Heavy Nuclides Research Articles

New prospects in synthesis and study of neutron rich heavy nuclei

The present limits of the upper part of the nuclear map are rather close to the beta stability line while the unexplored area of heavy neutron rich nuclides (also those located along the neutron closed shell N = 126 to the right hand side of the stability line) is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleogenesis. For elements with Z > 100 only neutron deficient isotopes (located to the left of the stability line) have been synthesized so far. The north-east area of the nuclear map can be reached neither in fusion-fission reactions nor in fragmentation processes widely used nowadays for the production of new nuclei. Multi-nucleon transfer processes in near barrier collisions of heavy ions seem to be the only reaction mechanism allowing us to produce and explore neutron rich heavy nuclei including those located at the superheavy island of stability. Neutron capture process can be also considered as an alternative method for the production of long-lived neutron rich superheavy nuclei. Strong neutron fluxes might be provided by nuclear reactors and nuclear explosions in laboratory frame and by supernova explosions in nature.

Penning trap mass measurements on nobelium isotopes

The Penning trap mass spectrometer SHIPTRAP at GSI Darmstadt allows accurate mass measurements of radionuclides, produced in fusion-evaporation reactions and separated by the velocity filter SHIP from the primary beam. Recently, the masses of the three nobelium isotopes No252−254 were determined. These are the first direct mass measurements of transuranium elements, which provide new anchor points in this region. The heavy nuclides were produced in cold-fusion reactions by irradiating a PbS target with a Ca48 beam, resulting in production rates of the nuclei of interest of about one atom per second. In combination with data from decay spectroscopy our results are used to perform a new atomic-mass evaluation in this region.Received 19 March 2010DOI:https://doi.org/10.1103/PhysRevC.81.064312©2010 American Physical Society

New ideas on the production of heavy and superheavy neutron rich nuclei

A new way is proposed to discover and examine unknown neutron-rich heavy and superheavy (SH) nuclei at the “north-east” part of the nuclear map. These nuclei can be produced neither in fusion reactions nor in fragmentation processes widely used nowadays for production of new nuclei. The present limits of the upper part of the nuclear map are very close to stability while the unexplored area of heavy neutron-rich nuclides to the east of the stability line (also those located along the neutron closed shell N = 126 ) is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleo-genesis. A novel idea is proposed for the production of these nuclei via low-energy multi-nucleon transfer reactions using a gain given by the shell effects. The estimated yields of neutron-rich nuclei are found to be rather high in such reactions and several tens of new nuclides can be produced, for example, in near-barrier collision of 136Xe with 208Pb. This finding may spur new studies at heavy ion facilities and should have significant impact for future experiments.

軽水炉における使用済みＭＯＸ燃料からのアクチニド崩壊熱の核データ由来の誤差評価

On the basis of the background of the global warming and energy resource problems, the programs for utilization of nuclear power are now being revised upward around the world. This situation leads to expansion in utilization of uranium-plutonium mixed oxide (MOX) fuels in existing thermal reactors. The MOX fuels include larger amounts of heavy actinide nuclides as the initial components than conventional uranium fuels. This means that the decay heat of spent fuels becomes much larger than that of uranium fuels. In the present study, prediction accuracy is evaluated for actinide decay heat in one of the typical cases of spent MOX fuels in light water reactors on the basis of covariance data in JENDL-3.3. There are few studies on burnup sensitivity analyses focused on the actinide decay heat of spent fuels; the present study provides a good starting point for future works. The present results show that the errors of actinide decay heat are 35.1±0.9 kW/t (2.7%), 3.27±0.12 kW/t (3.7%), and 2.37±0.05 kW/t (1.9%) at a 68% confidence level in the cases of MOX fuels of 0.1, 10, and 50 year cooling after reactor shutdown, respectively. In order to improve the prediction accuracy of the MOX decay heat, it is important to reduce the uncertainty of the cross section of actinide nuclides including the capture reaction in Pu-238, -239, -242, Am-241, -243, and the fission in Pu-241 followed by capture in Cm-244.

Production of heavy isotopes in transfer reactions by collisions ofU238+U238

The dynamics of transfer reactions in collisions of two very heavy nuclei $^{238}\mathrm{U}+^{238}\mathrm{U}$ is studied within the dinuclear system (DNS) model. Collisions of two actinide nuclei form a superheavy composite system during a very short time, in which a large number of charge and mass transfers may take place. Such reactions have been investigated experimentally as an alternative way for the production of heavy and superheavy nuclei. The role of collision orientation in the production cross sections of heavy nuclides is analyzed systematically. Calculations show that the cross sections decrease drastically as the charged numbers of the heavy fragments increase. The transfer mechanism is favorable to synthesizing heavy neutron-rich isotopes, such as nuclei around the subclosure at $N=162$ from No ($Z=102$) to Db ($Z=105$).

