Analysis of fission excitation functions and the determination of shell effects at the saddle point

A method is proposed to deduce the shell correction energy corresponding to the fission transition state shape of nuclei in the mass region around 200, from an analysis of the first chance fission values of the ratio of fission to neutron widths, (Γ _f /Γ _n )₁. The method is applied to the typical case of the fissioning nucleus²¹²Po, formed by alpha bombardment of²⁰⁸Pb. For the calculation of the neutron width, the level densities of the daughter nucleus after neutron emission were obtained from a numerical calculation starting from shell model single particle energy level scheme. It is shown that with the use of standard Fermi gas expression for the level densities of the fission transition state nucleus in the calculation of the fission width, an apparent energy dependence of the fission barrier height is required to fit the experimental data. This energy dependence, which arises from the excitation energy dependence of shell effects on level densities, can be used to deduce the shell correction energy at the fission transition state point. It is found that in the case of²¹²Po, the energy of the actual transition state point is higher than the energy of the liquid drop model (LDM) saddle point by (3 ± 1) MeV, implying significant positive shell correction energy at the fission transition state. Further, the liquid drop model value of level density parametera is found to be a few per cent smaller for the saddle point shape as compared to its spherical shape.     

Measurement of fast neutron induced fission cross section of thorium using Lexan plastic track detector

Fission-track registration characteristics of Lexan solid state nuclear track detectors have been used to measure the fast neutron induced fission cross section of²³²Th. The fast neutrons (?14.2MeV) were produced with the help of an AN-400 model Van-de-Graaff accelerator at Banaras Hindu University laboratory using³H(²H,n)⁴He reaction and were used to irradiate the fissile target deposited on the plastic detector. The track densityT, registered on the plastic detector is related to the fission cross sectionσ _f, through the relationT=knσ _føt wheren is the number of fissile atoms per cm² in the deposit, ø is the neutron flux,k is fission track registration efficiency andt is the time of irradiation. The fission cross sectionσ _f of²³²Th, relative to the well measured fission cross section of²³⁸U, was found to be 0.36±0.04 barn.     

Measurement of mass- and time-dependence of delayed fission neutrons by an on-line mass separator

Delayed neutron emission for thermal neutron fission of²³⁵U is investigated by means of a helium-filled mass separator. At the focus of the separator, the neutron activity is detected in coincidence with theΒ decay preceding the neutron emission. Measurements of the activity build-up give the total delayed neutron yield and the yields for different times after fission. Measurements of the activity decay give the half-life distribution. The mass-dependence of yield and half-life was investigated for the whole fission product mass region. The splitting of the total yield of delayed neutrons between light and heavy fission products has been determined. Based on Keepin's value of the total number of delayed neutrons per fission, a yield of 1.05 and 0.53 neutrons per 100 fissions has been obtained for light and heavy fission products, respectively. The experimentally determined mass- and time-dependences of delayed neutron emission are compared with those calculated from the known precursors. Unidentified neutron activities in the mass regions 85–88 (half-life 1 sec) and 96–100 (half-life 1.5 sec) are discussed. By means of theoretical considerations the yield difference for delayed neutrons between light and heavy fission products is interpreted and the new neutron activity is assigned to the precursors⁹⁸Y or⁹⁹Y.     

Fission of <Superscript>232</Superscript>Th in a spallation neutron field

The spatial distributions of thorium fission reaction rate in a spallation neutron field of thick lead target bombarded by protons or deuterons with energy between 1.0 and 3.7 GeV were measured. Approximately a linear dependence of the thorium fission rate on the beam energy is observed. The mean fission cross section of ²³²Th 〈σ _f 〉 ≈ 123 mb and it does not depend on energy and type of the beam particles.     

Twin position sensitive ionization chamber,utilizing backgammon and time projection method

The anomalous dependence of average prompt fission neutron (PFN) multiplicity on the fission fragment total kinetic energy of ²⁵²Cf(sf) was investigated using digital signal processing (DSP). A twin Frisch-grid ionization chamber (TGIC) was used for fission fragment (FF) spectroscopy and PFN was registered with help of fast neutron detector (FND) on liquid scintillator base. The DSP apparatus was build up around the 8-channel fast synchronously sampling waveform digitizers (WFD). It was found that the main limitation of the experimental method, developed in ref. [1] for investigation of the PFN properties in neutron induced fission reaction ²³⁵U(n,f), ²³⁹Pu(n,f) etc is related to the FF energy loses of in the target backing. A modification of the method was suggested to overcome such a limitation.     

Fission process of superheavy nuclei

The process of fusion-fission of superheavy nuclei with Z=102?122 formed in the reactions with ²²Ne, ²⁶Mg, ⁴⁸Ca, ⁵⁸Fe and ⁸⁶Kr ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) using a time-of-flight spectrometer of fission fragments CORSET and a neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions of fission fragments, fission and quasi-fission cross sections, multiplicities of neutrons and gamma rays and their dependence on the mechanism of formation and decay of compound superheavy systems have been studied.     

