Symmetric and asymmetric fission in electron induced fission of 232Th

We have measured fragment kinetic energies in electron induced fission of ²³²Th for electron energies in the range 7 MeV ≦ E_e ≦ 66 MeV. The relative contribution of the distribution peak associated with high fragment kinetic energies decreases continuously with electron energy. This is interpreted as a relative increase of the symmetric fission yield as compared to the asymmetric fission yield; this fact in turn indicates a non-negligible increase in the average excitation of the fissioning nucleus, with the energy of the bombarding electrons, even above the giant dipole resonance.     

Prompt γ-rays emitted in the thermal-neutron induced fission of 233U and 239Pu

The average number and average energy of γ-rays emitted within ≈ 5 nsec after fission have been determined as functions of fragment mass and as functions of total kinetic energy. They were obtained from a four-parameter experiment that recorded, event-by-event, correlated of γ-rays and of fission-fragment pairs and the time, relative to fission, at which a γ-ray was detected. For ²³³U(n_th, f) the average total number and energy emitted per fission were found to be 6.31 ± 0.3 and 6.69 ± 0.3 MeV, respectively, giving an average photon energy of 1.06 ± 0.07 MeV. The results for ²³⁹Pu(n_th, f) given in the same order, are 6.88 ± 0.35,6.73 ± 0.35 MeV, and 0.98 ± 0.07 MeV.     

Multiparameter study of 1H, 3H and 4He in fast neutron induced fission of 235U

The emission probabilities per fission of α-particles, tritons and protons have been measured in fast neutron induced fission of ²³⁵U. The measurements were carried out at neutron energies of 120, 180, 230 and 550 keV. AΔE-E semiconductor detector telescope was used to identify different light charged particles and the fission fragments were detected with an ionization chamber. The three-parameter data corresponding to the pulse heights from the ΔE-E detectors and the ion-chamber were recorded event by event on magnetic tape and were analyzed off-line by computer. No significant variation in the most probable energy ($\text{E}$) and the standard deviation (σ_E) of the energy spectra of different light charged particles with incident neutron energy was observed, although $\text{E}{}_{\mathrm{\alpha }}$ was seen to have a slightly higher value beyond E_n = 230 keV. The yield of α-particles in fission induced by neutrons of E_n ～ 200 keV was found to be higher by about 20 % than that in thermal neutron induced fission. The yields of tritons and protons were found to increase significantly with neutron energy.     

Angular distributions of fragments in the fission of thorium induced by neutrons in the range 12–18 MeV

The angular distributions of fragments from neutron-induced fission of ²³²Th have been measured by means of glass detectors in the range 12.2 MeV ≦ E_n ≦ 18.3 MeV. The behaviour of the angular anisotropy is analysed and the contribution of the (n, 2n′f) process is discussed.     

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.     

Multiparameter studies of polar emission in 236U fission

The energy distributions and relative intensities of protons, deutrons, tritons and α-particles emitted along the fission axis during thermal neutron fission of ²³⁵U were measured simultaneously with both fission fragment energies. The mass distributions of fragments, the total kinetic energy (TKE), the dependence of the mean TKE on the fragment mass, as well as the mean kinetic energy dependence of polar particles on the fragment mass and energy were subsequently deduced from these data. Although some experimental results agree remarkably well with the hypothesis that polar particles are evaporated in flight from fission fragments, the general conclusion is that these particles are emitted according to some other mechanism.     

Ratio of the ternary to binary fission induced by thermal and resonance neutrons in 239Pu

Measurements were performed of the ratio of ternary to binary fission (T/B) in ²³⁹Pu by neutrons in the energy region from 0.020 eV to 50 eV. The ternary and binary fission time-of-flight spectra were recorded at a 8.1 m flightpath at the Geel Linac with large gold-silicon surface-barrier detectors on both sides of a 1 mg/cm² double-faced ²³⁹Pu target. From these time-of-flight spectra the ratios of the areas of the strongest isolated resonances in ternary and in binary fission were calculated; moreover in the region 0.02–1 eV ratios were calculated for about 40 zones throughout the spectrum. The T/B ratio does not vary significantly in the energy region below 1 eV; this is not the case in the region 1–50 eV, where the T/B value for the 15.5 eV resonance is significantly larger than for the other resonances. These T/B values seem to be correlated with the resonance spin J and with some other fission characteristics.     

