Stigma (letter) Stigma (letter) Stigma (ϛ) is a ligature of the Greek letters sigma (Σ) and tau (Τ), which was used in writing Greek between the Middle Ages and the 19th century. It is also used as a numeral symbol for the number 6. In this unrelated function, it is a continuation of the old letter digamma (originally Ϝ, cursive form ), which had served as a numeral since antiquity and was conflated with the σ-τ ligature in the minuscule handwriting of the Middle Ages. The στ ligature (, ) was one of many ligature forms that came into widespread use as
Commonly used gamma-emitting isotopes into a tumor such as a breast tumor, and the tumor is irradiated by gamma ray photons from the wire. At the end of the treatment the wire is removed. A rare but notable gamma source is sodium-24, this has a very short half-life but it emits photons with very high energies (>2 MeV). It could be used for radiography of thick steel objects if the radiography occurred close to the point of production. In common with Co-60 and Ir-192 it is formed by the neutron activation of the commonly found stable isotope. Americium-241 has been used as a source