R-process Path Research Articles

Spectroscopy of neutron-rich nuclei produced by multinucleon transfer reactions at KISS

Properties such as lifetimes and masses of neutron-rich nuclei are key parameters for elucidating the astrophysical rapid neutron capture process (r-process). Due to the lack of experimental nuclear data in the relevant extremely neutron-rich region, especially near and above N = 126, the predictions of theoretical nuclear models are crucial for simulating r-process nucleosynthesis. Experimental studies of the properties and structures of neutron-rich nuclei from near the β stability line up to the r-process path provide important inputs to these theoretical models, improving the accuracy of predictions for the nuclear properties involved in the rprocess. We are developing the KEK Isotope Separation System (KISS) at the RIKEN RIBF facility to produce and separate these nuclei to perform spectroscopy experiments. The nuclei of interest are produced by multinucleon transfer reactions, which have been recently attracting renewed interest as they provide a way to access neutron-rich nuclei that are difficult to produce by other methods such as complete fusion or fragmentation. We have performed nuclear and laser spectroscopy, lifetime and mass measurements of neutron-rich nuclei of refractory elements near the N = 126 region using 198,natPt, natTa, natW, and natIr targets irradiated with 136Xe beams. We also extend our spectroscopic studies into the neutron-rich actinide region using 238U beams. This paper introduces the KISS facility and provides an overview of the nuclear spectroscopy experiments carried out there. We also introduce the KISS-1.5 project, which was started in FY2024 to further extend the research into the neutron-rich region.

Sensitivity of the r-process rare-earth peak abundances to nuclear masses

The sensitivity of the r-process rare-earth peak abundances to nuclear masses has been studied in different astrophysical scenarios. The most impactful nuclei lie along the r-process paths at r-process freeze-out and at the time when the neutron-capture timescale is approximately 3 times of the β-decay timescale (τnγ≈3τβ), corresponding to the onset and completion of rare-earth peak formation, respectively. In astrophysical scenario with fission involved, the sensitivities for nuclei lying along the r-process path at freeze-out are masked by the large contribution of fission products to the rare-earth peak abundances. This work provides recommended targets for future researches and thus helps to increase the understanding of rare-earth peak formation mechanism and the efficacy of the rare-earth peak as an r-process site diagnostic.

Momentum Distribution Studies of Projectile Fragments from Peripheral Collisions Below the Fermi Energy

This paper presents our recent studies of multinucleon transfer in peripheral collisions in reactions below the Fermi regime. Our current focus is the study of the mass, angular and momentum distributions of the projectile-like fragments from the reaction of an 86Kr beam at 15 MeV/nucleon with a target of 64Ni. Experimental data from our previous work with the MARS spectrometer at the Cyclotron Institute of Texas A&M University were compared with model calculations. The dynamical stage of the reaction is described with either the Deep-Inelastic Transfer Model (DIT) or with the microscopic Constrained Molecular Dynamics model (CoMD). The de-excitation of the hot projectile-like fragments is performed with the GEMINI model. The momentum distributions are characterized by a quasi-elastic peak and a deep-inelastic peak. Two-body kinematics was employed to extract the total excitation energies of these regions. Through the thorough study of peripheral reactions in the Fermi energy regime we expect to gain valuable information that could lead to the understanding of how the rare isotopes in regions such as the r-process path and the neutron drip line are formed and the reaction mechanism(s) that take place.

MeV neutrino flash from neutron star mergers viar-process nucleosynthesis

AbstractDetection of kilonova AT2017gfo proves that binary neutron star mergers can be the dominant contributor to the production of heavy elements in our Universe. Neutrinos from the radioactive decay of heavy elements would be the most direct messengers of merger ejecta. Based on r-process nucleosynthesis calculations, we study the neutrinos emitted from the β-decay of r-process elements and find that about half of the β-decay energy is carried away by neutrinos. The neutrino energy generation rate remains approximately constant at the early stage (t ≲ 1 s) and then decays as a power-law function with an index of −1.3. This powers a short-lived fast neutrino burst with a peak luminosity of ∼1049 erg s−1 in the early stage. Observation of neutrinos from neutron star mergers will be an important step towards understanding the properties of extremely neutron-rich nuclei and r-process nucleosynthesis, since the dominant contribution to the early time neutrino production is from nuclides near the r-process path. The typical neutrino energy is ≲8 MeV, which is within the energy ranges of the water-Cherenkov neutrino detectors such as Super-Kamiokande and future Hyper-Kamiokande, but the extremely low neutrino flux and event rate in our local Universe challenge the detection of the neutrino flashes.

