An interpretation of the structure of subthreshold fission cross section of240Pu

The structure recently observed in the subthreshold fission cross section of²⁴⁰Pu is interpreted as being due to intermediate states of the fissioning nucleus associated with the second minimum of the potential energy as a function of deformation postulated byStrutinsky.     

Resonances in the isomeric and prompt fission probabilities of240Pu

The prompt and isomeric fission probabilities of²⁴⁰Pu have been studied using the²³⁹Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum.     

Vibrational resonances below 5 MeV in the fission of 240Pu

Using the ²³⁹Pu(d, pf) reaction, the next lower K^π = 0⁺ vibrational resonance below 5 MeV in the deformed potential of ²⁴⁰Pu was located at 4.55 MeV. The resultant spacing of fission vibrations near 5 MeV in the second minimum is 500 keV. A resonance at 4.70 MeV is interpreted as the K^π = 0^? vibration coupled to the 4.55 MeV K^π = 0⁺ state, which yields E(K^π = 0^?) = 150 keV relative to the ground state of the second minimum.     

Detailed spectroscopy in the superdeformed second minimum of 240Pu

Spectroscopic studies in the superdeformed shape isomer of ²⁴⁰Pu using γ-spectroscopy, conversion electron spectroscopy and transmission resonance spectroscopy have been performed. In a high-resolution and high-efficiency γ-spectroscopy experiment the out-of-band decays of several excited superdeformed rotational sequences with K=2^? and 1^? could be identified together with evidence for a weakly populated 0^? octupole band. Surprisingly, no low-lying collective quadrupole excitations could be observed. Complementary information could be obtained in conversion electron measurements in coincidence with isomeric fission, resulting in the first identification of the lowest ß-vibrational K=0⁺ band. For all rotational bands the variation of the moment of inertia with spin could be studied. A predominant population of negative parity states in the second well could be observed, which can be explained by the selective population and depopulation of the second minimum. Complementary transmission resonance measurements have been performed, yielding new information on the fine structure of high-lying (ß-)vibrational multi-phonon states. A new method could be established to determine the excitation energy of the fission isomer ground state from measured level densities.     

Hyperfine interactions and charge distributions of actinide ions recoiling into vacuum

The angular distribution of conversion electrons from the 4⁺→2⁺ transition in the ground-state rotational band of²⁴⁰Pu has been measuredvia the²³⁸U (α,2n)²⁴⁰Pu reaction. The electrons are emitted in flight from the Pu nuclei recoiling into vacuum atv/c?0.2%. The attenuation of the angular distribution by extranuclear fields is found to be less than 20% within 120 ps. The hyperfine interactions are discussed with particular attention to the high charge states of actinide recoil ions following internal conversion. Charge distributions ranging up to 45⁺ have been observed for²⁴⁰Cm nuclei in the²³⁹Pu(α,3n) reaction.     

Angular momentum dependence of moments of inertia and fission barriers of 240Pu

Minimization with respect to a constraint on angular momentum leads to a strong angular momentum dependence of nuclear properties. The second minimum of the potential energy surface of ²⁴⁰Pu becomes lower than the first one for J >24.     

An investigation of the tensor force effects on the spin-orbit splitting in spherical and very deformed nuclei

A contribution of the tensor force to the phenomenological spin-orbit (SO) potential in the single-particle Hamiltonian is studied for spherical and deformed rare-earth and actinide nuclei. It is found that the deformation dependence of the SO coupling constant, suggested by the second order tensor force contribution, enables to explain the observed single particle (s.p.) states in the second minimum of²³⁷Pu and²³⁹Pu using a SO strength weaker than usually applied. Consequences of the tensor contributions for the stability of theZ=114 superheavy nucleus are studied.     

Photofission of nuclei with allowance for wriggling vibrations of fissioning nuclei

It is confirmed that one source of the large relative orbital momenta L of fragments in spontaneous and stimulated low-energy nuclear fission is quantum transverse zero-point wriggling vibrations of the fissioning system near its scission point. The angular distributions of fragments of low-energy photofission of actinide nuclei, calculated using the quantum theory of fission, are compared. Vibrations are allowed for by using parameter C _w determined by Nix and Swiatecki. Agreement between the experimental and theoretical angular distributions for ²³⁴U, ²³⁶U, ²³⁸U, ²³⁸Pu, ²⁴⁰Pu, ²⁴²Pu nuclei is observed. The strong sensitivity of the theoretical angular distributions for ²³⁸Pu, ²⁴⁰Pu, ²⁴²Pu nuclei toward the choice of parameters of transient fissioning states at the external and internal fission barriers is demonstrated.     

Moments of inertia of fissioning isomers

The moments of inertia of ground states and fissioning isomers of ²³⁶U and ²⁴⁰Pu are calculated within the cranking model by modifying the pairing interaction. In both cases the measured and calculated values agree if the pairing strength is independent of the surface area.     

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.     

