Proton-rich Nuclei Research Articles

Study of the proton halo structure of nuclei 23Al and 27P using the binary cluster model

The neutron, proton, and matter densities of the ground state of the proton-rich 23Al and 27P exotic nuclei were analyzed using the binary cluster model (BCM). Two density parameterizations were used in BCM calculations namely; Gaussian (GS) and harmonic oscillator (HO) parameterizations. According to the calculated results, it found that the BCM gives a good description of the nuclear structure for above proton-rich exotic nuclei. The elastic form factors of the unstable 23Al and 27P exotic nuclei and those of their stable isotopes 27Al and 31P are studied by the plane-wave Born approximation. The main difference between the elastic form factors of unstable nuclei and their stable isotopes is caused by the variation in the proton density distributions, especially the details of the outer part. Moreover, the Glauber model is used to calculate the matter rms radii and reaction cross-section of these exotic nuclei. The calculated results of the mentioned nuclei give a good accordance with the experimental data.

Probing nuclear forces beyond the nuclear drip line: the cases of $$^{16}$$F and $$^{15}$$F

The unbound proton-rich nuclei $^{16}$F and $^{15}$F are investigated experimentally and theoretically. Several experiments using the resonant elastic scattering method were performed at GANIL with radioactive beams to determine the properties of the low lying states of these nuclei. Strong asymmetry between $^{16}$F-$^{16}$N and $^{15}$F-$^{15}$C mirror nuclei is observed. The strength of the $nucleon-nucleon$ effective interaction involving the loosely bound proton in the $s_{1/2}$ orbit is significantly modified with respect to their mirror nuclei $^{16}$N and $^{15}$C. The reduction of the effective interaction is estimated by calculating the interaction energies with a schematic zero-range force. It is found that, after correcting for the effects due to changes in the radial distribution of the single-particle wave functions, the mirror symmetry of the $n-p$ interaction is preserved between $^{16}$F and $^{16}$N, while a difference of 63\% is measured between the $p-p$ versus $n-n$ interactions in the second excited state of $^{15}$F and $^{15}$C nuclei. Several explanations are proposed.

Isobaric-multiplet mass equation in a macroscopic-microscopic approach

We study the a, b and c coefficients of the isobaric-multiplet mass equation using a macroscopic-microscopic approach developed by P. Moeller and his collaborators in ADNDT 59, 185 (1995) and ADNDT 109-110, 1 (2016). We show that already the macroscopic part of the finite-range liquid-drop model (FRLDM) describes the general trend of the a and b coefficients relatively well, while the staggering behavior of b coefficients for doublets and quartets can be understood in terms of the difference of average proton and neutron pairing energies. The sets of isobaric masses, predicted by the full macroscopic-microscopic approaches, are used to explore the general trends of IMME coefficients up to A=100. We conclude that while the agreement for a coefficients is quite satisfactory, the global approaches have less sensitivity to predict the staggering pattern observed for b coefficients of doublets and quartets. The best set of theoretical b coefficients for multiplets up to about A=100 is used to predict masses of proton-rich nuclei based on the known experimental masses of neutron-rich mirror partners, and subsequently to investigate their one- and two-proton separation energies. The estimated position of the proton-drip line is in fair agreement with known experimental data. These masses are important for simulations of the astrophysical rp-process.

Progress on decays of extremely proton-rich nuclei based on the HIRFL-RIBLL1 facility

Progress on decays of extremely proton-rich nuclei based on the HIRFL-RIBLL1 facility

Sensitivity of the NEXT experiment to Xe-124 double electron capture

Double electron capture by proton-rich nuclei is a second-order nuclear process analogous to double beta decay. Despite their similarities, the decay signature is quite different, potentially providing a new channel to measure the hypothesized neutrinoless mode of these decays. The Standard-Model-allowed two-neutrino double electron capture (2νEC EC) has been predicted for a number of isotopes, but only observed in 78Kr, 130Ba and, recently, 124Xe. The sensitivity to this decay establishes a benchmark for the ultimate experimental goal, namely the potential to discover also the lepton-number-violating neutrinoless version of this process, 0νEC EC. Here we report on the current sensitivity of the NEXT-White detector to 124Xe 2νEC EC and on the extrapolation to NEXT-100. Using simulated data for the 2νEC EC signal and real data from NEXT-White operated with 124Xe-depleted gas as background, we define an optimal event selection that maximizes the NEXT-White sensitivity. We estimate that, for NEXT-100 operated with xenon gas isotopically enriched with 1 kg of 124Xe and for a 5-year run, a sensitivity to the 2νEC EC half-life of 6 × 1022 y (at 90% confidence level) or better can be reached.

