Energy losses of fission fragments and the density effect in gases with different collision frequencies

Conclusions Our calculation of the trajectories of the fission fragments in the experiment of [4] showed that the density effect becomes much weaker when the nonlinear relation between the ion charge and the velocity of the fragments during the stopping process is taken into account. The calculation was based on measurements of the energy losses of fission fragments in air, which were performed in the present work, and modern data on energy losses in argon. The error of the experiment described in the present work is close to 10%. It can be asserted to the same degree of accuracy that the density effect does not influence the energy losses in the experimental pressure range from 300 to 8000 Pa. This work was supported by the Russian Fund for Fundamental Research, project No. 96-02-17443a. Main Science Center of the Russian Federation—A. I. Leipunskii Physics and Power Engineering Institute. Translated from Atomnaya énergiya, Vol. 83. No. 1, pp. 7–12, July, 1997.     

Determination of the ionization and charge-transfer cross sections in interactions of fission fragments with helium and argon atoms

Conclusions Experimental data on the differential cross section of electron emission from helium and argon atoms with respect to the electron energy during bombardment by fission fragments were obtained under conditions of low data acquisition rate and high background level due to concomitant particles. The results were compared with a theoretical calculation of the ionization and charge-transfer cross sections of helium and argon atoms by the method of classical trajectories in the collision studied and attest to a good agreement between experiment and the proposed theoretical model. This work was supported by the Russian Foundation for Fundamental Research under projection No. 96-02-17443a. Joint Institute of Nuclear and Thermal Power Production, V. A. Rykov, Main Science Center of the Russian Federation — A. I. Leipunskii Physics and Power Engineering Institute. Translated from Atomnaya énergiya, Vol. 84, No. 1, pp. 29–34, January, 1998.     

Differential ionization cross-sections for nitrogen and carbon dioxide molecules bombarded by fission fragments

Conclusions Previous experimental data on electron-emission cross-sections, doubly differentiated with respect to emission angle and electron energy, which were obtained from bombarding molecules of nitrogen and carbon dioxide gas with fission fragments, were used to calculate iodization cross-sections, differentiated only with respect to angle and only with respect to electron energy. The results were compared with theoretical [2–4] calculations and showed a large (factor of ∼10) difference between calculation and experiment. The calculated results depend strongly on the model equations for the electron velocity in the atom. The authors express their thanks to L. V. Egorovaya and A. I. Chusov for help in doing the work, and also to A. P. Budkin and Yu. V. Sokolov for discussions of the results. This work was supported by the Russian Fund for Fundamental Research, Project No. 96-02-17443a. Russian Federation State Science Center, A. I. Leipunskii Institute of Physics and Power Engineering, Obninsk. Translated from Atomnaya énergiya, Vol. 83, No. 4, pp. 273–276, October, 1997     

Measurement of the double-differential cross section for electronic emission accompanying the interaction of fission fragments with nitrogen and carbon dioxide molecules

Conclusions Experimental data on the double-differential (with respect to angle of emergence and energy of the electrons) cross section for electron emission from nitrogen and carbon dioxide molecules bombarded with fission fragments were obtained under conditions of low data acquisition rate and high background level due to accompanying particles. Russian Science Center of the Russian Federation — A. I. Leipunskii Physics and Power Engineering Institute. Translated from Atomnaya énergiya, Vol. 82, No. 5, pp. 365–369, May, 1997.     

Secondary electron emission coefficient for fission fragments with a fixed mass and kinetic energy

Conclusions The experimentally obtained secondary emission coefficient of electrons from thin aluminum oxide films is described within the framework of effective-charge models and a velocity-dependent expression for fission-fragment energy losses. At the same time, it was shown that secondary emission is related to energy lost to electrons, and not to the ionic charge of the fragments. This semiempirical formula can be used to calculate energy losses from heavy ions in heavy-element materials. In our opinion, the work of Brunelle et al. [3] cannot serve as a basis for identifying ion charges that penetrate both surfaces of a target. Russian Federation State Science Center, A. I. Leipunskii Institute of Physics and Power Engineering, Obninsk. Translated from Atomnaya énergiya, Vol. 83, No. 4, pp. 266–273, October, 1997.     

