrons whose isospin directions are strongly coupled to the in-plane net magnetic moment encounter an isospin mismatch when moving across the AFM domain boundaries, which generates a high resistance state. By rotating a magnetic field that aligns in-plane net moments and removes domain boundaries, the macroscopically ordered isospins govern dynamic transport through the system, which leads to the extremely angle-sensitive AMR. As this work establishes a link between isospins and magnetotransport in strongly spin-orbit-coupled AFM Sr2 IrO4 , the peculiar AMR effect provides a beneficial foundation for fundamental and applied research on AFM spintronics.

Decay of Zr isotopes and related nuclear structure effects using the collective clusterization approach

The reaction dynamics of various even-mass Zr isotopes is studied using the collective clusterization approach of the dynamical cluster decay model (DCM). The role of iso-spin (N/Z ratio) and deformation is explored by analysing the decay pattern of various Zr-nuclei formed in 16O induced reactions. Also, the entrance channel effect is investigated by studying the decay of the 92Zr* nuclear system formed in two different reactions at comparable excitation energy. In order to obtain a clear picture of the dynamics involved in the reactions under consideration, the behavior of the fragmentation potential and the preformation factor is analyzed. The DCM calculated cross-sections find nice agreement with available experimental data.

Fusion-evaporation residues andα-decay chains of the superheavy elementZ=115formed in the243Am+48Ca reaction using the dynamical cluster-decay model

The decay of the $Z=115$ superheavy nuclear system, formed in the ${}^{243}\mathrm{Am}+{}^{48}\mathrm{Ca}$ reaction, is studied by using the dynamical cluster-decay model. The calculated excitation functions of $2n$-, $3n$-, and $4n$-evaporation channels, for the excitation energy range ${E}_{\mathrm{CN}}^{*}=31$--47 MeV, are compared with the recent experimental data. The deformation effects are included up to ${\ensuremath{\beta}}_{2}$, within the hot optimum orientation approach, and a comparative analysis of spherical versus static and dynamic deformations is investigated explicitly for the $2n$-evaporation residue, as only $2n$ decay responds to spherical fragments. The $3n$ and $4n$ decay cross sections could be fitted only after the inclusion of deformation effects. The variation of preformation probability, barrier penetrability, and barrier modification is investigated in order to extract a better picture of the dynamics involved in the reaction under consideration. It is observed that, for the $3n$-evaporation channel, the barrier modification at ${E}_{\mathrm{CN}}^{*}=36.15$ MeV is the smallest and hence supports the experimental observation of maximum cross section (8.5 pb) at this energy. The role of isospin ($N/Z$ ratio) is also investigated for the decay of various isotopes of $Z=115$ formed in ${}^{48}\text{Ca}\phantom{\rule{0.16em}{0ex}}+{\phantom{\rule{0.16em}{0ex}}}^{241,243,245}$Am reactions. Furthermore, the evaporation cross sections of $2n$, $3n,$ and $4n$ channels are also estimated at the Bass barrier by interpolating the neck-length parameters fixed in reference to available data at above-barrier energies. Finally, the $\ensuremath{\alpha}$-decay chains are analyzed by using the preformed cluster model. It is shown that the present data of $\ensuremath{\alpha}$-decay half-lives support ``hot'' optimum orientations of nuclei, rather than the usual ``cold'' ones, within a constant empirical factor in penetrability.

Optimizing the rapidity limit for nuclear stopping in intermediate energy heavy-ion collisions

A systematic study regarding the role of participant matter and spectator matter in nuclear stopping using isospin-dependent quantum molecular dynamics model is presented. The simulations have been carried out with soft equation of state along with the reduced isospin-dependent cross section to study the effect of different types and sizes of rapidity distributions on nuclear stopping for the whole colliding geometry with density-dependent symmetry energy. In addition to that, we attempt to investigate the role of isospin in heavy-ion collisions by calculating the individual contribution of neutrons and protons in nuclear stopping for different systems having different isotopic content.

The role of isospin in fusion evaporation reactions

The isospin dependence of the level density, both through the level density parameter and the symmetry energy contribution has been evaluated. The effects on the most relevant observables in fusion evaporation reactions have been estimated by using the statistical model. We present here the results for the 134Sn +4 He reaction, whose measurement will be possible with the second generation RIB facilities SPES and SPIRAL2. Sizeable effects are predicted on the light particle energy spectra and multiplicities and evaporation residue distribution. In particular, the measure of evaporation residue yields and light particle multiplicities are expected to unfold the two effects. In addition, the same calculations have been carried out for reactions induced by stable beams, in order to search for clues of isospin effects. In this case, the direct comparison of the experimental light charged particle energy spectra for the two reactions 7Li+58,64Ni at the same excitation energy and angular momentum, is proposed as a good approach to observe isospin effects.