Predictions for the α decay of proton-rich nuclei in the range of 86⩽Z⩽98

The α decay half-lives of odd-A and odd-odd nuclei ranging in 65⩽Z⩽112 are investigated by employing the modified universal decay law and the unified fission model, both of them take into account the contribution of centrifugal potential. The good agreement with the experimental data indicates that the modified universal decay law and the unified fission model are useful tools to investigate these α-decays when they contain the effect of centrifugal potential. Relatively long half-lives are predicted by using the new mass table WS2016+RBFs to determine the Qα for many unknown proton-rich nuclei between Rn and Cf, sufficient to detect them if synthesized in a laboratory.

Isovector giant dipole resonances in proton-rich Ar and Ca isotopes * *This work is partly Supported by the National Natural Science Foundation of China (11975096, 11775014) and the Fundamental Research Funds for the Central Universities (2020NTST06)

The isovector giant dipole resonances (IVGDR) in proton-rich Ar and Ca isotopes have been systematically investigated using the resonant continuum Hartree-Fock+BCS (HF+BCS) and quasiparticle random phase approximation (QRPA) methods. The Skyrme SLy5 and density-dependent contact pairing interactions are employed in the calculations. In addition to the giant dipole resonances at energy around 18 MeV, pygmy dipole resonances (PDR) are found to be located in the energy region below 12 MeV. The calculated energy-weighted moments of PDR in nuclei close to the proton drip-line exhaust about 4% of the TRK sum rule. The strengths decrease with increasing mass number in each isotopic chain. The transition densities of the PDR states show that motions of protons and neutrons are in phase in the interiors of nuclei, while the protons give the main contribution at the surface. By analyzing the QRPA amplitudes of proton and neutron 2-quasiparticle configurations for a given low-lying state, we find that only a few proton configurations give significant contributions. They contribute about 95% to the total QRPA amplitudes, which indicates that the collectivity of PDR states is not strong in proton-rich nuclei in the present study.

Determination of the Zn60 level density from neutron evaporation spectra

Nuclear reactions of interest for astrophysics and applications often rely on statistical model calculations for nuclear reaction rates, particularly for nuclei far from $\beta$-stability. However, statistical model parameters are often poorly constrained, where experimental constraints are particularly sparse for exotic nuclides. For example, our understanding of the breakout from the NiCu cycle in the astrophysical rp-process is currently limited by uncertainties in the statistical properties of the proton-rich nucleus $^{60}$Zn. We have determined the nuclear level density of $^{60}$Zn using neutron evaporation spectra from $^{58}$Ni($^3$He, n) measured at the Edwards Accelerator Laboratory. We compare our results to a number of theoretical predictions, including phenomenological, microscopic, and shell model based approaches. Notably, we find the $^{60}$Zn level density is somewhat lower than expected for excitation energies populated in the $^{59}$Cu(p,$\gamma$)$^{60}$Zn reaction under rp-process conditions. This includes a level density plateau from roughly 5-6 MeV excitation energy, which is counter to the usual expectation of exponential growth and all theoretical predictions that we explore. A determination of the spin-distribution at the relevant excitation energies in $^{60}$Zn is needed to confirm that the Hauser-Feshbach formalism is appropriate for the $^{59}$Cu(p,$\gamma$)$^{60}$Zn reaction rate at X-ray burst temperatures.

Reaction dynamics of exotic and stable weakly-bound nuclei using a four-body continuum-discretized coupled-channels formalism

Reactions induced by four weakly-bound nuclei on a lead target, at energies around the Coulomb barrier, are described using the same four-body continuum-discretized coupled-channels formalism. The nuclei considered are the halo nuclei 11Li and 6He, the stable nucleus 9Be and the proton-rich nucleus 10C. The differences and similarities on the reaction dynamics are addressed.

