Measurement of Average Cross Section for 238U(n, 2n)237U Reaction

Accurate solution of the group diffusion equations for PWR cores requires explicit treatment of the non-homogeneous macroscopic parameters within each fuel assembly. It is argued that the response matrix approach is a convenient method to handle this problem provided all matrix elements for the non-homogeneous assemblies can be computed. This so called local problem is solved in this paper by a perturbation algorithm which leads to sensitivity coefficients for the power series expansions of the response matrix elements. A numerical study for 2 representative assemblies of the Indian Point Unit No. 2 (IP2) reactor is carried out and response matrices obtained by the perturbative method are compared with values computed by a finite difference program.     

Preparation of Uranium-233 with Low Uranium-232 Content

Abstract

Thorium dioxide, previously freed of uranium and protactinium contamination by means of ion-exchange, was irradiated for 70.5 and 140hr respectively in the core and graphite thermal column positions of the KUR (Kyoto University Reactor). After being cooled for more than 1½yr, the ²³³U produced was chemically purified by ion exchange, and the resulting ²³³U specimens were electro-deposited on a stainless steel plate for submission to α-spectrometric measurement with a silicon surface barrier detector. From the α-spectra thus obtained, the ²³²U to ²³³U atomic ratios of the specimens were evaluated and compared with the calculated values. This revealed that the rate of formation of ²³²U depended largely on the ²³²Th(n, 2n)²³¹Th reaction by fast neutrons, and consequently, on the neutron energy distribution prevailing at the position of irradiation. The purest ²³³U sample obtained from the thorium target, irradiated with pure thermal neutrons in the graphite thermal column, yielded a ²³²U to ²³³U atomic ratio of 3.02×10^?9, which corresponded to 6.62×10^?6 in α-activity ratio.     

~(56)Fe(n,xnγ)截面的符合测量方法研究

为降低瞬发γ射线法在测量重裂变核(n,xnγ)截面中的康普顿坪和本底水平,尝试采用符合方法来分析数据。在中国原子能科学研究院600kV高压倍加器上用两个Clover探测器直接测量56Fe(n,xnγ)5个能点的反应截面,然后采用符合技术分析数据,计算1 238.3keVγ射线截面。此结果与直接方法计算的截面结果基本一致,从而验证了符合测量方法的有效性。     

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.     

14.8 MeV中子诱发78Kr(n,2n)77Kr反应截面的测量

快中子诱发(n,2n)反应截面的测量在核反应机制研究和核技术应用等方面有着广泛的应用价值。本文在中国原子能科学研究院的高压倍加器上,基于活化法实验测量了78Kr(n,2n)77Kr在148 MeV能点的反应截面。样品靶为高纯78Kr气体样品,用十万分之一天平称重得到78Kr的质量,将两片高纯93Nb薄片分别固定在样品靶两侧以监测中子注量率。利用T(d,n)4He反应产生148 MeV中子,轰击距中子源约10 cm的样品靶大于4 h后,用准确刻度过效率的HPGe探测器测量活化产物 77Kr和92Nbm的活度。利用蒙特卡罗程序计算中子注量率修正、样品自吸收修正、样品几何修正等因子,得到了78Kr(n,2n)77Kr的反应截面,并将结果与文献值和评价数据库进行了比较。研究结果有助于提高78Kr(n, 2n)77Kr反应截面测量和评价的水平。     

Measurement of the 93Nb(n,n')93mNb Reaction Cross Section at 14.5 and 14.9MeV

Abstract

Cross sections at 14.5 and 14.9 MeV for the low threshold energy reaction of ⁹³Nb(n, n')^93mNb were measured by the activation method at the FNS (Fusion Neutronics Source) of JAERI. Although there are multiple experimental data for this reaction cross section from the threshold to 9 MeV, only one data at 14.3 MeV has been reported by Ryves et at. The correction for low energy neutron contribution was found very sensitive on the reaction rate determination. Careful treatments were performed to arrive at final error evaluation considering neutron spectrum calculated with a Monte Carlo Code and cross section curves available. The cross sections measured in the present work are larger by more than 30% than those in both IRDF-90 and JENDL Dosimetry File, which are based on the data of Ryves et al. On the other hand, the present data are lower by 10-15% than the evaluation by Odano et al. It is highly recommended to re-evaluate the cross section by taking the present data into account.     

