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2.
为校验次临界能源堆的概念设计,在R19.4/30.0 cm的贫铀球壳装置上采用活化法开展14 MeV中子学积分实验.布放6片贫铀活化片于球壳中与入射D离子束90°方向上的不同位置处活化,用HPGe探测器测量238U(n,γ)反应、238U(n,f)及235U(n,f)反应和238U(n,2n)各反应产物发射的特征γ射线,得到了相应的反应率.238U(n,γ)反应率的不确定度为3.6%-3.7%,238U(n,D和235U(n,f)反应率的不确定度为5.1%-5.9%,238U(n,2n)反应率的不确定为4.3%-4.7%.用MCNP5程序在ENDF66c数据库下进行模拟计算,238U(n,γ)反应率的计算值/实验值(C/E)为0.972-1.034,238U(n,f)和235U(n,f)反应率的C/E为0.983-1.058,238U(n,2n)反应率的C/E为0.979-1.019. 相似文献
3.
The theory of nuclear fission is reconsidered by introducing the charge asymmetry in the asymmetric two center shell model as a dynamical collective coordinate. The quantum mechanical fluctuations, which are accompanied with the collective motion as a function of the mass asymmetry, are responsible for the mass distributions in nuclear fission. Numerical calculations are carried out for the mass distributions and charge dispersions for the nuclear fission of the 236U and 238U nuclei. The present obtained theoretical calculations are in good agreement with the experimental measurements. 相似文献
4.
The reaction of 238U with 12C was studied radiochemically with the purpose of elucidating fast fission characteristics. From the difference in the mass
distribution below and above the critical energy where fast fission is predicted to set in, fast fission component was extracted
in far-asymmetric mass region and interpreted as the mass diffusion following the Fokker-Planck equation. Anomalous charge
dispersion widths in the corresponding mass region and a sudden increase of the whole mass distribution width at the critical
energy were also observed to support the above result. The reaction time of fast fission deduced from the width and position
of the mass distribution was 5×10 −21s as well by taking into account the effect of neutron emission during the diffusion process, which turned out to be more
than one order of magnitude longer than the corresponding life time of typical deep inelastic scattering but substantially
short compared to ordinary fusion-fission life time. Evaluation of the driving potential for mass drift required dinuclear
configuration be of an elongated or deformed form for fast fission in contrast to a more compact form for the deep-inelastic
process.
Received: 11 November 1997 相似文献
5.
The fragmentation of the nuclear system 238U- 238U is studied by treating the fragmentation coordinate quantum mechanically. The time dependent Schrödinger equation, the Hamiltonian of which is calculated from the microscopic asymmetric two center shell model, is solved by the finite difference method. In order to study the fragmentation mechanism, model calculations have been carried out, by assuming the collective fragmentation potential as an oscillator potential which is adjusted to the realistic potential. Effects arising from the dynamical treatment of the fragmentation coordinate are found to be important in the collision of 238U on 238U. The dependence of the fragmentation on the incident energy is discussed. 相似文献
6.
The fragmentation of the nuclear system 238U- 238U is studied by treating the fragmentation coordinate quantum mechanically. The time dependent Schrödinger equation, the Hamiltonian of which is calculated from the microscopic asymmetric two center shell model, is solved by the finite difference method.In order to study the fragmentation mechanism, model calculations have been carried out, by assuming the collective fragmentation potential as an oscillator potential which is adjusted to the realistic potential. Effects arising from the dynamical treatment of the fragmentation coordinate are found to be important in the collision of 238U on 238U. The dependence of the fragmentation on the incident energy is discussed. 相似文献
7.
Using the available experimental data, production cross sections of the most stable nuclei have been calculated for the proton-induced fission of 238U and 232Th at 12, 20, 35, 50, 96 MeV in case of 238U and at 8, 9.3, 12, 19.55, 32.2, 44.7, 53 MeV for 232Th. The analysis has been carried out for the fission fragment mass ranges corresponding to N/Z ratios in the ranges 1.33 ± 0.09 and 1.32 ± 0.08 for 238U and 232Th respectively. Results have been compared with the ones generated indirectly by employing GEF nuclear reaction code, version 2017/1.1. From the production cross sections point of view, for the same energy, 232Th is found to be a better target than 238U for producing nuclei A around the symmetric mass peak, while 238U comes out to be a better one than 232Th for producing the fission fragments around the asymmetric peaks of the mass distribution. 相似文献
8.
Actinides have widely entered the environment as a result of nuclear accidents and atmospheric weapon testing. These radionuclides,
especially uranium, are outstanding radioactive pollutants, due to their high radiotoxicity and long half-lives. In addition
to this, since depleted uranium (DU) has been used in the Balkan conflict in 1999, there has been a concern about the possible
consequences of its use for the people and environment. Therefore, accurate, precise and simple determination methods are
necessary in order to evaluate the human dose and the concentration and effects of these nuclides in the environment. The
principal isotopes of uranium e.g.
235U and 238U are of primordial origin and 234U present in radioactive equilibrium with 238U. 236U occurs in nature at ultra trace concentrations with a 236U: 238U atom ratio of 10 −14. Concentrations of uranium in soil samples were determined using inductively coupled plasma mass spectrometry (ICP-MS) and
isotope ratios of uranium were measured using a thermal ionisation mass spectrometer. Radioactive dis-equilibrium of 234/ 238U, depletion of 235/ 238U and significant evidence of 236U/ 238U were noticed in soil samples.
相似文献
9.
The yields of products originating from 238U photofission are measured at the bremsstrahlung endpoint energies of 50 and 3500 MeV. Charge and mass distributions of fission fragments are obtained. Symmetric and asymmetric channels in 238U photofission are singled out on the basis of the model of multimode fission. This decomposition makes it possible to estimate the contributions of various fission components and to calculate the fissilities of 238U in the photon-energy regions under study. 相似文献
10.
The fission fragment mass distribution followed by neutron emission is studied for the 238U( 18O, f) reaction using the asymmetric two-center shell model. Within the thermodynamic approach, excitation energy carried by the compound nucleus is dissipated in the emission of a pair of neutrons in several consecutive steps. Therefore, we have considered 2–12 (in step of 2) neutron emission channels in our formalism. The mass distribution corresponding to 8-neutron emission channel compares reasonably well with the experimental data. The observed fine structure dips corresponding to shell closure ( Z = 50 and N = 82 of individual fission fragment arise mainly due to shell structure in the mass parameters. However, an exact location and magnitude of the dip at A = 124 in the mass distribution depends on how the temperature modifies masses and, also, on the precise information of pre- and post-neutron emission data. This suggests a possible importance of extending these calculations to get new insight into an understanding of the dynamical behaviour of fragment formation in the fission process. 相似文献
11.
The influence of the damping shell corrections with increasing excitation energy on the fission fragment angular anisotropies is considered. In the framework of the statistical approach to nuclear fission, experimental data on fission fragment angular anisotropies obtained in the 4He + 238U reaction is analyzed. Information about the energy dependence of the shell corrections is obtained from this analysis. 相似文献
12.
Energy shell corrections are derived for the nucleus 238U by means of the new shell correction technique developed for finite depth single-particle potentials and exploiting only the bound states. The results are compared with those obtained by the traditional method. 相似文献
13.
Cross sections for nuclear reactions at beam energies near and below the spherical Coulomb barrier V c were measured in the very heavy collision systems 238U + 238U and 238U + 197Au. The most probable reaction channel with mass transfer is the one-neutron transfer. Its excitation function is understood in terms of Rutherford trajectories together with the quantal process of neutron tunnelling over large distances. In addition, the exchange of up to 15 nucleons is observed down to 0.90 V c . The excitation functions for the multi-nucleon transfer products have much steeper slopes than that for one-neutron transfer, and are steeper for 238U + 197Au than for 238U + 238U, suggesting that nuclear contact is established in the associated collisions. The angular distribution for one selected multi-nucleon transfer product, 227Th, shows that its formation occurs in more central collisions within contact times shorter than about 10 ?21 s. There is no evidence for very longlived di-nuclear systems in the these reactions. 相似文献
14.
The monitoring and evaluation of radioactive content in samples of PM 10 aerosols have been investigated. The specific radioactivity concentrations (SRC) of 234U, 235U, 238U and 232Th were determined using inductively coupled plasma-sector field mass spectrometry in 13 samples collected in Mexico City and 8 samples collected in Cuernavaca in the centre of Mexico. The SRC of the radioisotopes analysed in PM 10 were larger than those reported in PM 2.5. The enrichment factor was greater than 5, indicating anthropogenic influences in both sites. The activity ratios of these isotopes in the samples were determined. The 235U/ 238U ratio showed variations with respect to the natural value, while the 234U/ 238U and 232Th/ 238U ratios did not show any secular equilibrium in all sites, corroborating that the increase of uranium is not influenced by natural sources. The annual dose results obtained have no impact on health. 相似文献
15.
Two-step photoionization of an atomic beam and quadrupole mass analysis have been used for the precise measurement of the isotope shift between uranium isotopes 235 and 238 and the hyperfine structure of 235U. For the 5915 Å ground-state transition 15 hfs components were found. The residual atomic beam was isotopically enriched by factors 2.5 and 10 for 235U and 238U, respectively. 相似文献
16.
The decay of the 238U superdeformed shape isomer has been reinvestigated by detecting for the first time simultaneously the fission and the gamma-back decay. An electrostatic deflection system has been used to transport the 238mU recoils, produced in a 238U( d, pn) reaction with a pulsed beam of 18 MeV deuterons, in front of a detector set-up consisting of three ion-implanted solidstate detectors and a Ge(Li) gamma-detector. The gamma-back decay has been measured in coincidence with conversion electrons of the 2 + 0 + transition deexciting the first rotational state in 238U. Two gamma-transitions of 2.513 MeV and 1.878 MeV have been observed with half-lives consistent with the result obtained for the decay by delayed fission T
1/2=(298±18) ns.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday 相似文献
17.
Fragment mass distributions are presented obtained in the heavy-ion reactions 22Ne+ 249Cf, 32S+ 238U, 40Ar+ 232Th and 56Fe+ 208Pb leading to composite systems with equal nuclear charge number Z=108. The experiments were performed at the heavy-ion cyclotron U 300 of the Laboratory of Nuclear Reactions in Dubna. The spectrometer DEMAS was used to measure the time-of-flight values and the laboratory angles of the correlated fragments. The shape of the mass distributions strongly depends on the initial mass asymmetry. When decreasing the bombarding energy down to values near the Coulomb barrier, the mass distributions obtained in the reactions 32S+ 238U and 40Ar+ 232Th exhibit relative maxima of M≈205 interpreted to be due to stabilizing effects of nuclear shells during the fragmentation process. 相似文献
18.
Fission product cross sections of intermediate-energy fission of 238U were used in order to construct the charge and mass yield distributions. Enriched target of 238U was irradiated by proton beam with energy 660 MeV for several hours at the LNP Phasotron, Joint Institute for Nuclear Research
(JINR), Dubna, Russia. The charge distribution of the fission fragments was analyzed for calculation of isobaric cross sections.
The mass yield curves were expanded into symmetric and asymmetric components according multimodal fission approach. The fissility
values of actinides were calculated at given proton energy. The obtained results have been compared to the same data for targets
237Np and 241Am. 相似文献
19.
Photofission of 238 U by bremsstrahlung photons is studied at four energies of an electron accelerator: 19.5, 29.1, 48.3, and 67.7 MeV. The yields of the fission fragments after the emission of prompt neutrons are obtained using the gamma-ray spectroscopic technique. The mass distributions of photofission are obtained at different upper energies of the bremsstrahlung spectrum. The ratio the symmetric-fission mode to the asymmetric mode is obtained from the mass distribution. The symmetric mode becomes 3–4 times greater than the asymmetric as the excitation energy of the 238U nucleus increases from 12 to 16 MeV. 相似文献
20.
Bremsstrahlung induced photofission of 197Au and 238U has been measured for energies Emaxγ = 0.8 to 2.2 GeV using catcher foil techniques. The mass distributions of the fission fragments of uranium were measured as a function of the maximum bremsstrahlung energy. The peak-to-valley and forward-backward ratios have been determined for 238U. 相似文献
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