Distribution of specific ionization along a track as a function of the initial energy of U235 fission fragments

By means of a telescope consisting of 11 pulse ionization chambers and a two-dimensional pulse height analyzer, measurements were made of the distribution of specific ionization energy loss along a track as a function of the initial energy of fission fragments. Studies were made of U²³⁵ fission fragments produced by thermal neutrons in the initial energy ranges 78–115.5 MeV (light fragments) and 34–88 MeV (heavy fragments). The data obtained made it possible to discover the dependence of specific ionization on fission fragment velocity. Those relationsMps indicated some differences in the nature of ionization energy losses for light and heavy fragments.Translated from Atomnaya Énergiya, Vol. 19, No. 3, pp. 244–250, September, 1965     

Nuclear safety in handling radioactive wastes containing fissile materials

Nuclear safety in disposal and reprocessing of solid and liquid radioactive wastes produced during the production of nuclear reactor fuel elements was investigated. The results of this work made it possible to determine the parameters which must be limited in handling wastes in order to avoid the occurrence of a self-maintaining fission chain reaction. The numerical values of these parameters are obtained for the new and conventional technology for handling wastes. 3 tables, 5 references. State Science Center of the Russian Federation-A. I. Leipunskii Physics and Power-Engineering, Institute. Translated from Atomnaya énergiya, Vol. 88, No. 5, pp. 362–370, May, 2000.     

<Superscript>243</Superscript>Am neutron fission cross section and resonance parameters

The SVZ-100 lead moderation time neutron spectrometer at the Institute of Nuclear Physics of the Russian Academy of Sciences was used to measure the fission cross section for ²⁴³Am in the neutron energy range E_n = 0.3 eV – 10 keV. The resonance fission integrals and the area and fission width of the resolved resonances were calculated. The properties of the intermediate-structure resonances were evaluated. The results were compared with existing data and recommended evaluations.     

The Radium Institute and the Russian Academy of Sciences

The contribution of the Radium Institute to the establishment and development of Russian atomic science and technology is examined in a historical review based on the scientific and organizational, relations with the Russian Academy of Sciences. The basic scientific achievements obtained at the Institute when it was part of the Academy system are noted: creation of the radium industry in Russia, discovery of the spontaneous fission of uranium, construction and startup of the first cyclotron in Europe, determination of the laws govering the distribution of trace elements in solution (gas)-solid phase systems development and adoption of the first Soviet technology for obtaining weapons plutonium. The integrated nature of the research, characteristic for the Institute from the moment the Institute wa formed to the present is shown. Scientific schools in nuclear physics, radiochemistry, and radiogeochemistry were established at the Institute, and are still operating, in connection with the work of outstanding Russian scientists. Science and Industrial Association “V. G. Khlopin Radium Institute”. Translated from Atomnaya énergiya, Vol. 86, No. 4, 322–327, April, 1999.     

Simultaneous statistical analysis of large arrays of correlated neutron measurements

The problems due primarily to the limitations of modern computers, preventing adoption of modern methods for estimating neutron data in everyday practice and making it difficult to use the estimated data in applied programs, are discussed. Possible solutions of these problems are presented. Specifically, an identity transformation of the statistical functional to be minimized is found: this transformation makes it possible to avoid the construction, storage, and numerical inversion of superlarge covariation matrices of measurement uncertainties. The transformation makes it possible to perform statistical analysis of an arbitrarily large number of measurements. An efficient method of parameterization of the cross sections of neutron reactions at high neutron energy is proposed. The method is based on the use of the Adler-Adler resonance formula for fitting the cross sections. The parameters in the formula in this case do not have a clear physical interpretation. The method makes it possible to decrease substantially the volume of information to be stored in estimated data files in ENDF-6 format. For three cross sections (total, fission, and radiative capture), the proposed parameterization is universal, and it makes it possible to represent the cross section in a single manner from the thermal point up to an arbitrarily high neutron energy. The results of this work are illustrated for estimation of the cross section of the reaction²⁴¹Am (n,f). 1 figure, 1 table, 27 references. State Science Center of the Russian Federation—A. I. Leipunskii Physics and Power-Engineering Institute. Translated from Atomnaya énergiya, Vol. 88, No. 3, pp. 197–203, March, 2000.     