Roles of tensor and isoscalar pairing interactions in β-decay calculations for possible r-process waiting point nuclei with N ~ 82 and 126* *Supported by the National Natural Science Foundation of China (11575120, 11822504), and Science Specialty Program of Sichuan University (2020SCUNL210)

In this study, we adopt the self-consistent Hartree-Fock-Bogoliubov (HFB) theory with the proton-neutron quasi-particle random phase approximation (pnQRPA) based on the Skyrme force for calculation of the β − decay half-lives for nuclei with N ~ 82 and 126 on possible r-process paths. In the calculations, the Skyrme interaction (e.g., SKO') is adopted, and the tensor interaction is added self-consistently in both HFB and QRPA calculations. We systematically study how the half-life is changed by varying the strength of the triplet-even (TE) and triplet-odd (TO) components as well as the IS pairing. We find that a variation in strength of the IS pairing of approximately 20% does not produce a substantial effect on β-decay rates with or without the tensor force, while a strength variation of the TO tensor force considerably affects the change in the β-decay half-lives for the very neutron rich N ~ 82 and 126 isotonic chains. In addition, with the inclusion of the tensor force, the GT decay becomes dominant for very neutron-rich nuclei.

Nuclear Mass Predictions of the Relativistic Density Functional Theory with the Kernel Ridge Regression and the Application to r-Process Simulations

The kernel ridge regression (KRR) and its updated version taking into account the odd-even effects (KRRoe) are employed to improve the mass predictions of the relativistic density functional theory. Both the KRR and KRRoe approaches can improve the mass predictions to a large extent. In particular, the KRRoe approach can significantly improve the predictions of the one-nucleon separation energies. The extrapolation performances of the KRR and KRRoe approaches to neutron-rich nuclei are examined, and the impacts of the KRRoe mass corrections on the r-process simulations are studied. It is found that the KRRoe mass corrections for the nuclei in the r-process path are remarkable in the light mass region, e.g., A<150, and this could influence the corresponding r-process abundances.

Trends in the Structure of Nuclei near 100Sn

Inevitable progress has been achieved in recent years regarding the available data on the structure of 100Sn and neighboring nuclei. Updated nuclear structure data in the region is presented using selected examples. State-of-the-art experimental techniques involving stable and radioactive beam facilities have enabled access to those exotic nuclei. The analysis of experimental data has established the shell structure and its evolution towards N = Z = 50 of the number of neutrons, N, and the atomic number, Z, seniority conservation and proton–neutron interaction in the g9/2 orbit, the super-allowed Gamow–Teller decay of 100Sn, masses and half-lives along the rapid neutron-capture process (r-process) path and super-allowed α decay beyond 100Sn. The status of theoretical approaches in shell model and mean-field investigations are discussed and their predictive power assessed. The calculated systematics of high-spin states for N = 50 isotopes including the 5− state and N = Z nuclei in the g9/2 orbit is presented for the first time.

Neutron transfer reactions on the ground state and isomeric state of a Sn130 beam

The structure of nuclei around the neutron-rich nucleus 132Sn is of particular interest due to the vicinity of the Z = 50 and N = 82 shell closures and the r-process nucleosynthetic path. Four states in 131Sn with a strong single-particle-like component have previously been studied via the (d,p) reaction, with limited excitation energy resolution. The 130Sn(9Be,8Be)131Sn and 130Sn(13C,12C)131Sn single-neutron transfer reactions were performed in inverse kinematics at the Holifield Radioactive Ion Beam Facility using particle-gamma coincidence spectroscopy. The uncertainties in the energies of the single-particle-like states have been reduced by more than an order of magnitude using the energies of gamma rays. The previous tentative Jpi values have been confirmed. Decays from high-spin states in 131Sn have been observed following transfer on the isomeric component of the 130Sn beam. The improved energies and confirmed spin-parities of the p-wave states important to the r-process lead to direct-semidirect cross-sections for neutron capture on the ground state of 130Sn at 30 keV that are in agreement with previous analyses. A similar assessment of the impact of neutron-transfer on the isomer would require significant nuclear structure and reaction theory input. There are few measurements of transfer reaction on isomers, and this is the first on an isomer in the 132Sn region.

