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.     

Cross section for U233 and U235 fission by neutrons with energies between 3 and 800 Kev

The fission cross sections for fission of U²³³ and U²³⁵ by neutrons with energies between 3 and 800 Kev have been measured. The neutrons were obtained from a van de Graaff generator using the reaction T(p,n)He³. When the neutron energy is varied between 3.4 and 780 Kev the fission cross section drops from 7.5 to 1.9 barns for U²³³ and from 4.8 to 1.05 barns for U²³⁵.This paper is based on the results of work carried out in 1953–1954.     

A measurement of the U233, U235, Pu239 total cross sections. As well as the fission cross section of U235 for resonance neutrons

The measurement of the interaction cross sections between resonance neutrons and fissionable isotopes are of definite importance both for reactor calculations and for the construction of various nuclear models. In this work we present the results of measurements of the U²³³, U²³⁵, and Pu²³⁹ total cross sections, as well as the fission cross section of U²³⁵. The measurements were performed on a mechanical neutron spectrometer (neutron chopper) with a resolution of 0.1–0.2 sec/m in the neutron energy region from 3–5 to about 500 ev. The resonance parameters are calculated to energies of about 30–50 ev, for which the levels may still be considered resolved. For these resonances the neutron widths g_n are determined; the total widths are determined for sufficiently excited levels when the uncertainty in this determination is no greater than about 50%. For U²³⁵ resonance in the energy region from 2.5 to 20 ev, in addition to the above parameters, the ratio of the fission width _f to the total width is found.     

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.     

Effective cross sections for fission of U233, U235, Pu239 and Pu240 by neutrons with energies from 30 kev to 5 Mev

Measurements have been made of the absolute values of the effective fission cross sections in U²³³, U²³⁵, Pu²³⁹ and Pu²⁴⁰, using as photoneutron sources Sb + Be ( 30 kev), Na + D₂O ( 250 kev), Na + Be ( 900 kev), a source with a simulated fission-neutron spectrum and Po--Be. The fission cross sections for 30 kev-neutrons are 3.06 ± 0.16, 2.21 ± 0.12 and 1.79 ± 0.11 barns for U²³³, U²³⁵ and Pu²³⁹, respectively. As the neutron energy Is increased from 30 kev to 250 kev the cross sections in U²³³ and U²³⁵ fall off by approximately 35% and then remain almost constant while the cross section in Pu²³⁹ falls off by 12% and then increases. The effective cross sections for fission in Pu²⁴⁰ for 900-kev and 5-Mev neutrons is 1.4 barns and the threshold for fission in Pu²⁴⁰ lies within the limits 250 and 900 kev.In conclusion the authors wish to express their gratitude to P. E. Spivak for help in formulating the problem and a discussion of the results.