L. B. Pikelner

Parity nonconservation in neutron resonances

Abstract Experiments to observe the parity nonconserving effects in 81Br, 111Cd, 117Sn, 127I, 139La and 238U are described. The measurements were performed with a beam of polarized neutrons from the pulsed reactor IBR-30 at Dubna. A dependence of total cross sections on neutron helicity was discovered in the following resonances: 0.88 eV 81Br, 4.53 eV 111Cd, 1.33 eV 117Sn, and 0.75 eV 139La. The effect has a resonance character and is in agreement with theoretical predictions made in the framework of the model of mixing compound states. The paper contains experimental estimates for the mixing coefficients and matrix elements.

Determination of 147Sm and 148Sm resonance parameters

Abstract Cross sections of S-neutron capture and scattering by 147,148 Sm were measured by the gamma-multiplicity method in the resolved resonance region. The pulsed booster IBR-30 and multisectional scintillation detector system were used. The values of Γ γ for 26 resonances of 147 Sm and values of Γ γ and Γ n for some resonances of 148 Sm were obtained. The 147 Sm resonances were identified by spin using gamma multiplicity.

Isomer-shift analogue in neutron resonances

Abstract For the first time, the recently predicted chemical shift of neutron resonances, to be regarded as an analogue to the Mossbauer isomer shift, has been experimentally observed studying the 6.67 eV resonance of 238 U. The experimental shifts were determined by a chi-square fitting technique from the time-of-flight transmission spectra of metallic uranium and four uranium compounds measured at the Dubna IBR-30 pulsed reactor. A computational method has been applied to estimate, and compensate for, the influence of the crystal-lattice vibrations on the experimental values thus obtained. The electron density differences at the nucleus have been calculated for the various sample pairs using available data on chemical X-ray shifts in uranium compounds, on Mossbauer isomer shifts in isovalent neptunium compounds and on free-ion electron densities. The resonance shift results lead to the conclusion that the mean-square charge radius of 238 U diminishes by 1.7 −0.8 +1.2 fm 2 upon capturing the resonance neutron.

Nb and Rb neutron resonances and radiation widths of medium-mass nuclei

Abstract The neutron resonance parameters for niobium and rubidium in the energy range up to 400 eV and up to 1300 eV, respectively were determined by measuring the transmission, capture, scattering and self-indication. The obtained values of the radiation widths for niobium s and p wave levels coincide within the measurement error. The radiation width for the magic nucleus of Rb 87 is less than that for the nonmagic Rb 85 . The experimental data on the radiation widths of medium nuclei show a minimum in the region of the closed shell with the number of neutrons equal to 50.

Investigations of parity violation and interference effects in 235U fission induced by resonance neutrons

Abstract These investigations were performed to test the modern theory and to obtain first estimates of nuclear matrix elements of weak interaction and the main parameters of p -resonances. Combined investigations of the space parity non-conservation effect of 235 U fission fragment emission and the interference effects of the forward-backward and left-right asymmetries were carried out over the range of neutron energies from 0.02 eV to 90 eV. The experimental results are found to be in good agreement with the theory. The first estimates of the nuclear matrix element of the weak interaction for three p -resonances and the main parameters of some low energy p -wave resonances are obtained from a combined theoretical description of all experimental data.

Investigation of spin dependence of neutron cross sections and of strength functions for rare earth nuclei in experiments with polarized neutrons and nuclei

Abstract Experiments on the spin dependence of the interaction of resonance neutrons (up to 100 keV) with the rare earth nuclei 141 Pr, 159 Tb, 165 Ho, 167 Er and 169 Tm are reported. The measurements were performed with polarized neutrons and nuclei. The spin dependence of S-wave strength functions was investigated, and the imaginary part of the spin-spin potential ( W ss = 0.10 ± 0.06 MeV) was estimated in optical-model calculations with a potential in the form of a rectangular well. The energy dependence of the difference of strength functions for two J -states shows possible intermediate states in the formation of the compound nucleus. The J -values of about 230 resonances were determined.

Magnetic moments of neutron resonances in rare-earth nuclei

Abstract The magnetic dipole moments μ J of compound states (neutron resonances) in holmium and terbium have been obtained by the neutron resonance energy shift method. This shift is due to the polarization of nuclei at ultra-low temperature in h.f. magnetic fields inside ferromagnetic samples. Shifts of resonances in neutron transmission were measured by the time-of-flight method in a booster mode of operation of the pulsed reactor IBR-30 with the 40 MeV Linac. The μ J results are −1.8±0.7 and 3.9±1.9 n.m. for the 3.93 and 12.7 eV resonances in 165 Ho and −0.2±1.0, 4.3±3.7 and −1.7±4.4 n.m. for the 3.35, 4.99 and 11.1 eV resonances in 159 Tb. The g -factors deduced from these data and those reported earlier were analysed. The mean value is g = 0.34±0.22 . The intrinsic dispersion Δdg = 0.51±0.20 was obtained from the statistical treatment of data. Both g and Δdg values are compared with theoretical predictions. The results point out to the distinct fluctuations of magnetic moments of neutron resonances.

Magnetic moments of compound states in dysprosium

Abstract Magnetic moments of neutron resonances in dysprosium have been measured by determining the neutron transmission through a polarized target.

Measurements of epithermal neutron depolarization using a polarized rare earth target

Abstract We described a facility for the production and analysis of the polarization of epithermal neutrons and show its application to the measurement of neutron depolarization in magnetic materials. The polarization of the neutrons is analyzed using the spin dependent transmission of neutrons at energies of compound nuclear resonances in a polarized rare earth target. The technique is applicable to the characterization of magnetic domains in thick samples or materials having large internal magnetizations. We illustrate the use of the technique in measuring the orientational dependence of depolarization in a single crystal of ferromagnetic holmium.

Nature of parity violation in neutron interaction with lead

AbstractThe effect of parity violation in the interaction of thermal neutrons with lead was discovered in a number of studies. According to the existing theory, this effect is explained by the mixing of compound states characterized by different parities (s-and p-wave resonances). In view of the absence of a p-wave resonance in the region of thermal neutron energies, it is of importance to reveal a level below the neutron binding energy, a so-called negative resonance. The energy dependence of the cross section for radiative neutron capture on lead was measured in the present study, and it is shown that, for the 207Pb isotope, the results of this measurement deviate from the