Characteristic X ray emission in gadolinium following neutron capture as an improved method of in vivo measurement: A comparison between feasibility experiment and Monte–Carlo simulation

We have extended our previous experimental and Monte–Carlo work on the detection of Gd by in vivo prompt gamma neutron activation analysis to include X ray emission. In this paper we incorporate the characteristic K X ray emission that occurs due to internal conversion from the de-excitation of the 155Gd(n,γ)156Gd∗ and 157Gd(n,γ)158Gd∗ reactions. The experimental Gd K X ray intensities are compared with the Monte–Carlo model and demonstrate excellent agreement. The experiment was consistently higher than simulation by 5%. For the detection system used, the Gd Kα X rays are about 1.5 times as intense as the most dominant prompt gamma ray from the 157Gd(n,γ) reaction. The partial elemental cross section for Kα X ray emission is ∼1.35 times larger than that of the most dominant prompt gamma ray from neutron capture of 157Gd alone. The use of the K X rays was found to improve the sensitivity of the proposed system to measure Gd retention after exposure to a Gd-based MRI contrast agent. The detection limit in phantoms was ∼30% better when the X ray signal was incorporated into the analysis method, reducing the detection limit from 0.89 to 0.64 ppm Gd.