Synthesis Of Heavy Elements Research Articles

Fusion-fission Study at JAEA for Heavy-element Synthesis

Abstract Fission fragment mass distributions were measured in the heavy-ion induced fission using 238 U target nucleus. The mass distribu- tions changed drastically with incident energy. The results are explained by a change of the ratio between fusion and qasifission with nuclear orientation. A calculation based on a fluctuation dissipation model reproduced the mass distributions and their inci- dent energy dependence. Fusion probability was determined in the analysis. Evaporation residue cross sections were calculated with a statistical model in the reactions of 30 Si+ 238 U and 34 S+ 238 U using the obtained fusion probability in the entrance channel. The results agree with the measured cross sections of 263,264 Sg and 267,268 Hs, produced by 30 Si+ 238 U and 34 S+ 238 U, respectively. It is also suggested that the sub-barrier energies can be used for heavy element synthesis.

In-beam fission study for Heavy Element Synthesis

Fission fragment mass distributions were measured in heavy-ion induced fissions using 238 U target nucleus. The measured mass distributions changed drastically with incident energy. The results are explained by a change of the ratio between fusion and qasifission with nuclear orientation. A calculation based on a fluctuation dissipation model reproduced the mass distributions and their incident energy dependence. Fusion probability was determined in the analysis. Evaporation residue cross sections were calculated with a statistical model in the reactions of 30 Si + 238 Ua nd 34 S + 238 U using the obtained fusion probability in the entrance channel. The results agree with the measured cross sections for seaborgium and hassium isotopes.

Structures of exotic131,133Sn isotopes and effect onr-process nucleosynthesis

Background: Four strong single-particle bound levels with strikingly similar level spacings have recently been measured in Sn-131 and Sn-133. This similarity has not yet been addressed by a theoretical nuclear structure model. Information on these single-particle bound levels, as well as on resonant levels above the neutron capture threshold, is also needed to determine neutron capture cross sections-and corresponding capture reaction rates-on Sn-130,Sn-132. The Sn-130(n,gamma) rate was shown in a recent sensitivity study to significantly impact the synthesis of heavy elements in the r-process in supernovae. Purpose: Understand the structure of bound and resonant levels in Sn-131,Sn-133, and determine if the densities of unbound resonant levels are sufficiently high to warrant statistical model treatments of neutron capture on Sn-130,Sn-132. Method: Single-particle bound and resonant levels for Sn-131,Sn-133 are self-consistently calculated by the analytical continuation of the coupling constant (ACCC) method based on a relativistic mean field (RMF) theory with BCS approximation. Results: We obtain four strong single-particle bound levels in both Sn-131,Sn-133 with an ordering that agrees with experiments and spacings that, while differing from experiment, are consistent between the Sn isotopes. We also find at most one single-particle level in the effective energy range for neutron captures in the r-process. Conclusions: Our RMF + ACCC + BCS model successfully reproduces observed single-particle bound levels in (131,13)3Sn and self-consistently predicts single-particle resonant levels with densities too low for widely used traditional statistical model treatments of neutron capture cross sections on Sn-130,Sn-132 employing Fermi gas level density formulations.

Nuclear Correlations and therProcess

We show that long-range correlations for nuclear masses have a significant effect on the synthesis of heavy elements by the rprocess. As calculated by Delaroche etal. [Phys. Rev. C 81, 014303 (2010)], these correlations suppress magic number effects associated with minor shells. This impacts the calculated abundances before the third r-process peak (at mass number A≈195), where the abundances are low and form a trough. This trough and the position of the third abundance peak are strongly affected by the masses of nuclei in the transition region between deformed and spherical. Based on different astrophysical environments, our results demonstrate that a microscopic theory of nuclear masses including correlations naturally smoothens the separation energies, thus reducing the trough and improving the agreement with observed solar system abundances.

