Neutron-proton Pairing Research Articles

Thermodynamics Properties of a System with Finite Heavy Mass Nuclei

The thermodynamics property of finite heavy mass nuclei, with the number of protons greater than the number of neutron is investigated. The core of the nucleus contains the neutron-proton pair that interacts harmonically; the excess neutron(s) reside(s) on the surface of the nucleus and introduce the anharmonic effect. The total energy is evaluated using ladder operator method and the quantum mechanical statistical expression of energy. The total energy, heat capacity and entropy are found to depend on the occupation number of states and the number of excess neutrons. At temperature near absolute zero the specific heat and entropy are lowest because a decreases in temperature leads to a decrease in particle interaction and energy.

Particle-number projected electric quadrupole moment of even-even proton-rich nuclei in the isovector pairing case

The effect of the particle-number projection on the electric quadrupole moment (Q 2 ) of even-even proton-rich nuclei is studied in the isovector neutron-proton (np) pairing case. As a first step, an expression of the electric quadrupole moment, which takes into account the isovector np pairing effect and which conserves the particle-number, is established within the Sharp-BCS (SBCS) method. This expression does generalize the one used in the pairing between like-particles case. As a second step, Q 2 is calculated for even-even proton-rich nuclei using the single-particle energies of a Woods-Saxon mean-field. The obtained results are compared with the results obtained in the pairing between like-particles case. It is shown that the np pairing effect, as well as the projection one, is maximal when N=Z.

Proton-neutron pairing amplitude as a generator coordinate for double- β decay

Proton-neutron pairing amplitude as a generator coordinate for double- β decay

NEUTRON–PROTON PAIRS IN NUCLEI

A review is given of attempts to describe nuclear properties in terms of neutron–proton pairs that are subsequently replaced by bosons. Some of the standard approaches with low-spin pairs are recalled but the emphasis is on a recently proposed framework with pairs of neutrons and protons with aligned angular momentum. The analysis is carried out for general j and applied to N=Z nuclei in the 1f7/2 and 1g9/2 shells.

Solving nuclear shape conundrum at HIE-ISOLDE

It is the purpose of this paper to illustrate some of the low-energy nuclear physics that we want to pursue at the new HIE-ISOLDE radioactive-ion-beam facility at CERN. The University of the Western Cape leads an experimental proposal and co-authors two Letters of Intent, in collaboration with European institutions, at HIE-ISOLDE. Timely topics such as the "Exploration of K-isomerism using unique high-K isomeric beams – CERN-INTC-I-101" and "Shape changes and proton-neutron pairing around the N = Z line – CERN-INTC-I-102" are addressed in these Letters of Intent. Our experimental proposal aims at performing a multi-step Coulomb-excitation of radioactive 70Se ion beams using the 208Pb(70Se,70Se*)208Pb* reaction at a bombarding energy of 5.5 MeV/u. The physics goal is a precise measurement of the (2+1 || Ê2 || 2+1) diagonal matrix element, related to the spectroscopic quadrupole moment, in 70Se. Full simulations presented in this work show distinct angular distributions for plausible values of the spectroscopic quadrupole moment; with a predicted uncertainty of approximately ±0.1 eb. Additional diagonal and transitional matrix elements will also be obtained. These results will shed light onto the origin of rarely-found oblate shapes and shape coexistence in this region of rapidly-changing shell structure.

Relation between Wigner energy and proton-neutron pairing

The linear term proportional to $|N-Z|$ in the nuclear symmetry energy (Wigner energy)is obtained in a model that uses isovector pairing on single particle levels from a deformed potential combined with a $\vec T^2$ interaction. The pairing correlations are calculated by numerical diagonalization of the pairing Hamiltonian acting on the six or seven levels nearest the $N=Z$ Fermi surface. The experimental binding energies of nuclei with $N\approx Z$ are well reproduced. The Wigner energy emerges as a consequence of restoring isospin symmetry. We have found the Wigner energy to be insensitive to the presence of moderate isoscalar pair correlations.

Nuclearβ+/EC decays in covariant density functional theory and the impact of isoscalar proton-neutron pairing

Self-consistent proton-neutron quasiparticle random phase approximation based on the spherical nonlinear point-coupling relativistic Hartree-Bogoliubov theory is established and used to investigate the $\beta^+$/EC-decay half-lives of neutron-deficient Ar, Ca, Ti, Fe, Ni, Zn, Cd, and Sn isotopes. The isoscalar proton-neutron pairing is found to play an important role in reducing the decay half-lives, which is consistent with the same mechanism in the $\beta$ decays of neutron-rich nuclei. The experimental $\beta^+$/EC-decay half-lives can be well reproduced by a universal isoscalar proton-neutron pairing strength.

