Transuranic Nuclei Research Articles

Element abundance patterns in stars indicate fission of nuclei heavier than uranium

The heaviest chemical elements are naturally produced by the rapid neutron-capture process ( r -process) during neutron star mergers or supernovae. The r -process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments so must be extrapolated by using nucleosynthesis models. We examined element abundances in a sample of stars that are enhanced in r -process elements. The abundances of elements ruthenium, rhodium, palladium, and silver (atomic numbers Z = 44 to 47; mass numbers A = 99 to 110) correlate with those of heavier elements (63 ≤ Z ≤ 78, A > 150). There is no correlation for neighboring elements (34 ≤ Z ≤ 42 and 48 ≤ Z ≤ 62). We interpret this as evidence that fission fragments of transuranic nuclei contribute to the abundances. Our results indicate that neutron-rich nuclei with mass numbers >260 are produced in r -process events.

Shell effect on fission fragment mass distribution at Ecn* up to 70 MeV: Role of multichance fission

Mass distributions of fission fragments produced from the $^{249}\mathrm{Bk}$ compound nucleus (CN), populated by complete fusion of $^{11}\mathrm{B}$ with $^{238}\mathrm{U}$, were measured for the excitation energies in the range of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}36.7--69.7$ MeV. Nearly flat tops observed for these distributions indicate the presence of asymmetric fission, contrary to the pure Gaussian mass distributions expected at such high excitation energies. A fit to the mass distribution using three Gaussian functions provides an estimate of individual contributions from symmetric and asymmetric modes of fission. A significant contribution of the asymmetric component observed at CN excitation energies above 40 MeV can be understood only by invoking ``multichance fission'' in the calculations using the semiempirical model code gef. A systematic analysis of the mass distributions for several heavy transuranic nuclei reveals that the manifestation of the fragment shell effect in the integral mass distributions are visible even up to an initial compound nucleus excitation of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}70$ MeV due to their influence on the distributions of higher chance fissions.

MOSCAB (Materia OSCura A Bolle)

This kind of detector (Geyser) has never been realized for the Elementary Particle Physics (it was constructed once in BERN A in 1964 by Hahn and Reist to detect transuranic nuclei. The Geyser is substantially a Vessel consisting of a “FLASK” containing a superheated liquid (f.i. some kind of freon) and a “NECK” (containing partially a separation liquid and partially the freon vapor). The Geyser was realized in Milano by setting the two different parts of the detector (flask and neck, filled in our case by freon C3 F8, at different temperatures. The part of the overheated liquid was kept at higher temperature (f.i. 25 degree) and the gaseous part was kept at lower temperature (f.i. 18 degrees).

Radioactive decay products in neutron star merger ejecta: heating efficiency and γ-ray emission

The radioactive decay of the freshly synthesized $r$-process nuclei ejected in compact binary mergers power optical/infrared macronovae (kilonovae) that follow these events. The light curves depend critically on the energy partition among the different products of the radioactive decay and this plays an important role in estimates of the amount of ejected $r$-process elements from a given observed signal. We study the energy partition and $\gamma$-ray emission of the radioactive decay. We show that $20$-$50\%$ of the total radioactive energy is released in $\gamma$-rays on timescales from hours to a month. The number of emitted $\gamma$-rays per unit energy interval has roughly a flat spectrum between a few dozen keV and $1$ MeV so that most of this energy is carried by $\sim 1$ MeV $\gamma$-rays. However at the peak of macronova emission the optical depth of the $\gamma$-rays is $\sim 0.02$ and most of the $\gamma$-rays escape. The loss of these $\gamma$-rays reduces the heat deposition into the ejecta and hence reduces the expected macronova signals if those are lanthanides dominated. This implies that the ejected mass is larger by a factor of $2$-$3$ than what was previously estimated. Spontaneous fission heats up the ejecta and the heating rate can increase if a sufficient amount of transuranic nuclei are synthesized. Direct measurements of these escaping $\gamma$-rays may provide the ultimate proof for the macronova mechanisms and an identification of the $r$-process nucleosynthesis sites. However, the chances to detect these signals are slim with current X-ray and $\gamma$-ray missions. New detectors, more sensitive by at least a factor of ten, are needed for a realistic detection rate.

Synthesis of neutron-rich transuranic nuclei in fissile spallation targets

A possibility of synthesizing neutron-rich superheavy elements in spallation targets of Accelerator Driven Systems (ADS) is considered. A dedicated software called Nuclide Composition Dynamics (NuCoD) was developed to model the evolution of isotope composition in the targets during a long-time irradiation by intense proton and deuteron beams. Simulation results show that transuranic elements up to 249Bk can be produced in multiple neutron capture reactions in macroscopic quantities. However, the neutron flux achievable in a spallation target is still insufficient to overcome the so-called fermium gap. Further optimization of the target design, in particular, by including moderating material and covering it by a reflector could turn ADS into an alternative source of transuranic elements in addition to nuclear fission reactors.

