The article presents the main trends in the development of nuclear physics research conducted at the Kurchatov Institute after commissioning of the first pool-type water-water research reactor in 1957 in the Soviet Union.

Определение поглощенной в биологическом образце дозы при смешанном гамма-нейтронном облучении

A method is proposed and results on determination of absorbed doses under mixed gamma–neutron irradiation of biological samples on a horizontal channel of the IR-8 research reactor at the National Research Center Kurchatov Institute are presented.

Experimental data have been presented for the spatial deflection and energy profiling of reactor neutron beams using flexible polyvinylchloride tubes, the inner surfaces of which are covered by a liquid fluoropolymer thin film.

В работах [1-2] было обнаружено исключительно высокое пропускание ультрахолодных нейтронов (УХН) полихлорвиниловыми трубками промышленного производства.Полихлорвинил (ПВХ) представляет собой высокомолекулярный полимер (СН 2 -CHСl) n с высоким молекулярным весом молекул в пределах от 30 000 до 100 000 amu. Граничная скорость полихлорвинила V lim = 2.9 m/s, поэтому нейтроны, падающие на гладкую поверхность с нормальной компонентой скорости V n < V lim , зеркально отражаются.Величину V lim можно увеличить, если на поверхность полихлорвинила нанести тонкий (∼ 10 µm) слой вещества, обладающего более высокой граничной скоростью, но сохраняющего зеркальность поверхности.В качестве такого покрытия

The properties of a convergent neutron lens in the form of an assembly of bent glass capillaries have been experimentally investigated. Each capillary is a conical tube with a length of 22.5 cm, an entrance diameter of 0.25 mm, and an exit diameter of 0.17 mm. The spectrum of thermal neutrons incident on the lens entry is formed on the beam of the IR-8 reactor by means of a diamond microcrystalline filter and investigated with a time-of-flight spectrometer. The spatial distribution of the neutron beam at the exit from the lens is measured using the position-sensitive image plate detector. The measured focal length of the lens is 75–80 mm. The possibility of focusing neutrons with an increase in the local beam density at the lens axis by a factor of 4.9 is demonstrated.

Показана возможность транспортировки, отклонения и оптимизации пучков тепловых нейтронов при помощи гибких полихлорвиниловых трубок с внутренним фторполимерным покрытием. Приведены результаты экспериментальных исследований по транспортировке тепловых нейтронов с помощью прямых и изогнутых трубок.

The possibility of transporting, deflecting, and optimizing thermal neutron beams using flexible polyvinyl chloride tubes with internal fluorine polymer coating is demonstrated. Experimental results on transportation of thermal neutrons using straight and bent tubes are presented.

Experimental results on the transmission of ultracold and cold neutrons through polyvinyl chloride tubes are presented. The dependence of the neutron transmission coefficient on the length and curvature of the tubes is studied. The possibilities of used tubes to create mobile and miniature sources of neutron and γ radiation as well as neutron-capture therapy are discussed.

Corrections have been introduced into the result τβ = 885.4 ± 0.9stat ± 0.4syst s of our measurements of the neutron lifetime. The corrected value is τβ = 881.6 ± 0.8stat ± 1.9syst s.

The transmission of ultracold neutrons (UCNs) through flexible polyvinyl chloride (PVC) tubes with lengths of up to 3 m and an internal diameter of 6–8 mm has been studied. High UCN transmission is found even for arbitrarily bent tubes (single bend, double bend, triple bend, figure eight, etc.). The transmission can be improved significantly by coating the inner surface of the tube with a thin layer of liquid fluorine polymer. The prospects of these neutron guides in fundamental and applied research are discussed.