Nucleon-content effect on the fission lifetime of excited nuclei

It was shown previously that the fission lifetime of a nucleus excited to about 100 MeV depends strongly and nonmonotonically on the initial value of its angular momentum L ₀. This result was obtained on the basis of a refined version of the combined dynamical and statistical model. The present study is devoted to a theoretical analysis of the dependence of the fission time on the nucleonic composition of the nucleus involved. The respective calculations were performed within the same model. The dependence of the average fission time 〈t _f 〉 on the initial fissility parameter (Z ²/A)₀ appears to be of a resonance type and is similar to its dependence on L ₀. This dependence of the average fission time on (Z ²/A)₀ stems both from statistical calculations and from a dynamical simulation of the fission mode with allowance for friction. The conditions under which the average fission time reaches a maximum are specified. The dependence of the average fission time on (Z ²/A)₀ remains nonmonotonic in the fusion-fission reaction as well, in which case the distribution of compound nuclei with respect to the initial angular momentum is broad.     

Anomalies of neutron and γ-ray emission in light charged particle accompanied fission of 252Cf

Correlations between the neutron and γ-ray emission and the kinetic energy of light charged particles (LCPs), such as protons, deuterons, tritons and alphas, from the fission of ²⁵²Cf were studied in a four parameter experiment. A ⊘ 16 cm × 5 cm liquid scintillator with n-γ discrimination and a ⊘ 10 cm × 10 cm NaI(Tl) crystal were employed to detect the neutrons and the γ rays, respectively, while a thin CsI(Tl) crystal, which had the ability to separate the LCPs, was used to determine the LCP energy. The experimental results show that for the α particle accompanied fission, the average total number of neutrons emitted per fission increases in the case of varying the alpha particle energy E_α from 7 to 11 MeV and then falls off linearly in a first approximation with increasing E_α, and the average total energy and the average total number of γ-rays per fission as well as the average γ-ray energy as functions of E_α show similar correlation features. For the hydrogen-ion accompanied fission, the correlations of the neutron and γ-ray emission with hydrogen ion (t, d and p) energy are very similar to those for the alpha-particle accompanied fission. The correlations of the neutron and γ-ray emission in the low LCP energy region, in contrast to that observed in the high LCP energy range, are called anomalies. On the basis of the results obtained by the three-point charge model and the liquid drop model calculation with shell and pairing correction, the anomalous behavior of the neutron and γ-ray emission is explained tentatively.     

Shell effects in fusion-fission of heavy and superheavy nuclei

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z=82?122 formed in the reactions with ⁴⁸Ca and ⁵⁸Fe ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) and at the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL) using the time-of-flight spectrometer of fission fragments CORSET and the neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments, fission, quasi-fission and evaporation residues cross sections, multiplicities of neutrons and γ quanta and their dependence on the mechanism of formation and decay of compound systems have been studied.     

Isotopic invariance of fission fragment charge distributions for actinide nuclei at excitation energies above 10 MeV

The fragment mass and energy distributions from the proton-induced fission of compound nuclei ²³³Pa, ^{234,236,237,239}Np, ^{239,240,241,243}Am, and ²⁴⁵Bk at proton energy E _p =10.3 and 22.0 MeV have been experimentally studied. It was revealed that the shapes of the asymmetric fission mass distributions are mainly defined by the proton numbers of compound nuclei and demonstrate only a weak dependence on the neutron ones. The detailed study of the fission fragment mass yields for compound nuclei Np and Am isotopic chains has shown that the asymmetric fission fragment charge distributions calculated within the unchanged charge density hypothesis for nuclei with equal Z _C practically coincide.     

Nuclear dissipation,fission probability and neutron multiplicity prior to fission

We discuss the effects of nuclear dissipation on fission probabilities that are characteristic of a diffusion model of the fission process. Reproducing the experimental fission probabilities at low excitation energies fixes the ratioa _f/a_n of the level density parameters for a given strength of the reduced dissipation coefficientβ. These low energy constraints ona _f/a_n andβ balance the effects of transients on neutron multiplicities prior to fission at higher excitation energies. For the competitive decay of¹⁵⁸Er formed in the reaction¹⁶O+¹⁴²Nd at 207 MeV we show that dueto transients only the multiplicity of pre-fission neutrons is enhanced with respect to the prediction of the statistical model in a manner consistent with our earlier general analysis.     

Measuring fluence of fast neutrons with planar silicon detectors

The results of measurements of 1-MeV (Si) equivalent fast neutron fluence with silicon planar detectors are reported. The measurement method is based on the linear dependence of the reverse detector current increment on the neutron fluence: ΔI = α _I × Φ × V. This technique provides an opportunity to measure the equivalent fluence in a wide dynamic range from 108 to 10¹⁶ cm^–2 with an unknown neutron energy spectrum and without detector calibration. The proposed method was used for monitoring in radiation resistance tests of different detector types at channel no. 3 of IBR-2 and for determining the fluence of fission and leakage neutrons at the KVINTA setup.     