ACTINIDE AND ULTRA-HEAVY ABUNDANCES IN THE LOCAL GALACTIC COSMIC RAYS: AN ANALYSIS OF THE RESULTS FROM THELDEFULTRA-HEAVY COSMIC-RAY EXPERIMENT

The LDEF Ultra-Heavy Cosmic-Ray Experiment (UHCRE) detected Galactic cosmic rays (GCRs) of charge Z ≥ 70 in Earth orbit with an exposure factor of 170 m2 sr yr, much larger than any other experiment. The major results include the first statistically significant uniform sample of GCR actinides with 35 events passing quality cuts, evidence for the existence of transuranic nuclei in the GCR with one 96Cm candidate event, and a low 82Pb/78Pt ratio consistent with other experiments. The probability of the existence of a transuranic component is estimated as 96%, while the most likely 92U/90Th ratio is found to be 0.4 within a wide 70% confidence interval ranging from 0 to 0.96. Overall, the results are consistent with a volatility-based acceleration bias and source material which is mainly ordinary interstellar medium material with some recent contamination by freshly synthesized material. Uncertainty in the key 92U/90Th ratio is dominated by statistical errors resulting from the small sample size and any improved determination will thus require an experiment with a substantially larger exposure factor than the UHCRE.

A future plan in observing ultra-heavy nuclei (Z = 30–110) of cosmic rays with large-scale collector at the lunar base

Lunar-based measurement of galactic cosmic ray (GCR) nuclei with a high precision is a challenging approach in cosmic ray research for the coming 20 years. This approach focuses to measure the elemental composition of Pt- and Pb-groups, actinide and possibly trans-uranic nuclei of Pu and Cm. The observation covers a wide range of scientific themes including the study on the origin of GCR nuclei, the characteristic time, heating and acceleration mechanism of GCR particles. A large-scaled particle telescope is required in order to measure those nuclides with high precision. Solid state nuclear track detectors (SSTDs) with a geometric factor of about 1000 m 2sr allow us to measure them easily. Fluorescent nuclear track detector such as Al 2O 3 doped with C and Mg is the best candidate at present among SSTDs for a lunar-based experiment which is currently the focus of an international program of scientific investigation. A permanent sunshine region near crater at lunar polar region is thought to be an excellent site. A two-year-exposure by the large-scaled telescope would result in the detection of about 30,000 actinides in GCRs.

Magnetic dipole properties in deformed nuclei

The magnetic dipole response in deformed nuclei is investigated. Spurious states due to the rotation of the whole nucleus are removed by restricting the residual interaction in such a way that it commutes with the total angular momentum as a ground state expectation value in the quasi-particle random phase approximation. By this one guarantees that the spurious state due to rotations lies at energy zero and that it is a solution of the quasi-particle random phase equation (QRPA) and can easily be removed. The Hamiltonian is chosen to be a deformed Saxon Woods potential with a pairing force and an isoscalar and isovector generalized quadrupole-quadrupole and spin-spin interaction. The generalized isoscalar quadrupole operator is defined as the commutator of the independent quasi-particle Hamiltonian with the total angular momentum. The restoration of rotational symmetry mentioned above determines the isoscalar coupling constant. The generalized isovector quadrupole-quadrupole force does not commute with the total angular momentum. But one can find a linear “isovector” combination of the proton and the neutron total angular momentum which produces by a commutator with the independent quasiparticle Hamiltonian an “isovector” quadrupole operator which yields an isovector quadrupole-quadrupole force which always commutes with the total angular momentum. Thus one gets no restriction of the isovector quadrupole-quadrupole force constant. We fit it to the isovector giant quadrupole resonance ( E(E2) = 130 A 1 3 MeV ). The Hamiltonian contains in addition a spin-spin force, which always commutes with the total angular momentum and therefore does not spoil the restoration of rotational symmetry. The proton-proton, neutron-neutron and proton-neutron spin force constant is determined by solving the nuclear matter problem with the Reid soft core interaction within the Brückner theory and translating the result with the help of the Migdal force into finite nuclei. This yields an almost pure repulsive isovector spin-spin force. This Hamiltonian predicts a high lying ΔN = 2 scissors mode which lies between 17 and 26 MeV. It is spread over very many 1 + states with an average spacing of 15 keV. This highlying scissors mode consists of several hundred states and the overlap with an artificially constructed scissors mode is of the order of 38%, while the low lying scissors mode which shows up as 1 + states around 3 MeV has an overlap with the scissors mode of more than 50 %. In the same energy range as the high lying scissors mode lies also the isovector giant quadrupole resonance. Even at 180°, that means at completely backward angles, the E2 is comparable with the M1 excitation. This is due to the transversal quadrupole transition probability which is not disappearing in electron scattering at 180°. The same Hamiltonian is nicely describing the low lying scissors mode around 3 MeV in the rare earth and the transuranic nuclei. It gives the energies and the reduced magnetic dipole transition probabilities. It also can reproduce the formfactors measured by inelastic electron scattering. The Hamiltonian also reproduces the spin-flip states between 5 and 9 MeV excitation energy, which can be measured by inelastic proton scattering. One obtains in this region two maxima. The lowest one around 6 MeV is mainly of isoscalar nature and is weaker than the spin flip maximum at 7 to 8 MeV, which is mainly of isovector nature. The detailed form of this two-spin flip maxima depend very sensitively on the ratio of the isoscalar and isovector strength of the spin-spin force. In conclusion one can say: There exist a high lying scissors mode, but it is strongly fragmented over several hundred 1 + states between 17 and 26 MeV and even in electron scattering at backward angles the cross section for the excitation of the isovector giant quadrupole resonance is of comparable size. Thus it will be difficult to establish experimentally the existence of this high lying scissors mode.