Nuclear pygmy modes and the dynamics of the nuclear skin

The information on pygmy resonances reveals new aspects on the isospin dynamics of the nucleus with important astrophysical consequences. In this connection, the precise knowledge of nuclear response functions plays a key role in the determination of photonuclear reactions cross sections which are of importance for the synthesis of heavy neutron-rich elements. For that purpose, a theoretical method based on density functional theory and multi-phonon approach is applied for investigations of nuclear excitations with different multipolarities and energies in stable and exotic nuclei. The possible relation of low-energy modes to the properties of neutron or proton skins is systematically investigated for isotonic and isotopic chains. Our studies of dipole and quadrupole response functions and the corresponding transition densities indicate new pygmy dipole and pygmy quadrupole resonances, describing oscillations of the nuclear skin. Also, the presence of skins is found to affect the magnetic response of nuclei.

THE EXTREME OVERABUNDANCE OF MOLYBDENUM IN TWO METAL-POOR STARS

We report determinations of the molybdenum abundances in five mildly to extremely metal-poor turnoff stars using five Mo II lines near 2000A. In two of the stars, the abundance of molybdenum is found to be extremely enhanced, as high or higher than the neighboring even-Z elements ruthenium and zirconium. Of the several nucleosynthesis scenarios envisioned for the production of nuclei in this mass range in the oldest stars, a high-entropy wind acting in a core-collapse supernova seems uniquely capable of the twin aspects of a high molybdenum overproduction confined to a narrow mass range. Whatever the details of the nucleosynthesis mechanism, however, this unusual excess suggests that very few individual nucleosynthesis events were responsible for the synthesis of the light trans-Fe heavy elements in these cases, an unexpected result given that both are only moderately metal-poor.

Effect of collective neutrino flavor oscillations on νp-process nucleosynthesis

The vp process is a primary nucleosynthesis process which occurs in core collapse supernovae. An essential role in this process is being played by electron antineutrinos. They generate, by absorption on protons, a supply of neutrons which, by (n,p) reactions, allow to overcome waiting point nuclei with rather long beta-decay and proton-capture lifetimes. The synthesis of heavy elements by the vp process depends sensitively on the \bar{\nu}_e luminosity and spectrum. As has been shown recently, the latter are affected by collective neutrino flavor oscillations which can swap the \bar{\nu}_e and \bar{\nu}_{\mu,\tau} spectra above a certain split energy. Assuming such a swap scenario, we have studied the impact of collective neutrino flavor oscillations on the vp-process nucleosynthesis. Our results show that the production of light p-nuclei up to mass number A=108 is very sensitive to collective neutrino oscillations.

Introduction to shell structure in exotic nuclei

The nuclear shell model is a benchmark for the description of the structure of atomic nuclei. The magic numbers associated with closed shells have long been assumed to be valid across the whole nuclear chart. Investigations in recent years of nuclei far away from nuclear stability at facilities for radioactive ion beams have revealed that the magic numbers may change locally in those exotic nuclei leading to the disappearance of classic shell gaps and the appearance of new magic numbers. These changes in shell structure also have important implications for the synthesis of heavy elements in stars and stellar explosions. In this review a brief overview of the basics of the nuclear shell model will be given together with a summary of recent theoretical and experimental activities investigating these changes in the nuclear shell structure.

Three-Body Forces and the Limit of Oxygen Isotopes

The limit of neutron-rich nuclei, the neutron drip line, evolves regularly from light to medium-mass nuclei except for a striking anomaly in the oxygen isotopes. This anomaly is not reproduced in shell-model calculations derived from microscopic two-nucleon forces. Here, we present the first microscopic explanation of the oxygen anomaly based on three-nucleon forces that have been established in few-body systems. This leads to repulsive contributions to the interactions among excess neutrons that change the location of the neutron drip line from (28)O to the experimentally observed (24)O. Since the mechanism is robust and general, our findings impact the prediction of the most neutron-rich nuclei and the synthesis of heavy elements in neutron-rich environments.

