Neutron-rich Rare Isotopes Research Articles

Collectivity at the prolate-oblate transition: The [formula omitted] lifetime of 190W

The neutron-rich rare isotope 190W is discussed as a candidate for a prolate-oblate transitional nucleus with maximum γ-softness. The collectivity of this isotope is assessed for the first time by the measurement of the reduced E2 transition probability of its first 2+ state to the ground state. The experiment employed the FAst TIming Array (FATIMA), comprised of 36 LaBr3(Ce) scintillators, which was part of the DESPEC setup at GSI, Darmstadt. The 41+ and 21+ states of 190W were populated subsequently to the decay of its 127(12) μs isomeric Jπ=10− state. The mean lifetime of the 21+ state was determined to be τ=274(28) ps, which corresponds to a B(E2;21+→01+) value of 95(10) W.u. The results motivated a revision of previous calculations within an energy-density functional-based interacting boson model-2 approach, yielding E2 transition properties and spectroscopic quadrupole moments for tungsten isotopes. From comparison to theory, the new data suggest that 190W is at the transition from prolate to oblate structure along the W isotopic chain, which had previously been discussed as a nuclear shape-phase transition.

Projectile fragments from peripheral collisions of 25 MeV/nucleon 64Ni on 64Ni

We study the yields and the momentum distributions of projectile-like fragments from peripheral collisions of 64Ni (25 MeV/nucleon) on 64Ni. The experimental data were obtained in previous works with the BigSol separator system at the Cyclotron Institute of Texas A&M University. Projectile fragments were collected and analyzed at forward angles (1.5°-3.0°). The production cross sections and the momentum distributions (p/A) of the fragments were systematically studied and compared with the Deep-Inelastic Transfer (DIT) model and the Constrained Molecular Dynamics (CoMD) model followed by the deexcitation model GEMINI. Special attention is given to the possibility to produce neutron-rich rare isotopes of elements at the beginning of the astrophysical r-process. Moreover, efforts to reconstuct the excitation energy distributions are ongoing in hopes to shed light to the reaction mechanisms of peripheral collisions at this energy (25 MeV/nucleon).

Recent progress of ISOL facility at RAON

The RAON (Rare Isotope Accelerator complex for ON-line experiments) is currently in the stage of commissioning under the Rare Isotope Science Project (RISP) launched in 2011. RAON will utilize an advanced rare isotope beams produced with a high power target by the Isotope Separation On-Line (ISOL) facility, aiming to deliver high purity and intense, neutron-rich rare isotope beams to the post-accelerator and experimental facilities. The RAON ISOL facility consists of a target/ion source module surrounded by movable shielding blocks in a bunker, remote handling facilities for the target operation, a pre-mass separator, a RFQ cooler buncher, an EBIS charge breeder, and an A/q separator. The installation and alignment of the ISOL facility were completed in June 2021. The target/ion source module allows us to bombard a thick target with a 70 MeV proton beam of RAON, producing a variety of rare isotope beams. Rare isotope beams extracted from the target/ion source can be transported to a pre-mass separator at energies of up to 60 keV, and will be cooled in a RFQ-CB. Cooled ion beams can be sent to two different experimental facilities, such as a Mass Measurement System and a Collinear Laser Spectroscopy in ISOL experimental hall. Alternatively, for post-acceleration of ion beams, the singly charged ion beam of interest can be bunched to 108 ions in RFQ-CB and then delivered to the EBIS charge breeder through the EBIS branch system. The preparation of multi charged ion beams for the post-acceleration using the superconducting LINAC of SCL3 will be carried out through the EBIS charge breeder and A/q separator to match the energy of 10 keV/u with A/q < 6 with the requirement of RAON Injector.The first commissioning experiment of the ISOL system started in March 2021 using the 133Cs and 120Sn stable beams produced from the target container combined with the surface ion source and laser Ion source. The stable beam experiments have demonstrated the overall functioning of the RAON ISOL system, and we are planning to carry out the first RI beam test using the SiC target with 70 MeV proton of cyclotron in the coming months.

β Decay of ^{61}V and its Role in Cooling Accreted Neutron Star Crusts.

