Basic results of investigations of scission neutrons in nuclear fission at low excitation energies

To estimate the main characteristics of neutrons emitted shortly before the scission of a fissioning nucleus, various experiments sensitive to the presence of these scission neutrons in thermal-neutron-induced fission of ²³⁵U and spontaneous fission of ²⁵²Cf were performed. The results of the experiments were analyzed within theoretical calculations allowing for various possible neutron-emission mechanisms, including the possibility of the emergence of neutrons from the scission of a nucleus.     

Die Spektren der Photoneutronen aus O18 und N14

Water enriched to 97% in O¹⁸ and liquid nitrogen were irradiated with 30,5 MeV bremsstrahlung. The energy distribution of emitted neutrons between 2,5 MeV and 20 MeV was measured via the recoil protons in a stilbene scintillator. The energy spectrum of photoneutrons from N¹⁴ shows maxima at 3,0, 6,5 and 11 MeV, whereas that of neutrons from O¹⁸ contains only indication of structure. For both nuclei emission of neutrons seems to proceed mainly through transitions into excited levels of the final nuclei.     

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.     

Mean energies of prompt fission neutrons in the neutron-induced fission of <Superscript>235</Superscript>U at primary energies in the range <Emphasis Type="Italic">E</Emphasis><Subscript>n</Subscript> < 20 MeV

The spectra of neutrons accompanying the induced fission of ²³⁵U target nuclei are described theoretically. It is confirmed that a third neutron source must be introduced in order to reproduce the shape of experimental distributions at high energies of primary neutrons (previously, a third source was used in describing the spectra of neutrons emitted in ²³²Th and ²³⁸U fission). On the basis of experimental results and their analysis, the mean energy of fission neutrons is estimated as a function of the bombarding-neutron energy up to E _n = 20 MeV.     

Die Energieverteilung der Neutronen aus einer Po-α-Be-Quelle

The neutrons from a Po-α-Be source were detected via the recoil protons in a stilbene scintillator. The pulse height spectrum of the scintillations produced in the stilbene crystal by the recoil protons is analysed to yield the incident neutron energy distribution. Theγ-ray background was eliminated through pulse shape discrimination. Separately the spectrum of neutrons coincident withγ-rays, i.e. of neutrons from the Be⁹(α,n)C^12* reaction, has been determined. The resulting spectra are compared to previous measurements. The structure of the spectra is not compatible with an isotropic angular distribution of the neutrons relative to theα-direction. Furthermore it could be shown that the fraction of reactions leading to the 7,65 MeV state of C¹² is only about 10^?3.     

Prompt-fission-neutrons spectra of <Superscript>238</Superscript>U

The spectra of prompt fission neutrons from the reaction ²³⁸U(n, F) for neutrons of energy in the range E _n ≤ 20 MeV are interpreted within the statistical model. It is shown that exclusive spectra of prefission neutrons emitted in (n, xnf) reactions play a decisive role in describing the observed promptfission-neutron spectra and determine the average energies of prompt-fission neutrons. The dependence of the effect of prefission neutrons on the fissility of a target nucleus is demonstrated for the reactions ²³²Th(n, F), ²³⁵U(n, F), and ²³⁹Pu(n, F).     

Beschleunigungsreaktionen von Neutronen

Acceleration of neutrons by isomeric nuclei. The inelastic neutron acceleration (INNA) by long living isomeric nuclei is studied. The theoretical estimates of the INNA cross-sections for thermal neutrons for a number of isomers give the values in the range 0.1 - 10²b. Inelastic acceleration of thermal neutrons was observed by ^152mEu and ^180mHf, and confirmed the high value of the INNA cross-sections. As a result of the high probability of acceleration some isomeric nuclei ^113mIn, ^115mIn, are not usual moderators but accelerators of neutrons. The use of the INNA reaction for the study of high excited levels and the nature of various nuclear hindrances are discussed.     

Emission of fast neutrons in deep-inelastic collisions of 16O on Ni

Angular correlations of neutrons with projectile-like fragments (PLF) were measured for the very asymmetric mass systems ¹⁶O+^58,64Ni at 6 MeV/amu. The emission of two fast neutron components has been observed: one is due to quasielastic and the other to deep-inelastic collisions (DIC). In both cases these neutrons seem to be ejected quasifree at the initial step of the reaction process. This is the first experimental proof for the emission of preequilibrium neutrons in DIC.     

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.     

Search for T-odd left-right asymmetry of prompt neutron emission in binary fission of the 233U and 239Pu nuclei by slow polarized neutrons

We report preliminary results of measuring the T-odd left-right asymmetry of prompt neutron emission in binary fission of the ²³³U and ²³⁹Pu nuclei by slow polarized neutrons. Assuming that about 35% of prompt neutrons are emitted from the “neck,” one can conclude that the emission asymmetry of scission neutrons in ²³³U is an order of magnitude lower than the asymmetry of α-particle emission in ternary fission.     

Prescission neutrons in the fission of 235U and 252Cf nuclei

Experimental data obtained previously for the energy-angular distribution of neutrons originating from the fission of ²⁵²Cf (spontaneous fission) and ²³⁵U (thermal-neutron-induced fission) nuclei are analyzed, the angle being measured with respect to the direction of fission-fragment motion. A regularity common to all independent experiments is revealed: at an angle of about 90°, there exists an excess of neutrons (30% for ²⁵²Cf and 60% for ²³⁵U) that does not admit explanation within the model of neutron emission from fully accelerated fragments. Two possible explanations of this experimental fact—neutron emission during the acceleration process and the existence of an additional source of neutrons (predominantly, prescission neutrons)—are considered. It is shown that the latter conjecture describes the observed features for both nuclei more adequately. The total yield of prescission neutrons and their energy and angular distributions are determined.     

