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.     

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.     

Difference in the electronic and fission decay modes for muonic atoms

A simple model is presented which predicts a difference in the electronic and fission decays of ground-state muonic heavy atoms. It is interpretable in terms of the relative population of two nuclear states: the isomeric fission state and the ground state. The application of the model to ²³⁸U indicates there could be a significant population of the isomeric state at the end of the muonic atomic cascade.     

裂变产物衰变链设计

根据核科学参考文献库,对半衰期、缓发中子发射几率等进行了更新评价,采用系统学研究和理论研究相结合的评价方法对裂变产物中不确定的基态进行了自旋指定,对部分存在问题的数据进行了必要修正;对衰变子核存在同质异能态的放射性核素,根据衰变纲图和内转换系数重新计算得到退激至子核同质异能态和基态的分支比数据。在此基础上,结合现有核结构和衰变数据库,研制了裂变产物衰变链设计专用衰变数据库,按照衰变规律,建立了裂变产物衰变路径和衰变信息的完整衰变链,本工作为裂变产物分析和裂变产额研究提供了一个便利参考。     

New isomeric states in 152,154,156Nd produced by spontaneous fission of 252Cf

Isomeric states have been observed in fission-fragments produced by spontaneous fission of ²⁵²Cf. These states are found in neutron rich nuclei of different structure and deformations. About 50 isomeric nuclei have been observed using coincidences between γ-rays identified in EUROGAM II and fission fragments detected in photovoltaic cells (SAPhIR). Lifetimes in the range from 20 ns to 2μs have been measured. Presented calculations based on HFB +D1S force on new measured isomeric states in the ^152,154,156Nd show evidence for K-isomers. Received: 3 November 1997 / Revised version: 15 December 1997     

Emission properties of fission fragments and their isomeric ratios

The properties of neutron emission from fragments formed in the spontaneous fission of ²⁵²Cf and in the thermal-neutron-induced fission of ²³⁵U are analyzed on the basis of the statistical model of nuclear reactions. Upon extracting the mean excitation energies of fission fragments from experimental data on the mean multiplicities of neutrons, the observables of neutron emission can be described over wide ranges of total kinetic energies and masses. The observed values of mean fragment spins are also reproduced. A method for calculating the isomeric ratios of the independent yields of fission fragments that is based on the cascade-evaporation model of excited-nucleus decay is employed to describe experimental data on ²³⁵U fission induced by thermal neutrons and on ²³⁸U fission induced by alpha particles. The effect exerted on the isomeric ratios for fission fragments by two different assumptions on the spin distributions of primary-fragment populations—the assumption of the distribution associated with rotational degrees of freedom and the assumption of the distribution associated with the internal degrees of freedom of fully accelerated fragments—is investigated.     

Isomeric decay studies around 204Pt and 148Tb

Relativistic energy projectile fragmentation of ²⁰⁸Pb has been used to produce a range of exotic nuclei. The nuclei of interest were studied by detecting delayed gamma rays following the decay of isomeric states. Experimental information on the excited states of the neutron-rich N=126 nucleus, ²⁰⁴Pt, following internal decay of two isomeric states, was obtained for the first time. In addition, decays from the previously reported isomeric I=27ħ and I=(49/2)ħ states in ¹⁴⁸Tb and ¹⁴⁷Gd, respectively, have been observed. These isomeric decays represent the highest spin discrete states observed to date following a projectile fragmentation reaction, and opens further the possibility of doing `high-spin physics' using this technique.     

High-spin excitations of 84 35Br49 and 85 35Br50: Mapping the proton sub-shells towards 78Ni

The ^{84, 85}Br nuclei have been produced as fission fragments in the fusion reaction ¹⁸O + ²⁰⁸Pb at 85MeV bombarding energy and studied with the Euroball IV array. The high-spin states of the odd-odd ⁸⁴Br nucleus have been identified for the first time, the observed structure being built on the known 6^- isomeric state. High-spin states of ⁸⁵Br have been observed up to 5.4MeV excitation energy and spin I ∼ 21/2. From angular correlation analysis, spin values have been assigned to most of the ⁸⁵Br excited states up to 4MeV. None of these excited states has been found to exhibit a delayed decay having T _1/2 > 10ns. All the observed states in ^{84, 85}Br can be described by various proton excitations involving at least the two sub-shells ( πf _5/2 and πp _3/2) located just above the Z = 28 shell closure.     

