Techniques for Measuring the $^{239}$Pu(n,f) /$^{235}$U (n,f) Cross Section Ratio using the NIFFTE Time Projection Chamber*

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) aims to measure the $^{239}$Pu (n,f) cross-section to better than 1\% across the 250~keV to 20~MeV incident neutron energy range through the use of a purpose-built Time Projection Chamber (TPC). Prior to the 2013 LANSCE run cycle, the active area of the TPC was doubled and now provides 4$\pi$ coverage with nearly 6000 independent readout channels. The current status of the $^{239}$Pu/$^{235}$U (n,f) cross-section measurement, including techniques for handling the data-rates associated with the large spontaneous alpha activity of $^{239}$Pu in the fission TPC will be discussed.