Fission Product Yields of U, U, U and Pu in Fields of Thermal Neutrons, Fission Neutrons and 14.7-MeV Neutrons

The yields of more than fifteen fission products have been carefully measured using radiochemical techniques, for ²³⁵U(n,f), ²³⁹Pu(n,f) in a thermal spectrum, for ²³³U(n,f), ²³⁵U(n,f), and ²³⁹Pu(n,f) reactions in a fission neutron spectrum, and for ²³³U(n,f), ²³⁵U(n,f), ²³⁸U(n,f), and ²³⁹Pu(n,f) for 14.7 MeV monoenergetic neutrons. Irradiations were performed at the EL3 reactor, at the Caliban and Prospero critical assemblies, and at the Lancelot electrostatic accelerator in CEA-Valduc. Fissions were counted in thin deposits using fission ionization chambers. The number of fission products of each species were measured by gamma spectrometry of co-located thick deposits.     

Fission Product Data Measured at Los Alamos for Fission Spectrum and Thermal Neutrons on Pu, U, U

We describe measurements of fission product data at Los Alamos that are important for determining the number of fissions that have occurred when neutrons are incident on plutonium and uranium isotopes. The fission-spectrum measurements were made using a fission chamber designed by the National Institute for Standards and Technology (NIST) in the BIG TEN critical assembly, as part of the Inter-laboratory Liquid Metal Fast Breeder Reactor (LMFBR) Reaction Rate (ILRR) collaboration. The thermal measurements were made at Los Alamos' Omega West Reactor. A related set of measurements were made of fission-product ratios (so-called R-values) in neutron environments provided by a number of Los Alamos critical assemblies that range from having average energies causing fission of 400-600 keV (BIG TEN and the outer regions of the Flattop-25 assembly) to higher energies (1.4-1.9 MeV) in the Jezebel, and in the central regions of the Flattop-25 and Flattop-Pu, critical assemblies. From these data we determine ratios of fission product yields in different fuel and neutron environments (Q-values) and fission product yields in fission spectrum neutron environments for ⁹⁹Mo, ⁹⁵Zr, ¹³⁷Cs, ¹⁴⁰Ba, ^141,143Ce, and ¹⁴⁷Nd. Modest incident-energy dependence exists for the ¹⁴⁷Nd fission product yield; this is discussed in the context of models for fission that include thermal and dynamical effects. The fission product data agree with measurements by Maeck and other authors using mass-spectrometry methods, and with the ILRR collaboration results that used gamma spectroscopy for quantifying fission products. We note that the measurements also contradict earlier 1950s historical Los Alamos estimates by ∼5-7%, most likely owing to self-shielding corrections not made in the early thermal measurements. Our experimental results provide a confirmation of the England-Rider ENDF/B-VI evaluated fission-spectrum fission product yields that were carried over to the ENDF/B-VII.0 library, except for ⁹⁹Mo where the present results are about 4%-relative higher for neutrons incident on ²³⁹Pu and ²³⁵U. Additionally, our results illustrate the importance of representing the incident energy dependence of fission product yields over the fast neutron energy range for high-accuracy work, for example the ¹⁴⁷Nd from neutron reactions on plutonium. An upgrade to the ENDF library, for ENDF/B-VII.1, based on these and other data, is described in a companion paper to this work.     

1.4MeV-5MeV中子诱发~(238)U裂变产额测量

~(238)U裂变产额测量工作在核数据测量中有着重要意义,本工作利用2.5MeV质子静电加速器产生的1.4MeV-5MeV单能中子诱发238U裂变,通过对裂变产物放射性的测量对裂变产物核素~(135)I、~(133)I、~(105)Ru和~(91)Sr的产额进行了测定。照射过程中中子通量用活化法确定。分析了影响实验测量的多个因素,包括用MCNPX程序对中子在靶头及样品中的多次散射和自屏蔽效应进行了修正,对γ射线在样品中的自吸收进行修正等。得到产额数据典型误差为3.5%,最后把测量结果与已有的裂变产额数据进行比对。     

