In gamma decay, the nucleus becomes more stable without changing its identity. During this process, a nucleus releases excess energy in the form of gamma radiation, but the number of protons and neutrons (and hence the element) remains unchanged.
In contrast:
- Alpha decay involves the emission of an alpha particle (2 protons and 2 neutrons), resulting in a change of the element.
- Beta-plus decay (or positron emission) changes a proton into a neutron, leading to a change in the element.
- Beta-minus decay involves the conversion of a neutron into a proton, also changing the element.
So, the answer is gamma decay.