A study of the de-excitation of primary fission fragments from the neutron-induced fission of 235U

Two separate three-dimensional experiments have been performed in which the energies of coincident fragment pairs and γ-rays or internal conversion electrons, emitted within ≈ 1.6 nsec of the thermal-neutron-induced fission of ²³⁵U, were recorded event by event. The fragment kinetic energies were used for mass identification. The self-consistency of the values of electron energy, γ-ray energy and fragment charge, and its agreement with X-ray selection data, were used to identify the atomic numbers of the fragments. The analysis of the γ-ray and conversion electron spectra resulted in the assignment of many transitions to new isotopes as well as improvement in or confirmation of many assignments from the ²⁵²Cf spontaneous fission data. Limited information on the multipolarities of the transitions in even nuclei is presented. The relative yield of electrons per fragment indicates softness to deformation in the mass region 100–110. Data are presented supporting the assignment of a 193 keV transition as the 2⁺ → 0⁺ transition in ⁹⁸Sr. An examination of the 2⁺ → 0⁺ level systematics of neighbouring even nuclei suggests a transition from vibrational to rotational behaviour in the light fragments between neutron numbers 58 and 60.     

Fission-fragment energy correlation measurements for the spontaneous fission of 244Cm

Fission-fragment mass and kinetic energy distributions and their correlations have been measured for the spontaneous fission of ²⁴⁴Cm. About 3.54 × 10⁵ fission events were collected. The global mass distribution shows shoulders at μ_H ≈ 134 and μ_H ≈ 143?144. The peak/valley ratio is 86. The dip ΔE_K value at symmetry is 13.4 ± 1.5 MeV. In the yields for high-kinetic-energy selected events, the masses 139 and 144 dominate. The results of ²⁴⁴Cm are compared with the other isotopes of Cm and discussed in terms of the structures present in the potential energy surface of the fissioning system caused by the structures in the nascent fragments.     

Correlation between fragment mass-distribution fine structure,charge distribution and nuclear structure for thermal-neutron-induced fission of 233U and 235U

Masses corresponding to observed fine-structure peaks in the fragment mass distributions for thermal-neutron-induced fission of ²³³U and ²³⁵U are shown to correspond to average measured masses for even-even nuclear charge splits. Evidence is presented that the yield enhancement for even-Z fragments is not restricted just to fission events with higher-than-average total kinetic energy. The anomalously high yield of fragments with mass 134 in ²³⁵U(n, f) as opposed to ²³³(n, f) is tentatively correlated with rapidly changing nuclear structure properties as a function of the mass of the complementary light (Z = 40) fragments.     

Scattemng of 14.1 MeV neutrons from 209Bi

Differential cross sections for the elastic and inelastic scattering of 14.1 MeV neutrons from ²⁰⁹Bi have been measured with a time-of-flight system which had an energy resolution of 650 keV. For elastic scattering from ²⁰⁹Bi, an optical-model analysis gave the best-fit potential parameters. The absolute cross sections for excitation of collective (2.66 and 4.36 MeV) states are reproduced by the results of distorted-wave calculations under the assumption of a core (²⁰⁸Pb) excitation model using deformation parameters obtained from (p, p') reactions.     

Search for coulomb fission induced by 84Kr ions on 238U

We have searched for Coulomb fission induced by ⁸⁴Kr ions on a ²³⁸U target at energies ranging from the interaction barrier down to 37 MeV below (408–458 MeV lab). No event attributed to Coulomb fission was detected; it was deduced that the cross section for this reaction is lower than 0.3 mb/sr near the interaction barrier. This value was compared to theoretical predictions. However, fission events originating from transfer reactions at the interaction barrier have been detected.     

Gamma rays of primary fission products from 235U(n,f) and 239Pu(n,f)

Gamma rays of primary fission products in thermal-neutron-induced fission of ²³⁵U and ²³⁹Pu were investigated. Isotopic assignments of several lines were made by comparing fission yield ratios and relative γ-ray intensities of the two fissioning nuclei. Differences between ²³⁵U and ²⁵²Cf fission product γ-ray lines are discussed.     