0.57、1.0和1.5MeV中子诱发235U裂变时95Zr、140Ba和147Nd产额的绝对测量

用HPGeγ能谱法绝对测量0.57、1.0和1.5 MeV中子诱发235U裂变的裂变产物95Zr、140Ba和147Nd的产额。P型同轴HPGe探测器的体积为110 cm3,对60Co 1 332.5 keVγ射线的分辨率为1.8 keV。使用双裂变电离室监测裂变率,应用SPAN程序解析γ谱。对影响产额测量的主要因素进行了分析,并用MCNPIVB程序模拟计算了非主中子的影响。实验测得0.57 MeV中子诱发时,95Zr、140Ba、147Nd的产额分别为(6.64±0.21)%、(6.37±0.22)%、(2.37±0.08)%;1.0 MeV中子诱发时,95Zr、140Ba、147Nd的产额分别为(6.49±0.19)%、(6.35±0.18)%、(2.25±0.07)%;1.5 MeV中子诱发时,95Zr、140Ba、147Nd的产额分别为(6.59±0.19)%、(6.25±0.18)%、(2.37±0.07)%。对95Zr、140Ba、147Nd的产额随入射中子能量的变化趋势作了简要分析与讨论。     

14 MeV中子穿过组合材料引发238U绝对裂变率的测量

为检验计算方法与参数，用小型平板铀裂变室测量了铁球壳表面沿赤道方向不同位置14 MeV中子穿透实验模型后引起的。^238U绝对裂变反应率，并与MCNP/4A和ENDF/B-V库数据计算结果相比较。实验总误差为6．1％。     

Fission Product Yields from Fission Spectrum n+Pu for ENDF/B-VII.1

We describe a new cumulated fission product yield (FPY) evaluation for fission spectrum neutrons on plutonium that updates the ENDF/B-VI evaluation by England and Rider, for the forthcoming ENDF/B-VII.1 database release.¹ We focus on FPs that are needed for high accuracy burnup assessments; that is, for inferring the number of fissions in a neutron environment. Los Alamos conducted an experiment in the 1970s in the Bigten fast critical assembly to determine fission product yields as part of the Interlaboratory Reaction Rate (ILRR) collaboration, and this has defined the Laboratory's fission standard to this day. Our evaluation includes use of the LANL-ILRR measurements (not previously available to evaluators) as well as other Laboratory FPY measurements published in the literature, especially the high-accuracy mass spectrometry data from Maeck and others. Because the measurement database for some of the FPs is small — especially for ⁹⁹Mo — we use a meta-analysis that incorporates insights from other accurately-measured benchmark FP data, using R-value ratio measurements. The meta-analysis supports the FP measurements from the LANL-ILRR experiment. Differences between our new evaluations and ENDF/B-VI are small for some FPs (less than 1-2%-relative for ⁹⁵Zr, ¹⁴⁰Ba, ¹⁴⁴Ce), but are larger for ⁹⁹Mo (4%-relative) and ¹⁴⁷Nd (5%-relative, at 1.5 MeV) respectively. We present evidence for an incident neutron energy dependence to the ¹⁴⁷Nd fission product yield that accounts for observed differences in the FPY at a few-hundred keV average energy in fast reactors versus measurements made at higher average neutron energies in Los Alamos' fast critical assemblies. Accounting for such FPY neutron energy dependencies is important if one wants to reach a goal of determining the number of fissions to accuracies of 1-2%. An evaluation of the energy-dependence of fission product yields is given for all A values based on systematical trends in the measured data, with a focus on the energy dependence over the fast neutron energy range from 0.2-2 MeV. Based on these trends, we present an evaluation of the FPY data at 0.5 and 2.0 MeV average incident neutron energies. This new set of ENDF/B-VII data will enable users to linearly interpolate between the pooled FPY data at ∼0.5 MeV and our new data at 2 MeV to obtain FPYs at other energies.     