Insight into the reaction dynamics of proton drip-line nuclear system 17F+58Ni at near-barrier energies

The mechanism of reactions with weakly-bound proton-rich nuclei at energies near the Coulomb barrier is a long-standing open question owing to the paucity of experimental data. In this study, a complete kinematics measurement was performed for the proton drip-line nucleus 17F interacting with 58Ni at four energies near the Coulomb barrier. Thanks to the powerful performance of the detector array, exhaustive information on the reaction channels, such as the differential cross sections for quasielastic scattering, exclusive and inclusive breakup, as well as for fusion-evaporation protons and alphas, was derived for the first time. The angular distributions of quasielastic scattering and exclusive breakup can be described reasonably well by the continuum-discretized coupled-channels calculations. The inclusive breakup was investigated using the three-body model proposed by Ichimura, Austern, and Vincent, and results indicate the non-elastic breakup is the dominant component. The total fusion cross sections were determined by the fusion-evaporation protons and alphas. Based on the measured exclusive breakup data, the analysis of the classical dynamical simulation code PLATYPUS demonstrates that the incomplete fusion plays a minor role. Moreover, compared with 16O+58Ni, both the reaction and total fusion cross sections of 17F+58Ni exhibit an enhancement in the sub-barrier energy region, which mainly arises from couplings to the continuum states. This work indicates that the information of full reaction channels is crucially important to comprehensively understand the reaction mechanisms of weakly bound nuclear systems.

New investigations on the 32S(3He,d)33Cl reaction at 9.6 MeV bombarding energy

The 32S(3He,d)33Cl one-proton transfer reaction is a powerful tool to investigate the spectroscopy of low-lying states in the proton-rich 33Cl nucleus. However, the extraction of firm differential cross-section data at various angles to benchmark and constrain theoretical models is made challenging by the presence of competitive reactions on target contaminants. In this paper we report on a recent measurement using a new generation hodoscope of silicon detectors, capable to detect and identify emitted deuterons down to energies of the order of 2 MeV. The high angular segmentation of our hodoscope combined with a suitable target to control possible contaminants, allowed to unambiguously disentangle the contribution of various states in 33Cl, in particular the 2.352 MeV state lying just few tens of keV above the proton separation energy.

Nuclear structure of proton drip-line nuclei as an input to nuclear astrophysics

The observation, and the theoretical interpretation of the decay of proton rich nuclei at the proton drip line, are a probe to the nuclear structure at the extremes of stability, with strong implications to nuclear astrophysics models. Nuclear levels, and proton separation energies, can be assigned in these studies, establishing information that could not be obtained otherwise due to very short lifetimes of these nuclei.

Prediction of the Spin-Parities and the Magnetic Moments for the Ground States of Proton-rich Nuclei with Z = 21–30

The present paper reports on the spin-parities and the magnetic moments for the ground states of 44 proton-rich isotopes with Z = 21–30 and A = 36–57, which are important for studies of either reaction rates in X-ray bursts or nuclear structure. These nuclear properties were calculated based on the single-particle shell model. The spins of the concerned nuclei were compared to available experimental data adopted from the NuDat database to evaluate the variations in the astrophysical rates of the rp-process reactions. We found discrepancies, due to the deformed nuclear structure, between the present results and those reported in the NuDat database. The spin uncertainties result in large variations, 13%–200%, in the astrophysical rates of the rp-process reactions. In particular, the spin uncertainties of the 44V and the 46–49Mn isotopes significantly affect the astrophysical rates of the reverse reactions of the proton captures 43Ti(p, γ)44V(p, γ)45Cr, 45Cr(p, γ)46Mn(p, γ)47Fe, 47Mn(p, γ)48 Fe, 47Cr(p, γ)48 Mn(p, γ)49 Fe, and 48Cr(p, γ)49 Mn(p, γ)50 Fe. Moreover, the magnetic moments of most of the isotopes were predicted for the first time. The results show that the magnetic moments are in the order of μp(1f7/2) > μp(2p3/2) > μn(1d3/2) > μp (1f7/2)for thenuclei having an unpaired nucleon in the proton/neutron shells. The present study suggests that reliable calculations and/or measurements for the properties of proton-rich nuclei are highly demanded.

N = Z even–even proton-rich nuclei and nuclear structure

Nuclear systems near nucleon drip lines have great importance in studying nucleon–nucleon force properties. However, such unstable systems have few experimental data. Due to technical development in recent years, new experimental data assembly provides huge information to analyze possible observation of new phenomena. In the aim of exploring proton-rich nuclei near closed shells, we have studied the even–even Z = N nuclei in $$28<Z<50$$ , $$50<N<82$$ mass region. This study is based on the investigation of the core polarization and the monopole effect originated from the interactions between the magic core and the valence nucleons. In this context, this work includes the calculation of the energy spectra of Zr, Mo, Ru, Pd and Cd nuclei. These calculations were performed in the framework of the nuclear shell model using NuShellX@MSU code. Using recent single-particle energies, the three-body effect consideration introduces some modifications on the effective interaction. By means of the modified interaction, some nuclear properties of the studied nuclei have been evaluated. The results show a reasonable agreement with the available experimental data. The monopole effect is an important component of nucleon–nucleon interaction, in which its consideration gives a reasonably agreement with available data.