Towards a better understanding of deep subthreshold photofission of238U

Deep subthreshold photofission data of²³⁸U are analysed. Spectroscopic properties of highly deformed states from experiments in the literature are introduced into a model calculation based on the doorway state formalism which is extended to fission at energies as low as the isomeric level. It is shown that starting from the results of a detailed analysis of the photofission measurements near the top of the fission barrier it is possible to describe the process in the energy range covered by the intermediate well in the double-humped deformation potential. In particular, the so-called “shelf effect” in the low energy photofission is reproduced in terms of delayed isomeric fission through a few undamped vibrational states localised in the pairing gap region above the isomer.     

Nonuniform character of the population of spin projections K for a fissile nucleus at the scission point and anisotropies in the angular distributions of fragments originating from the induced fission of nuclei

It is shown that the emergence of anisotropies in the angular distributions of fragments originating from the spontaneous and induced fission of oriented actinide nuclei is possible only if nonuniformities in the population of the projectionsM (K) of the fissile-nucleus spin onto the z axis of the laboratory frame (fissile-nucleus symmetry axis) appear simultaneously in the vicinity of the scission point but not in the vicinity of the outer saddle point of the deformation potential. The possibilities for creating the orientation of fissile nuclei for spontaneous and induced fission and the effect of these orientations on the anisotropies under analysis are considered. The role of Coriolis interaction as a unique source of the mixing of different-K fissile-nucleus states at all stages of the fission process is studied with allowance for the dynamical enhancement of this interaction for excited thermalized states of the nucleus involved that is characterized by a high energy density. It is shown that the absence of thermalization of excited states of the fissile nucleus that appear because of the effect of nonadiabaticity of its collective deformation motion in the vicinity of the scission point is a condition of conservation of the influence that transition fission states formed at the inner and outer fission barriers exerts on the distribution of the spin projections K for lowenergy spontaneous nuclear fission. It is confirmed that anisotropies observed in the angular distributions of fragments originating from the fission of nuclei that is induced by fast light particles (multiply charged ions) are due to the appearance of strongly excited equilibrium(nonequilibrium) states of the fissile nucleus in the vicinity of its scission point that have a Gibbs (non-Gibbs) distribution of projections K.     

Effect of entrance channel parameters on fission fragment angular momentum in medium energy fission

Independent isomeric yield ratios of¹³²I were radiochemically determined in alpha particle induced fission of²³⁸U in the energy range 25–44 MeV. Fission fragment angular momenta were deduced from the measured isomeric yield ratios using spin dependent statistical model analysis. It was seen that angular momentum of¹³²I increases with increase of excitation energy and angular momentum of the fissioning nucleus. Comparison of the present data on¹³²I in²³⁸U(α,f) with the literature data for the same product in²³⁸U(p, f) and²³⁸U(γ, f) at various excitation energies show that fragment angular momentum strongly depends on the input angular momentum in the range of excitation energy considered. Experimental fragment angular momentum at all excitation energies were seen to be in agreement with the theoretical values calculated based on thermal equilibration of the various collective rotational degrees after considering the occurence of multichance fission. Thus, strong effect of input angular momentum as well as the statistical equilibration among the various collective rotational degrees of freedom in medium energy fission is corroborated.     

Fragment mass and kinetic energy distribution in isomeric fission of 240Pu

Fragment mass and kinetic energy distributions have been measured for isomeric fission of ²⁴⁰Pu. The mass distribution is asymmetric with the average heavy fragment mass nearly equal to that found for ground state spontaneous fission of ²⁴⁰Pu, but slightly lower than for n_th + ²³⁹Pu-fission. The average total fragment kinetic energy appears to be higher in isomeric fission (179.5_?0.7^+1.5 MeV) than in spontaneous fission from the ground state (176.8 ± 1.8 MeV).     

