238U Reaction Research Articles

Synthesis and decay properties of the heaviest nuclei

The formation and decay properties of the heaviest nuclei with Z=112–116 and 118 were studied in the reactions 238U, 242,244Pu, 243Am, 245,248Cm and 249Cf + 48Ca. The new nuclides mainly undergo sequential α-decay, which ends with spontaneous fission. The total time of decay ranges from 0.5 ms to ∼1 day, depending on the proton and neutron numbers in the synthesized nuclei. The atomic number of the new elements 115 and 113 was confirmed also by an independent radiochemical experiment based on the identification of the neutron-rich isotope 268Db (TSF∼30 h), the final product in the chain of α-decays of the odd–odd parent nucleus 288115. The comparison of the decay properties of 29 new nuclides with Z=104–118 and N=162–177 gives evidence of the decisive influence of the structure of superheavy elements on their stability with respect to different modes of radioactive decay. The investigations connected with the search for superheavy elements in Nature and prospects of superheavy element research are also presented.The experiments were carried out at the Flerov Laboratory of Nuclear Reactions (JINR, Dubna) in collaboration with the Analytical and Nuclear Chemistry Division of the Lawrence Livermore National Laboratory (USA).

Differential and Angle-Integrated Cross-Section Measurement for the 6Li(n,t)4He Reaction at En = 1.05, 1.54, and 2.25 MeV

Differential cross sections and angle-integrated cross sections of the 6Li(n,t)4He reaction were measured at En = 1.05, 1.54, and 2.25 MeV by using the gridded ionization chamber method. A 6LiF sample and a 238U sample were set back-to-back inside the twin chamber for measurement. Neutrons were produced through the T(p,n)3He reaction. The absolute neutron flux for En = 2.25 MeV was determined by the 238U(n,f) reaction, and those for En = 1.54 MeV and En = 1.05 MeV were determined by a calibrated BF3 long counter. Present results are compared with existing data.

An International Evaluation of the Neutron Cross Section Standards

Abstract Work is reported here on the process and present results of an international evaluation of the neutron cross section standards. The evaluations include the H(n,n), 3He(n,p), 6Li(n,t), 10B(n,α), 10B(n,α1γ), 197Au(n,γ), 235U(n,f), and 238U(n,f) standard reactions as well as the 238U(n,γ) and 239Pu(n,f) reactions. This evaluation was performed to include new experiments on the standards that have been made since the ENDF/B-VI evaluation was completed and to improve the evaluation process. Evaluations have been completed for the 6Li(n,t), 197Au(n,γ) and 238U(n,γ) cross sections. Also below 20 MeV the H(n,n), 235U(n,f), 238U(n,f) and 239Pu(n,f) cross sections are completed. Many of the cross sections being evaluated are used in neutron dosimetry for fluence determination. The general trend observed for the evaluations is an increase in the cross sections for most of the reactions from fractions of a percent to several percent compared with the ENDF/B-VI results.

Opportunistic mass measurements at the Holifield Radioactive Ion Beam Facility

A technique for measuring mass differences has been developed at the Holifield Radioactive Ion Beam Facility (HRIBF) that requires no specialized equipment. Mass differences are measured as position differences between known and unknown-mass isobars, dispersed at the image of the energy-analyzing magnet following the 25 MV tandem post-accelerator, and identified by an energy-loss measurement. The technique has been demonstrated on neutron-rich 77–79Cu and 83–86Ge isotopes produced using the isotope separator online (ISOL) method with the 238U(p,fission) reaction, where a mass accuracy of 500 keV was achieved. These nuclides are well suited to the measurement technique, as they readily migrate out of the production target and to the ion source and comprise the most neutron-rich elements of the isobarically mixed beam. Because modest precision mass values can be obtained with only a few tens of counts of the nuclide of interest among orders of magnitude more of the isobaric neighbors closer to stability, the sensitivity of this technique makes it appropriate for initial mass measurements far from stability.

