裂变碎块角分布与鞍点结构

从Jacob和Wick所推导的具有自旋粒子的散射矩阵元一般表示式出发,采用适当近似后,分析了中子引起Th²³⁰,Th²³²,U²³⁴,U²³⁶,U²³⁸裂变的阈能附近碎块角分布的实验数据。分析结果说明:1)阈能附近碎块角分布可以用裂变是通过鞍点上少数转动带(每一带具有确定的宇称)来解释;2)这些转动带与Nilsson的渐近能谱图所预言的基本上一致。同时利用对称裂变鞍点比非对称裂变鞍点高的观点,讨论了裂变碎块角分布各向异性与质量比之间的关系。     

Intranuclear-Cascade model calculation of photofission probabilities for actinide nuclei

We have calculated the fission probabilities for ²³⁷Np, ^{233, 235, 238}U, ²³²Th, and ^natPb following the absorption of photons with energies from 68 MeV to 3.77 GeV using the RELDIS Monte Carlo code. This code implements the cascade-evaporation-fission model of intermediate-energy photonuclear reactions. It includes multiparticle production in photoreactions on intranuclear nucleons, pre-equilibrium emission, and the statistical decay of excited residual nuclei via competition of evaporation, fission, and multifragmentation processes. The calculations show that in the GeV energy region the fission process is not solely responsible for the entire total photoabsorption cross-section, even for the actinides. The fission probabilities are 80-95% for ²³³U, ²³⁵U, and ²³⁷Np, 70-80% for ²³⁸U, and only 55-70% for ²³²Th. This is because certain residual nuclei that are created by deep photospallation at GeV photon energies have relatively low fission probabilities. The results of those model calculations are in reasonable agreement (at the 10% level) with recent experimental data on relative photofission cross-sections for ²³⁷Np and ^{233, 235, 238}U (but not for ²³²Th or ^natPb) from the Saskatchewan and Jefferson Laboratories over a very wide range in photon energy. Using our calculated fission probabilities plus the total photoabsorption cross-sections per nucleon, estimated from previous cross-section data for nuclei from C to Pb, we can infer absolute photofission cross-sections for the actinide nuclei and compare them with the SAL and JLab results. The resulting discrepancies, however, clearly demonstrate the need for direct measurement of the total photoabsorption cross-sections for the heavy actinides.     

Fragment angular distributions from fission of238U,209Bi and197Au by 140-MeV4He Ions

The fission fragment angular distributions from reactions of 140-MeV⁴He ions with²³⁸U,²⁰⁹Bi and¹⁹⁷Au have been studied. From the anisotropies in the angular distributions, values for? _eff, the effective moment of inertia at the fission saddle point, have been estimated and compared with results obtained at lower bombarding energies. The derivation of? _eff values has included corrections for the effects of incomplete fusion mechanisms on the orbital angular momentum distribution of the fissioning nuclei and for neutron evaporation prior to fission. The results are also compared with heavy-ion-induced fission data for systems of similar fissility. Also, examination of the forward-backward symmetry of the²³⁸U angular distribution substantiates other results which show that the fraction of fission reactions which follow complete fusion of the target and projectile is less than 0.5 for 140-MeV⁴He-ion bombardment of²³⁸U.     

Fission within a statistical multistep model

A statistical multistep model for nuclear reactions is extended to the fission channel by using a statistical hypothesis. Results are presented for the neutron-induced fission of²³⁸ U.     

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.     

Th232与U238中子俘获截面的计算

本文采用复合核机制,计算了裂变道开放前Th²³²,U²³⁸中子俘获截面。探索了光学模型、辐射宽度、统计涨落因子等因素的影响。     

Total cross sections for fission of heavy nuclei by intermediate-energy protons

The results of measuring total cross sections for proton-induced fission of ^natPb, ²⁰⁹Bi, ²³²Th, ²³³U, ²³⁵U, ²³⁸U, ²³⁷Np, and ²³⁹Pu nuclei at proton energies from 200 to 1000 MeV are reported. The measurements were carried out in steps of ≈100 MeV. Complementary fission fragments were detected in coincidence by two parallel-plate avalanche gas counters placed in the beam line on both sides of the thin target close to it. Energy and isotope dependences of the fission cross sections are analyzed.     

Testing QED with resonance conversion

Various phenomena: hyperfine shift of ²⁰⁹Bi⁸²⁺, nuclear decay of ²²⁹Th and ¹⁶⁹Yb⁶⁹⁺ are considered as cross-invariant channels of the internal conversion process from the viewpoint of testing QED. The way of accelerating nuclear decay in laser field via merging the nuclear and laser photons is discussed. Discovering this fundamental process would give a convincing test of both QED along with our experimental abilities.     