Fission of muonic 232Th and 239Pu

The neutron emission is studied following the formation of muonic atoms of ²³²Th and ²³⁹Pu. Energy and time distributions are measured. Various processes which contribute to the measured spectra are considered. A collective resonance model of the muon capture is used to calculate the nuclear excitation function. The probability of the radiationless nuclear excitations and the influence of the presence of the bound atomic muon on the fission barrier are discussed. The existing data for the $\text{Γ}{}_{\mathrm{n}}\text{Γ}{}_{\mathrm{f}}$, are analysed. As a result of the analysis the rates of the prompt and delayed fission events (due to the radiationless mu-atomic transitions and the nuclear muon capture, respectively) are deduced from the experimental data to be 0.006/muon and 0.045/muon for ²³²Th and 0.10/muon and 0.49/muon for ²³⁹Pu, respectively. The increase of the fission barrier for muonic atoms is confirmed. The experimental neutron rates can be consistently explained only if it is assumed that in both nucleides the K_α radiationless transitions do not induce fission. The increase of the fission barrier for ²³⁹Pu is hence deduced to be not less than 1.2 MeV. The fate of the atomic muon after the nuclear fission is briefly discussed. Its influence on the interpretation of the present results is found to be small.     

Octupole vibrations in 226Ra

The low-lying negative parity band in ²²⁶Ra is described by a phenomenological model for octupole vibrations coupled to a static deformed core. The energies of the whole band are reproduced by adjusting the intrinsic energies of the octupole vibrations.     

The mass and kinetic energy distributions in the fission of 237Np, 232Th, 238U, 235U and 209Bi induced by 110 MeV electrons

Fission of ²³²Th, ²³⁷Np, ²⁰⁹Bi, ²³⁵U and ²³⁸U induced by 110 MeV electrons has been studied by means of surface barrier detectors. The resulting mass and kinetic energy distributions are presented. Comparison with the liquid drop model predictions shows reasonable agreement in the case of ²⁰⁹Bi. The data are analysed in terms of a two component model of fission and the mean total kinetic energies of the components are shown to depend linearly on $\text{Z}{}_1\text{Z}{}_2\text{(A}{}_1{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{+ A}{}_2{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}\text{)}$. Interesting differences are found when the present results are compared with the recent photofission experiments of Areskoug et al. and features in both sets of data correlate with changes of fragment deformation implied by the calculations of Wilkins et al.     

Multiparticle break-up of 10B induced by fast neutrons

The reaction $\text{n}\text{+}{}^{10}\text{B}\text{→ α + α +}\text{d}\text{+}\text{n}$ induced by 14.4 MeV neutrons has been investigated in a kinematically complete experiment. Several final-state interactions have been observed in the sequential decay of the n + ¹⁰B system. The angular distribution of the ¹⁰B(n, ⁶Li₁)⁵He₀ reaction, leading to two unstable particles and consequently measurable only in a kinematically complete experiment, has been measured. It has been analysed in the framework of DWBA as a pick-up process with the transfer of the five-nucleon cluster ⁵Li. The presence of quasifree scattering and quasifree reaction with the deuteron as spectator has also been observed.     

The polarization of MeV neutrons elastically scattered from 4He

The analyzing power of ⁴He for neutron elastic scattering has been measured at four angles between 20° and 80° (lab) throughout the energy range 1.5–6.0 MeV using a doublescattering method. The intense flux of polarized neutrons was generated via the reactions Pb(γ, n) → ¹²C(n, $\text{n}$)¹²C, and the magnitude of the polarization of the neutron beam measured absolutely in a separate double-scattering experiment. Neutron energies were determined with a nanosecond time-of-flight spectrometer, and the generalized neutron spin-precession method was used to minimize systematic uncertainties.     

Transfer reactions induced by 12C ON 18O and 26Mg AT 46 MeV

It is argued that a recent claim that there is an error of a factor of two in the Law- Campbell theory of K-electron shake-off in β^? decay is wrong.     

Scattering of 1 and 2 MeV neutrons from oriented 165Ho

The “deformation effect” in the neutron-¹⁶⁵Ho differential scattering cross section, defined as the relative variation of the cross section induced by the orientation of the target, has been measured at 1 and 2 MeV incident neutron energy, in the scattering angular interval from 30° to 130°. A polycrystalline holmium target has been employed, oriented normally to the scattering plane, with a nuclear alignment parameter $\text{B}{}_2\text{B}{}_{\mathrm{<mtext>2\; </mtext><mtext>max</mtext>}}\text{= 0.24}$. The observed effect has an oscillating character as a function of the scattering angle with a maximum value of 0.12 ± 0.02 at 2 (emMeV). The effect has been theoretically calculated by an adiabatic coupled-channel calculation performed by using the optical model and deformation parameters deduced from the analysis of previous experiments. The agreement between the experimental and the theoretical results is good, particularly at 2 MeV energy where the compound nucleus contribution in the scattering is negligible.