Scaling and isospin effects in quasielastic lepton–nucleus scattering in the relativistic mean field approach

The role of isospin in quasielastic electron scattering and charge-changing neutrino reactions is investigated in the relativistic impulse approximation. We analyze proton and neutron scaling functions making use of various theoretical descriptions for the final-state interactions, focusing on the effects introduced by the presence of strong scalar and vector terms in the relativistic mean field approach. An explanation for the differences observed in the scaling functions evaluated from ( e , e ′ ) and ( ν , μ ) reactions is provided by invoking the differences in isoscalar and isovector contributions.

Role of isospin in the nuclear liquid–gas phase transition

We study the thermodynamics of asymmetric nuclear matter using a mean-field approximation with a Skyrme effective interaction, in order to establish its phase diagram and more particularly the influence of isospin on the order of the transition. A new statistical method is introduced to study the thermodynamics of a multifluid system, keeping only one density fixed, the others being replaced by their intensive conjugated variables. In this ensemble, phase coexistence reduces to a simple one-dimensional Maxwell construction. For a fixed temperature under a critical value, a coexistence line is obtained in the plane of neutron and proton chemical potentials. Along this line, the grand potential presents a discontinuous slope, showing that the transition is first order except at the two ending points where it becomes second order. This result is not in contradiction with the already reported occurrence of a continuous transformation when a constant proton fraction is imposed. Indeed, the proton fraction being an order parameter in asymmetric matter, the constraint can only be fulfilled by gradual phase mixing along the first-order phase-transition line, leading to a continuous pressure.

Isotopic scaling of heavy projectile residues from the collisions of25MeV/nucleon 86Krwith124Sn,112Snand64Ni,58Ni

The scaling of the yields of heavy projectile residues from the reactions of 25 MeV/nucleon 86Kr projectiles with 124Sn,112Sn and 64Ni, 58Nitargets is studied. Isotopically resolved yield distributions of projectile fragments in the range Z=10-36 from these reaction pairs were measured with the MARS recoil separator in the angular range 2.7-5.3 degrees. The velocities of the residues, monotonically decreasing with Z down to Z~26-28, are employed to characterize the excitation energy. The yield ratios R21(N,Z) for each pair of systems are found to exhibit isotopic scaling (isoscaling), namely, an exponential dependence on the fragment atomic number Z and neutron number N. The isoscaling is found to occur in the residue Z range corresponding to the maximum observed excitation energies. The corresponding isoscaling parameters are alpha=0.43 and beta=-0.50 for the Kr+Sn system and alpha=0.27 and beta=-0.34 for the Kr+Ni system. For the Kr+Sn system, for which the experimental angular acceptance range lies inside the grazing angle, isoscaling was found to occur for Z<26 and N<34. For heavier fragments from Kr+Sn, the parameters vary monotonically, alpha decreasing with Z and beta increasing with N. This variation is found to be related to the evolution towards isospin equilibration and, as such, it can serve as a tracer of the N/Z equilibration process. The present heavy-residue data extend the observation of isotopic scaling from the intermediate mass fragment region to the heavy-residue region. Such high-resolution mass spectrometric data can provide important information on the role of isospin in peripheral and mid-peripheral collisions, complementary to that accessible from modern large-acceptance multidetector devices.

The role of isospin on the N=Z line

The role played by isospin in nuclear structure phenomena encountered on the N=Z line is discussed. New results on Coulomb energy differences (CED) at high spin for odd-A nuclei in the f 7/2 shell are presented and interpreted in the framework of a simple Cranked Shell Model treatment involving an exact numerical diagonalisation. Results for the CED between the A=46 even-even mirror pairs are also discussed. The CED between the T=1 states in N=Z odd-odd nuclei and their isobaric analogues are suggested as a possible probe of np pairing on the N=Z line. First results from a numerical diagonalization of IBM-4 are cited.