DYNAMICAL DIPOLE MODE IN FUSION HEAVY-ION REACTIONS

The prompt γ-ray emission, associated with the dynamical dipole mode decay, was investigated in the 32,36 S + 100,96 Mo and 36,40 Ar + 96,92 Zr fusion-evaporation reactions in the energy range E lab= 6 - 16 MeV / nucleon . The above reaction pairs populate, through entrance channels having different charge asymmetries, a compound nucleus in the 132 Ce mass region at excitation energies of 117, 174 and 284 MeV with identical spin distribution. By studying the differential γ-ray multiplicity spectra of the considered systems, the features of the dynamical dipole mode as a function of the beam energy were extracted while the γ-ray angular distributions were used to prove its pre-equilibrium character. The experimental findings were compared with theoretical predictions performed within a BNV transport model and based on a collective bremsstrahlung analysis of the entrance channel reaction dynamics. As a fast cooling mechanism on the fusion path, the prompt dipole radiation could be of interest for the synthesis of super heavy elements through hot fusion reactions providing a way to cool down the hot fusion paths, so ending up with a larger survival probability. To shed light on this hypothesis, the dynamical dipole mode investigation was extended to a heavier compound nucleus, 192 Pb , by means of the 40 Ca + 152 Sm and 48 Ca + 144 Sm reactions at E lab= 11 and 10.1 MeV/nucleon, respectively. Preliminary results of this measurement, concerning both fusion-evaporation and fission events are presented.

Dynamical dipole mode in heavy-ion fusion reactions

The dynamical dipole mode, excited in charge asymmetric heavy-ion collisions, was investigated in the mass region of the 192Pb compound nucleus, formed by using the 40,48Ca + 152,144Sm reactions at ∼11 MeV/nucleon. Preliminary results of this measurement, concerning both fusion-evaporation and fission events are presented. As a fast cooling mechanism on the fusion path, the dynamical dipole mode could be useful for the synthesis of super heavy elements through “hot” fusion reactions.

Uranium fluoride and metallic uranium as target materials for heavy-element experiments at SHIP

In this contribution we describe the production and application of uranium targets for synthesis of heavy elements. The targets are prepared from uranium fluoride (UF 4) and from metallic uranium with thin carbon foils as backing. Targets of UF 4 were produced by thermal evaporation in a similar way as the frequently applied targets out of Bi, Bi 2O 3, Pb, PbS, SmF 3, and NdF 3, prepared mostly from isotopically enriched material [Birgit Kindler, et al., Nucl. Instr. and Meth. A 561 (2006) 107; Bettina Lommel, et al., Nucl. Instr. and Meth. A 561 (2006) 100]. In order to use more intensive beams and to avoid scattering of the reaction products in the target, metallic uranium is favorable. However, evaporation of metallic uranium is not feasible at a sustainable yield. Therefore, we established magnetron sputtering of metallic uranium. We describe production and properties of these targets. First irradiation tests show promising results.

Pulse amplitude analysis of rare events measured in heavy-ion-induced nuclear reactions

A complex analysis has been performed on the energy amplitude signals corresponding to rare events observed in nuclear reactions aimed at the synthesis of super heavy elements (SHE). In a few cases, the values of the amplitudes are anomalous. The reaction products in the analyzed experiments were detected by a PIPS detector characterized by a significant pulse height defect (PHD) associated with the detection of evaporation residues (EVR) and fission fragments (FF). After correction, the observations of anomalous EVR energy values provide a reason to reject an entire event.

Synthesis of transactinide nuclei using radioactive beams

The prospects for the synthesis of transactinide nuclei using radioactive beams are evaluated quantitatively for a modern radioactive beam facility. A simple formalism for calculating the complete fusion cross sections that reproduces the known heavy element production cross sections over six orders of magnitude is used to calculate the production rates for transactinide nuclei with $Z\ensuremath{\leqslant}120$. All possible projectile and target combinations are evaluated. Exciting new possibilities for studies of the atomic physics, chemistry, and nuclear spectroscopy of the heaviest elements should be realized at a modern radioactive beam facility. The synthesis of new heavy elements is best undertaken at stable beam accelerators.

Detection of ultra rare α decays of super heavy nuclei

Three approaches to the measurement of a rare α decaying products produced in heavy-ion induced nuclear reactions are described. One is based on a chemical extraction and following deposition of the nuclides under investigation onto the surface of the detector, whereas the second one is associated with long–lived products implanted into silicon detectors by using the electromagnetic separation technique. The third approach relates with an application of real-time mode detection of correlated energy–time–position recoil-α sequences from 48Ca induced nuclear reactions with actinide targets, like 242, 244Pu, 245, 248Cm, 239Am and 249Cf. Namely with this technique, it has became possible to provide a radical suppression of backgrounds in the full fusion (3–5n) reactions aimed to the synthesis of super heavy elements with Z=113–116.