Relative Yield and Period of Individual Groups of Delayed Neutrons in 233U, 235U, and 239Pu Fission by Epithermal Neutrons

The relative yield of delayed neutrons and the half-life of their precursor nuclei in fissioning of ²³³U, ²³⁵U, and ²³⁹Pu by epithermal neutrons are measured on a setup which is based on a KG-2.5 electrostatic accelerator. The experimental samples are irradiated in a polyethylene cube, one face of which is irradiated by a neutron beam consisting of T(p, n)³He neutrons. A procedure is developed for averaging the group parameters of delayed neutrons from different series of measurements, taking account of their correlation. A comparative analysis of the data of this work and in ENDF/BVI in terms of the average half-life of the precursor nuclei of the delayed neutrons and using the dependence of the reactivity on the asymptotic period of the reactor is performed.It is shown that the data of this work agree, to within the uncertainty limits, with the recommended experimental data and differ substantially from the ENDF/BVI data.     

Calculation of Prompt Fission Product Average Cross Sections for Neutron-Induced Fission of 235U and 239Pu

Yield-weighted average cross sections of neutron radiative capture, (n,2n), and (n,3n) reactions over prompt fission products (FPs) from ²³⁵U and ²³⁹Pu are calculated. The prompt fission production yields are taken from the ENDF/B-VII.0 library. The FPs for each fissile material exist over a range of approximately 1000 neutron-rich nuclides. Several nuclear reaction codes are utilized for calculating the cross sections on each individual fission product—EMPIRE-2.19, TALYS-1.0, GNASH, and CoH. The influence of the FP isomers on the average cross sections is examined with TALYS. We investigate the dependence of the average cross sections on the number of FPs taken for averaging. It is shown that the average capture cross section is much more sensitive to the number of FPs included, compared with the (n,2n) and (n,3n) reactions. An intercomparison of the calculated cross sections with the different reaction codes is carried out. In the capture reaction, EMPIRE predicted lower cross section than TALYS and CoH owing to different default assumptions used in the γ-ray strength function modeling. Moreover, the preequilibrium models implemented in each code give different predictions for the neutron-emission reactions, although the differences are relatively small. We also discuss a difference between the macroscopic and microscopic calculation options in TALYS for the pre-equilibrium model, optical potential model, and γ-ray strength function. The predictive capability of the reaction codes for the capture reaction is examined by comparing their calculations with the ENDF data, which are based on measurements. Compared with the historic Foster and Arthur's evaluation, our new (n,2n) predictions are similar, although our capture predictions are almost an order of magnitude higher. Recommended cross sections for use in applications have been tabulated in ENDF-formatted files.     

特定实验条件下中子引发~(238)U裂变率测量可行性研究

用小型平板裂变室测量中子穿透实验模型后引发238U绝对裂变率,掌握小空间测量中子技术。通过对有无反射壳测量结果的比较,得到中子对反射壳反射因数。实验结果与MCNP模拟计算结果比较,以此来检验计算方法与参数。     

14.8 MeV中子诱发232Th裂变产额测量

本工作采用直接γ能谱法测量了14.8 MeV中子诱发²³²Th裂变的62个产物核的累积产额相对值,通过链产额之和200%归一的方法得到了47个质量链的链产额实验数据,数据精度好于10%。     

Discrepancies in Estimates of the Number of Secondary Fission Neutrons for 239Pu

The energy dependence of the total number of secondary neutrons v ²³⁹Pu is analyzed. The results of variations of different national libraries are compared: ENDF/B-5, ENDF/B-6.5, JEF-2.2, JENDL-3.2, CENDL-2, and BROND-2. It is concluded that the required accuracy has not been attained for the entire energy range.     

Mutual Separation of U,Pu and Fission Products with Tri-iso-octylamine

Uranium and Pu were extracted with tri-wo-octylamine (CC1₄) from irradiated U dissolved in sulfuric acid, from which Pu was back-extracted with sulfurous acid, and U with I M NH₄AC-1.5%H₂O₂. The decontamination factors were estimated to be 10² for U fraction (γ and β) activities) and >10³ for Pu fraction.     

