Pu239(n,2n) Pu238 cross section measurement using a recoil method

A new measurement of the $^{239}\mathrm{Pu}(n,2n)$ $^{238}\mathrm{Pu}$ cross section was performed at 7.1, 7.7, and 9.3-MeV incident neutron energy using an activation technique to help resolve inconsistencies in the existing database above the threshold of the reaction. The method is based on the collection of $^{238}\mathrm{Pu}$ nuclei produced in the $(n,2n)$ reaction. Although never used before to obtain a neutron cross section, this recoil technics overcomes the difficulty of having deposits of $^{239}\mathrm{Pu}$ of extremely high purity. The collection efficiency must be determined using previously published data at higher energy, and our measurements are relative to these points. The neutrons were produced via the $^{2}\mathrm{H}(d,n)$ $^{3}\mathrm{He}$ reaction. The incident neutron flux was determined via the $^{27}\mathrm{Al}$($n$,\ensuremath{\alpha}) $^{24}\mathrm{Na}$ standard reaction. The present measurements are consistent with the previous data and disagree with the jeff3.3 evaluation.