Vol 80, No 5 (2017)

Experimental results on the excitation of isomeric nuclear states in the reactions ¹¹¹Cd(γ, γ′)^111mCd, ¹¹³In(γ, γ′)^113mIn, and ¹¹⁵In(γ, γ′)^115mIn studied by the activation method at photon energies of up to 8 MeV are presented. The respective experiments were performed at the LUE-8 linear electron accelerator of the Institute for Nuclear Research (INR, Moscow). A method for calculating cross sections per equivalent photon was developed with the aid of a simulation on the basis of the GEANT-4 code. The data obtained in this way are discussed from the point of view of the possibility of studying collective nuclear excitations of various multipolarity near the threshold.

The root-mean-square radii of short-lived 0⁺-states in ¹⁶O were obtained for the first time from an analysis of α+¹⁶O diffraction scattering. Data on elastic and inelastic α+¹⁶O scattering were analyzed on the basis of the modified diffraction model in the range of projectile energies between a few tens of MeV units to 400 MeV. No case of a significant enhancement of the radius with respect to its ground-state value was observed. In particular, this concerns the 15.1-MeV 0₆⁺ state, which lies near the threshold for breakup to four alpha particles and for which we did not confirm a giant radius predicted by the alpha-particle-condensate model. This result disproves the hypothesis that the ¹⁶O nucleus in the 0₆⁺ state has a rarefied structure and appears to be an analog of the known Hoyle state at 7.65 MeV (0₂⁺) in ¹²C.

It is shown that the multiplicities and angular and energy distributions of neutrons and photons evaporated from thermalized fragments originating from the spontaneous and low-energy induced fission of nuclei, the relative yields of ground and isomeric states of final fragments, and the features of delayed neutrons emitted upon the beta decay of the above fragments can successfully be described by employing nonequilibrium distributions of spins and relative orbital angular momenta of fission fragments formed in the vicinity of the scission point for the fissile nucleus being studied. It is also shown that these distributions, which are characterized by large mean values of the spins and orbital angular momenta directed orthogonally to the symmetry axis of the fissioning nucleus are successfully constructed upon simultaneously taking into account zero-mode transverse wriggling and bending vibrations of a fissile compound nucleus in the vicinity of its scission point, the wriggling vibrations being dominant. It is confirmed that the zero-mode wriggling vibrations considered immediately above are directly involved in the formation of the angular distributions of fragments originating from the spontaneous and low-energy fission of nuclei. This makes it possible to describe successfully such distributions for photofission fragments.

It is shown that the wave functions for isobaric analog, double isobaric analog, configuration, and double configuration states may simultaneously have components corresponding to nn, np, and pp halos. The difference in the halo structure between the ground and excited states of a nucleus may lead to the formation of halo isomers. A halo structure of both Borromean and tango types can be observed for np configurations. The structure of ground and excited states with various isospins in halo-like nuclei is discussed. The reduced probabilities B(Mλ) and B(Eλ) for gamma transitions in ⁶⁻⁸Li, ⁸⁻¹⁰Be, ^8,10,11B, ¹⁰⁻¹⁴C, ¹³⁻¹⁷N, ^15−17,19O, and ¹⁷F nuclei are analyzed. Particular attention is given to the cases where the ground state of a nucleus does not have a halo structure, but where its excited state may have it.

The ⁶Li + n seven-nucleon nuclear system is studied at low energies within a microscopic approach based on the multichannel algebraic version of the resonating group model. The partial and total cross sections for the ⁶Li(n, t)⁴He reaction are calculated. The contributions of the various partial cross sections to the energy dependence of the total cross section are considered. The results of the calculations are found to be in a good agreement with experimental data.

On the basis of the theory of diagonal two-proton two-step virtual decays of spherical nuclei that was developed earlier and the superfluid model of the nucleus, the total and partial widths for the two-proton decay of the ⁴⁵Fe parent nucleus in the ground state to the ground state of the ⁴³Cr daughter nucleus were calculated along with the angular distribution of protons emitted in this decay. The calculated features of this mode of ⁴⁵Fe decay were shown to be highly sensitive to the choice of form for nucleon shell potentials. It is also shown that there exists a potential such with which one can construct a successful simultaneous description of the experimental total width and the angular distribution of emitted protons for the aforementioned two-proton mode of decay of the ⁴⁵Fe nucleus.

The radial distributions of proton and neutron densities in the even–even isotopes ⁴⁰⁻⁷⁰Cа and ⁴⁸⁻⁷⁸Ni and the analogous distributions of neutron densities in the even–even isotopes ⁹²⁻¹³⁸Mo were calculated on the basis of the mean-fieldmodel involving a dispersive optical potential. The respective root-mean-square radii and neutron-skin thicknesses were determined for the nuclei under study. In N > 40 calcium isotopes, the calculated neutron root-mean-square radius exhibits a fast growth with increasing N, and this is consistent with the prediction of the neutron-halo structure in calcium isotopes near the neutron drip line.

