The effects of X-ray exposure are modeled to fit the Nd surface density data in Fig. . The total Nd surface density under X-rays-on conditions, 𝑛 !" #$#,$& , was fit to the Nd surface density that desorbs from the surface, 𝑛 !" "'(,$& , subtracted from the Nd surface density that adsorbs to the surface, 𝑛 !" )",$& ,