Yields of neutron-rich isotopes around Z = 28 produced in 30 MeV proton-induced fission of 238U

Heavy ^65-70Co, ^68-74Ni, ^70-76Cu and ^74-81Ga isotopes were produced at the LISOL facility by means of 30 MeV proton-induced fission of ²³⁸U. Production rates were deduced and compared to two types of cross-section calculations: the empirical model (V. Rubchenya, private communication) and the PROFI code. Comparison with experimental data favors the latter model. Yields using different beam-target combinations and different energies are calculated and discussed. Received: 11 February 2002 / Accepted: 26 April 2002     

Precision determination of charge dispersion and distribution in the proton-induced fission of238U at 13.9MeV excitation

Since the balance energy, i.e. the energy of vanishing in-plane flow in heavy-ion collisions might be sensitive to in-medium effects we present here a study of these inmedium effects by comparing QMD calculations using phenomenological interactions and using realistic interactions as determined in the framework of Brückner theory. The influence of a self-consistent mean field and of in-medium cross sections are discussed as well. In addition, for the reactions²⁰Ne+²⁷Al,⁴⁰Ar+⁴⁵Sc and⁸⁴Kr+⁹³Nb a comparison with experimental data is given.     

Symmetric and asymmetric fission modes in proton-induced fission at 660 MeV of <Superscript>238</Superscript>U

Fission product cross sections of intermediate-energy fission of ²³⁸U were used in order to construct the charge and mass yield distributions. Enriched target of ²³⁸U was irradiated by proton beam with energy 660 MeV for several hours at the LNP Phasotron, Joint Institute for Nuclear Research (JINR), Dubna, Russia. The charge distribution of the fission fragments was analyzed for calculation of isobaric cross sections. The mass yield curves were expanded into symmetric and asymmetric components according multimodal fission approach. The fissility values of actinides were calculated at given proton energy. The obtained results have been compared to the same data for targets ²³⁷Np and ²⁴¹Am.     

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

Inclusive⁴He and⁴H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u²³⁸U with²³⁸U and^?197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For²³⁸U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For²³⁸+²³⁸U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In²³⁸U+¹⁹⁷Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.     

Abrupt changes of the characteristics of the proton-induced fission of238U around 14-MeV excitation

An experiment combining the conventional radio-chemical stacked target technique and rapid measurements using a gas-jet transport system were carried out with a purpose of precise determination of charge dispersion and distribution for the proton-induced fission of²³⁸U at two incident energies of 9.4 and 12.4 MeV. We determined the charge dispersion width as 1.01±0.05 charge unit at 17.9-MeV excitation and 0.70±0.04 charge unit except for the symmetric mass region at 14.8-MeV excitation. Fundamental differences were found also in the mass distribution characteristics between 17.9-MeV and 14.8-MeV excitations. This abrupt change of the fission characteristics strongly supports the existence of different fission mechanisms below and above around 14-MeV excitation.     

Electromagnetic fission of238U at 600 and 1000 MeV per nucleon

Electromagnetic fission of²³⁸U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsäcker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.Communicated by V. Metag     

Neutron multiplicity in the fission of 238U and 235U with neutrons up to 200 MeV

Prompt-fission-neutron multiplicities were measured for 238U(n,f) and 235U(n,f) from 0.4 to 200 MeV. The data are of great importance in connection with accelerator-coupled nuclear reactor systems incinerating actinides. We report that fission induced by 200 MeV neutrons produces approximately 10 more prompt neutrons than fission induced by reactor neutrons. Most neutrons are evaporated from the fission fragments and the prefission compound nucleus, as the preequilibrium emission of energetic neutrons accounts for a maximum of 15% of the prompt neutrons at 200 MeV.     

Isomeric yield ratios of <Superscript>238</Superscript>U and <Superscript>232</Superscript>Th photofission products

The isomeric ratios of fragments of photofission of ²³⁸U and ²³²Th have been determined experimentally at the bremsstrahlung end-point of 50 and 3500 MeV. Results of measurements enabled us to estimate the mean angular momenta of primary fragments and to compare them with the published data.     

Cumulative fission yield measurements with 14.7 MeV neutrons on 238U

The fission yield data in the 14 MeV energy neutron induced fission of ²³⁸U play an important role in decay heat calculations and generation-IV reactor designs. In order to accurately measure fission product yields (FPYs) of ²³⁸U induced by 14 MeV neutrons, the cumulative yields of fission products ranging from ⁹²Sr to ¹⁴⁷Nd in the ²³⁸U(n, f) reaction with a 14.7 MeV neutron were determined using an off-line γ-ray spectrometric technique. The 14.7 MeV quasi-monoenergetic neutron beam was provided by the K-400 D-T neutron generator at China Academy of Engineering Physics (CAEP). Fission products were measured by a low background high purity germanium gamma spectrometer. The neutron flux was obtained from the ⁹³Nb (n, 2n)^92mNb reaction, and the mean neutron energy was calculated using the cross-section ratios for the ⁹⁰Zr(n, 2n)⁸⁹Zr and ⁹³Nb(n, 2n)^92mNb reactions. With a series of corrections, high precision cumulative yields of 20 fission products were obtained. Our FPYs for the ²³⁸U(n, f) reaction at 14.7 MeV were compared with the existing experimental nuclear reaction data and evaluated nuclear data, respectively. The results will be helpful in the design of a generation-IV reactor and the construction of evaluated fission yield databases.     