Hyperdeformed states in the third minimum of the fission potential

The fission probability and the angular distribution of the associated fission fragments have been measured for the²³⁵U(d, pf)²³⁶U reaction slightly above the top of the inner fission barrier with the aim of disclosing new hyperdeformed states lying in the third minimum of the fission potential. The results are analyzed together with the data obtained from the literature for²³⁴U. The experimental resonances are interpreted as being rotational bands with rotational parameters of ?/2Θ=1.6±0.6 and 1.8±0,8 keV for²³⁴U and²³⁶U, respectively. TheK=1, 2 and 4 values of the resonances suggest that these rotational bands are built on some higher-lying excited states in the third minimum of the potential well.     

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 isomerism induced by helium ions

Delayed fission events due to isomeric fission have been produced using He³ and He⁴ beams from the MP Tandem Van de Graaff accelerator. The isomeric nuclei which recoil from the target and subsequently fission in flight were detected by a square cone array of polycarbonate foil detectors mounted along the beam direction. From the distribution of fission tracks along the detector foils, half-lives in the nanosecond region were deduced.     

Yields of products from fast neutron-induced fission of 233U measured by means of an isotope separator on-line (ISOL) system

A comprehensive study has been carried out of the yield pattern of fission products formed in fast neutron-induced fission of ²³³U. The isotope separator on-line facility at Studsvik to the R2-0 nuclear reactor was used for rapid separation of the fission products. At a target temperature of 2250 ^°C fission products of the elements from zinc (Z = 30) to barium (Z = 56) are released, with the exception of yttrium, zirconium, niobium, molybdenum, technetium, ruthenium and rhodium. The individual isotopes are then available for study, implying that an almost complete mapping of the yield distribution can be made. In the analysis, the delay between production and measurement and the overall separator efficiency for three consecutive elements (the one under study and its parent and grand parent) are taken into account. Independent and/or cumulative yields have been obtained for 203 nuclear species, among them 59 isomeric states. Received: 19 April 2000 / Accepted: 19 June 2000     

Possible evidence for microsecond shape isomerism in Tl isotopes

The decay of isomeric states in ^201mTl, ^202mTl and ^204mTl is investigated with a Ge(Li) detector. The isomeric states are produced with LINAC bremsstrahlung (24–32 MeV). Life-times and precise energies have been determined. In the ^201mTl case, shape isomerism is proposed, based on calculations of the total potential energy surfaces and calculations of the lifetime in a Nilsson model.     

Spontaneous fission of isomeric states of actinide nuclei

Spontaneous fission half-lives of K-isomeric states are calculated on the basis of microscopic- macroscopic method. The isomeric state is assumed to be a 2-quasiparticle excited state with high angular momentum.The calculations were performed for nuclei with 96 < Z < 110 and 144 < N < 158. It was shown that in the case of K-isomeric states (if they exist) the spontaneous fission half-life time may be comparable to the spontaneous fission half-life time of the ground state. Therefore it was suggested that in measurements of fission half-lives it may be very important to distinguish between both possible components (or more) of the fission decay.     

Neutron-induced fission cross sections of short-lived actinides with the surrogate reaction method

Neutron-induced fission cross sections for ^242,243Cm and ²⁴¹Am have been obtained with the surrogate reaction method. Recent results for the neutron-induced cross section of ²⁴³Cm are questioned by the present data. For the first time, the ²⁴²Cm cross section has been determined up to the onset of second-chance fission. The good agreement at the lowest excitation energies between the present results and the existing neutron-induced data indicates that the distributions in spin and parity of states populated with both techniques are similar.     

High-spin states in154Er

High-spin states in¹⁵⁴Er have been populated by bombarding¹⁴⁷Sm and¹⁴⁸Sm with¹²C particles. Excitation functions, lifetimes, angular distributions andγ-γ coincidences were measured. AT _1/2=35ns isomeric state atE _x～3 MeV has been found and is interpreted as a two-quasi-particle state with aligned angular momenta. A cascade of intense individual lines from states with spin up to at least 26 (excitation energy up to 8.543 MeV) was found to feed the isomeric state. The level sequence above this yrast trap exhibits an irregular pattern which cannot be easily interpreted in terms of collective modes.