Synthesis and decay properties of superheavy elements

Abstract A fundamental outcome of modern nuclear theory is the prediction of the "island of stability" in the region of hypothetical superheavy elements. A significant enhancement in nuclear stability at approaching the closed shells with Z = 114 (possibly 120 and 122) and N = 184 is expected for the nuclei with large neutron excess. For this reason, for the synthesis of nuclei with Z = 112-116 and 118, we chose the reactions 238U, 242,244Pu, 243Am, 245,248Cm, and 249Cf + 48Ca, which are characterized by fusion products with a maximal neutron excess. The formation and decay properties of the heaviest nuclei were registered with the use of a gas-filled recoil separator installed at a 48Ca-beam of the heavy-ion cyclotron. The new nuclides mainly undergo sequential α-decay, which ends with spontaneous fission (SF). The total time of decay ranges from 0.5 ms to ~1 d, depending on the proton and neutron numbers in the synthesized nuclei. The atomic number of the new elements 115 and 113 was confirmed also by an independent radiochemical experiment based on the identification of the neutron-rich isotope 268Db (T SF ~ 30 h), the final product in the chain of α-decays of the odd-odd parent nucleus 288115. The comparison of the decay properties of 29 new nuclides with Z = 104-118 and N = 162-177 gives evidence of the decisive influence of the structure of superheavy elements on their stability with respect to different modes of radioactive decay. The investigations connected with the search for superheavy elements in Nature are also presented. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions (JINR, Dubna) in collaboration with the Analytical and Nuclear Chemistry Division of the Lawrence Livermore National Laboratory (USA).

Very heavy fission fragments produced in the spallation reaction 238U+ p at [formula omitted

Fission fragments from 1 A GeV 238U ions impinging a hydrogen target are investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing, and Δ E-ToF techniques. On the basis of the kinematics of the fragments, binary fission can be assigned as production process up to very heavy fragments. Fission fragments are separated and identified as far as 184Re for the first time. Cross sections are measured and found in the range of 500 μb to 5 μb. Isobaric cross sections decrease smoothly with increasing masses. Beyond tungsten, fission fragments are hidden in the more abundant spallation evaporation residues. 283 isotopes of elements gadolinium to rhenium in the mass range 147 to 184 are observed and cross sections measured. All the fragments are formed by fission of excited parent nuclei following emission of a cascade of secondary neutrons.

Production of 234,235Np and 236Pu in bombardment of 236U with protons in the energy range from 17 to 40 MeV

Abstract The production of 235Np and 236Pu by the reactions 236U(p,2n) 235Np and 236U(p,nβ−) 236Pu using the K-130 cyclotron of the University of Jyväskylä was investigated. The cross sections for the reactions were determined. Thick-target yield curves were derived based on the cross sections. The results are discussed and compared with previous data on other reactions leading to the formation of the same end products. The thick-target yield of 235Np in the 236U(p,2n) reaction is about 50 percent higher than the yield obtained in the 238U(p,4n) reaction leading to this nuclide at comparable particle energies. The purity of 236Pu produced in the 236U(p,nβ−) reaction is more than ten times better than for earlier known as the purest reaction 237Np(p,2n+pnβ−) 236Pu.

New elements from Dubna

We have studied the dependence of the production cross-sections of the isotopes 282,283112 and 286–288114 on the excitation energy of the compound nuclei 286112 and 290114. The maximum cross-sections of the xn-evaporation channels for the reaction 238U(48Ca,xn)286-x 112 were measured to be: σ 3n = 2.5 -1.1 +1.8 pb and σ 4n = 0.6 -0.5 +1.6 pb; for the reaction 242Pu(48Ca,xn)290-x 114: σ 2n ∼ 0.5 pb, σ 3n = 3.6 -1.7 +3.4 pb and σ 4n = 4.5 -1.9 +3.6 pb. In the reaction 233U(48Ca, 2-4n)277–279 112 we measured an upper cross-section limit of σ xn ≤ 0.6 pb. An increase of σ ER in the reactions of actinide targets with 48Ca can be due to the expected increase of the survivability of the excited compound nucleus upon closer approach to the closed neutron shell N = 184. The observed nuclear decay properties of the nuclides with Z = 104–118 are compared with theoretical nuclear mass calculations and the systematic trends of α-decay properties. As a whole, they give a consistent pattern of decay of the 18 even-Z neutron-rich nuclides with Z = 104–118 and N = 163–177.

