Comparison of non-compound neutron and proton emission in 16O induced reactions on 238U at 310 MeV

The emission of energetic neutrons and protons in coincidence with fission fragments was measured for the reactions ²³⁸U(¹⁶O, nf) and ²³⁸U(¹⁶O, pf). Larger cross sections were observed for the emission of high-energy protons than for the emission of high-energy neutrons. The differences in spectral shapes and the implications for the coalescence model are discussed.     

High-energy γ-rays in α-accompanied spontaneous fission of 252 C f

An experiment was performed on prompt γ-ray emission in binary and α-particle accompanied spontaneous fission of ²⁵² C f using the Darmstadt-Heidelberg 4π NaI Crystal Ball spectrometer. The enhancement in γ-ray yield, denoted as the “high-energy component”, which appears between 3.5 and 8 MeV and in the region of near-symmetric fragment mass splits, was observed to be equally pronounced in both fission modes. Analyzing the fragment mass dependence of the mean γ-ray multiplicity in both fission modes clearly identifies the disintegration of equilibrated fission fragments in a narrow mass range around the double-magic ¹³²Sn as the source of these γ-rays.     

Fission and emission of H and He in the reactions of 215 MeV16O with181Ta,208Pb and238U

The reactions of 215 MeV¹⁶O with¹²C,¹⁸¹Ta,²⁰⁸Pb and²³⁸U have been studied. Inclusive measurements for⁴He emission are given from each target, and for fission and^1,2,3H from Ta, Pb and U. For H/He a high-energy, forward-peaked component is observed with characteristics similar to those reported by others. At backward angles a low-energy, nearly-isotropic component is also observed for⁴He that cannot be accounted for by emission from fully accelerated fission products. The spectral shapes for this evaporative component are compared with statistical model calculations, and information is obtained concerning the effective barriers to emission. For the reactions of¹⁶O with¹²C, complete fusion seems to be overwhelmed by incomplete fusion. Fission angular distributions and cross sections are also presented and discussed.     

Measurements of evaporation residues and fission for 40Ar + 110Pd

Fusion cross sections for the reaction ⁴⁰Ar + ¹¹⁰Pd have been measured in the bombarding energy range 164–262 MeV. Evaporation residues and fission fragments have been detected by ΔE-E telescopes. We compare the fusion cross sections to various theoretical models with special attention to the high-energy data. An analysis is also given for entrance-channel spin zones for evaporation residues and fission.     

Neutron emission and fragment yield in high-energy fission

The KRIS special library of spectra and emission probabilities in the decays of 1500 nuclei excited up to energies between 150 and 250 MeV was developed for correctly taking into account the decay of highly excited nuclei appearing as fission fragments. The emission of neutrons, protons, and photons was taken into account. Neutron emission fromprimary fragments was found to have a substantial effect on the formation of yields of postneutron nuclei. The library was tested by comparing the calculated and measured yields of products originating from the fission of nuclei that was induced by high-energy protons. The method for calculating these yields was tested on the basis of experimental data on the thermal-neutroninduced fission of ²³⁵U nuclei.     

Cross sections and barriers for nuclear fission induced by high-energy nucleons

The cross sections for the fission of ²³²Th, ^235,238U, ²³⁷Np, and ²³⁹Pu target nuclei that was induced by 20- to 1000-MeV neutrons and protons were calculated. The respective calculations were based on the multiconfiguration-fission (MCFx) model, which was used to describe three basic stages of the interaction of high-energy nucleons with nuclei: direct processes (intranuclear cascade), equilibration of the emerging compound system, and the decay of the compound nucleus (statistical model). Fission barriers were calculated within the microscopic approach for isotopic chains formed by 15 to 20 nuclei of the required elements. The calculated fission cross sections were compared with available experimental data. It was shown that the input data set and the theoretical model used made it possible to predict satisfactorily cross section for nuclear fission induced by 20- to 1000-MeV nucleons.     

Fission of highly excited fragments from collisions of 750 A.MeV 238U-ions on 208Pb

Fragments of relativistic 750 A.MeV U-projectiles were investigated by using the fragment separator FRS for magnetic selection of reaction products including ray-tracing and ΔE-ToF techniques. For elements between Ge and Sb, measurements of isotopic yield distributions and velocities revealed three processes: fragmentation, low-energy fission, and high-energy fission. The last of these regimes is presently reported. First and second moments of distributions of mass numbers, atomic numbers and velocities of the corresponding fragments allowed us to identify ¹⁰¹ ₄₃Tc₅₆ as the most probable fragment of a high energy symmetric fission reaction. Moreover, we could deduce a hypothetical mean fissioning fragmentation product ²⁰⁸Rn and its highly excited pre-fragmentation parent ²²⁷Ra produced in a primary abrasion reaction at an excitation energy of about 290 MeV. Received: 26 January 1998 / Revised version: 16 March 1998     

The study of prompt and delayed muon induced fission

Mass yield and total kinetic energy release (TKE) distributions of fragments from prompt and delayed muon induced fission, separately, have been measured for the isotopes²³⁵U,²³⁸U,²³⁷Np and²⁴²Pu. The distributions from prompt muon induced fission are compared with the corresponding distributions from hadronic reactions and from spontaneous fission (s.f.). The distributions from the delayed muonic fission processes are compared to the distributions for neutron and proton-induced fission. No mass distributions measured in the prompt muonic fission process show any signature, which can be attributed to the presence of the muon. Differences observed between the TKE distributions of prompt muon induced and hadron induced fission can be explained by the screening effect of the negative charge of the muon bound in the orbit of one of the fission fragments. The observed yield of symmetric muon induced fission was found to be defined merely by the value of the excitation energy.     