Measurement of 235U Fission Fragment Range in Thin Lavsan Films

The main objective of this work is to study experimentally the range and number distribution of ²³⁵U fission fragments over the thickness of thin lavsan films. A BR-10 reactor was used to irradiate a stack of lavsan films and the first experimental determination of the differential distribution of the number of ²³⁵U fission fragments over the film thickness is made. The results obtained are compared with the average values computed with the SRIM-2000 computer code. It is shown that the SRIM-2000 program makes it possible to describe satisfactorily the experimental data on the interaction of fission fragments with polymers. The data obtained are used as a basis to determine the optimal thickness of a polymer film for producing track membranes by the reactor method.     

Nuclear fission in heavy elements by high-energy particles

The results of a radiochemical investigation of nuclear fission in uranium, thorium, and bismuth by protons with an energy of 680 Mev are presented. Using an interpolation method a complete chart of the fission residue products is obtained. It is noted that there is a predominance in the production of nuclei with excess neutrons (58–64%); it is also shown that isotopes with maximum yield lie mainly in the neutron-ex cess region. The probability for symmetric fission is largest in bismuth. The cross sections for fission in uranium and thorium are 55–60% of the geometric cross section; in bismuth it is 5%. The charge distribution of fragments in fission induced by high energy protons is constant and independent of the mass number of the fission fragments and the atomic number of the fissioning nucleus. An analysis of the main features of the fission process seems to indicate that fission in uranium and thorium is due to a combined barrier-emission mechanism.     

Statistical Estimation of Distributions of Physical Quantities in Nuclear Fission

Making use of a model based on the statistical theory, calculations were performed to obtain the mass yields, the most probable charges, the kinetic energies and the prompt neutron yields of fission fragments, and the mass yields of fission products from thermal-neutron-induced fission of ²³³U, ²³⁵U, ²³⁹Pu, ²⁴¹Pu and from the spontaneous fission of ²⁵²Cf. The calculations are further extended to fast-neutron-induced fission. The scission-point distance is treated as a parameter varying with the mass number of the heavy fragments of fission. This proved successful in approaching the calculated curves closer to the observed values.

It is possible to predict unknown physical quantities in nuclear fission with use made of the method developed in the present work.     

适用于研究低能裂变碎片电荷分布的探测器装置

为研究²⁵²Cf自发裂变碎片电荷分布，建立了由屏栅电离室和ΔE-E粒子望远镜构成的探测器系统。在该系统中，将薄的屏栅电离室作为碎片的ΔE探测器，E探测器是金硅面垒半导体探测器。通过分析实验测量的4参数关联数据，得到了²⁵²Cf自发裂变碎片质量数、动能及碎片在气体ΔE探测器中的能量沉积分布等物理量。用多高斯（multi-Gaussian）分布函数对ΔE探测器的能量响应函数进行最小二乘法拟合，得到了在固定质量数A^*_L和动能条件下轻碎片的电荷分布。结果表明：该探测器系统的电荷分辨能力Z/ΔZ约为40∶1；建立起来的测量技术可用于测定²³⁵U(n,f)和²³⁹Pu(n,f)反应碎片的电荷分布。     

Modeling of fission gas swelling in LWR UO2 fuel under irradiation

An understanding of the behavior of fission gas in uranium dioxide (UO₂) fuel is necessary for the prediction of the performance of fuel rods under irradiation. A mechanistic model for matrix swelling by the fission gas in LWR UO₂ fuel is presented. The model takes into account intragranular and intergranular fission gas bubbles behavior as a function of irradiation time, temperature, fission rate and burn-up. The intragranular bubbles are assumed to be nucleated along the track of fission fragments, which play the dual role of creator and destroyer of intragranular bubbles. The intergranular bubble nuclei is produced until such time that a gas atom is more likely to be captured by an existing nucleus than to meet another gas atom and form a new nucleus. The capability of this model was validated by a comparison with the measured data of fission gas behavior such as intragranular bubble size, bubble density and total fuel swelling. It was found that the calculated intragranular bubble size and density are in reasonable agreement with the measured results in a broad range of average fuel burn-ups 6–83 GW d/tU. Especially, the model correctly predicts the fuel swelling up to a burn-up of about 70 GW d/tU.     

