Yu.V. Melikov

Determination of the lifetime of excited compound nuclei in fission using the shadow effect

Abstract The results of three experiments on the observation of the shadow effect in the fission of 238U nuclei in a UO2 single crystal induced by 1.7 and 3.3 MeV neutrons and 25 MeV α-particles are discussed. Comparison is made of the shapes of the fission-fragment angular distributions in the vicinity of two 〈111〉 crystallographic axes directed along and normal to the incident beam. It was found that the displacement of a fissionable nucleus from a lattice site in response to the momentum of the incident particle affects the shape of the shadow. The values of the lifetime of the 239U compound nucleus were found to be (3.5±1.0) × 10−16sec at an excitation energy of 6.5 MeV and (2.0±0.8) × 10−16sec at 8.1 MeV.

The (α, 2α) reaction on 6Li, 7Li, 9Be and 12C: (I). Experimental data

Abstract The (α, 2α) reaction on 6 Li, 7 Li, 9 Be and 12 C nuclei induced by 25 MeV α-particles was studied by two-dimensional energy analysis. Two-dimensional energy spectra have been obtained at various angular positions of the detectors in the reaction plane. It is shown that for low-filled distributions, the problem of two-dimensional analysis can be reduced to a one-dimensional analysis. The spectra obtained indicate that the (α, 2α) reaction proceeds both through a two-stage channel via known quasi-stationary states of the target nucleus and through a one-stage channel in which all three particles are produced simultaneously. The probabilities of the two processes are comparable; the cross section of the one-stage process decreases with increasing binding energy of the α-particle in the initial nucleus. The α-α angular correlation functions corresponding to the one-stage mechanism have been obtained for symmetrical and asymmetrical arrangements of the detectors with respect to the incident beam.

Lifetime-effect relations in the blocking technique for a thick crystal

Abstract The change in blocking dip profiles due to compound nuclei recoil displacement during the lifetime (lifetime-effect relations) has been studied experimentally using the 27Al(p, α) 24Mg resonance reaction at an energy Er=633 keV and treated theoretically using the multistring model and the Fokker-Planck equation. The influence of the crystal thickness traversed by outgoing α-particles and the radiation defects present in the crystal on the lifetime-effect relations has been investigated. The importance of the inclusion of defects in calculations is demonstrated.

A study of the time delay of excited shape isomers by the blocking technique

Abstract A method based on the blocking effect is used to study the fission lifetime of the excited 235,238,239Np nucleus. It is demonstrated that only the double-humped fission barrier model with the inclusion of the lifetime of excited shape isomers of the fissile nucleus can describe well the experimental results.

Lifetime measurements of nuclei formed in neutron emission: (II). Lifetime of neptunium isotopes

Abstract The lifetimes of the fissionable neptunium isotopes 235,236,238,239 Np have been measured using the crystal-blocking technique. These isotopes were produced by deuteron bombardment of 235 UO 2 and 238 UO 2 single crystals with following emission of neutrons. Analysis of the experimental results was performed in the framework of the statistical model of nuclear reactions and the double-humped fission barrier model with allowance for the lifetime of the second-well states.

Blocking study of the time characteristics of decay of excited heavy nuclei

Abstract The results of the fission time experiments with the reactions 232 Th (d, x nf ) , 235,238 U (d, x nf ) , 235,238 U (α, x nf ) ( p , x nf ) are reported. Measurements were made with the single crystals ThO 2 and UO 2 , using the blocking technique. The fission times of 232,233 Pa, 235,236,238,239 Np and 237,238,240,241 Pu in the excitation energy range of 6 to 12 MeV were determined. For the fissi of the Pa and Np isotopes a new physical phenomenon was observed — the time delay of the induced fission relative to the decay time of excited nuclei via any other channel. The time delay is connected with the nuclear lifetime of the excited states in the second potential well. The relation of the time delay with the density of the second-well excited states and with the double-humped fission barrier parameters was studied.

Study of the intermediate states of a fissionable nucleus by the blocking technique

Abstract A method based on the blocking effect is used to study the fission lifetime of the excited 238Np nucleus. Effects associated with the complicated structure of the fission barrier are experimentally demonstrated.

Lifetimes of the nuclei formed by deuterons bombardment of 235U

Abstract The lifetime effect of fissioning nuclei formed by the bombardment of 235 UO 2 single crystal with deuterons with energies from 10–14 MeV was observed. The analysis showed that this effect was due to the fission of 235 Np nuclei, created by evaporation of two neutrons from the compound nuclei 237 Np. The lifetime of 235 Np nuclei was estimated using the ratio Γf/Γn for nuclei 237 Np, 236 Np, 235 Np which in turn were calculated in the frame of the statistical theory of nuclear reactions.

Lifetime of fissionable nuclei produced in the alpha-bombardment of uranium isotopes

Abstract The lifetimes of the fissionable plutonium isotopes were measured using the shadow (blocking) effect. These isotopes were produced by bombarding of 235 UO 2 and 238 UO 2 single crystals with an α-particle beam and following emission of neutrons. The analysis of the experimental results in the framework of the statistical model of nuclear reactions was performed.

Measurement of nuclear lifetimes using the asymmetry of blocking patterns

Abstract The 27 Al(p,α) 24 Mg resonance reaction at E p = 505 keV was studied by analysis of the asymmetry of the shoulders of a (111) planar dip as a function of reaction depth. The displacement of the compound nucleus was obtained by means of analytical calculations based on a model of particle motion in a channel with α “loose” wall structure. The value of τ = (5 ± 0.4)×10 −16 s is obtained.