Influence of multichance fission on fragment angular anisotropy in the 232Th( [formula omitted]) and 238U( [formula omitted]) reactions at intermediate energies

Fission fragment angular distributions have been measured for the 232Th( n , f ) and 238U( n , f ) reactions in the neutron energy range 20–100 MeV. The fragment angular anisotropy for 232Th was found to be systematically larger than that for 238U. The obtained results have been analyzed in the framework of the statistical saddle-point model combined with pre-equilibrium and Hauser–Feshbach calculations of partial fission cross sections. The calculations have revealed that fission following multiple neutron emission takes place in both reactions, resulting in a considerable contribution of high chances to the total fission fragment angular anisotropy. This gives grounds to expect that the observed difference is mainly due to nuclei fissioning at the end of the neutron evaporation chain.

The role of orientation of nucleus symmetry axis in fusion dynamics

The dependence of the fusion excitation function on the orientation angles of the symmetry axis of deformed projectile- and target-nucleus relative to the beam direction is studied. The averaged over all orientations values of the fusion–fission cross sections obtained for 16O + 238U and 60Ni + 154Sm reactions are compared with the experimental data. The capture and fusion cross sections for the 16O + 238U reaction are nearly the same while those are different for the 60Ni + 154Sm reaction due to sufficient contribution of the quasifission events. The large values of the fusion cross section are obtained in the range 60 ° < α i < 90 ° of the orientation angles of the projectile- and target-nucleus. This phenomenon is explained by the dependence of the intrinsic fusion and quasifission barriers on the orientation angles of reactants.

On-line production of Rb and Cs isotopes from uranium carbide targets

A series of on-line mass separation experiments have been performed at the IRIS facility to measure the yield and release of Rb and Cs neutron-rich isotopes produced by fission reaction of 238U. A 1 GeV proton beam was used to bombard uranium carbide targets with the densities of 11 g/cm3 and 1.5 g/cm3 held at temperatures in the range (2000-2230) °C. The release curves of Rb and Cs long-lived isotopes were measured from both kinds of targets. The overall production efficiency was determined making use of experimentally measured cross-sections of that isotope production. Comparison of the experimental yields of Rb and Cs isotopes with the calculated ones after corrections for losses due to finite release times suggests that the diffusion is the dominating process reducing the efficiency for short-lived isotopes. When normalized to the same thickness, an enhancement for the high-density rod target of the measured isotope yields is observed when going far from stability. This is possibly explained by the reactions induced by secondary neutrons. A significant odd-even effect with higher yields of Cs even neutron isotopes has been observed, confirming a similar effect obtained in earlier experiments.

Uranium symmetric/asymmetric neutron-induced fission up to 200 MeV

The symmetric SL-mode and asymmetric lumped (S1 + S2)-mode fission cross-sections of 235U(n,F) and 233U(n,F) reactions are calculated up to $E_{\rm {n}} = 200$ MeV within a statistical model. For each U nuclide, emerging in $(\rm {n},x\rm {nf})$ reactions a separate triaxial outer fission barrier is assumed for the SL-mode. To reproduce the measured branching ratio of symmetric and asymmetric fission events for the 238U(n,F) reaction, more fissions coming from neutron-deficient nuclei were assumed. The damping of the triaxial collective modes contribution to the level density at the SL-mode outer saddle was essential for the branching ratio description. These assumptions allow to reproduce observed fission cross-sections of 235U(n,F) and 233U(n,F) reactions. The calculated branching ratio sensitivity to the target nuclide fissility is investigated.

Prompt fission neutron multiplicity and spectrum model for 30–80 MeV neutrons incident on 238U

The improved Los Alamos model is developed for the first time in order to provide prompt fission neutron multiplicity, prompt fission neutron spectra and other quantities at high incident neutron energies where the fission of secondary compound nuclei formed by charged particle emission occurs. In this model (exemplified by the n+ 238U reaction up to 80 MeV incident energy) the fission of the secondary nuclei formed by proton emission, neutron evaporation from the nuclei formed by proton emission, deuteron emission, alpha emission and neutron evaporation from the nuclei formed by alpha emission is taken into account. Input model parameters and related excitation energy dependences are determined using available experimental information and systematics as well as total and partial neutron induced fission cross-sections and their ratios obtained separately from a recent evaluation performed up to medium energies. Our present model predictions are in good agreement with the measured prompt neutron spectra and multiplicities.

