Application of a two-step dynamical model to calculating properties of fusion-fission reactions

The two-step model of fusion-fission reactions that was proposed previously by the present authors is extended in such a way as to describe the multiplicity of light particles emitted from nuclearfission fragments. Calculations are performed for the reaction induced by ⁴⁸Ca + ²⁴⁴Pu collisions. The mass distribution of fragments, their mass-energy distribution, and the total multiplicity of neutrons and gamma rays are obtained for E _lab = 230, 238, 249, and 255 MeV. It is shown that the model reproduces qualitatively relevant experimental data. In order to attain quantitative agreement, it is necessary to take into account the angular momentum carried away by particles from the nucleus undergoing fission and various types of gamma rays emitted by the nucleus and its fission fragments.     

Investigation of the 208Pb(18O, f) fission reaction: Mass-energy distributions of fission fragments and their correlation with the gamma-ray multiplicity

The mass-energy distributions of fragments originating from the fission of the compound nucleus ²²⁶Th and their correlations with the multiplicity of gamma rays emitted from these fragments are measured and analyzed in ¹⁸O + ²⁰⁸Pb interaction induced by projectile oxygen ions of energy in the range E _lab = 78–198.5 MeV. Manifestations of an asymmetric fission mode, which is damped exponentially with increasing E _lab, are demonstrated. Theoretical calculations of fission valleys reveal that only two independent valleys, symmetric and asymmetric, exist in the vicinity of the scission point. The dependence of the multiplicity of gamma rays emitted from both fission fragments on their mass, M_γ(M), has a complicated structure and is highly sensitive to shell effects in both primary and final fragments. A two-component analysis of the dependence M_γ(M) shows that the asymmetric mode survives in fission only at low partial-wave orbital angular momenta of compound nuclei. It is found that, for all E _lab, the gamma-ray multiplicity M_γ as a function of the total kinetic energy (TKE) of fragments, M_γ(TKE), decreases linearly with increasing TKE. An analysis of the energy balance in the fission process at the laboratory energy of E _lab = 78 MeV revealed the region of cold fission of fragments whose total kinetic energy is TKE ～Q _max.     

Probing fission time scales with neutrons and GDR gamma rays

The time scales for nuclear fission have been explored using both pre-and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV ¹⁶O + ¹⁷⁶Yb and ²⁰⁸Pb and 104-MeV ⁴He + ¹⁸⁸Os and ²⁰⁹Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF2 array. The pre-and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, γ, and fission time scales. The γ values ranged from 7 to 20, while the fission times were (31–105)×10^?21 s.     

Spatial distribution of albedo particles on altitudes ∼500 km

Spatial distributions of energetic charged particles, neutrons and gamma rays at altitude 500 km (below the radiation belts of the Earth), obtained by the measurements of two apparatuses on board the Intercosmos-17 satellite, are presented. The latitudinal dependences, [i.e. the variation of flux with vertical cut-off rigidity of the measurement point], for neutrons (E _n = 1 –30 MeV), gamma rays (E =0·15–6 MeV), secondary electrons (E _e > 100 MeV) and for primary protons coming from the west and the east, respectively, are given. The main characteristic, the ratioN _p/N _e of the counting rate of the particles in the polar regionN _p(R_vert< <0·1 GeV/c) and on the equatorN _e(R_vert > 16 GeV/c), is obtained for the various types of particles. This value is 10 for neutrons, 3.7 for gamma rays, 1·8 for electrons, 11 for protons in westward direction, 10 for protons in eastward direction. The latitude profile of neutrons and gamma rays is in a good agreement with calculations assuming their production by nuclear reactions of primary cosmic rays with nuclei of the atmosphere. The weakening of rigidity dependence of protons coming from east in comparison with those coming from the west, is interpreted as the cause of additional proton albedo flux. The equality of albedo electron fluxes (E_e = 100–3500 MeV) from these directions is observed. With the use of the shadowing effect the obtained data on electron-positron component are consistent with the flux of albedo positrons (E_e + > 3·5 GeV) of the value (0·5±0·2) m^–2. s^–1. ster^–1. The possibility of abundance of albedo positrons above electrons at these altitudes for the energy intervalE=0·2÷0·3 GeV is indicated.     

Investigation of65Cu by means of the average resonance proton capture method

The⁶⁴Ni(p, γ)⁶⁵Cu reaction has been studied in the proton energy rangeE _p =2.05–2.55 MeV. The gamma-ray spectra were recorded with a three-crystal pair spectrometer at proton energy differences of 19 keV covering the proton energy range. An average gamma-ray spectrum was formed by adding all the individual spectra after proper adjustment as a result of the alterations in proton energy. The intensities of the gamma rays to final states with knownJ ^π-values were tested against theoretical calculations based on the Hauser-Feshbach theory. The gamma-ray strength function for energies lower than 9 MeV has been extracted from the experiment.     

