Honors College Thesis

The Neutron Capture Cross Section of ²⁰⁸Pb

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  • In this project we measure the neutron capture cross section of ²⁰⁸Pb. We began by calibrating our silicon detector with ²⁰⁴Tl. Two experiments were then performed, the first in December of 1999 and the second in March of 2000. Both experiments were performed in the nuclear reactor of the Radiation Center at Oregon State University. The first was done in the rabbit fast transfer facility , with a neutron flux of 1.1 x 10¹³ n/cm²/s and the second was done in the thermal column of the reactor, with a flux of 7.35 x 10¹⁰ n/cm²/s. The December sample was prepared by rolling a thin foil between steel rollers, and had a large number of gamma and beta ray contaminants. It was not successful. The March sample was not rolled out, and was irradiated with a far lower flux and no fast neutrons. Its gamma spectrum was clean , except for two contaminants. However, this sample showed considerable internal absorption of low energy beta particles. We used the March data to calculate the cross section of ²⁰⁸Pb, and found that σ = 0.091 +/- 0.018 millibarns. This is considerably smaller (54%) than the previously published value of about 0.2 millibarns.
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