β-decay half-lives of neutron-rich nuclei and matter flow in the r-process

The β-decay half-lives of neutron-rich nuclei with 20≤Z≤50 are systematically investigated using the newly developed fully self-consistent proton–neutron quasiparticle random phase approximation (QRPA), based on the spherical relativistic Hartree–Fock–Bogoliubov (RHFB) framework. Available data are reproduced by including an isospin-dependent proton–neutron pairing interaction in the isoscalar channel of the RHFB+QRPA model. With the calculated β-decay half-lives of neutron-rich nuclei a remarkable speeding up of r-matter flow is predicted. This leads to enhanced r-process abundances of elements with A≳140, an important result for the understanding of the origin of heavy elements in the universe.

Systematic study of the isovector pairing effect on the moment of inertia of proton-rich nuclei in the region 30 ≤ Z ≤ 40

A systematic study of the isovector neutron-proton (np) pairing effect on the moment of inertia is performed at zero temperature. This study is based on a recently established expression obtained using the framework of the quantum perturbation theory and the Inglis cranking method, at the limit when the temperature is nil.We considered even–even proton-rich nuclei such as 30 ≤ Z ≤ 40 and N – Z = 0, 2, 4 using the single-particle energies and eigen-states of a deformed Woods-Saxon mean-field. The obtained results are compared to their homologues of the conventional BCS theory (i. e. when only the pairing between like-particles is considered).

Proton-neutron pairing correlations in the nuclear shell model

Systematic shell-model calculations in 2p1f shell nuclei are reported, to assess the relative importance of isoscalar and isovector pairing in the presence of nuclear deformation and spin-orbit effects. Results are presented for three N = Z nuclei, 44Ti, 46V and 48Cr.

High-spin structure in40K

High-spin states of $^{40}$K have been populated in the fusion-evaporation reaction $^{12}$C($^{30}$Si,np)$^{40}$K and studied by means of $\gamma$-ray spectroscopy techniques using one AGATA triple cluster detector, at INFN - Laboratori Nazionali di Legnaro. Several new states with excitation energy up to 8 MeV and spin up to $10^-$ have been discovered. These new states are discussed in terms of J=3 and T=0 neutron-proton hole pairs. Shell-model calculations in a large model space have shown a good agreement with the experimental data for most of the energy levels. The evolution of the structure of this nucleus is here studied as a function of excitation energy and angular momentum.

Four-nucleonα-type correlations and proton-neutron pairing away from theN=Zline

We study the competition between $\ensuremath{\alpha}$-type and conventional pair condensation in the ground state of nuclei with neutrons and protons interacting via a charge-independent pairing interaction. The ground state is described by a product of two condensates, one of $\ensuremath{\alpha}$-like quartets and the other one of pairs in excess relative to the isotope with $N=Z$. It is shown that this ansatz for the ground state gives very accurate pairing correlation energies for nuclei with the valence nucleons above the closed cores ${}^{16}$O, ${}^{40}$Ca, and ${}^{100}$Sn. These results indicate that $\ensuremath{\alpha}$-type correlations are important not only for the self-conjugate nuclei but also for nuclei away from the $N=Z$ line. In the latter case $\ensuremath{\alpha}$-like quartets coexist with the collective Cooper pairs formed by the nucleons in excess.

High-lying excited states in Gamow Teller strength and their roles in neutrino reactions

The Gamow Teller (GT) transition strengths deduced from charge exchange reactions (CEXRs) are very helpful for understanding the nuclear reaction induced by neutrinos, in particular, by the solar neutrino. For further study of supernovae (SNe) neutrinos in the cosmos, one needs to study high-lying GT states around a few tens of MeV region as well as other multipole transitions because of the high energy tail in the neutrino spectra emitted from the neutrino sphere. In this report, we address the importance of the high-lying GT excited states, whose data now become available from various CEXR experiments. For example, GT(± strengths up to 70MeV are successfully extracted by 90Zr(n, p) and 90Zr(p, n) reactions. Our discussions are extended to investigate roles of the high-lying states beyond a few low-lying states known in the old experiment on the reaction induced by SNe neutrinos particularly on 40Ar target. The nucleus was originally exploited to identify the solar neutrino emitted from 8B produced in the pp-chains on the Sun, and now lots of applications for more energetic neutrino detection are under progress. The expected large difference between the cross-sections of \( \nu_{e}^{}\) and \( \bar{{\nu}}_{e}^{}\) reactions on 40Ar , whose differences were anticipated because of the large Q-value in the \( \bar{{\nu}}_{e}^{}\) reaction, is significantly diminished compared to previous results. Our calculations are carried out by the Quasi-particle Random Phase Approximation (QRPA), which takes the neutron-proton pairing into account to the standard proton-neutron QRPA (pnQRPA) where only proton-proton and neutron-neutron pairing correlations are considered.

