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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.

The scientific interest in the resonance neutron induced capture and fission reactions on 239 Pu is continuously rising during the last decade. From a practical point of view, this is because more precise data on capture and fission cross sections, fission fragment mass and kinetic energy distributions, variation of prompt fission neutron and gamma yields in the resonance neutron region, are needed for the modelling of new generation nuclear power plants and for nuclear spent fuel and waste transmutation. From a heuristic and fundamental point of view, such a research improves our knowledge and understanding of the fission phenomena itself. To achieve these goals more powerful neutron sources and more precise fission product detectors have to be used. At the Joint Institute for Nuclear Research (JINR) Frank Laboratory of Neutron Physics (FLNP), where already half a century the thermal and resonance neutron induced nuclear reactions are studied, a new electron accelerator driven white spectrum pulsed neutron source IREN has been built and successfully tested. The improved characteristics of this facility, in comparison with those of the former pulse neutron fast reactor IBR-30, will allow measuring some of the neutron-nuclear reaction data with better precision and accuracy. A new experimental setup for detecting gamma rays (and neutrons) has been designed and is under construction. It will consist of 2 rings (arrays) of 12 NaI(Tl) detectors each (or 1 array of 24 detectors) with variable ring diameter and distance between both rings. Such a setup will make possible not only to measure the multiplicity, energy and angular anisotropy of prompt fission gammas, but also to separate the contribution of prompt fission neutrons by their longer time-of-flight from the fissile target to the detectors. The signals from all the 24 detectors will be recorded simultaneously in digitized form and will be stored on the hard disk of the personal computer for further off-line analysis. The measurement of the prompt gamma-ray emission from 239 Pu resonance neutron induced fission is one of the most probable candidates for the first experiments to be performed at IREN using the newly designed gamma-ray detector.

In JINR at the IBR-2 pulsed reactor [Ananiev, Blokhincev, Bulkin, et al., IET(ras.) 5 (1977) 17.] the “Kolkhida” setup intended for studies of neutron optics phenomena in interactions of polarized neutrons with polarized nuclei has been constructed. In particular, studies of nuclear precession of neutron spin in a wide energy range from thermal to neutron resonance energies are planned. The setup also makes it possible to investigate magnetic properties using polarized neutrons. In the given paper we present the description of the setup, its key parameters, as well as the result of computer simulation of the experiment on neutron paramagnetic resonance shift.

The yield of delayed neutrons, vd, from thermal-neutron-induced fission of 245Cm is measured. Experiments aimed at studying the properties of delayed neutrons from the fission of some reactor isotopes and initiated in 1997 were continued at the upgraded Isomer-M facility by a method according to which a periodic irradiation of a sample with a pulsed neutron beam from the IBR-2 reactor was accompanied by recording emitted neutrons in the intervals between the pulses. The accuracy of the resulting total delayed-neutron yield vd = (0.64 ± 0.02)% is two times higher than that in previous measurements. This work was performed at the Frank Laboratory of Neutron Physics at the Joint Institute for Nuclear Research (JINR, Dubna).

The delayed-neutron yield from thermal-neutron-induced fission of the 237Np nucleus was measured using a sample periodically exposed to a pulsed neutron beam with subsequent detection of neutrons during the time intervals between pulses. The experiment was realized on an Isomer-M setup mounted in the IBR-2 pulsed reactor channel equipped with a mirror neutron guide. The setup and the experimental procedure are described, the background sources are thoroughly analyzed, and the experimental data are presented. The total delayed-neutron yield from 237Np fission induced by thermal neutrons is ν d = 0.0110 ± 0.0009. This study was performed at the Frank Laboratory of Neutron Physics (JINR, Dubna).

The 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 \({1 \mathord{\left/ {\vphantom {1 {\sqrt E }}} \right. \kern-\nulldelimiterspace} {\sqrt E }}\) law, thereby suggesting the presence of a strong negative resonance. The parameters of this resonance are estimated.

The low-energy-gamma-ray spectra from neutron resonance capture with natural samples of Sb, Tb, Ho, and Ta were measured using the HPGe detector at the IBR-30 pulsed reactor at the JINR, Dubna. The resonance-spin-memory effect in the spectra from the odd-odd compound nuclei of 122Sb, 169Tb, and 166Ho was found to be quite distinct. For the 182Ta compound nucleus, it proved to be rather weak.

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.

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.

Some precision experiments require several tens of hours for measurements. In this case, it is important to take into account possible drift of the detection system parameters in time. An express analysis program has been developed for early detection of significant distortions in recorded spectra, their filtration, and correction of results.

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.