The interpretation of observations of cooling neutron star crusts in quasipersistent x-ray transients is affected by predictions of the strength of neutrino cooling via crust Urca processes. The strength of crust Urca neutrino cooling depends sensitively on the electron-capture and β-decay ground-state-to-ground-state transition strengths of neutron-rich rare isotopes. Nuclei with a mass number of A=61 are predicted to be among the most abundant in accreted crusts, and the last remaining experimentally undetermined ground-state-to-ground-state transition strength was the β decay of ^{61}V. This Letter reports the first experimental determination of this transition strength, a ground-state branching of 8.1_{-3.1}^{+4.0}%, corresponding to a log ft value of 5.5_{-0.2}^{+0.2}. This result was achieved through the measurement of the β-delayed γ rays using the total absorption spectrometer SuN and the measurement of the β-delayed neutron branch using the neutron long counter system NERO at the National Superconducting Cyclotron Laboratory at Michigan State University. This method helps to mitigate the impact of the pandemonium effect in extremely neutron-rich nuclei on experimental results. The result implies that A=61 nuclei do not provide the strongest cooling in accreted neutron star crusts as expected by some predictions, but that their cooling is still larger compared to most other mass numbers. Only nuclei with mass numbers 31, 33, and 55 are predicted to be cooling more strongly. However, the theoretical predictions for the transition strengths of these nuclei are not consistently accurate enough to draw conclusions on crust cooling. With the experimental approach developed in this work, all relevant transitions are within reach to be studied in the future.

Ion-optical design of High energy FRagment Separator (HFRS) at HIAF

High Intensity heavy-ion Accelerator Facility (HIAF) project is being constructed by the Institute of Modern Physics, Chinese Academy of Sciences (IMP, CAS). An in-flight High energy FRagment Separator (HFRS) at HIAF was designed to study the properties of rare isotopes far away from the line of beta stability and their involved nuclear reactions of astrophysics interests. HFRS is a two-stage magnetic system which consists of Pre-Separator and Main-Separator. The maximum magnetic rigidity of 25 Tm was designed in particular for experiments with very neutron-rich rare isotopes beams and studies of hypernuclei and Δ-resonances in exotic nuclei. A dispersive mode and an achromatic mode were designed for different experimental requirements. The HFRS device can be used to separate and purify the rare isotopes produced by the projectile fragmentation of all primary beams up to Uranium or by the projectile fission of Uranium ions via Bρ-ΔE-Bρ method. The detailed HFRS ion-optical design including the first order and the higher order, and the magnet specification were presented in this paper.

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.

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.

Neutron-rich rare isotope production with stable and radioactive beams in the mass range A~40-60 at 15 MeV/u

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Rare Isotope Production in peripheral heavy-ion collisions in the energy range 15-25 MeV/nucleon

In this contribution we summarize recent efforts to describe the production of rare isotopes with beams of 15–25 MeV/nucleon expected from low-energy facilities. We first present calculated production cross sections of proton-rich nuclides from collisions of stable beams of mass A∼60–80. Our calculations are performed with the phenomenological deep-inelastic transfer (DIT) model and the microscopic con- strained molecular dynamics model (CoMD). De-excitation of the excited quasipro- jectiles from the dynamical stage of the reaction is performed with the statistical multifragmentation model (SMM). In addition to the efforts on proton-rich nuclides, we investigated the possibility of producing neutron-rich rare isotopes in the mass range A∼180–200, i.e. near the third r-process peak of A=195. We performed calcu- lations for a 208Pb (15MeV/nucleon) beam and find that the multinucleon transfer mechanism leads to very neutron-rich nuclides in this mass range. We believe that our continued progress on the study of multinucleon transfer reactions using heavy- ion beams of 15–25 MeV/nucleon, can provide new opportunities in rare isotope research in the near future, as planned at the KOBRA facility of RISP in Korea.

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.

Neutron-rich rare isotope production with stable and radioactive beams in the mass range A ∼ 40–60 at beam energy around 15 MeV/nucleon

We studied the production of neutron-rich nuclides in multinucleon transfer collisions of stable and radioactive beams (RIB) in the mass range A ∼ 40–60. We first presented our experimental cross section data of projectile fragments from the reaction of 40Ar(15 MeV/nucleon) with 64Ni, 58Ni and 27Al. We then compared them with calculations based on either the deep-inelastic transfer model or the constrained molecular dynamics model, followed by the statistical multifragmentation model. An overall good agreement of the calculations with the experimental data is obtained. We continued with calculations of the reaction of 40Ar (15 MeV/nucleon) with 238U target and then with reactions of 48Ca (15 MeV/nucleon) with 64Ni and 238U targets. In these reactions, very neutron-rich rare isotopes are produced with large cross sections. These nuclides, in turn, can be assumed to form RIB and interact with a subsequent target (preferably 238U), leading to the production of extremely neutron-rich and even new isotopes (e.g. 60Ca) in this mass range. We conclude that multinucleon transfer reactions with stable or RIB at energy around 15 MeV/nucleon offer an effective route to access extremely neutron-rich rare isotopes for nuclear structure or reaction studies.

