Whether a gamma ray can penetrate lead depends on a probabilistic function. The proportion of gamma rays that penetrate lead decreases exponentially as the thickness of the lead is increased. For example, for gamma rays with an energy of 100 keV, 50% will penetrate lead shielding of 0.27 mm thickness. For each additional 0.27 mm of lead added, the proportion of penetrating gamma rays will decrease by a further 50%.