We report measurements of the depolarization of epithermal neutrons (1.7–59 eV) in magnetic domains of a 2.0-cm-diam cylindrical single crystal of ferromagnetic holmium. The neutrons were polarized by a dynamically polarized proton target and polarization analyzed using a statically polarized rare-earth spin filter. Based on the dependence of the depolarization on neutron energy and crystal orientation, we determined the domains to be laminar, or needlelike, with the long axis slightly deflected from the c crystalline axis and having an average width of 59 μm.

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.

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.

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) was observed. Unfortunately, the statistical accuracy of the PNC effect measurements is not enough for observation of these resonance effects in other cases of more high energies. Results of simultaneous analysis of all three asymmetry effects for all three nuclei are presented. The satisfactory combined description of the experimental points is received. As a result of theoretical evaluation of these data main parameters and the estimates of nuclear matrix elements of the weak interaction for some p‐resonances in the low energy range were extracted.

The average size and shape of magnetic domains in a material can be determined from the precession of polarized neutrons traversing the material. Epithermal neutrons (0.5 eV

Fission gamma-ray multiplicity measurements in low energy 233U, 235U, 237Np and 239Pu fission resonances were made by using the Dubna pulsed reactor IBR-30 as a resonance neutron source. A large 6-section liquid scintillation detector and fission chambers with a combination of (233U-235U), (237Np-235U) and (239Pu-235U) targets were used for detecting 3 or more fission gamma-quanta in coincidence with a fission fragment pulse. The fission gamma-ray multiplicity data from these measurements are discussed relative to the prompt fission neutron multiplicity data available for low energy 233U, were measured over the neutron energy interval around 40 eV. Correlation between the fission gamma-ray multiplicities and the fission widths Гf of measured 237Np resonances was observed, which might be explained by the (n,γf) reaction.

The pulsed booster IBR-30 (JINR, Dubna) is used as a neutrons source for the experiments on the measurement of multiplicities of γ-rays following the radiative neutron capture in 147Sm in the energy range from 15 eV to 900 eV. The experiments were performed by the time-of-flight method using the multisectional 4π -detector of γ-rays, which allows one to obtain multiplicity spectra in individual resonances. The correlation was investigated between the multiplicity spectrum and spin of the resonance. Spins were assigned to ∼90 resonances and radiative widths of about 30 resonances were measured.

The polarization left‐right asymmetry and angular fore‐aft asymmetry of γ yields were investigated for the direct transition Eγ=9.04 MeV in 114Cd near the p‐wave resonance Ep=7 eV. The experiments were performed at the pulsed reactor IBR‐30 using an LMN‐polarizer of neutrons and NaJ (T1)‐detectors.

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.

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.

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.

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.

The analyzing power and the fore-aft asymmetry were studied in the 117Sn(n,γ) reaction near the 1.33 eV p-wave resonance, where the parity violated. The experiments were performed at the IBR-30 pulsed reactor with the NaI(T1) spectrometer. The neutron width amplitudes and were determined

Transmission spectra were measured with the time-of-flight technique at the Dubna pulsed reactor for samples of metallic U, UO 2 , U 3 O 8 , and UO 3 enriched in 235 U, and were compared in the regions of nine low-energy resonances to observe chemically induced shifts. After elimination of contributions caused by different Doppler broadenings, the shifts are interpreted as changes of the mean-square charge radius for nuclei capturing neutrons. The 〈 r 2 〉 of the compound-nucleus states show on average a weak diminution as compared with the ground-state value.

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.

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

The 6Li(n,n) cross section value and its dependence on neutron energy in the 0.6–80 keV region are measured at the IBR-30 pulsed reactor by the time-of-flight method. A constant value of 0.72±0.02 barn was found for the scattering cross section below 10 keV. The energy dependence of the 6Li(n,n) cross section together with other data were analysed in the frame of R-matrix formalism to clarify the properties of levels of the 7Li nucleus.

The results on parity-nonconserving (PNC) effects in p-wave neutron resonances are reported. The experiments were carried out at the IBR-30 pulsed reactor with the neutron beam polarized by trans-mission through a polarized proton target. The dependence of total cross section on longitudinal polarization vi/as found for the resonances at 0.88 eV 81Br, 5.53 eV 111Cd, 1.33 eV 117Sn and 0.75 eV 139La. The PNC effect has the resonance shape. These results were analysed together with thermal neutron data in the frame of the model of compound states mixed due to weak nucleon interaction.

Transmission spectra were measured with a time-of-flight spectrometer at the Dubna pulsed reactor for several chemical compounds of uranium. The spectra were compared in the regions of low-energy resonances to observe chemically induced shifts which are interpreted as an effect of mean-square charge radius changes of the nuclei by neutron capture.