Answer:
Gamma decay
Explanation:
There are 3 types of radioactive decay:
- Alpha decay: in this decay, a nucleus emits an alpha particle (consisting of 2 protons and 2 neutrons, so a nucleus of helium). The alpha particle has a large charge (2e) and a large mass (4u), so it is strongly ionizing, and therefore loses energy faster while moving through matter, therefore its penetrating power is low (it can be easily stopped by a thin sheet of paper or by the skin)
- Beta decay: this decay occurs when a neutron in a nucleus turns into a proton, emitting a beta particle (a fast-moving electron) alongside with an antineutrino. The beta particle has a lower charge (e) and a smaller mass than the alpha particle, so it has a moderate penetrating power, being able to penetrate more than the alpha particle (the beta particle can be stopped by a thin sheet of aluminium)
- Gamma decay: this decay occurs when an excited nucleus decays emitting a gamma ray photon (which is electromagnetic energy with very high energy and frequency). The gamma ray photon has no charge and no mass, therefore it has the most penetrating power, being able to travel a much large distance before being absorbed by matter (several metres of concrete are required to stop gamma radiation.
So, the description in the question refers to gamma decay.