Nuclear masses near the proton drip-line and their impact on nucleosynthesis in explosive stars

The interplay of the fundamental forces (here mainly the strong force and electromagnetic force) holds nucleons together to form atomic nuclei. However, in nature only less than ten percent of the total number of nuclei is stable. The majority of nuclei are unstable; that is, after having lived for a certain time, they change into the daughter nuclei by radioactive decay. The familiar decay types that can be found in most parts of the chart of nuclide are α-decay and β-decay. There are other important processes in which unstable nuclei can change into other ones. These occur, for example, in very neutronrich or proton-rich regions of the chart of nuclide. There are boundaries called drip-lines, which are the lines on the Z (proton)-N (neutron) plane where the nucleon separation energy is zero. The nuclear binding becomes weaker and weaker when moving from the stable region towards the drip-lines, and eventually no nucleus can exist beyond the drip-lines. Taking the proton-rich region as an example, near the proton drip-line, protons are only weakly bound and a proton radioactivity can occur. Also because of the small binding, nuclei near the proton drip-line may capture additional protons and become new, heavier nuclides if the capture conditions are present. Detailed knowledge on the nucleon-capture processes lies at the heart of the understanding of nucleosynthesis in the Universe——a very fundamental but unanswered question about how the heavy elements (i.e. those heavier than iron) are created. In order for nuclei to capture protons, a high temperature environment (usually above 1×10 9 K) is needed so that they can overcome the large coulomb barrier for charged particle reactions. A hydrogen rich

Validation of a new continuous Monte Carlo burnup code using a Mox fuel assembly

The reactivity of nuclear fuel decreases with irradiation (or burnup) due to the transformation of heavy nuclides and the formation of fission products. Burnup credit studies aim at accounting for fuel irradiation in criticality studies of the nuclear fuel cycle (transport, storage, etc…). The principal objective of this study is to evaluate the potential capabilities of a newly developed burnup code called “BUCAL1”. BUCAL1 differs in comparison with other burnup codes as it does not use the calculated neutron flux as input to other computer codes to generate the nuclide inventory for the next time step. Instead, BUCAL1 directly uses the neutron reaction tally information generated by MCNP for each nuclide of interest to determine the new nuclides inventory. This allows the full capabilities of MCNP to be incorporated into the calculation and a more accurate and robust analysis to be performed. Validation of BUCAL1 was processed by code-to-code comparisons using predictions of several codes from the NEA/OCED. Infinite multiplication factors ( k ∞) and important fission product and actinide concentrations were compared for a MOX core benchmark exercise. Results of calculations are analysed and discussed.

New way for the production of heavy neutron-rich nuclei

A new way is found to discover and examine unknown neutron-rich heavy nuclei at the ‘north-east’ part of the nuclear map. This ‘blank spot’ of the nuclear map can be reached neither in fusion–fission reactions nor in fragmentation processes widely used nowadays for the production of new nuclei. The present limits of the upper part of the nuclear map are very close to stability while the unexplored area of heavy neutron-rich nuclides along the neutron closed shell N = 126 (to the east of the stability line) is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleogenesis. A novel idea is proposed for the production of these nuclei via low-energy multi-nucleon transfer reactions. The estimated yields of neutron-rich nuclei are found to be rather high in such reactions and several tens of new nuclides can be produced, for example, in the near-barrier collision of 136Xe with 208Pb. This finding may spur new studies at heavy-ion facilities and should have significant impact on future experiments.

TRIGA-SPEC: A setup for mass spectrometry and laser spectroscopy at the research reactor TRIGA Mainz

The research reactor TRIGA Mainz is an ideal facility to provide neutron-rich nuclides with production rates sufficiently large for mass spectrometric and laser spectroscopic studies. Within the TRIGA-SPEC project, a Penning trap as well as a beamline for collinear laser spectroscopy are being installed. Several new developments will ensure high sensitivity of the trap setup enabling mass measurements even on a single ion. Besides neutron-rich fission products produced in the reactor, also heavy nuclides such as 235U or 252Cf can be investigated for the first time with an off-line ion source. The data provided by the mass measurements will be of interest for astrophysical calculations on the rapid neutron-capture process as well as for tests of mass models in the heavy-mass region. The laser spectroscopic measurements will yield model-independent information on nuclear ground-state properties such as nuclear moments and charge radii of neutron-rich nuclei of refractory elements far from stability. TRIGA-SPEC also serves as a test facility for mass and laser spectroscopic experiments at SHIPTRAP and the low-energy branch of the future GSI facility FAIR. This publication describes the experimental setup as well as its present status.