The differential cross sections for p¹⁰B scattering are calculated at the energies of 197, 600, and 1000 MeV within Glauber theory. The contributions of single and double collisions are taken into account in the multiple-scattering operator. The contributions of proton collisions with nucleons belonging to various (1s, 1p) shells are estimated in the single-scattering cross sections. A comparison with experimental data and with the result of calculations in the distorted-wave Born approximation (DWBA) at 197 MeV showed that the differential cross sections for p¹⁰B scattering are adequately described in the region forward scattering angles.

The reliability of experimental cross sections obtained for (γ, 1n), (γ, 2n), and (γ, 3n) partial photoneutron reactions using beams of quasimonoenergetic annihilation photons and bremsstrahlung is analyzed by employing data for a large number of medium-heavy and heavy nuclei, including those of ^63,65Cu, ⁸⁰Se, ^90,91,94Zr, ¹¹⁵In, ^112−124Sn, ¹³³Cs, ¹³⁸Ba, ¹⁵⁹Tb, ¹⁸¹Ta, ^186−192Os, ¹⁹⁷Au, ²⁰⁸Pb, and ²⁰⁹Bi. The ratios of the cross sections of definite partial reactions to the cross section of the neutron-yield reaction, F_i = σ(γ, in)/σ(γ, xn), are used as criteria of experimental-data reliability. By definition, positive values of these ratios should not exceed the upper limits of 1.00, 0.50, 0.33,... for i = 1, 2, 3,..., respectively. For many nuclei, unreliable values of the above ratios were found to correlate clearly in various photon-energy regions F_i with physically forbidden negative values of cross sections of partial reactions. On this basis, one can conclude that correspondent experimental data are unreliable. Significant systematic uncertainties of the methods used to determine photoneutron multiplicity are shown to be the main reason for this. New partial-reaction cross sections that satisfy the above data-reliability criteria were evaluated within an experimental–theoretical method [σ^eval(γ, in) = F_i^theor (γ, in) × σ^expt(γ, xn)] by employing the ratios F_i^theor (γ, in) calculated on the basis of a combined photonuclear-reaction model. It was obtained that cross sections evaluated in this way deviate substantially from the results of many experiments performed via neutron-multiplicity sorting, but, at the same time, agree with the results of alternative activation experiments. Prospects of employing methods that would provide, without recourse to photoneutron-multiplicity sorting, reliable data on cross sections of partial photoneutron reactions are discussed.

Some currently developed applications of cyclotrons (beam technologies) are discussed. They include new methods for producing radioisotopes for medical purposes and investigations into the radiation strength ofmaterials and the wear of units of variousmachines and devices bymeans of beams of radioactive nuclei.

We present a search for a non-Standard-Model invisible particle X⁰ in the mass range 0.1–1.8 GeV/c² in B⁺ → e⁺X⁰ and B⁺ → μ⁺X⁰ decays. The results are obtained from a 711 fb⁻¹ data sample which corresponds to 772 × 10⁶\(B\bar B\) pairs, collected at the Υ(4S) resonance with the Belle detector at the KEKB e⁺e⁻ collider. One B meson is fully reconstructed in a hadronic mode to determine the momentum of the lepton of the signal decay in the rest frame of the recoiling partner B meson. We find no evidence of a signal and set upper limits on the order of 10⁻⁶.

The studies of the production of the X(3872), either prompt or from B-hadron decays, and of the J/ψϕ mass spectrum in B-hadron decays have been carried out by using pp collisions at \(\sqrt s \) = 7 TeV collected with the CMS detector at the LHC. The cross-section ratio of the X(3872) with respect to the ψ(2S) in the J/ψπ⁺π⁻ decay channel and the fraction of X(3872) coming from B-hadron decays are measured as a function of transverse momentum (p_T), covering unprecedentedly high values of p_T. For the first time, the prompt X(3872) cross section times branching fraction is extracted differentially in p_T and compared with NRQCD predictions. The dipion invariant-mass spectrum of the J/ψπ⁺π⁻ system in the X(3872) decay is also investigated. A peaking structure in the J/ψϕ mass spectrum near threshold is observed in B^± → J/ψϕK^± decays. The data sample, selected on the basis of the dimuon decay mode of the J/ψ, corresponds to an integrated luminosity of 5.2 fb⁻¹. Fitting the structure to an S-wave relativistic Breit–Wigner lineshape above a three-body phase-space nonresonant component gives a signal statistical significance exceeding five standard deviations. The fitted mass and width values are m = 4148.0 ± 2.4(stat.) ± 6.3(syst.) MeV and Γ = 28+15 − 11(stat.) ± 19(syst.) MeV, respectively. Evidence for an additional peaking structure at higher J/ψϕ mass is also reported.

In this lecture notes we discuss various aspects of phenomenology of near-threshold resonances.