Relative measurements of the mean number of neutrons emitted in fission of U233, U235 and Pu239 by thermal neutrons and by neutrons characteristic of a fission spectra

The ratios of the mean number of neutrons () emitted in the fission of U²³³, U²³⁵ and Pu²³⁹ by thermal neutrons and by neutrons characteristic of a fission spectra have been measured. A method was employed in which the number of fission events was counted simultaneously with the number of coincidences between fragments and fission neutrons. It is found that the quantity increases by approximately 10% in all isotopes which were investigated.In conclusion the authors wish to thank P. E. Nemirovsky for the interest which he has taken in this work.     

14.8 MeV中子诱发~(238)U裂变产物产额测量

裂变产物产额作为裂变过程的一个重要参数,其准确测量对有关裂变的很多方面都有重要意义。为了准确测量中子诱发~(238)U裂变产物产额,利用中国工程物理研究院PD-300加速器上的T(d,n)4He反应,产生14.8MeV的中子,诱发~(238)U裂变。辐照过程中,通过金硅面垒半导体探测器监测中子通量的变化。使用Al片作为监测片计算整个照射过程中样品的平均中子通量。辐照结束后,利用高纯锗(High-Purity Germanium,HPGe)探测器测得裂变产物特征γ射线计数,计算得到裂变产物的产额,使用MCNPX软件对中子的多次散射和自屏蔽效应进行修正,并通过计算得到样品和监测片的自吸收修正、中子通量波动因子。得到了95Zr、127Sb、140Ba、147Nd、131I、103Ru等长半衰期产物的累积产额值,并将结果与以前的文献值做了比对,研究结果有助于~(238)U裂变产物产额的分析和评价。     

Number of neutrons emitted by individual fission fragments of U235

A large detector filled with a liquid organic scintillator containing cadmium has been used to measure the number of neutrons emitted by individual fragments in U²³⁵ fission by thermal neutrons. Using 4 geometry the dependence of the number of neutrons emitted by fragment pairs on the mass ratio has been measured. The excitation energy used in neutron evaporation is determined on the basis of the semiempirical Weizsäcker formula. A sharp asymmetry in the distribution of excitation energy between the heavy and light fragments is noted. The data which are obtained are found to be in disagreement with the statistical theory of fission proposed by Fong.The authors wish to express their gratitude to B. G. Erozolimskii, who was one of the people responsible for initiating this work; he also helped in its execution by a number of valuable comments and discussions. The authors also wish to thank A. A. Markov and A. A. Borodin for the design and construction of the multi-channel ratio analyzer and K. S. Mikhailov for advice and help in the preparation of the scintillator for the neutron detector.     

On the number of neutrons emitted by Pu239 on fission by thermal and superthermal neutrons

The average number of neutrons emitted on fission of Pu²³⁹ by thermal and superthermal neutrons (E_w from 0.15 to 0.5 ev) is measured.The results of this work were presented for discussions at the Geneva Conference on the Peaceful Uses of Atomic Energy in August, 1955.     

Determination of the mean number of secondary fission neutrons from the fragment mass distribution

A method is presented for computing the mean number of secondary fission neutrons ¯v from the fission-fragment mass distribution curves. The error of the method is estimated. It is shown that in those cases in which the fragment mass distribution curves are carefully studied, ¯v can be determined with satisfactory accuracy by this method. The value of ¯v is computed for fission in Th²³², U²³³, U²³⁵, U²³⁸, Pu²³⁹, Am²⁴¹ and Cf²⁵². The results are discussed and compared with results obtained by other methods. The partial values ¯v _m are computed for thermal-neutron fission of U²³³ and U²³⁵.     

Fragment yield in the fission of U233 and Pu239 by fast neutrons

A radiochemical method has been used to determine the absolute yields for a number of fragments in the fission of U²³³ and Pu²³⁹ by 14.5 Mev neutrons and fission-spectrum neutrons. Some regularities in changes in the fragment mass distribution are discussed. The dependence of the difference m between the average masses of the heavy and light fragments on the mass number A of the dividing nucleus is considered.In conclusion, the authors wish to express their deep gratitude to A. A. Malinkin, Yu. A. Vasil'ev, and V. I. Shamarukhin for the irradiation of the specimens, to P. N. Moskalev, N. V. Shuvanova, A. A. Egorova, and K. N. Borozdina for carrying out some of the chemical work, and also to V. V. Zakatilov and L. N. Sorokina for assistance in the physical measurements.