The status of neutron-capture therapy of malignant tumors and its problems – damage to healthy tissue as a result of neutron transport to the irradiation location and presence in the therapeutic beam of a background consisting of γ rays and fast neutrons – are presented. To solve these problems, the authors have proposed using ultracold neutrons with energy less than 10 –7 eV, whose unique capability is to undergo total reflection from the surface of a condensed substance at any angle of incidence. Numerous works have demonstrated that such neutrons can be transported along neutron guides. The cross section for inelastic scattering of neutrons by hydrogen-containing substances – water, ethyl alcohol, and biological tissue – has been measured in an IR-8 beam of ultracold and very cold neutrons. At temperature 200–300 K, the experimental data are in very good agreement with calculations, but as temperature decreases further a discrepancy appears, which could be due to the inaccuracy of the model spectra of the oscillations hydrogen-containing substances used in the calculations. The use of ultracold neutrons opens up new possibilities of neutron-capture therapy for treating malignant tumors localized in body cavities or organs.

The results obtained by measuring the cross sections for the inelastic scattering of very cold neutrons for a number of metals and polymers by the method of a neutron-irradiation analysis are presented. The method is based on simultaneously measuring events of inelastic scattering and neutron capture in the sample under investigation via recording gamma radiation with a semiconductor germanium detector. Neutron capture by a nucleus of the sample is accompanied by the prompt radiation of gamma rays having a known spectrum. Upon inelastic scattering, a neutron acquires thermal energy. Upon leaving the sample, this neutron is absorbed in a special converter that contains the isotope 10 B. The capture of the neutron by a 10 B nucleus is followed by the emission of a 477-keV gamma ray. The probabilities of capture and inelastic scattering are proportional to the respective neutron-interaction cross sections, and the ratio of the recorded detector counts corresponding to events of the two types does not depend on the spectrum of the incident flux of very cold neutrons or on the trajectory of neutron motion in the sample. The sought inelastic-scattering cross section at a fixed sample temperature is calculated by using this ratio and the known cross section for neutron capture by the sample isotope having a known gamma-radiation spectrum.

Results are presented that were obtained from experiments devoted to storing very cold neutrons in vessels whose walls are made from structures involving a spatial inhomogeneity of the average Fermi potential. The possibility of storing neutrons owing to diffusion reflection from the walls is shown, and prospects of elaborating the method are discussed.

The possibility of attaining the calculated probabilities of the losses of ultracold neutrons (UCN) stored in vessels whose walls are made from graphite, fluorine polymer oil, or heavy-water ice is tested experimentally. It is found that UCN hitting the walls of a graphite vessel undergo additional inelastic scattering not predicted by the theory. It is shown that this scattering may be due to the presence of surface hydrogen that provides a channel of UCN leakage slightly varying with temperature. For vessels whose walls are coated with fluorine polymer oil, additional inelastic UCN scattering is also observed and is found to be efficiently suppressed with decreasing temperature. The experimentally observed and calculated values of the probabilities of UCN losses are shown to be in good agreement for vessels whose walls are made from heavy-water ice.

Neutron-radiation analysis is used to investigate the subbarrier reflection of ultracold neutrons from the surface of a titanium-stabilized Fe-Ni-Cr stainless steel. A significant selective increase in the probability of neutron capture by nuclei of the medium in comparison to theory is discovered. An explanation is given for the effect, which is associated with the existence of titanium-containing clusters and structural defects that distort the form of the distribution of the effective interaction potential between ultracold neutrons and the material surface.

The subbarrier reflection of ultracold neutrons (UCNs) from stainless steel (an alloy of iron, nickel, chromium, and titanium) is investigated by means of neutron-radiation analysis. It is found that the increase in the probability of capture of UCNs by nuclei is large compared to the standard theory. The effect is selective, the enhancement factor varying From 3 for iron to 90 for titanium.

The results of measurements of the correlation coefficient A characterizing the correlation of neutron spin and electron momentum in the decay of polarized neutrons are described in detail. The work was carried out during the years 1969--1976. As a result of two series of measurements a value of the correlation coefficient A=-0.114 +- 0.005 was obtained, which implies a ratio ..gamma..=G/sub A//G/sub V/=-1.261 +- 0.012.