Radioactive tracer is also called "radioactive indicator". A tracer distinctively characterized by radioactivity in the stomach. Commonly used radioisotopes include carbon 14 (14C), phosphorus 32 (32P), sulfur 35 (35S), iodine 131 (131I), hydrogen 3 (3H) radioactive isotopes hydrogen (H), carbon (C), phosphorus (P), sulfur (S), and iodine (I) are widely used to trace paths in biochemical reactions. Radioactive tracers can also be used to track distribution in natural systems, such as cells or tissues. Radioactive tracers can also be used to identify where natural gas is emerging, which can then be extracted using hydraulic fracturing. Radioactive tracers form the basis of various imaging systems, such as Positron emission computed tomography (PET) and single-photon emission computed tomography (SPECT, or SPET). and Technetium scans. Radiocarbon dating uses the naturally occurring carbon-14 isotope as an isotope marker for any living thing created by nature.