Double binding energy differences: Mean-field or pairing effect?

In this Letter we present a systematic analysis on the average interaction between the last protons and neutrons in atomic nuclei, which can be extracted from the double differences of nuclear binding energies. The empirical average proton–neutron interaction Vpn thus derived from experimental data can be described in a very simple form as the interplay of the nuclear mean field and the pairing interaction. It is found that the smooth behavior as well as the local fluctuations of the Vpn in even–even nuclei with N≠Z are dominated by the contribution from the proton–neutron monopole interactions. A strong additional contribution from the isoscalar monopole interaction and isovector proton–neutron pairing interaction is seen in the Vpn for even–even N=Z nuclei and for the adjacent odd-A nuclei with one neutron or proton being subtracted.

Aligned neutron–proton pairs in N ∼ Z nuclei

It is argued that N ∼ Z nuclei with 90 ⩽ A ⩽ 100 can be interpreted in terms of aligned neutron–proton pairs with angular momentum J = 2j and isospin T = 0. Based on this observation, a version of the interacting boson model is formulated in terms of isoscalar high-spin bosons. To illustrate its possible uses, the model is applied to the 21+ isomer in 94Ag.

Coherence features of the spin-aligned neutron–proton pair coupling scheme

The seniority scheme has been shown to be extremely useful for the classification of nuclear states in semi-magic nuclei. The neutron–proton (np) correlation breaks the seniority symmetry in a major way. As a result, the corresponding wave function is a mixture of many components with different seniority quantum numbers. In this paper, we show that the np interaction may favor a new kind of coupling in N=Z nuclei, i.e. the so-called isoscalar spin-aligned np pair mode. Shell model calculations reveal that the ground and low-lying yrast states of the N=Z nuclei 92Pd and 96Cd may be mainly built upon such spin-aligned np pairs, each carrying the maximum angular momentum J=9 allowed by the shell 0 g9/2 which is dominant in this nuclear region.

Spin-aligned neutron-proton pair coupling in the era of large scale computing

Shell model calculations reveal that the ground and low-lying yrast states of the N = Z nuclei 9246Pd and 9648Cd are mainly built upon isoscalar spin-aligned neutron-proton pairs each carrying the maximum angular momentum J = 9 allowed by the shell 0g9/2 which is dominant in this nuclear region. This mode of excitation is unique in nuclei and indicates that the spin-aligned pair has to be considered as an essential building block in nuclear structure calculations. In this contribution we will discuss this neutron-proton pair coupling scheme in detail. It may help in understanding the intrinsic structure of the shell-model wave function. In particular, we will explore the competition between the normal monopole pair coupling and the spin-aligned coupling schemes.

Systematic study of proton-neutron pairing correlations in the nuclear shell model

A systematic study of proton-neutron pairing in 1f − 2p shell nuclei is reported, based on a model that includes deformation, spin-orbit effects and isoscalar and isovector pairing. Selected results are presented for 44Ti, 46V and 48Cr.

On the Photonuclear Rates Calculation for Nuclear Transmutation of Beta Decay Nuclides by Application of the Ginzburg-landau Theory

The main aim of this paper is to found which are the conditions the photonuclear reactions in order to accelerate the beta decay of radionuclides. Therefore, it was developed a new model where the photonuclear reaction at above the giant resonance cross section (GR) is viewed as an incident photon creating superconducting hot spot (hot belt) across nucleons from a neutron-proton(n-p) pair (a quasi-deuteron ) of the valence bosons of a unstable nuclide, accordingly with IBAF model and, followed by a thermally induced vortex crossing, which turns superconducting hot belt into the normal state (vacuum) resulting in a vortex assisted photon beta decay with the fermion (e + ,e - ) release (capture), and eventually the pair break-up by a reaction ) , ( ) , ( p or n γ γ . Since, a such model requires data on energies of vortex and on currents from inside of the nucleon, an analogue of a superconductor model was developed for the nucleon, and tested on the data of natural beta decay viewed as dark count. The results consist in obtaining a precise value of the threshold energy of the photons when the beta decay of the radionuclides is accelerated. Such efficient installations could be , for example, the laser ELI-NP in construction in Romania for the higher energy photons source.

Relation of photonuclear reactions to π-meson photoproduction process

Neutron-proton pair knocking-out by photons from iron (Fe56) nuclei is studied by measuring induced gamma activity of Mn54 isotope. Experiments are performed in a synchrotron beam at two maximum photon energies, 150 and 650 MeV. It was found that the yield of the reaction under study at the maximum energy of 150 MeV is almost zero, whereas a significant yield of gamma active Mn54 nuclei is measured at the energy of 650 MeV. The cross section of the neutron-proton pair production reaction on Fe56 at an energy of 650 MeV is estimated as 4 · 10−27 cm2.