Neutron multiplicity in deep inelastic collisions: 400 MeV Cu+Au system

We have detected in nine different positions of space the neutrons associated with the collision of ⁶³Cu on ¹⁹⁷Au at 400 MeV bombarding energy. The deep inelastic products were detected at two different angles: close to the grazing angle and 30° forwards of it. Their masses were measured using a time-of-flight technique. The neutrons were detected in coincidence with the fragments - the efficiency of the neutron detectors was measured relatively to a ²⁵²Cf source during beam time. The neutron threshold was set at 300 keV. Within an accuracy of 10% all the emitted neutrons are evaporated by the fully accellerated deep inelastic fragments. It is shown that the excitation energy is shared between the fragments in proportion to their masses and that the relaxation time for internal equilibration of the composite system is very short (～10^?22 s).     

Supra-atomic structure of radiation-induced defects in synthetic quartz from neutron scattering data

The nanostructure of synthetic quartz samples irradiated with fast reactor neutrons with energies E n > 0.1 MeV has been studied by small-angle neutron scattering. The fluences are varied from 10¹⁷ to 2 × 10²⁰ neutrons/cm². In the quartz samples irradiated with fluences higher than 10¹⁷ neutrons/cm², point, extended (dislocation loops), and volume defects, namely, thermal peaks up to 50 nm in radius, are observed over the entire volume. At a fluence of 2 × 10²⁰ neutrons/cm², the total fraction of the formed defect regions, where the material is in a noncrystalline state, exceeds 10% of the sample volume. The data on the formation of a metamikt glassy phase in the quartz sample have been obtained.     

Time-dependent calculations for neutron transfer and nuclear breakup processes in 11Li+9Be and 11Li+12C reactions at low energy

Neutron transfer and nuclear breakup processes in reactions with weakly bound nucleus at low energies are investigated in the framework of the time-dependent Schrödinger equation. The evolution of probability density of outer weakly bound neutrons of ¹¹Li in the collision with ⁹Be and ¹²C were studied. The probabilities and cross sections of outer neutrons removal (breakup processes and transfer to target nucleus) were calculated. The theoretical results have close similarity with experimental data for the two-neutron removal in reaction ¹¹Li+⁹Be.     

Die Polarisation der Neutronen aus der Reaktion13C(α,n)16O

The angular distribution of the polarization of neutrons from the reaction¹³C(α,n)¹⁶O has been determined between 1.38 and 2.26 MeV energy of the α-particles by left-right asymmetry induced by elastic scattering from⁴He. The neutrons were detected by a time-of-flight method between a helium gas scintillation counter and two plastic scintillation counters. From Simon and Welton theory analysis a set of spectroscopic data has been obtained for seven states of the compound nucleus¹⁷O.     

Detailed Investigations of Neutron–Neutron Angular Correlations in Slow-Neutron-Induced Fission of <Superscript>233</Superscript>U, <Superscript>235</Superscript>U,and <Superscript>239</Superscript>Pu

An experiment devoted to studying neutron–neutron angular correlations in the slow-neutroninduced fission of ²³³U, ²³⁵U and ²³⁹Pu nuclei was performed. The experimentally determined angular dependence of the number of neutron–neutron coincidences was compared with the results of a Monte Carlo simulation for various values of the neutron-detection threshold in the range between 490 and 2080 keV. It was found that the experimental angular distributions in question can be described well under the assumption that 2% to 5% of all prompt fission neutrons are emitted isotropically in the laboratory frame. Probably, such neutrons can be interpreted as so-called scission neutrons directly associated with the nuclear-rupture instant. Energy distributions of this component were also obtained from the present analysis.     

(n, α) Reactions on light nuclei atE n =13.9 MeV

The reactions¹⁴N (n, α)¹¹B and¹⁰B(n, α)⁷Li were studied with 13.9 MeV neutrons using a counter telescope. Differential cross sections were measured for the transitions to the ground states and to some excited states. The results provide evidence for a strong contribution from a direct reaction mechanism. Systematics of integrated cross sections for the ground state transitions of all known (n, α) reactions on light nuclei with 14 MeV neutrons is also given.     

The14N(n,p)14C reaction cross section for thermal neutrons

The¹⁴N(n,p)¹⁴C reaction cross section has been measured for thermal neutrons, making use of the neutron beam of the IBR-30 pulsed booster of the JINR, Dubna. Thermal neutrons were identified by a time-offlight method. Solid adenin targets were utilized. An ionization chamber was used for detecting the charged particles. The cross section was found to beσ _np ^th =(1.83±0.07)b.     

Proton transitions in22Na(n,p)22Ne reaction

The nuclear reactions induced by thermal and 2 keV neutrons on²²Na radioactive nucleus were studied. For the thermal neutrons a more accurate value of²²Na (n,p)²²Ne reaction cross section was obtained. A weak proton transition to the²²Ne ground state was observed. The upper limits of cross sections for thermal neutron induced (n,∞) reaction and (n, p) reaction with 2 keV neut·rons are given.