Ts. Panteleev

Measurements of delayed-neutron yields from thermal-neutron-induced fission of 235U, 233U, 239Pu, and 237Np

The experimental results on delayed-neutron yields from thermal-neutron-induced fission of some actinides in the IBR-2 pulsed reactor are presented. A method of periodic irradiation without displacement of the sample was used. The measurements of delayed-neutron total yields in thermal-neutron-induced fission of 239Pu, 233U, and 237Np and in cold-neutron-induced fission of 235U, 233U, and 239Pu were carried out. All values were obtained with the use of the value of β0 for (nth+235U) as a reference. Precise measurements of decay curves in the time interval 5–350 ms for 235U and 239Pu were performed.

Development of gas-filled position-sensitive detectors of thermal neutrons at the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research

The Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research is one of the leading centers of neutron research in Russia. Within the works on the modernization of detector systems, researchers of the laboratory have designed and fabricated linear and area gas-filled position-sensitive detectors of thermal neutrons and the area monitor detector with a low attenuation of the incoming beam. These devices and their main characteristics have been described.

Collinear cluster tripartition channel in the reaction 235U(nth, f)

Investigation of the 235U(nth, f) reaction using the miniFOBOS double-arm time-of-flight spectrometer of fission fragments confirmed manifestations of the earlier unknown many-body, at least ternary, decay involving almost collinear decay-product escape, which were first observed in the spontaneous fission of 252Cf(sf). The use of variables sensitive to the nuclear charge of fission fragments allowed the reliability of identification of decay events to be increased and new decay modes to be revealed.

A multisectional annular thermal-neutron detector for the study of diffraction on microsamples in axial geometry

A new gaseous annular sectional thermal-neutron detector for the new neutron diffractometer at the IBR-2 reactor has been designed and manufactured at the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research (JINR). The detector is designed to study small-volume samples at high pressure and consists of 16 sections, each one of which is divided into six independent detector elements. We present the main design features of the detector, detecting electronics, data-acquisition and visualization software, and software system for experiment management. The results of the first neutronographic experiments that were carried out with the use of the detector are also presented.

Heating of ultracold neutrons on beryllium surfaces

The temperature dependence of the loss factor for ultracold neutrons owing to heating at thermal energies on the surface of a beryllium sample is studied. The probability of heating ultracold neutrons is anomalously high throughout the entire measured temperature interval, but especially at low temperatures.

Measuring the nuclear charge of fission fragments using a large ionization chamber—part of the double-arm time-of-flight spectrometer

Identification of decay products is a relevant task in studying rare collinear decays of low excited heavy nuclei. A technique for measuring the nuclear charge of decay products detected by a wide-aperture ionization chamber—a part of the double-arm time-of-flight spectrometer—is described. Two versions of nuclear charge calibration using data of the reaction 235U(nth, f) have been developed to determine the charge of the decay products. Testing with simulation data shows that the use of charge parameterization based on the Bohr-Willer empirical equation in the calibration procedure makes it possible to determine the nuclear charge of the fission fragments over a wide energy range. The charges of light ions from He to C, predicted on the basis of this approach, appear to be overestimated by two charge units.

A One-Dimensional Position-Sensitive Detector for Thermal Neutrons

A one-dimensional position-sensitive detector of thermal neutrons has been developed to equip high-efficiency neutron spectrometers at the ИБР-2 research reactor. The detector is based on a multiwire proportional chamber filled with a mixture of 3He + CF4 and has an active area of 200 × 80 mm and a spatial resolution of 2 mm. The detector has been used to good effect in the REFLEX and HRFD spectrometers of the ИБР-2 reactor, as well as in the diffraction facility of the ИВВ-2M research reactor (Zarechnyi, Russia).

Study of delayed neutron decay curves from thermal neutron induced fission of 235U and 239Pu

Abstract The results of measurements of the delayed neutron decay curves obtained from the thermal neutron induced fission of 235 U and 239 Pu are presented. The data were obtained by the periodical irradiation method on the pulsed reactor IBR-2 during the time interval 5 to 730 msec following irradiation. A comparison of these newly measured decay curves with the curves calculated using several standard delayed neutron sets was then performed. Based on these measurements, a new 7-group delayed neutron model is proposed.

The existence of (n, γf) process on235U nucleus and uranium X-ray yields

The measurement of X-rays emitted before the fission of235U induced by resonance neutrons is described. Preliminary results show the dependence of the X-ray yield on fission widthΓf. It is explained on the bases of the existence of the two-step (n,γf) process. The estimate of the width of the (n,γf) process is of the order of 4 MeV and theγ-transitions taking place before fission of236U compound nucleus are mostly of the M1 type.

An attempt to observe experimetally the (n, γ f) process in resonance neutron induced fission of 235U

Abstract The multiplicity of promp γ-radiation in resonance neutron induced fission of 235 U has been measured. The energy of recorded γ-rays ranged from 0.4 to 2.0 MeV. To estimate the contribution of the (n,γ f) process, it was assumed that the multiplicity of prompt γ-rays from fission fragments is independent of the spin of the compound state. At the accuracy achieved, it was established that the difference of the average widths of the (n, γ f) process for fission through the compound states with spins Jπ = 3 − and 4 − does not exceed ≈ 3 meV.