Heavy element synthesis in the oldest stars and the early Universe

The first stars in the Universe were probably quite different from those born today. Composed almost entirely of hydrogen and helium (plus a tiny trace of lithium), they lacked the heavier elements that determine the formation and evolution of younger stars. Although we cannot observe the very first stars--they died long ago in supernovae explosions--they created heavy elements that were incorporated into the next generation. Here we describe how observations of heavy elements in the oldest surviving stars in our Galaxy's halo help us understand the nature of the first stars--those responsible for the chemical enrichment of our Galaxy and Universe.

Fusion near the Coulomb barrier for the synthesis of heavy and superheavy elements: A theoretical approach

The compound nucleus formation is considered as a two-step process of touching and subsequent tunneling of the projectile into the target. The deep minima in the potential energy curve are due to shell effects in the experimental binding energies and give possible target-projectile combinations for the synthesis of heavy and superheavy elements. The asymmetric channels thus obtained are in remarkable agreement with the known experimental channels. In our model, the colliding partners are first shown to be captured in the pocket behind the outer (touching) barrier and the composite system so formed finally tunnels through the inner (fusion) barrier to form the resulting compound nucleus. These calculations reveal the importance of the fusion barrier, which occur only for the asymmetric target-projectile combinations. The calculated fusion cross-sections show a reasonable comparison with the observed one-neutron evaporation residue cross-sections. An estimate of the excitation energy carried by the compound nucleus is also obtained from our model calculations.

Heavy element synthesis through disk winds in a collapsar

We investigate heavy element synthesis through thermally driven winds launched from an inner neutron-rich region of an accretion disk in a collapsar. Various heavy elements are found to be produced through the wind. There exist three types of winds characterized by their abundant nuclei: (i) 56 Ni, (ii) light ( Z = 30 − 45 ) neutron-rich nuclei, and (iii) light ( Z ⩽ 44 ) p-nuclei. We find that Y e , NSE > 0.49 , Y e , NSE ⩽ 0.45 , and 0.45 Y e , NSE ⩽ 0.49 for winds of types (i), (ii), and (iii), respectively, where Y e , NSE is the electron fraction of the wind at T = 9 × 10 9 K . The collasar winds may be a lighter element primary process, suggested from Galactic evolution of Sr, Y, and Zr. The p-nuclei, 92 Mo and 94 Mo, which are underproduced in oxygen layers in a supernova, can be abundantly synthesized in the collapsar winds.

The reaction of triple radiative capture αα(n, γ) 9Be studied in a β decay of 9Li

A new way of study the reaction of triple radiative capture αα(n,γ)9Be is demonstrated. At the GSI-ISOL facility, we have measured the γ decays of excited 1/2+, 5/2− and 1/2− states in 9Be, which are strongly populated in the β decay of 9Li. The respective partial γ-widths are of astrophysical interest in order to reliably estimate the αα(n,γ)9Be rates in SN type-II as a possible site of the synthesis of heavy elements in the Universe. The 9Li nuclei were implanted into a foil viewed by high-granularity arrays of β and γ detectors. The γ-ray spectra were measured in coincidence with the β-particles by selecting large β-γ angles in order to suppress the bremsstrahlung background. For the first time, the γ-rays from the 1/2+ and 5/2− states in 9Be were observed in 9Li decay, with the 1/2+-state energy and width measured to be 1689(10) and 224(7) keV, respectively, while the latter transition provided a reference point. The upper limit of the 1/2− partial γ-width is measured to be 7 ⋅ 10 6 MeV . An improved sensitivity experiment is feasible at facilities with the larger yield of 9Li.

Fusion and nucleosynthesis by hypervelocity impact using hybrid magnetic fields

The electromagnetic acceleration of a metallic ring carrying a high current, combined with the mutual interaction of a concentrical conductor carrying the same current, in the presence of a DC magnetic field, results in a converging beam of particles reaching a velocity above 1000 km/s. Lithium, gold, and uranium rings are considered and transient temperatures ranging from 108to 1010°K are expected. In the presence of deuterium and tritium fusion reactions will occur and the capture of the released neutrons may result in the synthesis of heavier elements. The considered device lends itself to a varied treatment allowing to consider velocities in the range of 2000 km/s.