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Expansion of the Surrogate Method to Measure (n,xn) Cross Sections and Fission Neutron Multiplicity Distributions

  • Author(s): Akindele, Oluwatomi
  • Advisor(s): Norman, Eric B
  • et al.

In addition to \textsuperscript{235,238}U and \textsuperscript{239}Pu, \textsuperscript{241}Pu is considered a major actinide in regards to nuclear fuel and systems. Despite contributing up to $\sim$10$\%$ of energy generated in nuclear reactors towards the end of a cycle, nuclear data for this isotope is limited. Due to its 14 year half-life, target manufacturing for this isotope is difficult, and as a result the (n,xn) cross section and fission neutron multiplicity as a function of incident neutron energy for $^{241}$Pu does not exist in the published literature. Using the surrogate ratio method, experimental difficulties associated with target manufacturing can be circumvented in measuring n + $^{241}$Pu reactions by using inelastic scattering of $\alpha$ particles incident on $^{242}$Pu to create the same compound nucleus. The NeutronSTARS detection array in Cave 4 of the Texas A$

amp;$M Cyclotron was commissioned specifically for experiments of this nature. The target chamber consists of three large area silicon detectors: two serve as a silicon telescope to determine the energy of the scattered alpha particle, and one fission detector to gate on fission events by detecting fission fragments. A segmented cylindrical array consisting of ~2.2 tons of gadolinium doped liquid scintillator is used to detect emitted neutrons. By relating the detected recoiled alpha energy to an equivalent neutron energy, detecting fission fragments to separate neutron evaporation events from fission, and detecting emitted neutrons in close time coincidence; an attempt to measure the (n,xn) cross section and prompt fission multiplicity for $^{241}$Pu was made. The prompt fission neutron multiplicity was fit to a skewed Gaussian distribution, while the average neutron multiplicity was recorded. Due to the large contribution of oxygen and carbon in the target, relatively low neutron detection efficiency, and inadequate background rejection for non-fission events; the (n,xn) cross section measurement was difficult to extract.

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