Physical separators for the heaviest elements

Reports on the production, separation and identification of elements with atomic numbers Z up to 118 can be found in the current literature. Such heavy nuclides are generally produced in complete fusion-evaporation reactions. In many studies, powerful physical recoil separators are used to isolate the few interesting events from an orders of magnitude more intense background of unwanted reaction products. In this overview, the physical separators currently in use for studies of nuclei with Z⩾100 are presented along with the most important techniques and methods of the field.

Target for the heaviest element production at RIKEN

A new gas-cooled rotating target system was developed for the heaviest element production. A target wheel with a diameter of 30.0 cm could be rotated up to 3000 rpm. A helium gas introduced into the target chamber mounted upstream of a gas-filled recoil separator, which enables an effective gas cooling of the target. The development has led to the successful production of heavy nuclides via cold fusion reactions of 208Pb( 64Ni,n) 271Ds, 209Bi( 64Ni,n) 272Rg, 208Pb( 70Zn,n) 277112, and 209Bi( 70Zn,n) 278113.

Plutonium and Minor Actinides Recycling in Standard BWR using Equilibrium Burnup Model

Plutonium (Pu) and minor actinides (MA) recycling in standard BWR with equilibrium burnup model has been studied. We considered the equilibrium burnup model as a simple time independent burnup method, which can manage all possible produced nuclides in any nuclear system. The equilibrium burnup code was bundled with a SRAC cell-calculation code to become a coupled cell- burnup calculation code system. The results show that the uranium enrichment for the criticality of the reactor, the amount of loaded fuel and the required natural uranium supply per year decrease for the Pu recycling and even much lower for the Pu & MA recycling case compared to those of the standard once- through BWR case. The neutron spectra become harder with the increasing number of recycled heavy nuclides in the reactor core. The total fissile rises from 4.77% of the total nuclides number density in the reactor core for the standard once-through BWR case to 6.64% and 6.72% for the Plutonium recycling case and the Pu & MA recycling case, respectively. The two later data may become the main basis why the required uranium enrichment declines and consequently diminishes the annual loaded fuel and the required natural uranium supply. All these facts demonstrate the advantage of plutonium and minor actinides recycling in BWR.

Recent developments and on-line tests of uranium carbide targets for production of nuclides far from stability

The capacity of uranium carbide target materials of different structure and density for production of neutron-rich and heavy neutron-deficient nuclides have been investigated. The yields of Cs and Fr produced by a 1 GeV proton beam of the PNPI synchrocyclotron and release properties of different targets have been measured. Yields and release efficiencies of Cs and Fr produced from a high density UC target material and from low density UCx prepared by the ISOLDE method at IRIS in the collaboration with PARRNe group from Orsay are compared. The yields from ISOLDE original target are presented for comparison as well.

The generalized liquid drop model alpha-decay formula: Predictability analysis and superheavy element alpha half-lives

The predictive accuracy of the generalized liquid drop model (GLDM) formula for alpha-decay half-lives has been investigated in a detailed manner and a variant of the formula with improved coefficients is proposed. The method employs the experimental alpha half-lives of the well-known alpha standards to obtain the coefficients of the analytical formula using the experimental Q α values (the DSR-E formula), as well as the finite range droplet model (FRDM) derived Q α values (the FRDM–FRDM formula). The predictive accuracy of these formulae was checked against the experimental alpha half-lives of an independent set of nuclei (TEST) that span approximately the same Z, A region as the standards and possess reliable alpha spectroscopic data, and were found to yield good results for the DSR-E formula but not for the FRDM–FRDM formula. The two formulae were used to obtain the alpha half-lives of superheavy elements (SHE) and heavy nuclides where the relative accuracy was found to be markedly improved for the FRDM–FRDM formula, which corroborates the appropriateness of the FRDM masses and the GLDM prescription for high Z, A nuclides. Further improvement resulted, especially for the FRDM–FRDM formula, after a simple linear optimization over the calculated and experimental half-lives of TEST was used to re-calculate the half-lives of the SHE and heavy nuclides. The advantage of this optimization was that it required no re-calculation of the coefficients of the basic DSR-E or FRDM–FRDM formulae. The half-lives for 324 medium-mass to superheavy alpha decaying nuclides, calculated using these formulae and the comparison with experimental half-lives, are presented.

Numerical Solution of Stiff Burnup Equation with Short Half Lived Nuclides by the Krylov Subspace Method

The Krylov subspace method is applied to solve nuclide burnup equations used for lattice physics calculations. The Krylov method is an efficient approach for solving ordinary differential equations with stiff nature such as the nuclide bunrup with short lived nuclides. Some mathematical fundamentals of the Krylov subspace method and its application to burnup equations are discussed. Verification calculations are carried out in a PWR pin-cell geometry with UO2 fuel. A detailed burnup chain that includes 193 fission products and 28 heavy nuclides is used in the verification calculations. Shortest half life found in the present burnup chain is approximately 30 s (106Rh). Therefore, conventional methods (e.g., the Taylor series expansion with scaling and squaring) tend to require longer computation time due to numerical stiffness. Comparison with other numerical methods (e.g., the 4-th order Runge-Kutta-Gill) reveals that the Krylov subspace method can provide accurate solution for a detailed burnup chain used in the present study with short computation time.