Determination of Moisture Content in Coke with <Superscript>239</Superscript>Pu–Be Neutron Source and BGO Scintillation Gamma Detector

A series of experiments has been conducted at the Frank Laboratory of Neutron Physics (FLNP) of the Joint Institute for Nuclear Research (JINR) in order to study the possibility of determining the moisture content of coke using a standard neutron source. The proposed method is based on a measurement of the spectrum of prompt γ rays emitted when samples are irradiated by fast and/or thermal neutrons. The moisture content is determined from the area of the peaks of characteristic γ rays produced in the radiative capture of thermal neutrons by the proton (E_γ = 2.223 MeV) and inelastic scattering of fast neutrons by ¹⁶O (E_γ = 6.109 MeV). The ²³⁹Pu–Be neutron source (〈E _n 〉 ~ 4.5 MeV) with an intensity of ~5 × 10⁶ n/s was used to irradiate the samples under study. A scintillation detector based on a BGO crystal was used to register the characteristic γ radiation from the inelastic fast neutron scattering and slow (thermal) neutron capture. This paper presents the results of humidity measurement in the range of 2–50% [1, 2].     

The behavior of HfB2 at neutron irradiation: a simulation study

The behavior of Hafnium di-boride (HfB₂) under neutron irradiation has been simulated in a wide range of energy from 0.025?eV up to 14?MeV. The simulation and the analysis have been carried out using Geant4 and its related database. From the radiation shielding perspective, it was observed that, under thermal neutron irradiation, HfB₂ scatters neutrons with a marginally higher energy than the incident neutrons and also produces prompt gamma rays up to 11?MeV. These results would indicate that, for high-energy neutron 14?MeV, not only is HfB₂ unacceptable as a reasonable neutron absorber but also produces 20?MeV prompt gamma rays.     

The Giant Dipole Resonance in the superdeformed143Eu nucleus

The γ-decay of the Giant Dipole Resonance (GDR) built on excited nuclear states has been measured for the nucleus¹⁴³Eu. The reaction¹¹⁰Pd(³⁷Cl, 4n)¹⁴³Eu at a beam energy of 165 MeV has been employed. This experiment aimed at searching the γ-decay of the GDR built on the superdeformed¹⁴³Eu states, populated at high spins. High-energy γ-rays were detected in 8 large BaF₂ scintillators in coincidence with discrete transitions measured with the NORDBALL array (in the configuration consisting of 17 HPGe detectors and a 2π multiplicity filter). The spectrum of high-energy γ-rays gated by low-energy transitions between states fed by the superdeformed states shows some excess of yield in the 7–10 MeV region with respect to that gated by transitions between states not populated by the superdeformed states. This excess should be due to the γ-decay of the the giant dipole oscillation along the superdeformed axis of the nucleus that is expected to have a frequency corresponding to ≈9–10 MeV (low-energy component of the GDR strength function). The measured excess, in spite of the large error bars, is found to be of the same order as predicted statistical model values.     

Investigation of the reaction <Superscript>208</Superscript>Pb(<Superscript>18</Superscript>O, <Emphasis Type="Italic">f</Emphasis>): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for ¹⁸O + ²⁰⁸Pb interactions at energies of the beam of ¹⁸O ions in the range E _lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of ¹⁸O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.     

Decay of a new nuclide:89m Mo

The half-life of the 1/2^? isomer in⁸⁹Mo has been determined in an experiment where an enriched⁹²Mo target was irradiated by 350 or 700 ms bursts of 60 MeV protons. Analysis of the gamma-ray spectra collected as a function of time between irradiations revealed gamma rays at 118.8 and 268.5 keV decaying with a 190±15 ms half-life. The hindrance ofE3 transitions inN=47 andZ=47 nuclides is discussed.     

Level structure of <Superscript>164</Superscript>Dy from the (<Emphasis Type="Italic">n</Emphasis>, <Emphasis Type="Italic">n</Emphasis>′γ) reaction

By employing a beam of reactor fast neutrons, gamma-ray spectra and angular distributions of gamma rays with respect to the neutron-beam axis were measured in the reaction ¹⁶⁴Dy(n, n′γ). The scheme of levels and gamma transitions in ¹⁶⁴Dy was composed on the basis of data published earlier and new levels, rotational bands, gamma transitions, and multipole-mixing parameters. It is concluded that the scheme of J = 0–4 levels is complete up to the excitation energy of 1.95 MeV. The commonly accepted rule in constructing K _n ^π = 0₂ ⁺ and 1₁ ⁺ rotational bands is found to be violated. The nature of the 2⁺ level at the energy of 1796.68 keV is discussed.     

Studies of the gamma-strength function and low energy levels in 65Ga using the (p, γ) average resonance method

The ⁶⁴Zn(p, γ)⁶⁵Ga reaction has been studied in the proton energy ranges E_p = 2.0–2.9 and 3.3–4.3 MeV. Using a three-crystal pair spectrometer, gamma-ray spectra of primary transitions were measured throughout this proton energy interval in steps of about 20 keV. An average gamma ray spectrum for each energy region was formed by adding all the relevant individual spectra after adjustment due to the change of the proton energy. In the excitation energy range 0.0–3.7 MeV 50 levels, of which 20 are new, were observed. The gamma strength function was deduced from the average intensities of the primary gamma-rays. The absolute value of the strength function was found to be 5–10 times smaller than that expected from the extrapolation of the lorentzian shape of the El giant resonance.     

