Yu.D. Mareev

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.

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.

Analysis of element and isotope composition of samples by neutron spectroscopy at the IREN facility

One of the applications of neutron spectroscopy for applied purposes is a nondestructive determination of the isotopic composition of samples under study. The method is based on the registration of neutron resonances and the measurement of the yield of reaction products in the resonances. The resonance energies are known practically for all stable nuclei and the set of energies does not coincide completely for any pair of isotopes. It allows determining the isotope composition. These measurements have been performed for a number of targets at the IREN Facility.

Investigation of parity violation and interference effects in the angular distributions of fragments originating from 233U fission induced by resonance neutrons

Three interference asymmetry effects in the angular distributions of fragments originating from 233U fission induced by resonance neutrons were measured. The energy dependences of the asymmetry factors being studied show sizable irregularities that are associated, according to modern theory, with the interference of s and p resonances at the stage of a compound nucleus. The basic features of weak p-wave resonances in the low-energy region were obtained from a global theoretical analysis of the asymmetry factors as functions of energy. The first estimates of nuclear matrix elements of weak interaction were derived for a few p-wave resonances.

Investigations of the Space Parity Violation and Interference Effects in the Fragment Angular Distributions of 235U, 233U, and 239Pu Fission by Resonance Neutrons

Investigations of the space parity nonconserving (PNC) asymmetry of 233U, 235U, and 239Pu fission fragment emission and parity conserving (PC) interference effects of left‐right and forward‐backward asymmetries were carried out on the neutron beams of the reactor IBR‐30 (JINR, Dubna) over the range of neutron energies from 0.02 eV to about 100 eV. All experimental results obtained have been found to be in a good mutual accordance within the frames of modern theoretical conceptions about the mechanisms of PNC and PC effects forming in fission process induced by slow neutrons. In case of the P‐even interference effects of asymmetry the evident mutual well‐marked irregularities in their neutron energy dependencies up to about 100 eV were observed. It is connected with the interference of s, p‐resonances at fission compound stage according to modern theory. As a remarkable result of the PNC effect measurements the resonance behavior of the PNC asymmetry coefficients in the low neutron energy region (En < 2 eV...

A spectrometer of polarized neutrons

A polarized neutron spectrometer intended for studies of interactions of nuclei and condensed matter with polarized neutrons with energies in the range from thermal energies to several electron volts was designed at the IBR-2 pulsed reactor (JINR, Dubna). Diffraction on magnetized Co(92%)–Fe(8%) single crystals was used to polarize neutrons and analyze the polarization. The attained neutron polarization was ∼95% within the entire energy range.