Effective Resonance Integral for Pu²⁴⁰ in Plutonium-Aluminum Alloy Rods

Reactivity measurements have been made in the Physical Constants Testing Reactor (PCTR) on a series of plutonium-aluminum rods with 0.65 in. diam and 31.00 in. long. The samples were contained in a cadmium cover during the measurements. The series of rods consisted of eight sets of two rods each. Two separate batches of plutonium with different isotopic compositions were used in the two rods within a set. The concentrations of plutonium in the alloys were adjusted in such a manner that (1) the two rods within a set differed effectively only in Pu²⁴⁰ content, and (2) the rod with the most Pu²⁴⁰ in one set had the same quantity of Pu²⁴⁰ as the rod with the least Pu²⁴⁰ in an adjacent set. The difference in reactivity measured between the rods in a set is the result of different resonance absorption rates in Pu²⁴⁰ in the two rods. The sensitivity of the reactor to epicadmium absorption in Pu²⁴⁰ was obtained by calibrating the system with three sets of rods that were all very nearly dilute in Pu²⁴⁰ concentration. The experimental data yield effective resonance integrals of Pu²⁴⁰ for the remaining rods relative to the dilute resonance integral. The value of the effective resonance integral for the rod with the highest concentration of Pu²⁴⁰ is a factor of about seven smaller than the dilute integral.