A method for optimizing experimental conditions and simulating spectra in the measurement of heavy nuclides in AMS

The sensitivity of accelerator mass spectrometry (AMS) is often limited by interferences from isobars, isotopes and other nuclides. In AMS measurements of heavy nuclides, where a high molecular fragment background can be present, it can be difficult to discriminate the nuclides of interest from the background particles online. In order to solve this problem, a method for optimizing the experimental conditions and simulating spectra in AMS measurements has been developed. Satisfactory agreement has been obtained between calculated and experimental spectra for 26Al, 36Cl, 41Ca and 99Tc measured with the HI-13 AMS system at China Institute of Atomic Energy (CIAE).

Numerical Solution of Stiff Burnup Equation with Short Half Lived Nuclides by the Krylov Subspace Method

The Krylov subspace method is applied to solve nuclide burnup equations used for lattice physics calculations. The Krylov method is an efficient approach for solving ordinary differential equations with stiff nature such as the nuclide bunrup with short lived nuclides. Some mathematical fundamentals of the Krylov subspace method and its application to burnup equations are discussed. Verification calculations are carried out in a PWR pin-cell geometry with UO2 fuel. A detailed burnup chain that includes 193 fission products and 28 heavy nuclides is used in the verification calculations. Shortest half life found in the present burnup chain is approximately 30 s (106Rh). Therefore, conventional methods (e.g., the Taylor series expansion with scaling and squaring) tend to require longer computation time due to numerical stiffness. Comparison with other numerical methods (e.g., the 4-th order Runge-Kutta-Gill) reveals that the Krylov subspace method can provide accurate solution for a detailed burnup chain used in the present study with short computation time.

FRAGMENTATION PRODUCTION CROSS-SECTIONS IN RELATIVISTIC COLLISIONS

In this study, the fragmentation production cross-sections of heavy residual nuclides in the reactions 238 U (1 A GeV ) + p and p (1.2 GeV ) + 232 Th were calculated and the production of more than 370 different isotopes for this incident reactions was presented. The calculations were made by using the Cascade-Exciton Model including pre-equilibrium effect, the Intranuclear Cascade Model, the empirical and the semi-empirical parametrization. The results of the cross-sections obtained were compared with available experimental data and examined the relation between each other. It is seen that the precision of this models to estimate residue production cross-sections is still far from the performance required for technical applications and this incident model should be improved to obtain a deeper understanding of the reaction mechanisms involved in the production of isotopes.

Formation of light isotopes by protons and deuterons of 3.65 GeV/nucleon on separated tin isotopes

We measure cross sections for residual nuclide formation in the mass range 7 ≤ A ≤ 96 caused by bombardment with protons and deuterons of 3.65-GeV/nucleon energy of enriched tin isotopes (112,118,120,124Sn). The experimental data are compared with calculations by the codes FLUKA, LAHET, CEM03, and LAQGSM03. Scaling behavior is observed for the whole mass region of residual nuclei, showing a possible multifragmentation mechanism for the formation of light products (7 ≤ A ≤ 30). Our analysis of the isoscaling dependence also shows a possible contribution of multifragmentation to the production of heavier nuclides, in the mass region 40 ≤ A ≤ 80.

Composite dark matter from the fourth generation

The hypothesis of a heavy stable quark of the fourth family can provide a nontrivial solution for cosmological dark matter if baryon asymmetry in the fourth family has a negative sign and an excess of Ū antiquarks with charge (−2/3) is generated in the early Universe. Excessive Ū antiquarks form (ŪŪŪ) antibaryons with electric charge −2, which are all captured by 4He and trapped in a [4He++(ŪŪŪ)−−] O-helium OHe “atom” as soon as 4He is formed in Big Bang nucleosynthesis. Interaction of O-helium with nuclei opens a new path to the creation of heavy nuclides in Big Bang nucleosynthesis. Due to the large mass of the U quark, OHe “atomic” gas decouples from baryonic matter and plays the role of dark matter in large-scale structure formation with structures on small scales being suppressed. Owing to nuclear interaction with matter, cosmic O-helium from the galactic dark matter halo is slowed in the Earth below the thresholds of underground dark matter detectors. However, an experimental test of this hypothesis is possible in the search for OHe in balloon-borne experiments and for U hadrons in cosmic rays and accelerators. OHe “atoms” might form anomalous isotopes and could cause cold nuclear transformations in matter, offering a possible way to exclude (or prove) their existence.