Nuclear fission

LLNL researchers are developing capabilities to increase the accuracy of data essential for nuclear reactor design and operation.

Accurate data for accurate models

Nuclear data play an important role in the modeling of nuclear physics application, and the accuracy of the data has a direct impact of the accuracy of the model. Nuclear cross sections—a measure of the probability that two particles will react—provide important data essential for the design and operation of nuclear reactors. Precise measurements of actinides, notoriously difficult to obtain, are especially important to support LLNL’s stockpile stewardship missions.

A collaboration for precision measurements

Our researchers lead a collaboration of two national laboratories and four universities—the Neutron Induced Fission Fragment Tracking Experiment (NIFFTE)—to design, construct, and operate a fission time projection chamber (TPC) for measuring fission cross-sections of major actinides to the highest achievable precision.

fission time projection chamber
A cutaway view of the fission TPC. The detector is dual sided for use with two targets for cross-section ratio measurements. The detector gas volume is approximately 2 L. Each of the two anodes is composed of nearly 3,000 segments.

TPCs are gas-filled devices that measure the trajectory and energy of charged particles in motion. When a positively charged particle, such as a fission fragment, enters a gas, it ionizes the gas, creating electron–ion pairs all along its trajectory. With the appropriate gas mixture and an electric field, the electrons drift at a constant velocity towards an anode to become the basic constituent of a measured signal. We segment the anode and measure the relative arrival time of the electrons at each segment to generate a 3D reconstruction of the charge cloud.

This 3D particle track reconstruction provides:

  • Track angular distributions for an improved detector efficiency correction
  • Particle identification for suppression of neutron induced backgrounds
  • Track vertexing for beam and target nonuniformity corrections

The unparalleled background rejection capabilities of the fission TPC allow the detailed systematic checks needed for precision cross-section measurements.