Feasibility of a Compton Suppression System for the X-ray Fluorescence (XRF)

Quantifying the plutonium contents in spent nuclear fuels or recovered U or U/TRU products is necessary in safeguards. The x-ray fluorescence (XRF) technique measures self- induced characteristic x-ray fluorescence from uranium and plutonium, and compares measured pulse heights of uranium and plutonium peaks to quantify elemental uranium and plutonium. Photon measurements of spent nuclear fuel using high resolution spectrometers show a large background continuum in the low energy x-ray region in large part from Compton scattering of energetic gamma-rays. The high Compton continuum can make measurements of plutonium x-rays difficult because the relatively small signal to background ratio produced. In pressurized water reactor (PWR) spent fuels with low plutonium contents (~1%), the signal to background ratio may be too low to get an accurate plutonium x-ray measurement. In the present study, to reduce the Compton continuum background, the Compton suppression system for XRF was proposed. In this study, the feasibility of a Compton suppression system was evaluated by the MCNP simulations and measurements of radiation source, by measuring relevant factor, the suppression ratio and the peak-to-total ratio. Elimination of Compton scattered events which could not make photo-peaks in main detector is made by using anti-coincidence electronics to reject gamma-rays that escaped from the main detector. Additionally, to verify and optimize the system for measurements of spent nuclear fuels, the MCNP simulations were performed using same geometry of the experiments. As a performance test of the Compton suppression system for XRF, the peak-to- total ratio was enhanced by a factor of two or more when the Compton suppression system was used.