Neutron-rich rare-isotope production from projectile fission of heavy nuclei near 20 MeV/nucleon beam energy

We investigate the possibilities of producing neutron-rich nuclides in projectile fission of heavy beams in the energy range of 20 MeV/nucleon expected from low-energy facilities. We report our efforts to theoretically describe the reaction mechanism of projectile fission following a multinucleon transfer collision at this energy range. Our calculations are mainly 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 deexcitation/fission of the hot heavy projectile fragments is performed with the Statistical Mul- tifragmentation Model (SMM). We compared our model calculations with our previous experimental projectile-fission data of 238U (20 MeV/nucleon)+208Pb and 197Au (20 MeV/nucleon)+197Au and found an overall reasonable agreement. Our study suggests that projectile fission following periph- eral heavy-ion collisions at this energy range offers an effective route to access very neutron-rich rare isotopes toward and beyond the astrophysical r-process path.

Positioning the neutron drip line and the r-process paths in the nuclear landscape

Exploring nucleon drip lines and astrophysical rapid neutron capture process (r-process) paths in the nuclear landscape is extremely challenging in nuclear physics and astrophysics. While various models predict similar proton drip line, their predictions for neutron drip line and the r-process paths involving heavy neutron-rich nuclei exhibit a significant variation which hampers our accurate understanding of the r-process nucleosynthesis mechanism. Using microscopic density functional theory with a representative set of non-relativistic and relativistic interactions, we demonstrate for the first time that this variation is mainly due to the uncertainty of nuclear matter symmetry energy $E_{\rm{sym}}(\rho_{\rm{sc}})$ at the subsaturation cross density $\rho_{\rm{sc}}=0.11/0.16\times\rho_0$ ($\rho_0$ is saturation density), which reflects the symmetry energy of heavy nuclei. Using the recent accurate constraint on $E_{\rm{sym}}(\rho_{\rm{sc}})$ from the binding energy difference of heavy isotope pairs, we obtain quite precise predictions for the location of the neutron drip line, the r-process paths and the number of bound nuclei in the nuclear landscape. Our results have important implications on extrapolating the properties of unknown neutron-rich rare isotopes from the data on known nuclei.

Excitation energies in neutron-rich rare isotopes as indicators of changing shell structure

The quest for a predictive model of atomic nuclei fuels experimental and theoretical research programs worldwide. The properties of rare isotopes emerge as crucial ingredients for the development of nuclear models valid also towards the nucleon driplines. Many important aspects of the interactions between the constituent protons and neutrons are amplified in the regime of large isospin and can best be isolated and characterized there. The energies of excited states offer a unique view into the structure of nuclei and are often some of the first quantities accessible by experiment. Excitation energies can be measured directly and in model-independent ways and thus are among the key observables that can guide our understanding of atomic nuclei.

Yield estimation of neutron-rich rare isotopes induced by 200 MeV/u 132Sn beams by using GEANT4

A so-called “two-step reaction scheme”, in which neutron-rich rare isotopes obtained from ISOL are post-accelerated and bombarded on a second target, is employed to estimate the production yields of exotic rare isotopes. The production yields of neutron-rich rare isotope fragments induced by 200MeV/u 132Sn beams bombarded on a 9Be target are estimated with Monte Carlo code, GEANT4. To substantiate the use of GEANT4 for this study, benchmark calculations are done for 80MeV/u 59Co, 95MeV/u 72Zn, 500MeV/u 92Mo, and 950MeV/u 132Sn beams on the 9Be target. It is found that 132Sn beams can produce neutron-rich rare isotopes with 45⩽Z⩽50 more effectively than 238U beams at the same energy per nucleon.

Systematic study of neutron-rich rare isotope production in peripheral heavy-ion collisions below the Fermi energy

Detailed calculations of the yields of projectilelike fragments (with focus on the neutron-rich isotopes) are presented for the interaction of $^{86}\mathrm{Kr}$ (15 MeV/nucleon) with $^{64}\mathrm{Ni}$, $^{58}\mathrm{Ni}$, and $^{124}\mathrm{Sn}$, $^{112}\mathrm{Sn}$, as well as $^{86}\mathrm{Kr}$ (25 MeV/nucleon) with $^{124}\mathrm{Sn}$ and compared with our recently published experimental data for these reactions. The calculations are based on a two-step approach: the dynamical stage of the collision was described with the microscopic constrained molecular dynamics (CoMD) model, as well as the phenomenological deep-inelastic transfer (DIT) model and its modified (DITm) version. The deexcitation of the hot projectile fragments was performed with the statistical multifragmentation model (SMM) and the binary-decay model gemini, which provided nearly similar results for the neutron-rich products from the reactions studied. An overall good agreement of the calculations with the experimental results, especially for near-projectile isotopes was observed using both the CoMD model and the DITm model for the dynamical stage. The successful description of the production of neutron-rich isotopes with the CoMD model is of particular importance, due to the predictive power of the microscopic approach that essentially does not depend on the reaction dynamics. Our studies to date suggest that peripheral heavy-ion collisions at this energy range (i.e., well above the Coulomb barrier, but below the Fermi energy), if induced by neutron-rich rare-isotope beams of adequate intensity may offer a unique route to access extremely neutron-rich rare isotopes toward the astrophysical $r$-process path and the presently uncharted neutron drip line.