R-Process with Magnetized Nuclei at Dynamo-Explosive Supernovae and Neutron Star Mergers

Nucleosynthesis at latge magnetic induction levels relevant to core-collapse supernovae and neutron star mergers is considered. For respective magnetic fields of a strength up to ten teratesla, atomic nuclei exhibit a linear magnetic response due to the Zeeman effect. Such nuclear reactivity can be described in terms of magnetic susceptibility. Susceptibility maxima correspond to half-filled shells. The neutron component rises linearly with increasing shell angular momentum, while the contribution of protons grows quadratically due to considerable income from orbital magnetization. For a case j = l + 1/2, the proton contribution makes tens of nuclear magnetons and significantly exceeds the neutron values which give several units. In a case j = l − 1/2, the proton component is almost zero up to the g shell. A noticeable increase in the generation of corresponding explosive nucleosynthetic products with antimagic numbers is predicted for nuclei at charge freezing conditions. In the iron group region, new seeds are also created for the r-process. In particular, the magnetic enhancement of the volume of 44Ti isotopes is consistent with results from observations and indicates the substantial increase in the abundance of the main titanium isotope (48Ti) in the Galaxy’s chemical composition. Magnetic effects are proven to result in a shift of the r-process path towards smaller mass numbers, as well as an increase in the volume of low-mass nuclides in peaks of the r-process nuclei.

Markov Chain Monte Carlo Predictions of Neutron-rich Lanthanide Properties as a Probe of r-process Dynamics

Abstract Lanthanide element signatures are key to understanding many astrophysical observables, from merger kilonova light curves to stellar and solar abundances. To learn about the lanthanide element synthesis that enriched our solar system, we apply the statistical method of Markov Chain Monte Carlo to examine the nuclear masses capable of forming the r-process rare-earth abundance peak. We describe the physical constraints we implement with this statistical approach and demonstrate the use of the parallel chains method to explore the multidimensional parameter space. We apply our procedure to three moderately neutron-rich astrophysical outflows with distinct types of r-process dynamics. We show that the mass solutions found are dependent on outflow conditions and are related to the r-process path. We describe in detail the mechanism behind peak formation in each case. We then compare our mass predictions for neutron-rich neodymium and samarium isotopes to the latest experimental data from the CPT at CARIBU. We find our mass predictions given outflows that undergo an extended (n,γ)⇄(γ,n) equilibrium to be those most compatible with both observational solar abundances and neutron-rich mass measurements.

Exploring the uncertainties in theoretical predictions of nuclear β-decay half-lives * *Partly supported by the National Natural Science Foundation of China under (11805004, 11875070), the Key Research Foundation of Education Ministry of Anhui Province (KJ2020A0485), and the Open fund for Discipline Construction, Institute of Physical Science and Information Technology, Anhui University

Nuclear -decay half-lives are predicted based on an empirical formula and the mass predictions from various nuclear models. It is found that the empirical formula can reproduce the nuclear -decay half-lives well, especially for short-lived nuclei with s. The theoretical half-life uncertainties from -decay energies and the parameters of the empirical formula are further investigated. It is found that the uncertainties of the half-lives are relatively large for heavy nuclei and nuclei near the neutron-drip line. For nuclei on the r-process path, the uncertainties for those with are about one order of magnitude, which are much larger than the uncertainties for those with and . However, theoretical uncertainties from the parameters of the empirical formula are relatively small for the nuclei on the r-process path, which indicates that the empirical formula is very suitable for predicting the -decay half-lives in r-process simulations.

Spin-orbit interaction near the neutron drip line and shell effects at N = 82 near the r-process path

We have studied the behavior of charge and neutron radii across the shell-closure at N = 82 near the neutron dripline in a comparative approach which includes Relativistic Mean Field (RMF) Theory and the nonrelativistic Skyrme-Hartree-Fock (SHF) Theory. We have investigated the extent and the effects of the spin-orbit interaction on the ground state properties of neutron-rich nuclei in the extreme region. RMF and SHF calculations have been performed for nuclei in the isotopic chains from Kr (Z = 36) to Sn (Z = 50) near the neutron dripline across the magic number N = 82. We present the results of shell-effects on charge radii, neutron radii, and the neutron skin thickness. We have examined the single particle energies (SPE) in the context of the kinks in the radii. It is shown that RMF theory produces a kink in charge radii, similar to what was seen experimentally for Pb (Z = 82) isotopic chains. This is in contrast to the Skyrme forces, which do not produce such a kink.

