Electrical and inertial parameters for Th,U, Pu isotopes

Nuclear properties of eight heavy deformed isotopes have been determined from muonic x-ray hyperfine structure. Trends observed for the electrical and inertial properties are discussed in terms of collective model predictions.     

Mass distribution in the bremsstrahlung-induced fission of Th,U and Pu

The yields of various fission products in the 10 MeV bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were determined using a recoil catcher and off-line γ-ray spectrometric techniques. From the yield data, mass yield distributions were obtained using charge distribution corrections. The higher yields of fission products around mass numbers 133–135, 138–140, 143–145 and their complementary products in the neutron and bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were interpreted based on nuclear structure effects. From the mass yield distribution, the peak-to-valley (P/V$P/V$) ratio was also obtained for the above fissioning systems. The present data, along with data from the literature on different bremsstrahlung- and mono-energetic neutron-induced fissions of ²³²Th and ²³⁸U are interpreted to examine the influence of excitation energy on the peak to valley ratio. For the same compound nucleus ²⁴⁰Pu^?, the data in the 10–30 MeV bremsstrahlung-induced fission of ²⁴⁰Pu were compared with similar data of thermal to 14 MeV neutron-induced fission of ²³⁹Pu and the spontaneous fission of ²⁴⁰Pu to examine the role of excitation energy due to bremsstrahlung radiation and mono-energetic neutrons.     

Symmetric and asymmetric yields in the photofission of232Th,235U,238U and239Pu

Measurements are presented for the mass and energy distributions of fragments from the photofission of²³²Th,²³⁵U,²³⁸U and²³⁹Pu. The experiments were performed for bremsstrahlung endpoint energies between 15 and 55 MeV, using silicon surface barrier detectors. The results are discussed with respect to the competition between the symmetric and asymmetric fission modes.     

Magnetic properties of the AnFe2 compounds (An = U,Np, Pu,and Am)

The magnetic properties of the actinide Laves compounds (cubic C-15 crystal structure) with iron have been studied by magnetization, neutron diffraction, and Mössbauer-effect techniques. All the compounds exhibit a spontaneous magnetization. Magnetic moments of 0.06, 1.09, 0.45, and −0.4 μ_B are associated with the U, Np, Pu, and Am ions, respectively. The corresponding iron moments are 0.6, 1.4, 1.5, and 1.7 μ_B. The ferrimagnetic nature of AmFe₂ suggests a partial occupancy of the Am²⁺ :f⁷ state, inasmuch as the Am³⁺ :f⁶ state has a nonmagnetic ground state.     

Microscopic structure of superdeformed states in Th, U, Pu and Cm isotopes with Gogny force

The structure properties of the even-even nuclei ^{226, 228, 230, 232, 234}Th, ^{230, 232, 234, 236, 238, 240}U, ^{240, 242, 244, 246}Pu, and ^{242, 244, 246, 248}Cm have been investigated at normal and superdeformed shapes in microscopic mean-field calculations based on Gogny force. Collective levels are predicted from constrained Hartree-Fock-Bogoliubov and configuration mixing calculations. Two quasiparticle states are also predicted from blocking calculations for neutron and proton configurations. Predictions are shown and compared with experimental data at superdeformed shapes. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: michel-g.girod@cla.tr     

Total spontaneous and isomer fission half-lives of U,U and Pu

Using the recently proposed coupled channel decay theory, we have reproduced here the observed absolute yield curves; total decay probabilities from the spontaneous and the isomeric fission states of ²³⁴U, ²³⁶U, and ²⁴⁰Pu, and the average kinetic energies. Our predicted yield curve for the decay of isomer fission state seems to be in agreement with preliminary observation.     

Fission of 232Th, 238U, 244Pu, 248Cm induced by Xe and Kr ions at coulomb barrier energies

Excitation functions and angular distributions have been measured for the fission of ²³²Th, ²³⁸U, ²⁴⁴Pu and ²⁴⁸Cm induced by ⁸⁶Kr and ¹³⁶Xe ions at energies in the vicinity of the Coulomb barrier. No large differences as a function of either target or projectile were found. These results suggest that the main process occuring is not Coulomb fission.     