0.57、1.0和1.5 MeV中子诱发235U裂变的99Mo产额绝对测量

用HPGe γ能谱法绝对测量了0.57、1.0和1.5 MeV中子诱发²³⁵U裂变产物⁹⁹Mo的产额,使用双裂变室测量了样品辐照过程中的裂变率,应用MCNP ⅣB模拟了铀样品中的中子能谱,并讨论了非主中子的各种来源对产额数据的影响。得到⁹⁹Mo在0.57、1.0和1.5 MeV的产额分别为6.61%、6.62%和6.28%。本工作与美国阿贡实验室的结果有15%以上的相对偏差,主要是由引用的衰变数据不同引起。对阿贡实验室数据进行校正后,本工作与阿贡实验室数据的相对偏差处于实验不确定度范围内。     

Inelastic Scattering of 14MeV Neutrons by Heavy Nuclei

Energy spectra of neutrons scattered inelastically from natural samples of Ta, W, Au, Hg, TI, Pb and Bi have been studied at an incident energy of 14MeV, using time-of-flight technique. Angular distributions were also measured for Ta and Bi. The energy spectra from 2MeV to 4MeV were found to be qualitatively consistent with the prediction from statistical theory taking account of effects of nuclear pairing and shell structure. The Fermi gas level density parameter and the nuclear temperature were obtained by considering the contribution of the (n,2n) reaction. The variation of level density parameter plotted against mass number was found to agree with the trend predicted theoretically by Newton. Angular distributions proved to be symmetric about 90° and the spin cut-off parameter was estimated to be about 5.8 for Ta and 4.3 for Bi. Cross sections for the (n,n′2n) and (n,2n) reactions were obtained.     

Inelastic Scattering of 14MeV Neutrons by Medium Weight Nuclei

Continuous spectra of neutrons inelastically scattered from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge and As have been measured at an incident energy of 14 MeV with good statistics. Time-of-flight technique was used. In the analyses there were used three sets of inverse-reaction cross section σ_i: (1) calculated with Perey-Buck potential, (2) with Bjorklund-Fernbach potential and (3) cross section assumed to be constant. The inverse-reaction cross section does not affect appreciably the values of nuclear temperature or nuclear level density parameter for these nuclei except Ti, V and Cr. The nuclear temperature was found to be nearly constant in the mass number region of 48–75. The level density parameter as a function of mass number follows the general trend suggested by Newton. Newton's coefficient is found to be 0.092, in agreement with values obtained in other experiments. The total inelastic scattering cross sections are derived.     

14 MeV中子辐照钒引起的长寿命放射性

用HPGe探测器测量了钒以及杂质由14MeV中子活化引起的放射性,得到了50V(n,α)和51V(n,α)反应截面比值在14MeV处为0 584,并由此得到了50V(n,α)反应截面为(8 7±0 7)×10-27cm2。测量了钒中杂质成分被活化后其长寿命放射性核素的半衰期。     

Multi-parametric Measurement of Prompt Neutrons and Fission Fragments for 233U(n th,f)

The multiplicity and the energy of prompt neutrons from the fragments for ²³³U(n _th, f)were measured as functions of fragment mass and total kinetic energy. Average neutron energy against the fragment mass showed a nearly symmetric distribution about the half mass division with two valleys at 98 and 145 U. This shape formed a contrast with a saw-tooth distribution of the average neutron multiplicity. It indicates that the shell-effects, which are pronounced for the fragments having the proton number or neutron number close to the magic-number of 50 or 82, affected the neutron emission process. The slope of the neutron multiplicity with total kinetic energy depended on the fragment mass and showed the minimum at about 130 U. The obtained neutron data were applied to determine the total excitation energy of the system, and the resulting value in the typical asymmetric fission lied between 22 and 25 MeV. The excitation energy agreed with that determined by subtracting the total kinetic energy from the Q-value within 1MeV, thus satisfied the energy conservation. In the symmetric fission, where the mass yield was drastically suppressed, the total excitation energy is significantly large and reaches to about 40MeV: suggesting that fragment pairs are preferentially formed in a compact configuration at the scission point.     

中子活化法测定~(232)Th裂变产物产额

采用中子活化法测量了~(232)Th的裂变产物及其累积产额。利用加速器T(d,n)~4He反应产生的14.9 MeV高注量中子长时间照射ThO_2样品,用高纯锗γ谱仪测量其特征γ谱,求得较长半衰期核素~(99)Mo、~(141)Ce、~(143)Ce、~(131)I、~(140)Ba等的裂变产额,实验结果的典型误差为4%。其中,利用MCNP程序对中子的多次散射效应和自屏蔽效应进行修正,同时考虑了中子注量波动及γ射线在样品中的自吸收影响。