Time features of delayed neutrons and partial emissive-fission cross sections for the neutron-induced fission of 232Th nuclei in the energy range 3.2–17.9 MeV

The energy dependence of the relative abundances of delayed neutrons and the energy dependence of the half-lives of their precursors in the neutron-induced fission of ²³²Th nuclei in the energy range 3.2–17.9 MeV were measured for the first time. A systematics of the time features of delayed neutrons is developed. This systematics makes it possible to estimate the half-life of delayed-neutron precursors as a function of the nucleonic composition of fissile nuclei by using a single parameter set for all nuclides. The energy dependence of the partial cross sections for emissive fission in the reaction ²³²Th(n, f) was analyzed on the basis of data obtained for the relative abundances of delayed neutrons and the aforementioned half-lives and on the basis of the created systematics of the time features of delayed neutrons. It was shown experimentally for the first time that the decrease in the cross section after the reaction threshold in the fission of ²³²Th nuclei (it has a pronounced first-chance plateau) is not an exclusion among the already studied uranium, plutonium, and curium isotopes and complies with theoretical predictions obtained for the respective nuclei with allowance for shell, superfluid, and collective effects in the nuclear-level density and with allowance for preequilibrium neutron emission     

Mass and energy dependence of pion-induced fission

Data for fission induced by pi meson beams from 80 to 500 MeV are presented for nuclei from Fe through Pu as measured by solid state track detectors. The general trends for binary fission withπ ⁺ are reproduced fairly well by a calculation in the ‘high excitation’ limit with standard level density and fission barrier parameters, butπ ^? data are underpredicted. A universal dependence of the binary fission probabilities with the fissility (Z±1)²/A is found to be valid for both pion beam charges for all beam energies below the delta resonance. Probabilities for observing three fragments withπ ⁺ are not reproduced by a ternary fission application of the model found to work for binary fission.     

Design and commissioning of a timestamp-based data acquisition system for the DRAGON recoil mass separator

The fission fragment mass distribution followed by neutron emission is studied for the ²³⁸U(¹⁸O,f) reaction using the asymmetric two-center shell model. Within the thermodynamic approach, excitation energy carried by the compound nucleus is dissipated in the emission of a pair of neutrons in several consecutive steps. Therefore, we have considered 2–12 (in step of 2) neutron emission channels in our formalism. The mass distribution corresponding to 8-neutron emission channel compares reasonably well with the experimental data. The observed fine structure dips corresponding to shell closure (Z = 50 and N = 82 of individual fission fragment arise mainly due to shell structure in the mass parameters. However, an exact location and magnitude of the dip at A = 124 in the mass distribution depends on how the temperature modifies masses and, also, on the precise information of pre- and post-neutron emission data. This suggests a possible importance of extending these calculations to get new insight into an understanding of the dynamical behaviour of fragment formation in the fission process.     

Systematics of neutron emission probabilities from delayed neutron precursors

A simple correlation based on the gross theory ofβ-decay is derived between the neutron emission probabilitiesP _n of delayed neutron precursors, theirβ-decay energies and the neutron binding energies of the daughter nuclei. The correlation is shown to be valid for delayed neutron precursors among the fission products. TheP _n-values of several expected but still unidentified neutron precursors are estimated together with their contributions to the delayed neutron groups in thermal-neutron induced fission of²³⁵U. Some aspects of theβ-strength function for transitions into highly excited states are discussed.     

P-odd asymmetries for the binary fission of oriented target nuclei, induced by cold polarized neutrons

P-odd asymmetries in binary fission of oriented target nuclei induced by cold polarized neutrons are described for the first time using methods of the quantum theory of fission. A spin matrix of the compound nucleus density is constructed with allowance for the interference of the fission amplitudes of various pairs of neutron resonances excited in the given nucleus during the capture of a cold polarized neutron by the oriented target nucleus. In the differential cross sections for this reaction we obtain not only P-odd T-even correlations with the angular dependence different from that of the earlier investigated correlations in the fission of unoriented target nuclei by polarized neutrons but also P-odd T-odd correlations that do not occur in the fission of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons.     

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.     

The study of prompt and delayed muon induced fission

The mean life times of negative muons bound to actinide nuclei have been measured by detecting the time difference between a stopped muon and the arrival of fragments from delayed fission after muon capture. The deduced capture ratesΛ _c are 1.392(4)·10⁷/s for²³⁷Np, 1.290(7)·10⁷/s for²⁴²Pu and 1.240(7)·10⁷/s for²⁴⁴Pu. The results are compared with published data for the fission and the neutron decay channels and for the electron decay of the bound muon. Including a former measurement ofΛ _c for²³⁹Pu, an isotopic dependence of the muon capture rates in the Pu isotopes is clearly observed.