Using n as the number of nucleons (in this case, protons), we can introduce the degree of ionization 0 ≤ ≤ x ≤ ≤ 1 so n e = x n , and the numerical density of neutral hydrogen is n h = ( 1 − − x ) n . With a cross section α α 0 (which has units of area) and the number of ionizing photons per area per second J , the ionization rate N I is