Capture cross sections of In¹¹⁵ and I¹²⁷ in the Kev region

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Virginia Polytechnic Institute


Design requirements of fast reactors and theoretical studies which involve the optical model of the nucleus have increased the needs for neutron capture cross sections in the kev region. An indirect method to obtain these cross sections has been developed and applied to In¹¹⁵ and I¹²⁷ . Previous measurements of capture cross sections from a few kev to a few hundred kev and attempts to obtain average resonance parameters from analyses of these cross sections have not been entirely successful because they did not extend over a large enough energy range or because the normalizations to obtain absolute values were not completely satisfactory.

Samples of In¹¹⁵, I¹²⁷, and Co⁵⁹ were activated in a well collimated beam of neutrons from the Virginia Polytechnic Institute UTR-10 nuclear reactor with varying thicknesses of B¹⁰ filters being interposed in the beam. The resulting data, representing activity versus filter thickness, were corrected for neutrons scattered from the beam and for neutrons with kinetic energies in excess of 100 kev. These corrected activation curves were then analyzed to obtain the average resonance parameters. Using the known thermal activation cross sections and the gamma ray strength function of I¹²⁷ as standards, the average s- and p- wave gamma ray strength functions <rγ/D₀> l = 0 and <rγ/D₀/> l = 1 have been calculated. From these average resonance parameters it was possible to calculate neutron capture cross sections for the two nuclides for neutron energies up to 100 kev. It was also possible to obtain the resonance integral of In¹¹⁵ I¹²⁷ relative to that of I and the s- and p- wave neutron strength functions of both nuclides.