Systematic study of proton radioactivity of spherical proton emitters with Skyrme interactions

Proton radioactivity is an important and common process of the natural radioactivity of proton-rich nuclei. In our previous work [J. H. Cheng et al., Nucl. Rhys. A 997, 121717 (2020)], we systematically studied the proton radioactivity half-lives of 53< Z <83 nuclei within the two-potential approach with Skyrme-Hartree-Fock. The calculations can well reproduce the experimental data of spherical nuclei. The present work is an extension of the previous work. We systematically study 32 proton radioactivity half-lives of spherical nuclei with the two-potential approach with Skyrme-Hartree-Fock (TPA-SHF) within 115 different versions of the Skyrme interactions. The calculated results indicate that the SLy7 Skyrme interaction gives the lowest r.m.s. deviation in the description of the experimental data of the spherical proton emitters among all the different versions of the Skyrme interactions. In addition, we use this model with the SLy7 Skyrme interaction to predict the proton radioactivity half-lives of 7 spherical nuclei in the same region, whose proton radioactivity is energetically allowed or has been observed but is not yet quantified.

The origin of the elements: a century of progress.

This review assesses the current state of knowledge of how the elements were produced in the Big Bang, in stellar lives and deaths, and by interactions in interstellar gas. We begin with statements of fact and discuss the evidence that convinced astronomers that the Sun is fusing hydrogen, that low-mass stars produce heavy elements through neutron capture, that massive stars can explode as supernovae and that supernovae of all types produce new elements. Nucleosynthesis in the Big Bang, through cosmic ray spallation, and in exploding white dwarfs is only ranked below the above facts in certainty because the evidence, while overwhelming, is so far circumstantial. Next, we highlight the flaws in our current understanding of the predictions for lithium production in the Big Bang and/or its destruction in stars and for the production of the elements with atomic number [Formula: see text]. While the theory that neutron star mergers produce elements through neutron-capture has powerful circumstantial evidence, we are unconvinced that they produce all of the elements past nickel. Also in dispute is the exact mechanism or mechanisms that cause the white dwarfs to explode. It is difficult to determine the origin of rare isotopes because signatures of their production are weak. We are uncertain about the production sites of some lithium and nitrogen isotopes and proton-rich heavy nuclei. Finally, Betelgeuse is probably not the next star to become a supernovae in the Milky Way, in part because Betelgeuse may collapse directly to a black hole instead. The accumulated evidence in this review shows that we understand the major production sites for the elements, but islands of uncertainty in the periodic table exist. Resolving these uncertainties requires in particular understanding explosive events with compact objects and understanding the nature of the first stars and is therefore primed for new discoveries in the next decades. This article is part of the theme issue 'Mendeleev and the periodic table'.

New isospin-breaking “USD” Hamiltonians for the sd shell

Two new USD-type Hamiltonians, USDC and USDI, have been developed that directly incorporate Coulomb and other isospin-breaking interactions. Starting from ab initio interactions, linear combinations of two-body matrix elements were constrained by experimental energy levels in sd-shell nuclei. With this method, binding energies and excitation energies of proton-rich nuclei in the shell can be added to the data set used in the fit. USDC is based on the same renormalized $G$ matrix used in the derivation of previous USD-type Hamiltonians, while USDI is derived from in-medium similarity renormalization group (IMSRG) interactions. Both contain an analytic Coulomb interaction with Miller-Spencer short-range correlations and an effective isotensor interaction. Also presented are modifications to these interactions, USDCm and USDIm, that have had the Coulomb interaction constrained to better reproduce the experimental $b$ coefficients of the isobaric mass multiplet equation. These Hamiltonians are used to provide new predictions for the proton-dripline and to examine isospin-level mixing and other properties of $sd$-shell nuclei. An empirical expression for the Thomas-Ehrman shift in loosely bound and unbound proton-rich states is presented, and several such states are examined.