Monte Carlo models for the production of β-delayed gamma-rays following fission of special nuclear materials

A Monte Carlo method for the estimation of β-delayed γ-ray spectra following fission is described that can accomodate an arbitrary time-dependent fission rate and photon collection history. The method invokes direct sampling of the independent fission yield distributions of the fissioning system, the branching ratios for decay of individual fission products and the spectral distributions for photon emission for each decay mode. Though computationally intensive, the method can provide a detailed estimate of the spectrum that would be recorded by an arbitrary spectrometer, and can prove useful in assessing the quality of evaluated data libraries, for identifying gaps in these libraries, etc. The method is illustrated by a first comparison of calculated and experimental spectra from decay of short-lived fission products following the reactions 235U(n th, f) and 239Pu(n th, f). For general purpose transport calculations, where detailed consideration of the large number of individual γ-ray transitions in a spectrum may be unnecessary, it is shown that an accurate and simple parameterization of a γ-ray source function can be obtained. These parametrizations should provide high-quality average spectral distributions that should prove useful in calculations describing photons escaping from thick attenuating media.

Dynamics of spallation reactions investigated from production cross sections of residual nuclei in collisions induced by 238U at 1 A·GeV on proton and deuterium

A systematic investigation of the isotopic cross sections of nuclear residues produced in the reactions 238U (1 A GeV)+p,d has been performed with the high-resolution magnetic spectrometer FRS at GSI. The large range of excitation energies induced in these reactions, together with the high fissility of uranium-like systems constitute an optimum scenario to study the dynamics of spallation reactions. In particular, some fundamental questions connected to the role of dissipation and the onset of a break-up stage at high temperatures can be addressed with these data.

Actinide symmetric/asymmetric nucleon-induced fission up to 200 MeV

The fission cross sections of the symmetric SL-mode and the asymmetric lumped (S1+S2)-mode of the 235U(n,F), 237Np(n,F) and 238U(p,F) reactions are calculated up to En=200 MeV within a statistical model. For each fissioning nuclide, emerging in (n,xnf) reactions, a separate triaxial outer fission barrier is assumed for the SL-mode. To reproduce the measured branching ratio of symmetric and asymmetric fission events, strong contribution of fission from neutron-deficient nuclei is assumed. Damping of the contribution of triaxial collective modes to the level density at the SL-mode outer saddle seems to be essential for the description of the branching ratio. The sensitivity of the calculated branching ratio to the fissility of the target nuclide and the incident particle is investigated.

Second experiment at VASSILISSA separator on the synthesis of the element 112

The upgraded separator VASSILISSA was used to confirm results of previous experiments on the synthesis of the heavy isotopes of the element 112 obtained in complete-fusion reactions of 48Ca and 238U. An additional reason was the non-observation of any events in the same reaction studied with the BGS separator at Berkeley. Limits of $280 \leq A \leq 286$ for the atomic mass number of the observed spontaneously fissioning isotope were measured. The obtained results on the decay mode, half-life and production cross-section are in agreement with those obtained in our first experiment.

Influence of the shell effects on fission fragment angular anisotropies

The influence of the damping shell corrections with increasing excitation energy on the fission fragment angular anisotropies is considered. In the framework of the statistical approach to nuclear fission, experimental data on fission fragment angular anisotropies obtained in the 4He + 238U reaction is analyzed. Information about the energy dependence of the shell corrections is obtained from this analysis.

Dynamical model of fission fragment angular distributions

A dynamical model of fission fragment angular distributions is developed. The experimental data on the angular anisotropy of fission fragments is analyzed for the 16O + 208Pb, 232Th, 238U, and 248Cm reactions at energies of the incident 16O ions ranging from 90 to 160 MeV. This analysis allows us to extract the relaxation time for the tilting mode. It was also demonstrated that the angular distributions are sensitive to the deformation dependence of the nuclear friction.

Fission-residues produced in the spallation reaction 238U + p at 1 A GeV

Fission fragments from 1 A GeV 238U projectiles irradiating a hydrogen target were investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing and Δ E–ToF techniques. The momentum spectra of 733 identified fragments were analysed to provide isotopic production cross sections, fission-fragment velocities and recoil momenta of the fissioning parent nuclei. Besides their general relevance, these quantities are also demanded for applications. Calculations and simulations with codes commonly used and recently developed or improved are compared to the data.