基于GEM工艺的裂变时间投影室性能模拟研究

基于GEM工艺的裂变时间投影室具有探测效率高和空间分辨率高的特点,可实现裂变产物的多参量测量。本文主要研究基于GEM工艺的裂变时间投影室在不同条件下的测量精度,使用Garfield++软件计算得到裂变时间投影室中不同的裂变产物质量数测量误差约为4～6 u,并通过时间信息的径迹重建研究了裂变碎片在不同工作气体中的角度分辨。研究发现,电子漂移时间长的工作气体中,裂变产物具有更好的角度分辨,并可依此在实验中选择合适的工作气体、气压和漂移电场强度来进行裂变碎片的测量。     

Concentration dependence of the separation factor of hydrogen isotopes in ternary and pseudoternary systems H2-H-X-Y-H

The concentration dependence of the separation factor for hydrogen isotopes in ternary and pseudoternary systems H₂ (gas)-H-X-Y-H, where X and I are different interstices in the metal matrix or intermetallic compound or different molecular fragments, is studied. Formulas are obtained for the limiting separation factors, and new features of the concentration dependence as compared with binary systems are noted. It is shown that the separation factors of hydrogen isotopes in complex systems can be calculated using the corresponding thermodynamic constants of binary systems. 1 figure, 1 table, 4 references. D. I. Mendeleev Russian Chemical Engineering University. Translated from Atomnaya énergiya, Vol. 87, No. 1, pp. 62–67, July 1999.     

Effects due to multiple scattering of fission fragments in layers of fissioning material

Computer simulation with the SRIM program is used to investigate the passage of fragments from ²³⁵U fission by thermal neutrons through 50–400 μg/cm² thick uranium layers. The calculations are performed in the two-fragment approximation: light ⁹⁷Mo and heavy ¹³⁹Ba. The energy spectra of the fission fragments emanating from the layer are calculated and the effects due to multiple scattering of the fragments in thin layers are estimated. __________ Translated from Atomnaya énergiya, Vol. 100, No. 1, pp. 57–63, January, 2005.     

Projected-length distributions of fission-fragment tracks from U and Th thin film sources in muscovite

Thin films of natural uranium and thorium deposited on muscovite were used as sources of neutron-induced fission fragments. Fragment energy loss in thin-source geometry is negligible. In this way, the observed fragment range results from the interaction of the fragment with the detector material. This characteristic enables the investigation of asymmetric fission and etching, through measurements of projected track length distributions in muscovite micas coupled with thin films. The means and standard deviations of the etchable length distributions of the heavy and light fission-fragment tracks were estimated by fitting a theoretical equation to the experimental data.The light fission-fragment accounts for ∼54% and the heavy fission-fragment for ∼46% of the etchable length of a full fission track. This average partition is the same for tracks from thermal-neutron-induced fission of ²³⁵U and fast-neutron-induced fission of ²³²Th. The mean etchable length of uranium fission tracks is ∼2.5% longer than that of thorium fission tracks. This difference is at the resolution limit of these measurements but correlates with the difference in the mean combined initial kinetic energies of the fission fragments. The mean etchable length of uranium fission tracks in muscovite is ∼5% shorter than their calculated latent track length, supporting earlier estimates of a length deficit of this magnitude. The length deficit and the standard deviation of the etchable length distribution of the light fission-fragment tracks are twice the equivalent values for the heavy fission-fragment tracks. This is interpreted in terms of a v_t-profile (track etch rate) that depends on the mass of the track-forming particle.     

Measurement of specific energy losses of individual fission fragments with a back-to-back δE−E detector system

The specific ionization of fission fragments of specified charge and $\text{E}\text{M}$ values in thermal neutron induced fission of ²³⁵U have been measured employing a back-to-back gridded ionization chamber ΔE-semiconductor detector E setup. The measurements were carried out for the case of a mixture of argon (95%) and methane (5%) gases at two gas pressures, 44 Torr and 270 Torr. The experimental data for the lower gas pressure were used to determine the effective charge parameter γ for the fragments at various velocities. These γ-values have been used to compute the fragment energy losses at the higher gas pressure and are compared with the measured energy loss values.