Cross sections for 209Bi, 232Th, 235U, 238U, and 237Np fission induced by intermediate-energy protons and deuterons

The cross sections for ²⁰⁹Bi, ²³²Th, ²³⁵U, ²³⁸U, and ²³⁷Np fission induced by protons of energy in the range 1.0–3.7 GeV and by deuterons of energy 1.0 GeV were measured. The results are compared with data from other experiments, with available estimates, and with the predictions of theoretical models.     

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.     

Absence of delayed fission in theβ −-decay of 2.3 min238Pa

We have searched for beta-delayed fission in the decay of 2.3 min²³⁸Pa produced in the²³⁸U(n,p) reaction with 14.7 MeV neutrons. Through microprocessor-controlled chemical separations of protactinium about 10⁹ atoms of²³⁸Pa were isolated and exposed to fission track detectors. From the absence of fission tracks an upper limit for the betadelayed fission probability of²³⁸Pa, i.e.P _βf <2.6 10^?8, is obtained at 95% confidence level. This rules out positive evidence for this decay mode of²³⁸Pa reported elsewhere. Simple theoretical estimates ofP _βf range from 10^?7 to 10^?9.     

RADIOCHEMICAL INVESTIGATION OF MASS DISTRIBUTIONS IN 12C-INDUCED FISSION OF 209Bi AND 238U

The yields of 15 and 17 fission, products have been obtained by radiochemical determinations in ¹²C-induced fission of ²⁰⁹Bi and ²³⁸U respectively. From these data, the mass distributions of fission fragments are calculated based on three different hypotheses for the charge distribution. The results seem to indicate that the hypothesis of equal charge displacement gives the best fit to the Gaussian distribution for the masses of the fission fragments. By comparison with the mass distributions for ²⁰⁹Bi (¹²C, f), ²³⁸U(¹²C, f) and ¹⁹⁷Au(¹²C, f) in our early measurement, we find that for fission parameter Z²/A>37, the width of the mass distributions increases with Z²/A rapidly.     

Charge distribution studies in the fast-neutron-induced fission of <Superscript>232</Superscript>Th, <Superscript>238</Superscript>U, <Superscript>240</Superscript>Pu and <Superscript>244</Superscript>Cm

Charge distribution studies for heavy-mass fission products were carried out in the fast-neutron-induced fission of ²³²Th, ²³⁸U, ²⁴⁰Pu and ²⁴⁴Cm using radiochemical and gamma-ray spectrometric techniques. The width parameter( σ_Z/σ_A), the most probable charge/mass ( Z _P/A _P), the charge polarization (ΔZ) and the slope of charge polarization [ δ(ΔZ)/δA ^′] as a function of the fragment mass (A ^′) were deduced. The average charge dispersion parameter ( 〈σ_Z〉) and proton odd-even effect ( δ_p) were also obtained for these fissioning systems. The 〈σ_Z〉 and δ_p values in the fissioning systems ²⁴¹Pu ^* and ²⁴⁵Cm ^* were determined for the first time. The δ(ΔZ)/δA ^′ value is also determined for the first time in the fissioning systems ²³⁹U ^* , ²⁴¹Pu ^* and ²⁴⁵Cm ^* . These data along with the literature data for even-Z fissioning systems such as ²³⁰Th ^* , ²³²Th ^* , ²³³U ^* , ²³⁴U ^* , ²³⁶U ^* , ²³⁸U ^* , ²³⁹Pu ^* , ²⁴⁰Pu*, ²⁴²Pu ^* , ²⁴⁶Cm ^* , ²⁵⁰Cf ^* and ²⁵²Cf(SF) are discussed in terms of nuclear structure effect and dynamics of descent from the saddle to the scission point. The role of the excitation energy in low-energy fission is also discussed. Received: 26 July 2002 / Accepted: 3 December 2002 / Published online: 3 April 2003 RID="a" ID="a"Present address: Emeritus Scientist (CSIR) Bhabha Atomic Research Centre, Nuclear Recycle Group, WIP Building, Trombay, Mumbai-400085, India; e-mail: rhiyer@magnum. barc.ernet.in Communicated by D. Schwalm     

NQR indications of unconventional magnetism in some bismuth-based diamagnets

An evidence that local ordered magnetic fields from 30 to 200 G exist in diamagnets α-Bi₂O₃, Bi₃O₄Br, Bi₄Ge₃O₁₂, Bi₂Al₄O₉ which comprise neither d- nor f-elements was earlier given by the zerofield and Zeeman-perturbed²⁰⁹Bi nuclear quadrupole resonance (NQR) spectra. With a view to find similar spectroscopic effects in other compounds, we examined the²⁰⁹Bi NQR Zeeman-perturbed spectra of the Bi₃B₅O₁₂ oriented single crystal as well as the zero-field spin echo envelopes in Bi₃B₅O₁₂ and Bi₂Ge₃O₉. Distinctive modulations were displayed by the zero-field²⁰⁹Bi spin echo envelopes in Bi₂Ge₃O₉ powder. The modeling of the spin echo envelopes within the density matrix formalism explained the observed effect by the presence of local ordered magnetic field of the order of 65 G at the Bi atoms. The zero-field modulations of the²⁰⁹Bi spin echo envelopes were also observed in Bi₃B₅O₁₂ indicating the presence of the internal source of line splitting. This finding and considerable deviation of the resonance intensity ratios from that in a pure NQR, found in the zero-field²⁰⁹Bi spectrum of the single crystal, were understood as indirect evidences that a local ordered magnetic field exists also in Bi₃B₅O₁₂. The zero-field²⁰⁹Bi spin echo envelopes assigned to the lines split by the local magnetic fields in α-Bi₂O₃ and Bi₄Ge₃O₁₂ were observed to display modulations on the appropriate curves.     