Neutron emission from the reaction 232Th(n, xn′f)

The energy distributions of neutrons accompanying the fission of ²³²Th are measured by the time-of-flight technique at the bombarding-neutron energies of E _n=14.6 and 17.7 MeV. The data obtained in this way are compared with the results of previous investigations. An excess of soft neutrons that was observed in the experimental spectra of neutrons from ²³⁸U fission at E _n=13.2, 14.7, 16.0, and 17.7 MeV in relation to the results of the calculations based on the model of two sources is also present in the spectra for ²³²Th. The discrepancy between the results of the calculations and experimental data disappears as soon as one assumes the presence of a third source that is related to neutron emission from nonaccelerated fragments.     

Gamma-ray multiplicity measurements in the28Si+64Ni reaction at 163.8 MeV

The²⁸Si+⁶⁴Ni reaction at 163.8 MeV incident energy is studied by measuring in coincidenceγ-rays and charged particles identified from Z=2 to Z=16. The transition from quasi-elastic to more damped reactions is observed when the difference between the detected charge and the projectile one is increased. The strong influence of the particle decay on the measuredγ-ray multiplicity is evidenced with the help of the statistical model computer code CASCADE. Dissipative events are well described in the rolling limit with excitation energy equally shared between the fragments. The overall agreement is lost for the fragments with the projectile charge which show a small value of theγ-multiplicity even for dissipative events. This is probably connected with the previously observed non statistical behavior of gamma rays emitted in coincidence with projectile-like fragments. In the alpha-spectrum measured in coincidence with gamma-rays, the deexcitation of fused systems is clearly separated from in flight emission of deep inelastic fragments. The low measured gamma-ray multiplicity for fusion events is qualitatively explained taking into account the effect of alpha-emission in the statistical decay.     

Investigation of 88Sr in the reaction Sr(n, n′γ)

With the aid of a beam of fast reactor neutrons, the gamma-ray spectrum and angular distributions of gamma rays with respect to the neutron-beam axis are measured in the reaction Sr(n, n′γ) along with the lifetimes of ⁸⁸Sr levels. Values of the multipole-mixture parameter δ are found for gamma transitions between levels.     

Investigation of the reaction 208Pb(18O, f): Fragment spins and phenomenological analysis of the angular anisotropy of fission fragments

The average multiplicity of gamma rays emitted by fragments originating from the fission of ²²⁶Th nuclei formed via a complete fusion of ¹⁸O and ²⁰⁸Pb nuclei at laboratory energies of ¹⁸O projectile ions in the range E _lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier (E _c.m. ? E _B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory.     

Measurement of mean lifetimes, γ-ray angular distributions and linear polarizations for low-lying levels of 50V

Levels of ⁵⁰V were populated with the ⁵⁰Ti(p, n)⁵⁰V reaction at E_p = 4.50, 4.62 and 4.80 MeV. Gamma rays were measured in singles. From Doppler shift attenuation measurements mean lifetimes have been deduced for 9 levels below E_x = 2 MeV. The J^π assignments for 8 of these levels follows from γ-ray angular distribution and linear polarization measurements. The transition strengths have been compared with calculations based on the wave functions of McCullen, Bayman and Zamick.     

Searches for the superheavy hydrogen isotope 7H in the absorption of stopped π − mesons

Experimental searches for the superheavy hydrogen isotope ⁷H were performed in reactions involving the absorption of stopped π ^? mesons on ⁹Be and ¹¹B nuclei. In the reaction ⁹Be(π ^?, pp)X, the missing-mass spectrum shows evidence for the formation of ⁷H states, that of E _r = 16 ± 1 MeV and Γ ? 2 MeV and that of E _r = 21 ± 1 MeV and Γ ? 5 MeV (E _r is the resonance energy with respect to breakup into a trition and four neutrons, and Γ is the observed level width).     

Preequilibrium GDR excitation and entrance channel angular momentum effects

The energy spectra of the γ-rays emitted in the ³⁵Cl + ⁹²Mo reaction at incident energy E = 260 MeV were measured in coincidence with the ejectiles produced in dissipative reaction events. The cumulative energy spectrum of the γ-rays coming from the decay of the ejectiles was calculated within the statistical model and its comparison to the experimental spectrum evidences an excess in the data for E _γ = 8 to 12 MeV. Such an excess, fitted with a Lorentz curve, is attributed to the preequilibrium GDR γ-decay of the intermediate dinuclear system. The centroid energy of the Lorentz curve corresponds to a dipole oscillation along the symmetry axis of the system and its width is found to be comparable to that of the ground state GDR low energy component of the deformed dinucleus. The small quantal dispersion Δ? = (10.3 ± 0.1)? of the entrance channel angular momentum, determined by analysing the dissipative fragment angular distribution in the framework of the Strutinsky model, is suggested to limit the broadening of the preequilibrium GDR width.