Gamma decay, also known as gamma radiation, does not directly change the atomic mass (mass number) of a nucleus. Gamma decay involves the emission of gamma rays, which are high-energy photons. These photons carry no electric charge and have no mass, so their emission does not affect the number of protons or neutrons in the nucleus.
However, gamma decay often accompanies other types of radioactive decay, such as alpha decay or beta decay, which can change the atomic mass of a nucleus. During alpha decay, an alpha particle (consisting of two protons and two neutrons) is emitted from the nucleus, leading to a decrease in the mass number by 4. In beta decay, either a beta minus (β-) particle (an electron) or a beta plus (β+) particle (a positron) is emitted, which can result in a change in the mass number as well.
Therefore, while gamma decay itself does not change the atomic mass of a nucleus, it may occur simultaneously with other types of radioactive decay that do affect the mass number.