Sub-Coulomb transfer and fission in collisions of129Xe,132Xe and136Xe with238U

Integral cross sections for fission and for one- and two-neutron transfer reactions in the system¹³²Xe+²³⁸U were measured radiochemically in the energy range 0.7≦E/E _Coul≦1. The excitation functions for fission and transfer are found to be essentially parallel below 0.85×E _Coul. Even at the lowest energies the transfer cross sections exceed the fission cross section by more than one order of magnitude. With the other projectiles¹²⁹Xe and¹³⁶Xe different transfer cross sections illustrating their sensitivity for the ground stateQ-values,Q _gg , are observed while the fission cross sections are the same as in the¹³²Xe +²³⁸U reaction. The fission data are interpreted in terms of a continuous transition between Coulomb fission and several transfer-induced fission processes.     

Influence of collective rotational degrees of freedom in medium energy fission

In the present work rms angular momenta have been deduced for the fission fragments corresponding to¹³¹Te^m,g and¹³³Te^m,g in²³²Th(α _{40 MeV},f) and²³⁸U(α _{40 MeV},f) systems from the radiochemically determined independent isomeric yield ratios and statistical model based analysis. For¹³¹Te and¹³³Te the rms angular momenta deduced are 5.9±1.0 and 7.9±1.2 ? respectively in²³²Th(α _{40 MeV},f) and 7.2±0.6 and 8.0±0.8 ? respectively in²³⁸U(α ₄₀ MeV,f). Comparison of the present data with the literature data for these fragments in the same compound nuclei²³⁶U^* and²⁴²Pu^* at lower excitation energies shows increase in the fragment angular momentum with increasing excitation energy and angular momenta of the fissioning nuclei. Fragment angular momentum deduced theoretically for asymmetric and deformed fragments on the basis of thermal equilibration of the collective rotational degrees e.g., rigid rotation, wriggling, tilting, bending and twisting modes considering the effect of multichance fission, are in good agreement with the experimental observations.     

Electronic Spectroscopy of UO

Spectra for gas-phase uranium oxide have been analyzed to obtain molecular constants and provide a partial atlas of the visible and near infrared bands for the isotopomers²³⁸U¹⁶O,²³⁸U¹⁸O,²³⁵U¹⁶O, and²³⁵U¹⁸O. Nineteen bands of the²³⁸U¹⁶O,²³⁸U¹⁸O, and²³⁵U¹⁸O isotopomers were rotationally analyzed. With few exceptions, Ω assignments were unambiguously determined from observations of the first lines in at least two rotational branches. Accurate term values and rotational constants are reported.     

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.     

Fission and the ion-ion interaction

A fission theory based on a preformation probability and the decay of the daughter pair through their associated ion-ion potential is presented. The preformation probability is estimated to be 10⁻⁵. The nuclear component of ion-ion potential is determined in a sudden approximation by superposing the ground state densities of the daughter pair and is found to be 60–70 MeV deep. Using observed kinetic energies, absolute lifetimes are calculated for both symmetric and asymmetric fission of U²³⁴, Pu²⁴⁰, Cm²⁴⁴, and Cf²⁴⁸ with good results. In addition, the absolute lifetimes associated with the decay of Pu²⁴⁰ into a series of other modes are presented. The relevance of this theory to the isotopic dependence of lifetimes, prompt neutron fission, and subthreshold fission is discussed in qualitative terms. Thus the nuclear fission seems to be a sudden process.     

Fission and gamma-ray decay of the238U shape isomer

The decay of the²³⁸U superdeformed shape isomer has been reinvestigated by detecting for the first time simultaneously the fission and the gamma-back decay. An electrostatic deflection system has been used to transport the^238mU recoils, produced in a²³⁸U(d, pn) reaction with a pulsed beam of 18 MeV deuterons, in front of a detector set-up consisting of three ion-implanted solidstate detectors and a Ge(Li) gamma-detector. The gamma-back decay has been measured in coincidence with conversion electrons of the 2⁺ 0⁺ transition deexciting the first rotational state in²³⁸U. Two gamma-transitions of 2.513 MeV and 1.878 MeV have been observed with half-lives consistent with the result obtained for the decay by delayed fissionT _1/2=(298±18) ns.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday