Fission decay properties of ultra neutron-rich uranium isotopes

The fission decay of highly neutron-rich uranium isotopes is investigated which shows interesting new features in the barrier properties and neutron emission characteristics in the fission process. ²³³U and ²³⁵U are the nuclei in the actinide region in the beta stability valley which are thermally fissile and have been mainly used in reactors for power generation. The possibility of occurrence of thermally fissile members in the chain of neutron-rich uranium isotopes is examined here. The neutron number N = 162 or 164 has been predicted to be magic in numerous theoretical studies carried out over the years. The series of uranium isotopes around it with N = 154–172 are identified to be thermally fissile on the basis of the fission barrier and neutron separation energy systematics; a manifestation of the close shell nature of N = 162 (or 164). We consider here the thermal neutron fission of a typical representative ²⁴⁹U nucleus in the highly neutron-rich region. Semiempirical study of fission barrier height and width shows that ²⁵⁰U nucleus is stable against spontaneous fission due to increase in barrier width arising out of excess neutrons. On the basis of the calculation of the probability of fragment mass yields and the microscopic study in relativistic mean field theory, this nucleus is shown to undergo exotic decay mode of thermal neutron fission (multi-fragmentation fission) whereby a number of prompt scission neutrons are expected to be simultaneously released along with the two heavy fission fragments. Such properties will have important implications in stellar evolution involving r-process nucleosynthesis.      

JMCT程序临界安全基准校验计算与分析

JMCT程序是北京计算数学与应用物理研究所自主开发的蒙特卡罗方法粒子输运计算程序。从国际临界安全实验数据库中选取棒栅类、U系、Pu系、233U系实验方案对JMCT程序进行验证,验证涵盖了常见裂变元素（如U,Pu等）从低富集度到高富集度的快、热能区,同时也包含了常见的中子毒物以及反射层材料。将JMCT程序的计算结果与基准实验值进行对比,并且与MCNP,MONK程序的计算结果进行了比较。结果表明,在检验的范围内,JMCT程序具有与国际通用蒙特卡罗方法粒子输运程序一样良好的计算精度。     

Further experimental evidence for fast fission

Fusion-fission products have been studied for three reactions: Ar + Au, Ar + Bi and Ar + U (5.25–7.5 MeV/u). By measuring symmetric fragmentation components (fission-like events), cross sections for fusion were deduced and compared with the predictions of static and dynamic models. With increasing projectile energy, the width of the mass distributions strongly increases for the two lighter systems. By contrast, for Ar + U it remains essentially constant at a very large value. These results clearly demonstrate that the large increase of the width of the mass distribution cannot be attributed simply to large values of the angular momentum. However, they can be explained by the occurence of a different dissipative process, fast fission, which can be expected if there is no barrier to fission. For the reaction Ar + Au, the total kinetic-energy distributions were also studied in detail. In this case fast fission occurs only at high incident energy. The average total kinetic energy (TKE) was found to be constant with increasing energy whereas the widths of the TKE distribution increase.     

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of⁴He in the reaction 334 MeV⁴⁰Ar+²³⁸U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the⁴He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed⁴He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the⁴He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average⁴He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.     

核电和物理学

20世纪三四十年代，原子物理学家们发现中子轰击铀原子核的裂变现象并首次实现可控裂变链式反应，把世界带进了原子能时代．半个世纪以来核电已达到全世界电力的16％．目前99％以上都是热中子反应堆核电站。在大规模发展核电的情况下，比如说百GWe,必须加快快中子增殖反应堆的发展和推广，方无核燃料匮乏之虞。     

Measurements of Fission and Radioactive Capture Reaction Rates Inside the Fuel of the Ipen/MB-01

This work presents the measures of the nuclear reaction rates along the radial direction of the fuel pellet by irradiation and posterior gamma spectrometry of a thin slice of fuel pellet of UO₂ at 4.3% enrichment. From its irradiation, the rate of radioactive capture and fission had been measured as a function of the radius of the pellet disk using a Ortec GMX HPGe detector. Lead collimators had been used for this purpose. Simulating the fuel pellet in the pin fuel of the IPEN/MB-01 reactor, a thin UO₂ disk is used, being inserted in the interior of a dismountable fuel rod. This fuel rod is then placed in the central position of the IPEN/MB-01 reactor core and irradiated during 1 h under a neutron flux of 5 ×10⁸ n/cm² s. In gamma spectrometry, 10 collimators with different diameters have been used; consequently, the nuclear reactions of radioactive capture that occurs in atoms of ²³⁸U and the fission that occurs on both ²³⁵U and ²³⁸U are measured in function of 10 different regions (diameter of collimator) of the UO₂ fuel pellet disk. Nuclear fission produces different fission products such as ¹⁴³Ce with a yield fission of 5.9% which decay is monitored in this work. Corrections in geometric efficiency due to introduction of collimators on HPGe detection system were estimated using photon transport of MCNP-4C code. Some calculated values of nuclear reaction rate of radioactive capture and fission along the radial direction of the fuel pellet obtained by Monte Carlo methodology, using the MCNP-4C code, are presented and compared to the experimental data showing very good agreement.     

Deuterium Permeation Through Reduced Activation V-4Cr-4Ti Alloy and V-4Cr-4Ti Alloy with AlN/Al Coatings

Physics of Atomic Nuclei - Vanadium alloys are considered as candidate structural materials for thermonuclear fusion reactors (FR) and fast neutron reactors. Some properties of vanadium alloys are...     

ITER中子通量监测器原型的研制

研制的用于国际热核实验堆(ITER)的中子通量监测器由裂变室探测器组合构成,包括两只不同     

Neutron yield when fast deuterium ions collide with strongly charged tritium-saturated dust particles

The ultrahigh charging of dust particles in a plasma under exposure to an electron beam with an energy up to 25 keV and the formation of a flux of fast ions coming from the plasma and accelerating in the strong field of negatively charged particles are considered. Particles containing tritium or deuterium atoms are considered as targets. The calculated rates of thermonuclear fusion reactions in strongly charged particles under exposure to accelerated plasma ions are presented. The neutron generation rate in reactions with accelerated deuterium and tritium ions has been calculated for these targets. The neutron yield has been calculated when varying the plasma-forming gas pressure, the plasma density, the target diameter, and the beam electron current density. Deuterium and tritium-containing particles are shown to be the most promising plasmaforming gas–target material pair for the creation of a compact gas-discharge neutron source based on the ultrahigh charging of dust particles by beam electrons with an energy up to 25 keV.     

Twin position sensitive ionization chamber,utilizing backgammon and time projection method

The anomalous dependence of average prompt fission neutron (PFN) multiplicity on the fission fragment total kinetic energy of ²⁵²Cf(sf) was investigated using digital signal processing (DSP). A twin Frisch-grid ionization chamber (TGIC) was used for fission fragment (FF) spectroscopy and PFN was registered with help of fast neutron detector (FND) on liquid scintillator base. The DSP apparatus was build up around the 8-channel fast synchronously sampling waveform digitizers (WFD). It was found that the main limitation of the experimental method, developed in ref. [1] for investigation of the PFN properties in neutron induced fission reaction ²³⁵U(n,f), ²³⁹Pu(n,f) etc is related to the FF energy loses of in the target backing. A modification of the method was suggested to overcome such a limitation.     

Systematics of neutron emission probabilities from delayed neutron precursors

A simple correlation based on the gross theory ofβ-decay is derived between the neutron emission probabilitiesP _n of delayed neutron precursors, theirβ-decay energies and the neutron binding energies of the daughter nuclei. The correlation is shown to be valid for delayed neutron precursors among the fission products. TheP _n-values of several expected but still unidentified neutron precursors are estimated together with their contributions to the delayed neutron groups in thermal-neutron induced fission of²³⁵U. Some aspects of theβ-strength function for transitions into highly excited states are discussed.     

Fast fission mechanism and duality of the diffusion process

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     

The hydrogen surface chemistry of carbon as a plasma facing material

Recent progress in plasma performance in experiments on controlled thermonuclear fusion will lead to next-generation fusion experiments with the ultimate goal to make a power-generating fusion reactor reality. A major issue in the design of fusion experiments is the selection of the first wall material. Due to its outstanding thermal properties and its low Z carbon is and will be a first choice material, either elemental or in compounds and composites. Given the fact that in magnetically confined hydrogen plasmas of high density and temperature substantial wall fluxes of H species occur, the hydrogen/carbon surface chemistry, in particular chemical erosion and H retention, becomes a concern. Although for more than a decade materials science studies on the H---carbon interaction have been performed, the elementary reaction steps of this interaction have become clear only recently. This report reviews work performed on the H/carbon surface chemistry under the aspects of chemical erosion. It presents in detail model studies directed towards identifying elementary reaction steps. Related fields, e.g. the H/C surface chemistry as relevant for the production of hard a-C:H coatings and low pressure diamond synthesis are covered in the review.     

激光等离子体中子发射研究

本文研究了脉宽(FWHM)为毫微秒量级、功率为10¹⁰瓦、10¹¹瓦的单束和脉宽为毫微秒、亚毫微秒的六束四种激光核聚变装置上,激光辐照含氘热核材料平面靶、微球靶和充氘气(D₂)玻壳微球靶中子发射特性。 关键词：     

Cumulative fission yield measurements with 14.7 MeV neutrons on 238U

The fission yield data in the 14 MeV energy neutron induced fission of ²³⁸U play an important role in decay heat calculations and generation-IV reactor designs. In order to accurately measure fission product yields (FPYs) of ²³⁸U induced by 14 MeV neutrons, the cumulative yields of fission products ranging from ⁹²Sr to ¹⁴⁷Nd in the ²³⁸U(n, f) reaction with a 14.7 MeV neutron were determined using an off-line γ-ray spectrometric technique. The 14.7 MeV quasi-monoenergetic neutron beam was provided by the K-400 D-T neutron generator at China Academy of Engineering Physics (CAEP). Fission products were measured by a low background high purity germanium gamma spectrometer. The neutron flux was obtained from the ⁹³Nb (n, 2n)^92mNb reaction, and the mean neutron energy was calculated using the cross-section ratios for the ⁹⁰Zr(n, 2n)⁸⁹Zr and ⁹³Nb(n, 2n)^92mNb reactions. With a series of corrections, high precision cumulative yields of 20 fission products were obtained. Our FPYs for the ²³⁸U(n, f) reaction at 14.7 MeV were compared with the existing experimental nuclear reaction data and evaluated nuclear data, respectively. The results will be helpful in the design of a generation-IV reactor and the construction of evaluated fission yield databases.     

Prompt fission neutron spectra of n+~(235)U above the(n,nf) fission threshold

Calculations of prompt fission neutron spectra(PFNS) from the235U(n, f) reaction were performed with a semi-empirical method for E n = 7.0 and 14.7 Me V neutron energies. The total PFNS were obtained as a superposition of(n,xnf) pre-fission neutron spectra and post-fission spectra of neutrons which were evaporated from fission fragments, and these two kinds of spectra were taken as an expression of the evaporation spectrum.The contributions of(n,xnf) fission neutron spectra on the calculated PFNS were discussed. The results show that emission of one or two neutrons in the(n,nf) or(n,2nf) reactions influences the PFNS shape, and the neutron spectra of the(n,xnf) fission-channel are soft compared with the neutron spectra of the(n,f) fission channel. In addition,analysis of the multiple-chance fission component showed that second-chance fission dominates the PFNS with an incident neutron energy of 14.7 Me V whereas first-chance fission dominates the 7 Me V case.     

Identification of a shape isomer in 235U

The shape isomer in 235U has been searched for in a neutron-induced fission experiment on 234U, which was performed at the isomer spectrometer NEPTUNE of the EC-JRC IRMM. A neutron source, with a tunable pulse frequency in the Hz to kHz range and its individually adjustable neutron pulse width in connection with an appropriate detector system turned out to be the ideal instrument to perform an isomer search, when decay half-lives above 100 micros are expected. From the delayed fission events observed for two different NEPTUNE settings and at mean incident neutron energies En=0.95 and 1.27 MeV the isomeric fission half-life could be determined to be T1/2=(3.6+/-1.8) ms. The corresponding cross section was determined to sigmaif=(10+/-8) microb. With these results an experimental confirmation for the existence of a superdeformed shape isomer in odd-uranium isotopes is given for the first time.     

Betterment in EDXRF analytical results for compositional characterization of mixed uranium thorium oxide samples with bead specimens compared with pressed pellet specimens

A comparative study on the energy dispersive X‐ray fluorescence analytical results of uranium determinations, in uranium and uranium–thorium mixed oxides, using specimens in the form of fused beads and pressed pellets, has been made. It was observed that in case of fusion bead specimens, the intensity of the analyte lines was approximately 1.6 times of that observed in pellet specimens under identical instrumental conditions. In case of uranium oxide samples, the analytical results with bead specimens were slightly better compared with the pellet specimens. However, in case of the uranium–thorium oxide mixtures, the average precision obtained with bead specimen was significantly better (1%, 1 s) in comparison with that achieved using pellet specimens (7%, 1 s). This difference may be due to the hardness of thorium oxide compared with uranium oxide, which affects the homogeneity of the pellet specimens prepared. In fusion bead method of sample preparation, even highly refractory material like ThO₂ forms uniform glass beads. Addition of internal standard further improves the analytical results, with reduction in the percent deviation of energy dispersive X‐ray fluorescence results from the expected values to 3% from 7% compared with that obtained using without internal standard. The fusion bead method of sample preparation will be very useful for characterization of sintered (U,Th)O₂ pellets, which are highly refractory and difficult to dissolve. Copyright © 2016 John Wiley & Sons, Ltd.     

Angular correlation of neutrons from thermal-neutron fission of 235U

The neutron-neutron angular correlation for neutrons with energies ? 1 MeV emitted in thermal-neutron-induced fission of ²³⁵U has been measured. The correlation could be adequately described by a simple model, assuming isotropic neutron emission in the fully accelerated fragment frames together with a 20% isotropic scission component and using binomial distributions to simulate the neutron multiplicity (instead of the usual gaussian form) for each fission fragment. The results give no indication of enhanced polar emission.