We review the experimental results on the P -even and P -odd angular correlations of fission fragments in the fission of the 235 U and 239 Pu nuclei induced by unpolarized and polarized resonance neutrons, and on the TRI and ROT effects in the ternary and binary fission of actinides induced by polarized thermal neutrons. Also reported are the measured yields of prompt and delayed neutrons per fission event. The experimental data are analyzed within a novel theoretical framework developed by the JINR—RNC KI Collaboration, whereby the reduction of the multidimensional phase space of fission fragments to the JπK -channel space is consistently validated and the role of resonance interference in the observed correlation effects is revealed.

The nuclear-physical characteristics obtained for a gallium target in a series of experiments using the time-of-flight method, the method of threshold indicators and a spectrometer based on a 3He proportional counter on the IREN setup (JINR, Dubna) are presented. The absolute values of the energy spectra of leakage neutrons in the energy range from 10 keV to 15 MeV were measured. The activation method was used to measure the neutron flux density on the surface of a gallium target: (2.882 ± 0.098) · 108 sec–1 · cm–2. The integral yield of neutrons obtained from the gallium target on the basis of these data was 1.54 · 1014 sec–1 with average current 4 mA. The time dependence of the activation level of the gallium target was measured. It was shown as a result of the analysis that the background level is reached on the surface of the irradiated gallium target in 5 days.

In connection with commissioning of the IREN pulsed resonance neutron source new electronics and appropriate software are developed for registration of time-of-flight spectra with small width of the channel (21 ns). The hardware-software system is intended for research of the IREN neutron beam characteristics, properties of new detectors, and also for performance of precision experiments under conditions of low intensity or registration of rare events.

The results of measurements of the fission cross-sections and resonance parameters σ0Γf and Γf of 234U and 237Np together with the preliminary results of 243Am fission cross-section measurements carried out in the framework of the program of studying subthreshold fission at the IBR-30 pulsed neutron source in Dubna are presented. The measurements were conducted by the time-of-flight technique. Fission events in 234U and 237Np were detected by two multisectional ionisation chambers containing 234U (about 100 mg) and 237Np (about 1.5 g) highly purified from isotopes with a large fission cross-section, such as 235U and 239Pu. Each chamber also contains a 235U target for neutron flux measurements. 243Am cross-section measurements are conducted with a back-to-back ionisation chamber containing about 2 mg of 243Am and 1 mg of 235U. The fission width is calculated in the region where the TOF resolution is sufficient to resolve most resonances at the energy up to 200 eV for 234U and below 50 eV for 237Np. The cross-sections in the energy region from 1 to 1000 eV are shown. The new upper limit for the thermal neutron induced fission of 234U is reported.

Construction and properties of a double ionization chamber (DIC) are described. Heavy ion fusion-evaporation reaction products obtained at the focal plane of the kinematic separator VASSILISSA enter the DIC volume through a thin plastic window. After their thermalization in the gas, they drift in the electric field to the DIC cathode. Their α-decay and/or spontaneous fission are detected in 4π-geometry in two ionization chambers, divided by the common cathode. Using DIC the α-decay of Bi and Pb isotopes produced in the fusion reactions 40Ar + 159Tb and 40Ca + 153Eu were detected. A new isomer, 189mBi was observed.

A detector designed to register γ-quanta emitted in spontaneous fission of nuclei is described. It was used together with a neutron detector with 3He counters designed to register rare events of spontaneous fission. An additional γ-ray detector used with the neutron detector permitted the increase of information on every event registered. It allowed a decrease of the background when detecting spontaneous fission rare events. For a detection efficiency for spontaneous fission ϵ = 0.8 (for 252Cf) the detector background was about one event, in which one neutron and not less than three γ-quanta were registered per day. Energy spectra and distributions over multiplicity of γ-quanta of spontaneous fission in 252Cf are presented.

There are two types of rare nuclear transformations occurring in nature, which are accompanied by multiple neutron emission: spontaneous fission of uranium nuclei and hypothetical super-heavy elements, and spontaneous nuclear decay induced by nucleon decay or ñN annihilation in the process of neutron oscillation. The design and characteristics of a detector for registration of multiple neutron events are discussed.

In the previous study of some stony meteorites and geothermal waters the multiple emission of neutrons was observed which might have arisen from spontaneous fission of superheavy elements (SHE). In this connection it is of interest to search for terrestrial samples in which an increase of concentration of the sought nuclide might have occurred in comparison with meteorites, which are the least differentiated substance in the solar system. The search can be carried out by means of a setup of weight 1 t. The detection of multiple emission of neutrons from samples of this weight permits observation of an admixture of SHE with a sensitivity 10/sup -15/ g/g with a length of measurement 20 days. The development and operation of this setup will permit analysis of an additional approach to the experimental problem of nuclear decays due to nonconservation of baryon number (the decay of the nucleon and the annihilation n-italic-tildeN resulting from the oscillation of a neutron bound in the nucleus).

The parameters are assessed of a detector designed to record nucleon decay and/or the oscillations of a neutron bound in the atomic nucleus by detecting multiple neutron emission events in massive samples. Calculations have been carried out for the detection efficiency and for the spatial distribution of the points at which the single neutrons emitted from a point source had to be detected, for different variants of the detector geometry. To evaluate the background characteristics of the detector, calculations have been performed for the multiplicities and spatial distributions of the points of formation of secondary neutrons occurring in the interactions of 0.3–1 GeV nucleons with infinite lead units. It is shown that the comparison of the spatial distribution of the points of neutron detection for nucleon decay or the neutron oscillation phenomena and the background processes allows one to suppress the cosmic-ray background of the detector by a factor of more than 10.