Deuterium's absorption cross section for thermal neutrons is 0.52 millibarns (5.2 × 10−32 m2; 1 barn = 10−28 m2), while those of oxygen-16 and oxygen-17 are 0.19 and 0.24 millibarns, respectively. 17O makes up 0.038% of natural oxygen, making the overall cross section 0.28 millibarns. Therefore, in D2O with natural oxygen, 21% of neutron captures are on oxygen, rising higher as 17O builds up from neutron capture on 16O. Also, 17O may emit an alpha particle on neutron capture, producing radioactive carbon-14.