Pu and Am determination in the environment—method development

A high resolution inductively coupled plasma mass spectrometric (HR-ICP-MS) method for the determination of plutonium isotopes, Am and the ²⁴⁰Pu/²³⁹Pu isotope ratio utilising modification of Pu-02-RC Plutonium in Soil Samples, Pu-03-RC Plutonium in Soil Residue—Total Dissolution Method, Pu-11-RC Plutonium Purification—Ion Exchange Technique, Pu-12-RC Plutonium and/or Americium in Soil or Sediments, HASL-300 was developed. Total plutonium concentrations (^239+240Pu) measured in environmental samples by this HR-ICP-MS method were in good agreement with recommended data obtained from a-spectrometry. It was achieved the decreasing of the time to analyze the samples over than 33%.     

Analyse von Hyperfeinstrukturen des Eu151, Am241 und einiger 3 d-Elemente mittels „Exchange Polarization“

It is shown how several discrepancies in the optical hfs of the Eu can be understood as consequences of the exchange polarization of the inner and outers-electrons by the spin of the half filled (4f ⁷)-subshell, an effect which should produce additional magnetic fields at the nucleus. Thus from the two different values of the electronic splitting constanta _6s in the two Eu-II ground states the polarization field from the 6s-shell (Δ H _6s ) is determined to be ca. +260 KG, and the formal splitting constantσ (??3 mK) of the (4f ⁷)-subshell yields ca. ?350 KG for the fieldΔ H _(1?5) from the five innern s-shells (n=1?5) in good agreement with the strength of the inner field obtained from recent Mössbauer effect studies.Δ H _(1?5) is deduced to be approximately equal in all sufficiently analysed ground and excited configurations of the neutral and ionised Eu atom ((4f ⁷) 6s, 6p, 5d, 6s ² and 6s 6p). Other elements with half filled subshells (Am, Mn) show similar features in their optical hfs. For Am⁺ ((5f ⁷) 7s) ca. ?2200 KG are found for the inner field (Δ H _(1?6)). For several 3d-elements it was found that the agreement between the calculated polarization fields and those following from experimental results is better than assumed so far.     

High-temperature magnetic properties of AnFe2 (An = Np,Pu, Am) Laves phases

Magnetization measurements have been performed on NpFe₂, PuFe₂, and AmFe₂ (cubic C15-type Laves phase) between 300 and 1100 K. The Curie temperatures were found to be 492 ± 2, 564 ± 2, and 613 ± 3 K, respectively. A correlation was found between the Curie temperature and the magnetic moment per iron atom determined in previous neutron-diffraction experiments. Critical exponents determined for NpFe₂ and PuFe₂ were found to be close to previously reported values for the chemically ordered metallic system Pd₃Fe. The overall magnetic behavior of the compounds studied implies the presence of local magnetic moments, in contrast to the itinerant magnetic behavior of UFe₂ noted in the previous paper.     

Pockets in heavy ion potentials for the deformed systems Th + Ta,U + Th and Th + Th

Using a realistic nucleon-nucleon interaction the optical potentials for the strongly deformed systems Th + Ta, U + Th and Th + Th are calculated at E_lab/A ≅ 6 MeV and for different orientations. Pockets exist in the total real potentials (nuclear plus Coulomb potentials) at surface contact. It has been found that the deepest pockets for the U + Th and Th + Th systems exist when the two colliding nuclei are touching with orientation angle β ≅ 45° while for the Th + Ta system the deepest pocket occurs at β = 90° (equator touching).     

