Implications on the chiral quark bag from the deuteron asymptoticD-state

Fission yields of^124–132In in the thermal neutron fission of²³⁵U were determined for the first time. Charge displacementsΔZ=Z _p?Z _UCD were calculated for the corresponding mass chains. Both fission yields and charge displacement values were compared with those obtained by systematics by Wahl et al. and Wolfsberg. It was found that the fission yields of the In isotopes obey the gaussian distribution. The displacement function of Wolfsberg seems to give the better representation of the experimental results. Half-lives of^124–131In were determined from beta decay curves.     

Fission of 232Th, 238U and 235U induced by negative muons

The absolute yields of prompt and delayed fission induced by negative muons in ²³²Th, ²³⁸U and ²³⁵U have been measured. The delayed fission yields are much lower than could be predicted from Γ_n/Γ_f systematics for 15–20 MeV nuclear excitation. The systematics of prompt fission yields are compared with recently obtained photofission data. It is suggested that prompt fission can be used for investigating the channel structure of the fission barrier.     

Calculated beta-ray spectra from decay of fission products produced by thermal-neutron fission of 235U

Beta-ray spectra from decay of fission products created by thermal-neutron fission of ²³⁵U have been calculated using summation techniques and using an independently created beta-ray data file. The calculated beta-ray spectra are in very good agreement for E_β ? 1 to 8 MeV with recently measured specta of beta rays for two very different exposure and counting time conditions: (a) an “instantaneous” spectrum following a 1.5 day exposure, and (b) an 8 s counting-time spectrum obtained 1.7 s following a 1 s exposure of ²³⁵U by thermal neutrons.     

Influence of shell effects on the angular distributions of fission fragments

A dynamical-statistical model is used to analyze the experimental angular distributions of fission fragments in the reactions α + ²³⁸U, ²³⁷Np at E _α = 20–100 MeV, as well as to determine the Am isotope fission probabilities and the shape isomer yields in the reactions d + ^242,240Pu at E _d = 20–30 MeV. Manifestations of shell effects are found in the fission barrier structure up to the excitation energies of 50–60 MeV.     

Dynamical approach to calculating angular distributions of fission fragments

A dynamical approach is proposed for calculating the angular distributions of fission fragments. The relaxation time for the degree of freedom associated with the projection of the total angular momentum of the nuclear system onto the symmetry axis and the coefficient of damping of the fission mode are the basic parameters of this approach. Experimental data on the anisotropy of the angular distributions of fission fragments and on the multiplicities of prescission neutrons are analyzed within the proposed model for ¹⁶O+²⁰⁸Pb (E _lab=110–148 MeV), ¹⁶O+²³²Th (120–160 MeV), ¹⁶O+²⁴⁸Cm (110–148 MeV), and ¹⁶O+²³⁸U (96–148 MeV). The relaxation time and the damping coefficient are estimated at τ_K=(5–6)×10^?21 s and β=4×10²¹ s^?1, respectively.     

Cross section for the subthreshold fission of 236U

The cross section for ²³⁶U fission in the neutron-energy range E _n = 0.001–20 keV was measured by using the INR RAS (Institute of Nuclear Research, Russian Academy of Sciences, Moscow) LSDS-100 neutron spectrometer of the lead slowing-down spectrometer type. The resonance fission areas of the resonances at 5.45 eV and 1.28 keV were found, and the fission widths of these resonances were evaluated. The cross section for the ²³⁸U(n, f) fission process was measured, and the threshold sensitivity of the LSDS-100 to small values of fission cross sections was estimated. The well-known intermediate structure in the cross section for the neutron-induced subbarrier fission of ²³⁶U was confirmed.     

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.     

Measurements of delayed-neutron yields from thermal-neutron-induced fission of 235U, 233U, 239Pu, and 237Np

The experimental results on delayed-neutron yields from thermal-neutron-induced fission of some actinides in the IBR-2 pulsed reactor are presented. A method of periodic irradiation without displacement of the sample was used. The measurements of delayed-neutron total yields in thermal-neutron-induced fission of ²³⁹Pu, ²³³U, and ²³⁷Np and in cold-neutron-induced fission of ²³⁵U, ²³³U, and ²³⁹Pu were carried out. All values were obtained with the use of the value of β₀ for (n _th+²³⁵U) as a reference. Precise measurements of decay curves in the time interval 5–350 ms for ²³⁵U and ²³⁹Pu were performed.     