Characteristics of rare isotope beams in the KoRIA facility

The Korea Rare Isotope Accelerator (KoRIA) facility plans to provide neutron-rich rare isotopes (RIs) and stable heavy ion beams for research in the fields of basic and applied sciences. In the facility, both the isotope separator on-line (ISOL) and the in-flight fragmentation (IFF) methods are employed for RI production by 238U fission, which is effective for producing neutron-rich RI beams. These ISOL and IFF systems are designed to obtain world-class RI beam intensity, which allows exotic nuclei near the neutron drip line in the nuclear chart, which have not yet been explored, to be studied. In this paper, the uniqueness of the KoRIA facility under consideration, compared to other facilities, is described, and we present the characteristics of the RI beams that will be available at KoRIA.

Approaching neutron-rich nuclei toward ther-process path in peripheral heavy-ion collisions at 15 MeV/nucleon

The production cross sections of projectilelike fragments from the interaction of 15 MeV/nucleon ${}^{86}$Kr with ${}^{64}$Ni and ${}^{58}$Ni and with ${}^{124}$Sn and ${}^{112}$Sn were measured using a magnetic separator with emphasis on the neutron-rich isotopes. Neutron pickup isotopes (with up to six neutrons picked up from the target) were observed with large cross sections. Calculations involving the the deep-inelastic transfer model as well as the microscopic constrained molecular dynamics model (CoMD) for the primary interaction stage coupled to the statistical deexcitation code gemini were used to describe the gross properties of the product distributions. 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. This enhancement in the cross sections is possibly associated with very peripheral collisions and longer interaction times of the neutron-rich ${}^{86}$Kr projectile with the neutron-rich targets. The large cross sections of such reactions well above the Coulomb barrier, but below the Fermi energy, involving peripheral nucleon exchange, suggest that both the $N\text{/}Z$ of the projectile and the $N\text{/}Z$ of the target may properly be exploited in the production of neutron-rich rare isotopes. These reactions offer a novel and attractive route to access extremely neutron-rich rare isotopes toward the the astrophysical $r$-process path and, possibly, the neutron drip-line.

Studies of the nuclear landscape and the nuclear equation of state using peripheral collisions near the Fermi energy

Studies of peripheral (quasi-elastic and deep-inelastic) collisions near the Fermi energy are presented with a two-fold goal: first, to produce very neutron-rich nuclei towards the neutron drip line and, second, to obtain insight into the underlying dynamics and the nuclear equation of state (EOS). Towards the first goal, our studies indicated an enhanced production of neutron-rich nuclides suggesting that peripheral reactions at Fermi energies offer a novel way to access very neutron-rich rare isotopes in unexplored regions of the nuclear chart. Towards the second goal, we have made comparisons of our experimental data to detailed calculations using the semiclassical microscopic model CoMD (Constrained Molecular Dynamics). The CoMD code implements an effective interaction with a nuclear-matter compressibility of K=200 (soft EOS) or K=380 (stiff EOS) with several forms of the density dependence of the nucleon-nucleon symmetry potential and imposes a phase space constraint to restore the Pauli principle during the collision. Present results from these comparisons are consistent with a soft equation of state (K=200) with a rather stiff density dependence of the symmetry potential (symmetry energy).

Neutron-rich rare isotope production in the Fermi energy domain and application to the Texas A&M radioactive beam upgrade

The production and separation of neutron-rich rare isotope beams in peripheral collisions around the Fermi energy is described in the present work. The experiments were performed at the Cyclotron Institute of Texas A&M University employing beams from the K500 Superconducting Cyclotron. Two magnetic separators were used: the MARS recoil separator and the Superconducting Solenoid Line (BigSol Line). Enhanced production cross sections of neutron-rich nuclides in comparison with high-energy fragmentation mechanisms were observed and attributed to the role of the N/Z and the nuclear periphery of the target. From a practical viewpoint, these reactions below or around the Fermi energy offer a promising way to access very neutron-rich rare isotopes. The experience obtained in the production of rare isotope beams (RIB) in this energy range will be applied in the ongoing RIB upgrade of the Cyclotron Institute that will involve production, stopping and re-acceleration of rare isotopes.