Approaching neutron-rich nuclei towards the r-process path in 86Kr-induced collisions at 15 MeV/nucleon

The production cross sections of projectile-like fragments from collisions of 15 MeV/nucleon 86Kr with 64,58Ni and 124,112Sn have been measured using a magnetic separator with emphasis on the neutron-rich isotopes. Neutron pick-up isotopes (with up to 6-8 neutrons picked-up from the target) were observed with large cross sections. The present results were also compared with our previous data of the same reactions at 25 MeV/nucleon. The data at 15 MeV/nucleon show enhanced production of neutron-rich isotopes very close to the projectile, relative to the corresponding data at 25 MeV/nucleon. The large cross sections of such reactions involving peripheral nucleon exchange, indicate that these reactions offer a novel route to access extremely neutron-rich rare isotopes towards the the astrophysical r-process path and the neutron-drip line.

Intermediate-Energy Fragmentation of Heavy-Element Beams: A Novel Approach Towards the Nuclear Drip-Lines

The fragmentation of heavy beams at intermediate energy and the opportunities offered by this in the exploration of heavy nuclei near the drip lines will be discussed. First, a study of 1 9 7Au projectile fragmentation at 30 MeV/nucleon will be presented in which an appreciable number of new p-rich nuclei were observed. Perspectives of proton radioactivity studies using the fragmentation approach will be discussed. Second, a study of the fission of 2 3 8U projectiles at 20 MeV/nucleon will be presented, where a number of new neutron rich nuclei were identified. Production rates of extremely η-rich nuclides from a typical projectile-fragmentation facility are given. A large number of nuclei along the astrophysical r-process path can be investigated and approach of the neutron dripline in the region Z=45-50 may be possible. The ability to produce and study these either p-rich or η-rich nuclei at intermediate (and possibly lower) energy facilities may open a variety of possibilities for experimental studies of exotic heavy nuclei.

β− -Decay Half-lives Using the ANN Model: Input for the R-Process

Full understanding of nucleosynthesis via the r-process continues to be a major challenge for nuclear astrophysics. Apart from issues within astrophysical modeling, there remain significant uncertainties in the nuclear physics input, notably involving the β- decay halflives of neutron-rich nuclei. Both the element distribution on the r-process path and the time scale of the r-process are highly sensitive to β− lifetimes. Since the majority of nuclides that lie on the r-process path will not be experimentally accessible in the foreseeable future, it is important to provide accurate predictions from reliable models. Toward this end, a statistical global model of the β−-decay halflife systema- tics has been developed to estimate the lifetimes of nuclides relevant to the r-process, in the form of a fully-connected, multilayer feedforward Artificial Neural Network (ANN) trained to predict the halflives of ground states that decay 100% by the β− mode. In predictive performance, the model can match or even surpass that of conventional models of β-decay systematics. Results are presented for nuclides situated on the r-ladders N=50, 82 and 126 where abundances peak, as well as for others that affect abundances between peaks. Also reported are results for halflives of interesting neutron-rich nuclides on or towards the r-process path that have been recently measured. Comparison with results from experiment and conventional models is favorable.

Influence of nuclear mass uncertainties on radiative neutron-capture rates

Neutron-capture rate plays a crucial role in the rapid neutron-capture process (r-process) and nuclear mass is one of the most important inputs for the estimations of neutron-capture rates. In this work, we employ ten nuclear mass models, including macroscopic, macroscopic-microscopic, and microscopic models, to calculate the radiative neutron-capture rates for the nuclei relevant to the r-process at various temperatures $T={10}^{5}--{10}^{10}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. It is found that the differences in the predictions of neutron-capture rate using these mass models get larger when moving to the neutron-rich and superheavy regions, whereas they generally become smaller with the increase of temperature. Taking the nuclei on the $N=126$ r-process path as examples, we find that the uncertainties of neutron-capture rates at $T={10}^{9}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ range from about one to four orders of magnitude. The influence of the experimental mass errors on neutron-capture rates is investigated as well.