Components of antineutrino emission in nuclear reactor

New ?_ee scattering experiments aimed at sensitive searches for the ν_e magnetic moment and projects to explore small mixing angle neutrino oscillations at reactors require a better understanding of the reactor antineutrino spectrum. Six components which contribute to the total ?_e spectrum generated in a nuclear reactor are considered. They are beta decays of the fission fragments of ²³⁵U, ²³⁹Pu, ²³⁸U, and ²⁴¹Pu and decays of beta emitters produced as a result of neutron capture in ²³⁸U and in accumulated fission fragments which perturb the spectrum. For antineutrino energies of less than 3.5 MeV and for each of the four fissile isotopes, the time evolution of ?_e spectra is given during fuel irradiation and after the irradiation is stopped. The relevant uncertainties are estimated. Small corrections to the ILL spectra are considered.     

Beta-delayed fission and neutron emission: Consequences for the astrophysicalr-process and the age of the galaxy

Beta-delayed neutron emission and beta-delayed fission probabilities (P _n andP _{β d j} ) were calculated for neutron-rich nuclei between the beta stability line and the neutron-drip line in the range 10≦Z≦100 and 75≦Z≦100, respectively. These results were obtained by applying recentβ-strength function calculations, fission barrier height predictions, and a neutron optical potential from infinite nuclear matter calculations. An area of ～100% fission probability is predicted around Z=94,N=168 extending beyond the well-known island of spontaneous fission in that region. Astrophysicalr-process calculations were performed including the calculatedP _n and Pβ d f values. This puts the method to determine the age of the Galaxy by the actinide chronometers on a reliable basis. An excellent overall agreement with the observedr-abundance distribution is obtained. The predictedr-process production ratios for the Chronometrie pairs²³²Th/²³⁸U,²³⁵U/²³⁸U and²⁴⁴Pu/²³⁸U result in an age of the Galaxy oft _G =(20.8± ₄ ² )×10⁹ a, which is by almost a factor two larger than earlier predictions by this method, but in accordance with recent astronomical observations from globular clusters. The predicted island of 100%β-delayed fission acts as a sink to ther-process with the consequence that no superheavy elements are produced in nature.     

Determination of fission barrier heights from β-delayed fission

Beta delayed fission (βDF) gives a possibility to investigate the fission barrier for nuclei far off beta stability. However, before any information on the fission barrier can be extracted, the effect of low-lying structures in the beta-strength function (S _β) on theβDF branching ratio has to be considered. This is in general not done. In this paper the lowlying structures that occur inS _β are discussed and microscopic calculations for the Gamow-Teller strength function are presented for²³²Th (β ^?-decay) and²³²Pu,²⁴⁰Cm,^244,248Cf and²⁴⁸Fm (β ⁺-decay). Using the calculated strength functionsβ ⁺DF branching ratios are calculated and compared with the experimental ones. The sensitivity of the results to different shapes ofS _β is investigated. It is concluded that, when the expected structures inS _β are considered, there are at present no indications fromβ ⁺ DF measurements that the errors in the fission barrier calculations are larger than the uncertainty given for those calculations. The difference in magnitude between theβ ^? DF and theβ ⁺DF branching ratios is also explained by the occurrence of low-lying structures inS _β.     

Electrofission of238U and232Th

Absolute electrofission cross sections for²³⁸U and²³²Th in the energy regionE _e =7 ?65 MeV and fission fragment angular distributions forE _e =7–30 MeV have been measured. The angular distributions show strong anisotropies for low energies. The relative dipole and quadrupole contributions as a function of excitation energy are discussed in terms of the low lying fission transition states above the fission barriers. The cross sections show significant deviations from the results of some earlier measurements, in particular in the energy region above the giant dipole resonance. From the difficulties of absolute electrofission cross section measurements and the ambiguities in their interpretation it is concluded that by this time the quantitative analysis of electrofission cross sections with respect to the contributions of the giant quadrupole resonances to the fission decay channel should be regarded as rather tentative.     