Progress of experimental studies on two-proton emission

<p indent=0mm>Radioactivity is one of the most significant characteristics of unstable nuclei, the familiar modes including α, β, γ decay and fission. Two-proton (2p) emission is an exotic radioactivity mode of nuclei in the proton drip-line area, which is very significant for studying the structure, mass and correlation between proton pair of extremely proton-rich nuclei and one of the frontiers in radioactive ion beam physics. In this article, progress of experimental studies on two-proton emission was briefly reviewed. Several experimental measurement methods were introduced. The identification method for the mechanism of two-proton emission was discussed. Two-proton emission was predicted to be existed in extremely proton-rich nuclei in 1960. For 2p emission from the ground state of a nucleus with long half-life, it was observed for the first time in 2002 due to the difficulty in the production of a candidate. Only few decay events were observed for the four proton drip-line nuclei with a long half-life ground state. For the detection of 2p emission process, the in-flight decay, implantation decay and optical time projection chamber (OTPC) plus CCD method are the three main experimental methods. The in-flight decay method is suitable for the candidate with very short half-life and almost inapplicable for the candidate with a half-life longer than 1 μs. The implantation decay method is suitable for the candidate with long half-life, especially for β-delayed decay nuclei. In this case, the measured decay signal will be very clear with low background due to the separation recording of the implantation and decay events. The OTPC plus CCD method could record a picture of 2p emission process directly for the candidate. Generally, there are three mechanisms for proton-rich nuclei to emit two protons: (1) ²He cluster emission; (2) three-body emission; (3) sequential emission. The ²He cluster emission is basically two protons constrained by the pair correlation into a quasi-bound singlet, i.e., ¹S₀ configuration. Such a quasi-bound state can only exist for very short time and then becomes separated after penetrating through the Coulomb barrier. Usually, the two-proton relative momentum and the opening angle distributions are used to identify the mechanism of 2p emission in experimental investigations. However, this method can only distinguish it being ²He cluster emission or not. For the three-body and sequential emissions, the key difference is the emission time between the two protons. The proton-proton momentum correlation function (also called HBT) method is a widely used analysis technique in nuclear physics, which can extract the emission time of a particle. It has been demonstrated that the three-body and sequential emissions could be distinguished by using the HBT method. The difficulty in the study of exotic radioactivity is the low production rate of the unstable nuclei in the drip-line area. The high-intensity heavy-ion accelerator facility (HIAF) under construction in China has an advantage of high beam intensity, the most frontier work in the field of exotic radioactivity for nuclei around the drip-line could be performed on it by adopting the superior detection technique and investigation method.

Effect of source size and emission time on the p–p momentum correlation function in the two-proton emission process

The effect of source size and emission time on the proton–proton (p–p) momentum correlation function ($$C_\mathrm{pp}(q)$$) has been studied systematically. Assuming a spherical Gaussian source with space and time profile according to the function $$S(r,t)\sim \exp (-r^2/2r_{0}^{2}-t/\tau )$$ in the correlation function calculation code (CRAB), the results indicate that one $$C_\mathrm{pp}(q)$$ distribution corresponds to a unique combination of source size $$r_0$$ and emission time $$\tau $$. Considering the possible nuclear deformation from a spherical nucleus, an ellipsoidal Gaussian source characterized by the deformation parameter $$\epsilon =\Delta {R}/R$$ has been simulated. There is almost no difference of $$C_\mathrm{pp}(q)$$ between the results of spherically and ellipsoidally shaped sources with small deformation. These results indicate that a unique source size $$r_0$$ and emission time could be extracted from the p–p momentum correlation function, which is especially important for identifying the mechanism of two-proton emission from proton-rich nuclei. Furthermore, considering the possible existence of cluster structures within a nucleus, the double Gaussian source is assumed. The results show that the p–p momentum correlation function for a source with or without cluster structures has large systematical differences with the variance of $$r_{0}$$ and $$\tau $$. This may provide a possible method for experimentally observing the cluster structures in proton-rich nuclei.

The Production of the Isotopes with Z = 98–106 in the Multinucleon Transfer Reaction 238U + 251Cf

The multinucleon transfer process in the reaction 238U + 251Cf is investigated by the dinuclear system model with GEMINI++ code. The time evolution of the fragment probability is studied. For the fragments with larger proton number, the time needed to reach the maximum probability is larger. The isotopic distributions become wider at larger evolution time, and more neutron-rich and proton-rich nuclei can be produced. On the neutron-rich side, the ΔU values becomes higher at larger incident angular momentum, resulting in lower cross sections of neutron-rich nuclei. New neutron-rich isotopes with Z = 98–104 are produced with cross sections larger than 1 nb. Three unknown Db isotopes and two unknown Sg isotopes are generated with cross sections larger than 1 pb.