Which of the following is not used in autoradiography?   

In autoradiography, radioactive isotopes (tritium, carbon-14, Phosphorous-32, Sulphur-35) are incorporated in intermediate or raw materials like carbon dioxide, uridine, thymidine, amino acids etc. for studying metabolic pathways including DNA to RNA to proteins pathway, otherwise known as Central Dogma of Molecular Biology. 

The tissues are sectioned or squashed at intervals and then coated with photographic emulsion silver bromide in order to locate the product. Using this technique, C${}^{14}$ has been used for working out the path of carbon assimilation by Calvin in 1955. Tritium in thymidine was employed by Taylor for studying chromosome replication. Tritium in uridine was used in studying RNA synthesis while tritium in amino acid like leucine is employed for study of protein synthesis.