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