Measure of the probability of the occurrence of a reaction. The cross section is the apparent surface which a Piece of matter on which radiation impinges, causing nuclear... Positively charged nucleus of an atom. Its diameter amounts ... exposes to an arriving particle. The cross section is indicated in surface units. Uncharged elementary particle with a mass of 1.67492716 ·10... cross sections are frequently measured in units called ‘barns’, symbol: b. 1 Unit used in nuclear physics to indicate probability of a re... is equal to 10-28 m2.