In inertial confinement fusion (ICF), deuterium-tritium (DT) fuel is brought to densities and temperatures where fusion ignition occurs. Mix of ablator material into the fuel may prevent ignition by diluting and cooling the fuel. MARBLE experiments at the National Ignition Facility (NIF) provide new insight into how mix affects thermonuclear burn. These experiments use laser-driven capsules containing deuterated plastic foam and tritium gas. Embedded within the foam are voids of known sizes and locations, which control the degree of heterogeneity of the fuel. Initially, the reactants are separated, with tritium concentrated in the voids and deuterium in the foam. During the implosion, mix occurs, leading to DT fusion reactions in the mixed region. Here we show that by measuring ratios of DT and deuterium-deuterium (DD) neutron yields for different macropore sizes and gas compositions, effects of mix heterogeneity on thermonuclear burn may be quantified and understood for the first time.