Measurement of the normalized <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mi mathvariant="normal">U</mml:mi><mml:mprescripts/><mml:none/><mml:mn>238</mml:mn></mml:mmultiscripts><mml:mo>(</mml:mo><mml:mi>n</mml:mi><mml:mo>,</mml:mo><mml:mi>f</mml:mi><mml:mo>)</mml:mo><mml:mo>/</mml:mo><mml:mmultiscripts><mml:mi mathvariant="normal">U</mml:mi><mml:mprescripts/><mml:none/><mml:mn>235</mml:mn></mml:mmultiscripts><mml:mo>(</mml:mo><mml:mi>n</mml:mi><mml:mo>,</mml:mo><mml:mi>f</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math> cross section ratio from threshold to 30 MeV with the NIFFTE fission Time Projection Chamber

The normalized $^{238}\mathrm{U}(n,f)/^{235}\mathrm{U}(n,f)$ cross section ratio has been measured using the NIFFTE fission Time Projection Chamber (fissionTPC) from the reaction threshold to $30\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The fissionTPC is a two-volume MICROMEGAS time projection chamber that allows for full three-dimensional reconstruction of fission-fragment ionization profiles from neutron-induced fission. The measurement was performed at the Los Alamos Neutron Science Center, where the neutron energy is determined from neutron time of-flight. The $^{238}\mathrm{U}(n,f)/^{235}\mathrm{U}(n,f)$ ratio reported here is the first cross section measurement made with the fissionTPC, and will provide new experimental data for evaluation of the $^{238}\mathrm{U}(n,f)$ cross section, an important standard used in neutron-flux measurements. Use of a development target in this work prevented the determination of an absolute normalization, to be addressed in future measurements. Instead, the measured cross section ratio has been normalized to ENDF/B-VIII.$\ensuremath{\beta}5$ at 14.5 MeV.