Measurements of Average Cross Section for 232Th(n, 2n)231Th Reaction to Neutrons with Fission-Type Reactor Spectrum and of Gamma-Ray Intensities of 231Th

The average cross section for the ²³²Th(n, 2n)²³¹Th reaction to neutrons with the energy spectrum close to that of fission neutrons was obtained in the core of the Kyoto University Reactor, KUR. The value obtained was 12.5±0.84 mb. This value agrees satisfactorily with Phillips' and with the calculated value obtained with the cross section in the U-K library and the Maxwellian fission neutron spectrum given by Leachman. A somewhat poorer agreement is seen with the calculated value obtained from Butler & Santry's cross section and Leachman's spectrum. The discrepancy amounts to 24 and to 39% respectively, for the average cross sections calculated with these two excitation functions and the fission neutron spectrum given by McElroy.

By making use of a Ge(Li) counter whose photopeak efficiency had been carefully calibrated, the absolute intensities were determined for eleven photopeaks observed on the γ-ray spectrum emitted by ²³¹Th.     

35MeV以下56Fe(n,p)56Mn反应截面及协方差评价

56Fe(n,p)56Mn通常作为标准反应来监测中子场通量,该反应截面数据的准确性直接影响到活化法测量结果的精确度,进而影响到实验待测物理量的精度。本文开展了56Fe(n,p)56Mn反应截面实验测量数据评价工作与协方差计算工作,首先系统分析EXFOR中现有的56Fe(n,p)56Mn反应截面实验测量数据,对实验数据进行了归纳总结分析,并从中子源、测量方法、探测器类型等方面对56Fe(n,p)56Mn直接测量实验数据进行评价。然后,拟合给出适用入射中子能量区间为295～35 MeV的激发曲线。随后,针对评价中重点推荐的实验数据开展了关联协方差矩阵的计算工作。最后,使用核反应计算程序TALYS对56Fe(n,p)56Mn激发曲线进行了调参计算并和评价数据进行了比较分析。该工作拓展了现有的中子活化反应截面实验数据的评价方法,结果提高了35 MeV以下中子诱发56Fe(n,p)56Mn反应的评价数据精度。     

Activation Cross Sections for (n, 2n) Reaction on Neodymium,Samarium, Gadolinium and Ytterbium at 14.6 MeV

Activation cross sections for the (n, 2n) reaction on Nd, Sm, Gd and Yb have been measured at 14.6 MeV by using a Ge(Li) γ-ray detector. The following cross sections (mb) have been obtained: ¹¹²Nd 1,675±160, ¹⁴³Nd 1,789±147, ¹⁵⁰Nd 1,720±128, ¹⁵⁴Sm 2,010±137, ¹⁶⁰Gd 2,173±152, ¹⁶⁰Yb 2,226 ±152. These (n,2n) cross sections are compared with the theoretical calculations performed by a new model including pre-equilibrium and statistical models. A good agreement between the experimental and calculated cross sections is obtained.     

Estimate of (n,p) Cross Section for Unstable Nuclide 60Co by Multi-Step Hauser-Feshbach Model with Pre-Equilibrium Correction

Abstract

The cross section of the (n, p) reaction for unstable target nuclide ⁶⁰Co was estimated by a multi-step Hauser-Feshbach model code with pre-equilibrium correction. Parameters used in the model calculations were determined from the fitting of the cross sections to existing experimental data of (n, p), (n, α) and (n, 2n) cross sections and particle emission spectra for the adjacent stable nuclide ⁵⁹Co. The present result for ⁶⁰Co(n, p)⁶⁰Fe was compared with the values estimated from semi-empirical formulae around 14 MeV, The prediction accuracy of the present calculation is considered to satisfy the requirement for the fusion reactor applications. The theoretical model calculations with a reliable parameter set are recommended rather than the semi-empirical formulae for the important activation cross sections of exotic target nuclei including unstable ones.