Measurement of the$^{ 235}$U(n,n')$^{235m}$U Integral Cross Section in a Pulsed Reactor

We will present the integral measurement of the neutron inelastic cross section of $^{235}$U leading to the 26-minute, E*=76.5 eV isomer state. Small samples (5-20 microgm) of isotope-enriched $^{235}$U were activated in the central cavity of the CALIBAN pulsed reactor at Valduc where a nearly pure fission neutron spectrum is produced with a typical fluence of 3x10$^{14}$ n/cm$^{2}$. After 30 minutes the samples were removed from the reactor and counted in an electrostatic-deflecting electron spectrometer that was optimized for the detection of $^{235m}$U conversion electrons. From the decay curve analysis of the data, the 26-minute $^{235m}$U component was extracted. Preliminary results will be given and compared to gamma-cascade calculations assuming complete K-mixing or with no K-mixing.