In-beam γ -ray study of the neutron-rich nuclei 240U, 246Pu, and 250Cm produced by the (18O, 16O) reaction

We have measured deexcitation γ rays in the neutron-rich nuclei of ²⁴⁰U, ²⁴⁶Pu, and ²⁵⁰Cm produced by the (¹⁸O, ¹⁶O) two-neutron transfer reactions, in coincidence with the ¹⁶O particles using Si ΔE−E detectors. The γ rays in these nuclei were identified by selecting the kinetic energies of ¹⁶O particles, which correspond to the excitation energies in the residual nuclei below the neutron separation energies. The ground-state bands of ²⁴⁰U, ²⁴⁶Pu, and ²⁵⁰Cm were established up to 12⁺ states and the K ^π = 0⁻ octupole band of ²⁴⁰U was established up to 9⁻ states. The systematics of the moments of inertia of the ground-state bands for actinide nuclei shows that the deformed subshell closure at N = 152 is sustained for ₉₆Cm isotopes and that it disappears for ₉₄Pu isotopes. The text was submitted by the authors in English.     

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.     

Photofission of 238Pu, 240Pu, and 242Pu in the energy range 5–10 MeV

The cross sections for the photofission of plutonium isotopes ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu in the energy range 5–10 MeV have been measured by using a beam of bremsstrahlung photons from the microtron installed at the Institute of Physics and Power Engineering (Obninsk). The energy regions below the fission barrier and above 6 MeV have been scanned with pitches of 0.05 and 0.1 MeV, respectively. In deriving the absolute cross section for ²³⁸Pu photofission, ²³⁸U photofission has been employed as a reference reaction. In measurements involving ²⁴⁰Pu and ²⁴²Pu nuclei, the neptunium isotope ²³⁷Np, which is characterized by a more regular dependence of the photofission cross section on excitation energy than ²³⁸U, has been chosen for the first time as a reference nucleus. The measured cross sections as functions of energy show resonance structures in the vicinity of the fission threshold that are consistent with those previously observed in the energy dependences of fissilities for corresponding direct reactions. The partial-wave cross sections for the J ^π K = 1^?0, 1^?1, and 2⁺0 photofission channels have been determined as functions of energy. At energies below some 5.5 MeV, the total cross section for photofission of each plutonium isotope being studied receives a significant contribution from quadrupole interaction. Within the one-dimensional model of a two-humped fission barrier, the parameters of the barriers for ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu have been extracted from data and have then been compared with estimates based on previous measurements.     

Excitation function and half-life for the fission isomer240mPu from the238U(α, 2n)240mPu reaction

The excitation function for the fission isomer^240mPu produced in the reaction of²³⁸U with α particles, has been measured at laboratory incident energies ranging from 20.1 to 27.3 MeV. A value of 3.8 _?0.4 ^+0.6 ns was obtained for the half-life of^240mPu. The results are compared with other measurements performed using different experimental methods.     

The shell structure and the asymmetry of nuclear fission for 140Pu in a statistical model

The dependence on excitation energy of the mass distribution of the fragments arising from the fission of ²⁴⁰Pu is studied in a statistical model. The level densities needed are calculated on the basis of the single-particle energies obtained from a deformed Woods-Saxon potential. First we investigate when and how the shell effects disappear in ²⁴⁰Pu with increasing excitation energy. Using the transition state method we then calculate some characteristic properties of the fragment mass distributions and compare the results with the experimental observations. Reasonable agreement is obtained.     

Plutonium isotopes concentration in the ground level air and rain samples from Kraków

The aim of the present work was to look at the background levels of plutonium in air and rain samples. Two sets of Petryanov type filters through which ca. 0.5 Mm³ each of air had passed and two large samples of collected rain (170 L and 182 L) were analysed for the content of plutonium alpha-emitters. In the article the radiochemical procedure applied to these samples is described. Obtained Pu activities in air were 0.5±0.1 nBqm^?3 and 0.7±0.1 nBqm^?3 for²³⁸Pu and 3.1±0.2 nBqm^?3 and 8.3±0.6 nBqm^?3 for^239+240Pu. Activities of rain samples were 7.5±0.7 mBqm^?3 and 14.3±0.8 mBqm^?3 for^239±240Pu and 0.4±0.1 mBqm^?3 and 2.1±0.2 mBqm^?3 for²³⁸Pu, respectively, for the two samples. The most interesting finding is the largely different²³⁸Pu to^239+240Pu activity ratios (from 0.03±0.01 to 0.18±0.01) measured in the samples. This result needs conformation, but as now it suggests different origins of plutonium isotopes in incoming to Kraków air.     

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%.     

Evaluation of 242Cm and 244Cm decay data

The decay data evaluation results are presented for the ²⁴²Cm and ²⁴⁴Cm radionuclides decaying to the levels in ²³⁸Pu and ²⁴⁰Pu, respectively. The evaluated data have been obtained using information published up to 2005. The text was submitted by the author in English.     

An investigation of the properties of single-particle states in the second minimum of 237Pu

In connection with the recent measurement of the magnetic moments of the fission isomeric states in ²³⁷Pu, we investigate the following subjects: (i) The spin alignment of the lowest-lying states in a potential minimum which are populated by (α, 2nγ) reactions, and the fragment angular distributions in fission from isomeric states. (ii) The single-particle level scheme in a very deformed Woods-Saxon potential. (iii) The spin polarizability as a function of the quadrupole deformation.