On the 34Si emission of 242Cm

A ³⁴Si-decay of ²⁴²Cm has been reported by Oglobin et al. But an interpretation of this ³⁴Si emission as a ternary high-energy particle emission accompanying the fission of ²⁴²Cm can be proposed as well. Received: 18 October 2000 / Accepted: 8 November 2000     

强辐射环境下高能脉冲裂变中子探测方法

在研究通道衰减、探测方法分离和探测器中子/射线本征分辨的基础上,研究了测量高能脉冲裂变中子数目的探测技术。基于电流型Si-PIN探测器,设计了减本底的背靠背探测结构,给出了测量强射线和低能散射中子干扰信号及有效扣除强辐射本底的实现方法,最终实现了高n/n和n/分辨测量和强裂变中子、射线混合场中的高能脉冲裂变中子数目探测,探测系统的信号/辐射本底比可达到10倍以上。     

An experimental study of mass asymmetry in subbarrier photofission of 238U

A measurement of symmetric and asymmetric photofission yields of²³⁸U in the energy region from 4.5 MeV to 6.5 MeV has been performed. As γ-source the bremsstrahlung from a microtron has been used. The fission yields are obtained from β-counting of chemically separated isotopes ¹¹¹Ag, ¹¹⁵Cd and ¹¹⁷Cd for symmetric fission and ¹³⁹Ba for asymmetric fission. Some structures in the yield curves at about 5.3 MeV and a pronounced maximum at about 6.0 MeV in the valley-to-peak ratio in the fission fragment yield distribution are observed. It is suggested that these effects are connected with the double-hump fission barrier concept and indicate differences between fission barriers for symmetric and asymmetric fission.     

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.     

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.     

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.     

Investigations of fission characteristics and correlation effects

We review the experimental results on the P-even and P-odd angular correlations of fission fragments in the fission of the ²³⁵U and ²³⁹Pu nuclei induced by unpolarized and polarized resonance neutrons, and on the TRI and ROT effects in the ternary and binary fission of actinides induced by polarized thermal neutrons. Also reported are the measured yields of prompt and delayed neutrons per fission event. The experimental data are analyzed within a novel theoretical framework developed by the JINR—RNC KI Collaboration, whereby the reduction of the multidimensional phase space of fission fragments to the JπK-channel space is consistently validated and the role of resonance interference in the observed correlation effects is revealed.     

Cold and hot binary and ternary fission yields in the spontaneous fission of 252Cf

The spontaneous fission (SF) of ²⁵²Cf has been studied via γ-γ-γ coincidence and light charged particle—γ-γ coincidence with Gammasphere. The yields of correlated Mo-Ba pairs in binary fission with 0–10 neutron emission have been remeasured with an uncompressed cube. The previous hot fission mode with 8–10 neutron emission seen in the Mo-Ba split is found to be smaller than earlier results but still present. New 0n binary SF yields are reported. By gating on the light charged particles detected in ΔE-E detectors and γ-γ coincidence with Gammasphere, the relative yields of correlated pairs in alpha ternary fission with zero to 6n emission are observed for the first time. The peak occurs around the α2n channel. A number of correlated pairs are identified in ternary fission with ¹⁰Be as the light charged particle. We observed only cold, 0n ¹⁰Be and little, if any, hot, xn ¹⁰Be channels.     

Observation of proximity- and non-equilibrium effects in ternary heavy ion reactions

Kinematically complete experiments have been performed on the two- and three-body exit channels in the reactions⁸⁴Kr+¹⁶⁶Er and¹²⁹Xe+¹²²Sn at 12.5 MeV/u. Three-body events occur with an unusually high probability. They arise from a fast two-step mechanism where a sequential fission-like process follows a deep inelastic collision with preferentially very large energy losses. Strong Coulomb proximity effects are observed in the three-body final state which, treated quantitatively in Coulomb trajectory calculations, establish a time-scale of 1·10^?21 s between the consecutive scission acts. The angular distribution of fission fragments is consistent with an orientation of the fission axis approximately collinear with the axis of the first scission, and the mass distribution of the fission is asymmetric with the heavier mass emitted preferentially opposite to the direction of the third particle. The high fission probability, the short time-scale, the near collinear orientation and the fission mass asymmetry together present consistent evidence for a new phenomenon of non-equilibrium fission.     

Fission reactions of 469-MeV56Fe+238U: Detection of4He/1H emission from pre- and post-fission sources

The emission of⁴He and¹H has been measured in coincidence with fission for reactions of 469-MeV⁵⁶Fe+²³⁸U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the⁴He/¹H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for⁴He and¹H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed⁴He/¹H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For⁴He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For¹H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of⁴He/¹H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.     

On symmetric tripartition in the spontaneous fission of 252Cf

Angular and energy distributions of ternary fission products of ²⁵²Cf have been measured with a position sensitive ionization chamber. Most of the observed triple coincidences are light-charged-particle accompanied fission events. The few events registered with relative angles near 120° allow to deduce an upper limit of 10⁻⁸ per binary fission for a symmetric ternary fragmentation, in contradiction to previous counting experiments yielding ternary-to-binary fission ratios between 10⁻⁴ and 10⁻⁶.