arXiv subjects

Jagjit Singh

Publications and source records attributed to Jagjit Singh.

At least 19 recordsLinked to original sources

arXiv2026

Influence of configuration-interaction on isospin impurities and isospin symmetry breaking corrections to superallowed $0^+\rightarrow 0^+$ beta decays

The symmetry-conserving density functional theory (DFT)-based no-core configuration-interaction (DFT-NCCI) framework is applied for the first time to investigate the impact of configuration interaction (CI) on the Coulomb (isospin) impurity, $\alpha_{\rm C}$, in the ground and excited states of $^{10}$C, $^{10}$B, and $^{14}$N, as well as on the isospin-symmetry-breaking (ISB) correction to the superallowed $0^+ \rightarrow 0^+$ $\beta$ decay of $^{10}$C. We demonstrate, among other findings, that within the DFT-NCCI framework CI has a negligible effect on the ground-state isospin impurities, which are dominated by a single doorway state. In contrast, CI significantly modifies the impurities in excited states, including the isobaric analogue $I=0^+,\,T=1$ state in $^{10}$B. Hence, it also has a non-negligible impact on the ISB correction to the superallowed $\beta$ decay of $^{10}$C. Our calculations yield $\bar{\delta}_{\rm C}=0.45(4)\%$ when the Coulomb interaction is taken as the sole source of ISB, and $\bar{\delta}_{\rm ISB}=0.46(6)\%$ when short-range charge-symmetry-breaking (CSB) terms are included in addition. Hence, no statistically significant dependence of the ISB correction on the short-range CSB interaction is observed for this decay. Comparison with our previous results reveals a strong sensitivity to the nuclear symmetry energy, which governs the strength of the isospin-restoring force and whose value in finite nuclei remains difficult to constrain because of its intricate dependence on the momentum-dependent terms of the effective interaction.

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arXiv2026

Deformation, halo, and bubble structure: A paradigm shift of exotic phenomena in light to medium mass nuclei

The emergence of exotic nuclear structures, such as deformation, one- and two-neutron halos, and bubble configurations, marks a paradigm shift in our understanding of light- to medium-mass nuclei far from stability, particularly near and within the island of inversion extending across $N=20-28$. In this review, we integrate microscopic structure calculations using the antisymmetrized molecular dynamics method with reaction theories such as the Glauber model for high-energy collisions, and highlight the use of the fully quantum mechanical finite-range distorted wave Born approximation for calculating both inclusive and exclusive Coulomb breakup observables for these medium mass systems. These theoretical frameworks enable precise probing of nuclear density profiles through observables such as total reaction cross sections, neutron removal cross sections, relative energy spectra, parallel momentum distributions, and angular distributions. Applications to several nuclei in the island of inversion reveal enhanced halo extensions, neutron-neutron correlations in Borromean nuclei, and central density depletions in bubbles, challenging traditional shell-model paradigms. Furthermore, the sensitivity of astrophysical reaction rates to these exotic inputs is explored, demonstrating their role in the refinement of r-process nucleosynthesis models and elemental abundance predictions. This unified approach not only bridges nuclear structure and reactions, but also highlights the driplines as frontiers for unraveling nuclear matter under extreme conditions, with implications for rare-isotope beam experiments and beyond.

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arXiv2026

Is $^{40}$Mg a Borromean halo nucleus? A case built on the electric-dipole response

We investigate the low-energy electric-dipole response of $^{40}$Mg using a $^{38}$Mg$+n+n$ three-body model. This model is implemented using a three-body hyperspherical formalism with an analytical transformed harmonic oscillator basis. In this study, two different neutron-neutron interactions are considered: a scalar Gaussian density-dependent central potential and a more realistic finite-range potential which includes central, spin-orbit, and tensor components. We examine how electric-dipole response is affected by the choice of the interaction.

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arXiv2025

The effect of inversion of $p$ and $f$ orbits on halo formation in heavy sodium isotopes

The role of the inversion of the $p$ and $f$ shell-model orbits in the emergence of halo structures in the ground states of neutron-rich $^{34,37,39}$Na is investigated. Families of two- and three-body models are constructed with effective core-neutron interactions, with parameter choices based on a combination of the available experimental data and systematic trends, as well as the GPT $n$-$n$ interaction and a phenomenological three-body force. Our results indicate a possible one-neutron halo in $^{34}$Na, while $^{37,39}$Na exhibit features of Borromean halos. The halo formation is driven by the weakening of the shell gap and inversion of the $2p_{3/2}$ and $1f_{7/2}$ orbits expected to occur somewhere near these masses. We further show that the electric dipole response provides a clear and sensitive probe of halo structure in these isotopes.

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arXiv2025

Determination of $S_{18}$ from $^{9}$C breakup reaction within a four-body reaction model

The astrophysical factor $S_{18}$ for the $^{8}$B($p$,$γ$)$^{9}$C has indirectly been measured with the proton removal reactions from $^9$C, elastic breakup of $^9$C off a heavy target, and transfer reactions. Quite recently, the elastic breakup cross section data were reanalyzed with the continuum-discretized coupled channels method (CDCC) assuming a $p+{\rm ^{8}B}$ two-body model for $^9$C and the $S_{18}$ was modified. It was not well justified, however, to treat $^8$B as an inert nucleus given its proton separation energy is only 137~keV. We reexamine the elastic breakup of $^9$C by the four-body CDCC with a $p+p+{\rm ^{7}Be}$ three-body model for $^9$C and evaluate $S_{18}$. To achieve this, we propose a method to disentangle the $p+{\rm ^{8}B}+{\rm ^{208}Pb}$ three-body channel in the four-body CDCC calculation, for the first time. We calculate the elastic breakup cross section of $^9$C off a $^{208}$Pb target at 65~MeV/nucleon. The obtained breakup cross sections are decomposed into the contributions of the $p+{\rm ^{8}B}+{\rm ^{208}Pb}$ and $p+p+{\rm ^{7}Be}+{\rm ^{208}Pb}$ channels by using the solution of the complex-scaled Lippmann--Schwinger equation. The breakup cross section to the $p+{\rm ^{8}B}+{\rm ^{208}Pb}$ channel reproduces well the shape of the experimental data in the low breakup energy region, which is important for determining $S_{18}$. By fitting the theoretical result to the experimental data, the asymptotic normalization coefficient of $^9$C for the $p+{\rm ^{8}B}$ configuration is determined and we obtain $S_{18}=38.4\pm1.1$ eVb. This result is smaller than the previous value obtained with the three-body CDCC by about 45\%. Thus, our new results suggest the necessity of taking into account the fragile nature of $^{8}$B in the $^{9}$C breakup.

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arXiv2025

Two-cluster approach to the properties of one- and two-neutron-halo nuclei

In this work, we present a new approximate method for obtaining simple wave functions for the ground state of exotic nuclei with a neutron halo. We model the system as a two-cluster structure, treating the core and halo as inert objects. The relative wave function is expressed as a combination of simple harmonic oscillator states, with the oscillator parameter determined from the separation energy. Since these wave functions lack the expected exponential decay, we introduce a simple multiplicative renormalization factor based on the nuclear root mean square radius. This approach, combining oscillator wave functions and the renormalization factor, is then applied to calculate dipole strength distributions $dB(E1,\varepsilon)/d\varepsilon$ and Coulomb dissociation cross section $dσ(E1,\varepsilon)/d\varepsilon$ for several $1n$- and $2n$-halo nuclei. The results show excellent agreement with the available experimental data.

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arXiv2024

Investigating the rate of $^{10}$Be(n,$γ$)$^{11}$Be radiative capture reaction within the FRDWBA framework

This study examines the radiative capture of a neutron by $^{10}$Be using the Coulomb dissociation approach within the FRDWBA theory. We analyze the elastic Coulomb breakup of $^{11}$Be on a $^{208}$Pb target at 72 MeV/A to determine the photodisintegration cross-section and radiative capture cross-section. Utilizing the Maxwell-averaged velocity distribution, we calculate the resulting radiative neutron capture reaction rate for the $^{10}$Be(n,$γ$)$^{11}$Be reaction. Comparative analyses are conducted with experimental data, theoretical results from direct radiative capture methods, and transfer reaction calculations. Additionally, we contrast our findings with the existing $^{10}$Be($α$,$γ$)$^{14}$C reaction rate and conclude the dominance of neutron capture over $α$ capture by $^{10}$Be.

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arXiv2024

Prediction of two-neutron halos in the $N=28$ isotones $^{40}$Mg and $^{39}$Na

The ground states of the nuclei $^{40}$Mg and $^{39}$Na are investigated using the hyperspherical formalism. Since they are located at the edge of the "big island of inversion", we concentrate on whether we are likely to find two-neutron Borromean halos in these nuclei. A three-body model with effective $n$-$n$ and $^{38}$Mg$+n$ interactions is built for $^{40}$Mg based on the available data. We also give predictions for the low-lying spectrum of $^{38}$Na$=^{37}$Na$+n$ and two-neutron separation energy of the $^{39}$Na nucleus. Depending on parameter choice, we report an increase in the matter radii in the range $0.1$-$0.5$ fm relative to those of the core nuclei. The results suggest a two-neutron halo structure in $^{40}$Mg for a subset of parameters, reinforcing the prediction of a Borromean halo nucleus. The calculations indicate that a two-neutron halo is even more likely for $^{39}$Na. As expected, the halo is linked to the disappearance of the shell gap in these nuclei due to the inversion of the $2p_{3/2}$ and $1f_{7/2}$ orbitals. We study the total cross section for scattering of these nuclei from a carbon target using a Glauber model and show that these provide a clear signal to assess the halo structure.

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arXiv2023

Calculation of Dynamical Response Functions Using a Bound-state Method

We investigate a method to extract response functions (dynamical polarisabilities) directly from a bound-state approach applied to calculations of perturbation-induced reactions. The use of a square-integrable basis leads to a response in the form of a sum of $δ$ functions. We integrate this over energy and fit a smooth function to the resulting stepwise-continuous one. Its derivative gives the final approximation to the physical response function. We show that the method reproduces analytical results where known, and analyse the details for a variety of models. We apply it to some simple models, using the Stochastic Variational Method as the numerical method. Although we find that this approach, and other numerical techniques, have some difficulties with the threshold behaviour in coupled-channel problems with multiple thresholds, its stochastic nature allows us to extract robust results even for such cases.

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arXiv2022

Exploring the halo character and dipole response in the dripline nucleus $^{31}$F

Lying at the lower edge of the `island of inversion', neutron-rich Fluorine isotopes ($^{29-31}$F) provide a curious case to study the configuration mixing in this part of the nuclear landscape. Recent studies have suggested that a prospective two-neutron halo in the dripline nucleus $^{31}$F could be linked to the occupancy of the $pf$ intruder configurations. Focusing on configuration mixing, matter radii and neutron-neutron ($nn$) correlations in the ground-state of $^{31}$F, we explore various scenarios to analyze its possible halo nature as well as the low-lying electric dipole ($E$1) response within a three-body approach. We use an analytical, transformed harmonic oscillator basis under the aegis of a hyperspherical formalism to construct the ground state three-body wave function of $^{31}$F. The $^{31}$F ground-state configuration mixing and its matter radius are computed for different choices of the $^{30}$F structure coupled to the valence neutron. The admixture of {$p_{3/2}$, $d_{3/2}$, and $f_{7/2}$} components is found to play an important role, favouring the dominance of inverted configurations with dineutron spreads for two-neutron halo formation. The increase in matter radius with respect to the core radius, $Δr \geqslant$ 0.30 fm and the dipole distributions along with the integrated $B(E1)$ strengths of $\geqslant$ 2.6 $e^2$fm$^2$ are large enough to be compatible with other two-neutron halo nuclei. Three-body results for $^{31}$F indicate a large spatial extension in its ground state due to the inversion of the energy levels of the normal shell model scheme. The increase is augmented by and is proportional to the extent of the $p_{3/2}$ component in the wave function. Additionally, the enhanced dipole distributions and large $B(E1)$ strengths all point to the two-neutron halo character of $^{31}$F.

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arXiv2021

Three-body description of $^9$C: Role of low-lying resonances in breakup reactions

The $^9$C nucleus and related capture reaction, ${^8\mathrm{B}}(p,γ){^9\mathrm{C}}$, have been intensively studied with an astrophysical interest. Due to the weakly-bound nature of $^9$C, its structure is likely to be described as the three-body (${^7\mathrm{Be}}+p+p$). Its continuum structure is also important to describe reaction processes of $^9$C, with which the reaction rate of the ${^8\mathrm{B}}(p,γ){^9\mathrm{C}}$ process have been extracted indirectly. We perform three-body calculations on $^9$C and discuss properties of its ground and low-lying states via breakup reactions. We employ the three-body model of $^9$C using the Gaussian-expansion method combined with the complex-scaling method. This model is implemented in the four-body version of the continuum-discretized coupled-channels method, by which breakup reactions of $^9$C are studied. The intrinsic spin of $^7$Be is disregarded. By tuning a three-body interaction in the Hamiltonian of $^9$C, we obtain the low-lying $2^+$ state with the resonant energy 0.781 MeV and the decay width 0.137 MeV, which is consistent with the available experimental information and a relatively high-lying second $2^+$ wider resonant state. Our calculation predicts also sole $0^+$ and three $1^-$ resonant states. We discuss the role of these resonances in the elastic breakup cross section of $^9$C on $^{208}$Pb at 65 and 160 MeV/A. The low-lying 2$^+$ state is probed as a sharp peak of the breakup cross section, while the 1$^-$ states enhance the cross section around 3 MeV. Our calculations will further support the future and ongoing experimental campaigns for extracting astrophysical information and evaluating the two-proton removal cross-sections.

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arXiv2021

Systematic study on the role of various higher-order processes in the breakup of weakly-bound projectiles

The virtual photon theory (VPT), which is based on first-order Coulomb dissociation restricted to the electric dipole ($E1$), has been successfully used to explain the breakup data for several cases. Our aim is to study the role of various higher-order processes that are ignored in the VPT, such as the nuclear breakup, interference between nuclear and Coulomb amplitudes, and multistep breakup processes mainly due to strong continuum-continuum couplings in the breakup of two-body projectiles on a heavy target at both intermediate and higher incident energies. For the purpose of numerical calculations, we employed eikonal version of three-body continuum-discretized coupled-channels (CDCC) reaction model. Our results for the breakup of $^{11}$Be and $^{17}$F on $^{208}$Pb target at 100, 250, and 520 MeV/A, show the importance of nuclear breakup contribution, and its significant role in the multistep processes. The multistep effect on Coulomb breakup for core-neutron projectile was found to be negligible, whereas it was important for core-proton projectile. Coulomb-nuclear interference (CNI) effect was also found to be non-negligible. Quantitatively, the multistep effects due to the nuclear breakup was found to depend on the incident energy through the energy dependence of the core-target and nucleon-target nuclear potentials. The nuclear breakup component, the CNI effect, and the multistep breakup processes are all found to be non-negligible; hence, the assumptions adopted in the VPT for the accurate description of breakup cross sections are not valid.

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arXiv2021

Scaling with deformation in probable $p$-wave halo $^{34}$Na

We investigate the electric dipole response of $^{34}$Na, a probable $p$-wave one-neutron halo nucleus, lying in the "island of inversion" and having a deformed structure. We use a semi-analytic approach to probe the dipole response and further compare the results obtained from a post form finite-range distorted wave Born approximation theory of Coulomb breakup. The effects of deformation are figured out on the peak positions of the electric dipole strength distribution which further constraint the one-neutron separation energy of the deformed projectile and it leads to a two-dimensional scaling of total $B(E1)$ strength with parameters: separation energy and deformation.

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arXiv2021

Exploring the structure of $^{29}$Ne

We apply a fully quantum mechanical Coulomb breakup theory under the aegis of post form finite-range distorted wave Born approximation to analyze the elastic Coulomb breakup of $^{29}$Ne on $^{208}$Pb at $244$\,MeV/u. We calculate several reaction observables to quantify its structural parameters. One-neutron removal cross-section is calculated to check the consistency of the ground state configuration of $^{29}$Ne with the available experimental data. A scrutiny of the parallel momentum distribution of the charged fragment reveals a full width at half maximum of $82$\,MeV/c, which is in good agreement with the experimental value and indicates a moderate halo for a nearly spherical $^{29}$Ne in the $^{28}$Ne$(0^+) \otimes 2p_{3/2}ν$ ground state. The energy-angular distributions and average momentum of the charged fragment point to the absence of post-acceleration effects in the breakup process, a desirable result for the elastic breakup.