Rare Isotope Production in peripheral heavy-ion collisions at beam energy of 15 MeV/nucleon

This paper presents our recent study on the production of neutron-rich rare isotopes with heavy-ion beams in the energy region of 15 MeV/nucleon. We present calculated production cross sections of neutron-rich nuclides from collisions of a 86Kr (15 MeV/nucleon) beam with 238U targets. Our calculations are based on a two-step approach: the dynamical stage of the collision is described with either the phenomenological Deep-Inelastic Transfer model (DIT), or with the microscopic Constrained Molecular Dynamics model (CoMD). The de-excitation of the hot heavy projectile fragments is performed with the Statistical Multifragmentation Model (SMM). We also performed calculations with a radioactive beam of 92Kr (15 MeV/nucleon) with a target of 238U and observed that the multinucleon transfer mechanism leads to very neutron-rich nuclides toward and beyond the astrophysical r-process path. In the future, we plan to experimentally investigate such reactions in the KOBRA spectrometer at the RISP facility in Korea. We conclude that the reaction mechanism at beam energies below the Fermi energy involving periph- eral nucleon exchange, constitutes a novel and effective route to access extremely neutron-rich isotopes toward the r-process path and the neutron drip-line.

Systematic Study of Neutron-Rich Rare Isotope Production in Peripheral Heavy-Ion Collisions at Beam Energies 15–25 MeV/nucleon

The production cross sections of projectile-like fragments from collisions of 86Kr projectiles with 64,58Ni and 124,112Sn targets at 15 and 25 MeV/nucleon are studied systematically with emphasis on the neutron-rich isotopes. Our recent experimental data are compared with calculations for the above collisions employing a hybrid approach. The dynamical stage of the projectile-target interaction was described with either the phenomenological deep-inelastic transfer (DIT) model or with the the microscopic constrained molecular dynamics model (CoMD). Subsequently, for the de-excitation of the projectile-like fragments, the statistical multifragmentation model (SMM) or the binary-decay code GEMINI were employed. An overall good agreement with the experimental results was obtained. We point out that our current understanding of the reaction mechanism at beam energies below the Fermi energy suggests that such nuclear reactions, involving peripheral nucleon exchange, can be exploited as a novel route to access extremely neutron-rich rare isotopes toward the r-process path and the hard-to-reach neutron drip-line. For this purpose, we believe that the use of re-accelerated neutron-rich radioactive beams may offer unique and exciting opportunities toward unexplored regions of the nuclear landscape.

Advances at TRIUMF-ISAC and decay of neutron-rich Cd studied with GRIFFIN

The β-decay half lives of nuclei near the r-process path are critical information required for abundance calculations, especially those near neutron number N = 82. Specifically, the nuclei below doubly-magic 132Sn are key, and play an important role in the formation and shape of the second r-process abundance peak. The half lives in this region are challenging to measure due to the significant β-delayed neutron decay branches and the population of isomeric states with half lives comparable to the ground states. However, by measuring the time distribution of γ rays, these complications can be eliminated. This requires, however, a very effcient γ-ray spectrometer since the production of isotopes in this region is very limited. The new GRIFFIN array at TRIUMF-ISAC provides the high effciency required for these measurements. Recent improvements in the quality of the beams produced at TRIUMF, employing the IG-LIS device, are outlined, as well as the current status of the ARIEL facility. The GRIFFIN spectrometer and its use are briefly described. The experiment to measure the half lives of 128-130Cd is outlined and the results given, and some examples of the power of GRIFFIN to expand decay schemes, specifically for the decay of 128Cd to 128In, are given.

Production and Study of Neutron-rich Nuclei Using the LICORNE Directional Neutron Source

We have recently successfully demonstrated a new technique for production and study of many of the most exotic neutron-rich nuclei at moderate spins. LICORNE, a newly developed directional inverse-kinematic fast neutron source at the IPN Orsay, was coupled to the MINIBALL high resolution -ray spectrometer to study nuclei the furthest from stability using the 238U(n; f) reaction. This reaction and 232Th(n; f) are the most neutron-rich fission production mechanisms achievable and can be used to simultaneously populate hundreds of neutron-rich nuclei up to spins of 16 ~. High selectivity in the experiment was achieved via triple -ray coincidences and the use of a 400 ns period pulsed neutron beam, a technique which is unavailable to other population mechanisms such as 235U(nth; f) and 252Cf(SF). The pulsing allows time correlations to be exploited to separate delayed rays from isomeric states in the hundreds of nuclei produced, which are then used to cleanly select a particular nucleus and its exotic binary partners. In the recent experiment, several physics cases are simultaneously addressed such as shape coexistence, the evolution of shell closures far from stability, and the spectroscopy of nuclei in the r-process path near N = 82. Preliminary physics results on anomalies in the 238U(n; f) fission yields and the structure of the 138Te and 100Sr nuclei will soon be published. A future project, -ball, to couple LICORNE with a hybrid escape-suppressed spectrometer to refine further the technique and achieve a large increase in the observational limit is discussed.