Computational Optimization of ^133mXe Production in the Washington State University TRIGA Reactor

Here we present a new method of irradiating ¹³²Xe capsules with neutrons to produce ^133mXe gas standards that are used for radiation detector calibration at radioxenon measurement laboratories in support of the Comprehensive Test Ban Treaty (CTBT). This method is designed to maximize the production of ^133mXe compared to ¹³³Xe, both of which are competing products from the ¹³²Xe(n, g) reaction. The ^133mXe is produced at a much higher fraction for high-energy neutron absorptions in ¹³²Xe (~50% for fast neutrons versus ~11% for thermal neutrons).

We performed “spectral tuning” of the Washington State University (WSU) TRIGA reactor neutron spectrum inside the ¹³²Xe ampules to maximize the number of fast neutrons and minimize the number of thermal neutrons available for ¹³²Xe absorption. Spectral tuning analysis, done with Monte Carlo simulations, provided valuable insights into a future final design for a ¹³²Xe irradiation capsule. With no spectral tuning, the fractional yield of ^133mXe in the WSU reactor was ~11.7%. By surrounding the ¹³²Xe capsule with a 0.5-cm-thick layer of tungsten and a 2.83-cm layer of europium (III) oxide and placing it in the reactor’s cadmium rotator tube next to the fuel elements, the fractional yield of ^133mXe can be increased to 24.6%, a 111% increase in yield. Thus, by improving the fractional yield of ^133mXe through spectral tuning, the CTBT will have better quality gas standards to use for radioxenon detector calibration to assist in the CTBT’s mission.