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arXiv2020

Electric dipole response of low-lying excitations in the two-neutron halo nucleus $\boldsymbol{^{29}}$F

The neutron-rich $^{28,29}$F isotopes have been recently studied via knockout and interaction cross-section measurements. The $2n$ halo in $^{29}$F has been linked to the occupancy of $pf$ intruder configurations. We investigate bound and continuum states in $^{29}$F, focusing on the $E1$ response of low-lying excitations and the effect of dipole couplings on nuclear reactions. $^{29}\text{F}$ ($^{27}\text{F}+n+n$) wave functions are built within the hyperspherical harmonics formalism, and reaction cross sections are calculated using the Glauber theory. Continuum states and $B(E1)$ transition probabilities are described in a pseudostate approach using the analytical THO basis. The corresponding structure form factors are used in CDCC calculations to describe low-energy scattering. Parity inversion in $^{28}$F leads to a $^{29}$F ground state characterized by 57.5% of $(p_{3/2})^2$ intruder components, a strong dineutron configuration, and an increase of the matter radius with respect to the core radius of $ΔR=0.20$ fm. Glauber-model calculations for a carbon target at 240 MeV/nucleon provide a total reaction cross section of 1370 mb, in agreement with recent data. The model produces also a barely bound excited state corresponding to a quadrupole excitation. $B(E1)$ calculations into the continuum yield a total strength of 1.59 e$^2$fm$^2$ up to 6 MeV, and the $E1$ distribution exhibits a resonance at $\approx$ 0.85 MeV. Results using a standard shell-model order for $^{28}$F lead to a considerable reduction of the $B(E1)$ distribution. The four-body CDCC calculations for $^{29}\text{F}+^{120}\text{Sn}$ around the Coulomb barrier are dominated by dipole couplings, which totally cancel the Fresnel peak in the elastic cross section. These results are consistent with a two-neutron halo and may guide future experimental campaigns.

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arXiv2020

The $\boldsymbol{^{29}}$F nucleus as a lighthouse on the coast of the island of inversion

The exotic, neutron-rich and weakly-bound isotope $^{29}$F stands out as a waymarker on the southern shore of the island of inversion, a portion of the nuclear chart where the effects of nuclear forces lead to a reshuffling of the single particle levels and to a reorganization of the nuclear structure far from stability. This nucleus has become very popular, as new measurements allow to refine theoretical models. We review the latest developments and suggest how to further assess the structure by proposing predictions on electromagnetic transitions that new experiments of Relativistic Coulomb Excitation should soon become able to measure.

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arXiv2020

Exploring two-neutron halo formation in the ground-state of $^{29}$F within a three-body model

Background$\colon$ The $^{29}$F system is located at the lower-N boundary of the "island of inversion" and is an exotic, weakly bound system. Little is known about this system beyond its two-neutron separation energy ($S_{2n}$) with large uncertainties. A similar situation is found for the low-lying spectrum of its unbound binary subsystem $^{28}$F. Purpose$\colon$ To investigate the configuration mixing, matter radius and neutron-neutron correlations in the ground state of $^{29}$F within a three-body model, exploring the possibility of $^{29}$F to be a two-neutron halo nucleus. Method$\colon$ The $^{29}$F ground-state wave function is built within the hyperspherical formalism by using an analytical transformed harmonic oscillator basis. The Gogny-Pires-Tourreil (GPT) nn interaction with central, spin-orbit and tensor terms is employed in the present calculations, together with different core$+n$ potentials constrained by the available experimental information on $^{28}$F. Results$\colon$ The $^{29}$F ground-state configuration mixing and its matter radius are computed for different choices of the $^{28}$F structure and $S_{2n}$ value. The admixture of d-waves with pf components are found to play an important role, favoring the dominance of dineutron configurations in the wave function. Our computed radii show a mild sensitivity to the $^{27}$F$+n$ potential and $S_{2n}$ values. The relative increase of the matter radius with respect to the $^{27}$F core lies in the range 0.1-0.4 fm depending upon these choices. Conclusions$\colon$ Our three-body results for $^{29}$F indicate the presence of a moderate halo structure in its ground state, which is enhanced by larger intruder components. This finding calls for an experimental confirmation.

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arXiv2019

Impact of uncertainties of unbound $^{10}$Li on the ground state of two-neutron halo $^{11}$Li

Recently, the energy spectrum of $^{10}$Li was measured upto 4.6 MeV, via one-neutron transfer reaction d($^{9}$Li, p)$^{10}$Li. Considering the ambiguities on the $^{10}$Li continuum spectrum with reference to new data, we report the configuration mixing in the ground state of the two-neutron halo nucleus $^{11}$Li for two different choices of the $^{9}$Li$+$n potential. For the present study, we employ a three-body (core$+$n$+$n) structure model developed for describing the two-neutron halo system by explicit coupling of unbound continuum states of the subsystem (core$+$n), and discuss the two-neutron correlations in the ground state of $^{11}$Li.

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