An infinite slab of moderator of thickness 2a contains at its center a thin sheet of 235 u of thickness t. derive the one-group criticality condition, using d and t to represent the diffusion parameters of the moderator σaf and to represent the macroscopic cross section of the ^235 u. plot the thickness t as a function of σaf using the criticality condition. explain the relationship between t and σaf in combinations that lead to a critical system. use eta = 1.80, d = 1.0 cm,| l = 56 cm, and a = 15 cm.