Microscopic description of fission in neutron-rich plutonium isotopes with the Gogny-D1M energy density functional

The most recent parametrization D1M of the Gogny energy density functional is used to describe fission in the isotopes ^232-280Pu . We resort to the methodology introduced in our previous studies (Phys. Rev. C 88, 054325 (2013) and Phys. Rev. C 89, 054310 (2014)) to compute the fission paths, collective masses and zero point quantum corrections within the Hartree-Fock-Bogoliubov framework. The systematics of the spontaneous fission half-lives t _SF , masses and charges of the fragments in plutonium isotopes is analyzed and compared with available experimental data. We also pay attention to isomeric states, the deformation properties of the fragments as well as to the competition between the spontaneous fission and α-decay modes. The impact of pairing correlations on the predicted t _SF values is demonstrated with the help of calculations for ^232–280Pu, in which the pairing strengths of the Gogny-D1M energy density functional are modified by 5% and 10%, respectively. We further validate the use of the D1M parametrization through the discussion of the half-lives in ^242–262Fm. Our calculations corroborate that, though the uncertainties in the absolute values of physical observables are large, the Gogny-D1M Hartree-Fock-Bogoliubov framework still reproduces the trends with mass and/or neutron numbers and therefore represents a reasonable starting point to describe fission in heavy nuclear systems from a microscopic point of view.     

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.     

Contribution of beta and gamma radiation activity to total dose intensity of fission products

A numerical analysis shows the contribution of beta and gamma radiation to the total dose intensity of fission products. The problem was solved for the case of an infinite plane covered with an ideally thin layer of isotope and for two different cases of absorption layers above this plane, 86 mg/cm² and 650 mg/cm². The different ages of the fission products were also considered. It was found that the beta radiation contribution to the total dose intensity predominates strongly in the case of the 86 mg/cm² absorption layer and for the 650 mg/cm² absorption layer it is comparable with the dose intensity of gamma radiation.     

Nuclear spectroscopy of neutron-richA=143 nuclei

A study of the beta decay of¹⁴³Cs and¹⁴³Ba fission products was undertaken by the use of two on-line mass-separators OSTIS and OSIRIS. Level schemes for¹⁴³Ba and¹⁴³La are deduced from gamma and conversion electron spectra,γ-γ andβ-γ coincidences.Q _β values and ground state beta feedings were also measured. The nuclei¹⁴³La and¹⁴³Ba are tentatively inserted in a systematic of odd-even nuclei aroundA=143 and even-odd nuclei with 87 neutrons respectively.     

Analysis of fragment mass distribution in asymmetric area at fission of <Superscript>235</Superscript>U induced by thermal neutrons

The fragment mass yields in fission of ²³⁵U induced by thermal neutrons for A = 145–160 and E_K = 50–75 MeV were measured using a mass spectrometer. The fine structure is observed at A = 153, 154 and E_K = 50–60 MeV. The obtained results were described in the framework of a model based on the dinuclear system concept. The analyzed correlation between the total kinetic energy and mass distribution of fission fragments is connected with the shell structure of the formed fragments of fission. From this correlation and the time dependence of the calculated mass distribution of the binary reaction products, one can conclude that the descent time from a saddle point to a scission point for the more deformed fragments is longer than that for fragments of more compact shape.     

Absorption by XeCl* excimer molecules of their own emission of the В–Х transition (λ = 308 nm) in a dense Ar–Xe–CCl<Subscript>4</Subscript> medium upon pumping by fast electrons and uranium-235 fission fragments

Luminescence of dense Ar–Xe–CCl₄ gas mixtures with a low CCl₄ content upon pumping by fast electrons and uranium-235 fission fragments is studied by spectroscopic methods. It is found that, in a cell with a resonator tuned to the В–Х transition of the XeCl* molecule (λ = 308 nm), the D-state population of the XeCl* excimer molecule (the D–X transition, λ = 235 nm) depends on the B-state population and increases by many times with increasing B-state population of the XeCl* molecule. The stimulated absorption coefficient k = 1.2 × 10^–16 of В–Х transition emission of the XeCl* molecule (λ_max = 308 nm), which leads to population of the D-state of this molecule, and the coefficient of amplification μ = 2.5 × 10^–4 cm^–1 of В–Х transition emission of the ХеCl* molecule (λ = 308 nm) are measured upon pumping by uranium- 235 fission fragments with the specific energy input into the gas medium of ~60 mJ/cm³ and a specific power of energy input of about 240 W/cm³.