Phase transition in an isospin dependent Lattice Gas Model

The role of isospin as a degree of freedom and as an observable in heavy ion reaction experiments is analyzed in the theoretical framework of an isospin dependent Lattice Gas Model. We show that the critical temperature is only slightly dependent on the isospin asymmetry and finite size effects go in the direction of increasing the τ critical exponent respect to the asymptotic limit of the three dimensional Ising universality class. An interesting signature of the liquid-gas phase transition and a possible observable to localize the multifragmenting system in the T− ρ plane is given by the isotopic composition of fragments. We show that in the coexistence region the vapor fraction is more asymmetric than the liquid fraction and that this signal should not be washed out by secondary decays.

Isospin Physics in Heavy-Ion Collisions at Intermediate Energies

In nuclear collisions induced by stable or radioactive neutron-rich nuclei a transient state of nuclear matter with an appreciable isospin asymmetry as well as thermal and compressional excitation can be created. This offers the possibility to study the properties of nuclear matter in the region between symmetric nuclear matter and pure neutron matter. In this review, we discuss recent theoretical studies of the equation of state of isospin-asymmetric nuclear matter and its relations to the properties of neutron stars and radioactive nuclei. Chemical and mechanical instabilities as well as the liquid-gas phase transition in asymmetric nuclear matter are investigated. The in-medium nucleon-nucleon cross sections at different isospin states are reviewed as they affect significantly the dynamics of heavy ion collisions induced by radioactive beams. We then discuss an isospin-dependent transport model, which includes different mean-field potentials and cross sections for the proton and neutron, and its application to these reactions. Furthermore, we review the comparisons between theoretical predictions and available experimental data. In particular, we discuss the study of nuclear stopping in terms of isospin equilibration, the dependence of nuclear collective flow and balance energy on the isospin-dependent nuclear equation of state and cross sections, the isospin dependence of total nuclear reaction cross sections, and the role of isospin in preequilibrium nucleon emissions and subthreshold pion production.

Role of isospin in neutron- and alpha-induced reactions

The effects of including isospin in proton-induced reactions have been previously discussed. The effects are most important for heavy nuclei, where a substantial enhancement in proton decay of the compound nucleus is predicted. A comparable study for neutrons and alpha particles shows effects which are largest for light nuclei. The most important consequences are an enhancement in alpha decay and a slight reduction in proton and neutron decay, with some effects persisting for higher stages of multistep reactions.

Isospin and F-spin in the interacting boson model

The role of isospin in the different versions of the interacting boson model is discussed and compared with other concepts such as F-spin which is also related to the neutron-proton degree of freedom.

Role of isospin in the statistical decay of the giant dipole resonance built on excited states

The statistical γ-decay of the giant dipole resonance built on excited states was investigated in several nuclei in the sd-shell region. The yield of high-energy γ-rays in self-conjugate nuclei populated by T = 0 entrance channels was found to be strongly inhibited compared to the yield from T ≠ 0 entrance channels and compared to that in nuclei with Z ≠ Z. This can be explained as due to good isospin purity in the compound nuclear states at high excitation energy.

Deep-lying hole states in the optical model

The strength function for deep-lying hole states in an optical potential is studied by the method of Green's functions. The role of isospin is emphasized. It is shown that, while the main trends of the experimental data on hole states in isotopes of Sn and Pd can be described by an energy independent optical potential, intermediate structures in these data indicate the specific nuclear polarization effects have to be included. This is done by introducing doorway states of good isospin into the optical model potential. Such states consist of neutron hole plus proton core vibrations as well as more complicated excitations that are analog states of proton hole plus neutron core vibrations of the parent nuclear system. Specific calculations for /sup 115/Sn and /sup 103/Pd give satisfactory fits to the strength function data using optical model and doorway state parameters that are reasonable on physical grounds.

Role of Isospin in Statistical Processes

Role of Isospin in Statistical Processes

The role of isospin in shell-model calculations of N = 28 nuclei

Abstract The shell-model description of N = 28 nuclei within the two lowest configurations is considered. The lowest configuration only involves proton-proton effective interactions, whereas the excited one involves proton-proton interactions and, if isospin is properly treated, also proton-neutron effective interactions. Thus, the two-nucleon matrix elements entering the description of the N = 28 nuclei also uniquely describe the N = 29 nuclei in their lowest configuration, and can therefore be obtained empirically by a fit to N = 28 and N = 29 spectra. It is shown that under certain circumstances the neglect of isospin in the excited configuration may not affect the energy level calculation seriously.