Very high spin states in spherical and transuranic nuclei

Deformation energy surfaces at very high spins of the nuclei in 'spherical' (Te-Sm) and transuranic (Po-Th) regions are calculated as a function of beta and gamma deformations in terms of the Strutinsky approach using the Woods-Saxon single-particle potential. In general, it is found that the nuclei in the 'spherical' region have the deepest energy minimum in the deformation energy surface at gamma =-60 degrees (oblate shape rotating around its symmetry axis) over a wide range of angular momenta (I approximately=30-70 h(cross)). The transuranic nuclei show a sudden change of their shape from nearly spherical to large prolate deformation.

Charge and Energy Spectra of Trans-Iron Cosmic Rays

A 22-${\mathrm{m}}^{2}$ detector array consisting of $G\ensuremath{-}5$ nuclear emulsion, fast-film \ifmmode \check{C}\else \v{C}\fi{}erenkov detector, and 40 sheets of plastic detectors was exposed for \ensuremath{\sim} 60 h at a mean atmospheric depth of \ensuremath{\sim}3.7 g/${\mathrm{cm}}^{2}$ and a rigidity of 2.5 GV. The detector was designed to determine both the charge and energy of trans-iron nuclei. For the first time it has been possible to determine cosmic-ray abundances in the difficult region $30Z\ensuremath{\le}42$ as well as to study the heavier nuclei. Over the entire periodic table from Ne on, the trend of cosmic-ray abundances is similar to that expected from sources with solar abundances, after nuclear reactions in interstellar space have been taken into account. The abundance ratios $[Z\ensuremath{\ge}90]:[81\ensuremath{\le}Z\ensuremath{\le}83]:[74\ensuremath{\le}Z\ensuremath{\le}80]$ are not inconsistent with solar abundances if Grevesse's photospheric abundances of Th and U, adjusted for radioactive decay, are adopted instead of meteoritic abundances. These ratios agree somewhat better with an $r$-process composition and a leakage lifetime of several million years. No trans-uranic nuclei were observed in the present flight. Serious consideration must be given to the possibility that most of the cosmic rays with $Z60$ originate from material of solar composition. One of the major new results of the study was the determination of an energy spectrum for the nuclei with $Z60$. If the integral-energy spectrum is given by $N(E)\ensuremath{\propto}{(E+{M}_{0}{c}^{2})}^{\ensuremath{-}\ensuremath{\eta}}$ where $E$ is the kinetic energy per nucleon, then the spectral index for high-$Z$ nuclei is $\ensuremath{\eta}=2.75\ifmmode\pm\else\textpm\fi{}0.53$ for $0.5 \frac{\mathrm{GeV}}{\mathrm{amu}}\ensuremath{\le}E\ensuremath{\le}2.0 \frac{\mathrm{GeV}}{\mathrm{amu}}$. In the same energy interval the index for helium nuclei is $\ensuremath{\eta}=1.3$. This may imply a different origin for high- and low-$Z$ cosmic rays.

Neutron strength functions from model equilibrium deformations for transuranic nuclei

Average neutron s-, p- and d-wave strength functions for 58 heavy, doubly even rotor nuclei from 220Em to 262(104) have been calculated with a strong coupling deformed nucleus model using single-particle potential equilibrium deformations derived from projected wave functions with and without a charge term, BCS functions and an independent-particle-model approximation. The optical-model potentials employed have been corrected for both the symmetry number and the Coulomb parameter. In comparison to existing strength function data, it is found that although the differences between the models is often less than the uncertainties assigned to the measurements, for s-waves, the general agreement is better for those model variants which include a Coulomb term in the potential.

The surface delta interaction in the transuranic nuclei

The Surface Delta Interaction (SDI) is employed to calculate the ernergy of gamma-vibrational and theK=0, 1, 2, and 3 Octupole vibrational band heads in the even-mass Transuranic nuclei. The quasi-particle Random Phase (RPA) and the quasi-particle Tamm Dancov approximations (TDA) are utilized to solve the Hamiltonian. The resulting energies and gamma transitions are compared with the Pairing plus Quadrupole (PQF) or Octupole (POF) force model and the available experimental data. The agreement between theory and experiment is satisfatory. Only theK=0, 1, and 2 Octupole bands are collective. TheK=3 negative parity state is practicaly a pure two quasi-particle state.