Structure of high-spin states in 232Th, 234U and 236U

Multiple Coulomb excitation of ²³²Th, ²³⁴U and ²³⁶U by 5.3 $\text{MeV}\text{u}$²⁰⁸Pb ions has been studied using γ-ray spectroscopy. Excitation of ground-band levels is observed up to spin I^π = 26⁺ (tentatively 28 ⁺) in ²³²Th and ²³⁴U and I^π = 28⁺ (tentatively 30⁺)in ²³⁶U. High-spin levels of the K^π = 0^? octupole-vibrational bands are also observed in these nuclei. The measured transition energies between ground-band levels suggest that at $\text{I≈ 28}\text{h}\text{?}$ several units of angular momentum are carried by single particles aligned with the rotation axis.This result can be understood in terms of a super band built on aligned two-quasiparticle configurations which crosses the ground-state rotational band at a rotational frequency of $\text{h}\text{?}\text{ω ? 0.25}\text{MeV}\text{(I ? 28}\text{h}\text{?}\text{)}$. The E2 transition-matrix elements deduced from the experimental γ-yields agree within their errors with the rigid-rotor predictions up to the highest spins observed.The experimental results are discussed using the concept of rotation alignment and are compared with predictions of the rotation-vibration model and the interacting-boson model.     

Measurements of delayed-neutron yields from thermal-neutron-induced fission of 235U, 233U, 239Pu, and 237Np

The experimental results on delayed-neutron yields from thermal-neutron-induced fission of some actinides in the IBR-2 pulsed reactor are presented. A method of periodic irradiation without displacement of the sample was used. The measurements of delayed-neutron total yields in thermal-neutron-induced fission of ²³⁹Pu, ²³³U, and ²³⁷Np and in cold-neutron-induced fission of ²³⁵U, ²³³U, and ²³⁹Pu were carried out. All values were obtained with the use of the value of β₀ for (n _th+²³⁵U) as a reference. Precise measurements of decay curves in the time interval 5–350 ms for ²³⁵U and ²³⁹Pu were performed.     

Fragments' mass and energy characteristics in the spontaneous fission of 236Pu, 238Pu, 240Pu, 242Pu,and 244Pu

The results of a systematic study of the mass and energy of fission fragments emitted in the spontaneous fission of ^{236,238,240,242,244}Pu are reported. A comparison of the fragments' mass and energy distributions demonstrates the occurrence of different fission modes with varying relative probability. These results are discussed in terms of the random neck rupture model as well as in terms of the scission point model, showing the influence of the neutron number of the fissioning system. Finally an improved method of analysis allows the investigation of the cold fission region.     

The low-temperature heat capacity of the (Th,Pu)O2 solid solution

The low-temperature heat capacity of the (Th_1−y,Pu_y)O₂ solid solution with y=0.03, 0.08, 0.30, 0.54 and 0.85 was determined from 4 K up to about 300 K with a PPMS instrument using the hybrid adiabatic relaxation method. In addition, the heat capacity of ²³⁹PuO₂ end member was also measured and compared with available literature data for ²⁴²PuO₂, observing an indistinguishable match. The obtained heat capacity values of the intermediate compositions are greater than the data obtained from Neumann–Kopp's molar additivity rule based on the ThO₂ and PuO₂ end-members, and a substantial excess capacity is observed for all intermediate compositions, particularly at the lowest temperatures.     

Transmutation of 129I, 237Np, 238Pu, 239Pu, and 241Am using neutrons produced in target-blanket system ‘Energy plus Transmutation’ by relativistic protons

Target-blanket facility ‘Energy + Transmutation’ was irradiated by proton beam extracted from the Nuclotron Accelerator in Laboratory of High Energies of Joint Institute for Nuclear Research in Dubna, Russia. Neutrons generated by the spallation reactions of 0.7, 1.0, 1.5 and 2 GeV protons and lead target interact with subcritical uranium blanket. In the neutron field outside the blanket, radioactive iodine, neptunium, plutonium and americium samples were irradiated and transmutation reaction yields (residual nuclei production yields) have been determined using γ-spectroscopy. Neutron field's energy distribution has also been studied using a set of threshold detectors. Results of transmutation studies of ¹²⁹I, ²³⁷Np, ²³⁸Pu, ²³⁹Pu and ²⁴¹Am are presented.