Electromagnetic and nuclear fission of238U in the reaction of 100, 500, and 1000 A·MeV208Pb with238U

The folding- and azimuthal-angle and velocity distributions for the²³⁸U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV²⁰⁸Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the²⁰⁸Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (≈1.5 b) is approximately constant between 100 and 1000 A·MeV.     

Study of the heavy products from84Kr+238U collisions at 522 MeV

Angular distributions and integral cross sections for heavy reaction products from⁸⁴Kr +²³⁸U collisions at 522 MeV were measured using catcher foil techniques and off-line X-ray identification of individual reaction product isotopes. There is no evidence for a fission-fusion mechanism being responsible for the formation of the “gold finger”. The data show that usual deep inelastic collisions account for the formation of all products with 70≦Z≦86. Compared to existing data atE≦605 MeV, the deduced mass distribution indicates reduced fission probabilities forZ≧80 fragments at 522 MeV. This is consistent with the expected dependence of fission probability on excitation energy and especially on angular momentum.     

Isomeric yield ratio of fission product148Pr in235U(n th,f)

The independent isomeric yield ratio of¹⁴⁸Pr in thermal neutron induced fission of²³⁵U has been determined experimentally. The fission product¹⁴⁸Pr isomers, extracted directly by on-line mass separation technique, have high-spin (J=4) to low-spin (J=1) isomer ratio of 0.14±0.04 using growth and decay analysis. Statistical model calculation of isomeric yeild ratio using constant initial r.m.s. angular momentumJ _rms can not reproduce either present results or other recent measurements of isomer ratios. TheJ _rms derived from isomer ratio data in all thermal fissioning systems indicate a wide spread ranging from 2? to 13?. No clear correlation betweenJ _rms and isomeric spins or number of neutrons of isomers is found, thus, more model refinements and experimental works should be done in order to evaluate independent isomeric yields correctly.     

Fragment mass dependence of angular anisotropy in 15 MeV proton-induced fission of 244Pu

Angular distributions of fission fragments with mass number A=97-159 have been measured by the radiochemical recoil-catcher method in the proton-induced fission of ²⁴⁴Pu with the incident energy of 15 MeV. Angular anisotropies of extreme asymmetric mass division products even up to the fragment mass ratio of A _H /A _L ∼ 1.85 are found not any different from those of the typical asymmetric mass division products with A∼ 138, which indicates that no clear evidence is observed for the existence of an additional saddle point configuration in the extreme asymmetric mass division. The correlation between the saddle point state evaluated from the angular anisotropy and the mass division mode is discussed.     

Recoil studies of fission products formed in the fission of U by protons of energies 25–85 MeV

The recoil properties of ten fission products with masses ranging from 72 to 136 formed in the fission of ²³⁸U with protons of energies 25–85 MeV have been determined radiochemically by the integral-range method. From the recoil properties of the products and the Monte Carlo cascade calculations the average kinetic energy, cascade deposition energy, and anisotropy parameter for each fission product has been calculated. The kinetic energy and the excitation energy of the primary fragments leading to the observed fission product, and the total kinetic energy and the total excitation energy of the primary fragment pair have also been calculated.

The results indicate that up to a bombarding energy of 40 MeV fission takes place predominantly by the compound nucleus mechanism, with an increasing contribution of the direct interaction mechanism as the bombarding energy increases. The kinetic energy deficit was found to decrease with increasing bombarding energy. The fission products formed from the symmetric mode of fission have a larger separation distance between the charge centres of their respective primary fragments than those for the asymmetric mode of fission.     

Nuclear excitation and prompt fission in muonic238U

A study of muonic²³⁸U has been performed in a combined (μ ^?,γ f) and (μ ^?,γγ) coincidence experiment to investigate the role of non-radiative transitions and their fission probabilities. An augmentation of the outer fission barrier ofΔE _b =(0.6±0.1) MeV due to the presence of the muon is deduced. A significant contribution to the prompt fission yield not only results from the (2p→1s) and (3d→1s) non-radiative transitions, but also from other radiationless transitions. Specifically, the measured fission probabilities of the transitions (2p→1s), (3d→1s), and (3p→1s) are (1.5±0.4)%, (5.7±1.7)%, and (5.3±1.9)%, respectively.     

Reaction and fission cross-sections of 750 A·MeV238U ions on Pb,Cu and Al-targets

Charge-loss and fission cross-sections of²³⁸U at 750 A·MeV were measured on Al, Cu and Pb targets. The charge-loss rate was obtained by the attenuation method. Fission was selected by detecting the pair of highly ionizing fragments. Since the neutron-loss cross sections were measured in a parallel experiment for the same projectiles, all cross sections contributing to²³⁸U collisions on nuclei are available now as function of the target mass number and can be compared with current models.     

Observation of the delayed fission induced by antiproton annihilation in 238U

Delayed fission following the annihilation of antiprotons at rest in ²³⁸U nuclei has been observed with a yield of approximately 10⁻³. By means of the recoil-distance technique a lifetime of about 10⁻¹⁰ s has been determined for the delayed fission; this is interpreted as due to the non-mesonic decay of heavy hypernuclei.