What type of particles consists of two protons and two neutrons?

Alpha particles are specifically defined as a type of particle that consists of two protons and two neutrons, which means they are essentially identical to the nucleus of a helium atom. These particles are emitted during certain types of radioactive decay processes, most notably alpha decay.

The structure of an alpha particle, with its two protons and two neutrons, gives it a relatively high mass compared to other types of radiation, and as a result, alpha particles have a limited range and a high ionization potential. This means they can effectively cause damage to materials and biological tissues but cannot penetrate very far, as they are stopped by barriers as thin as a sheet of paper.

The other types of particles mentioned in the choices do not share this specific composition: beta particles are electrons or positrons emitted during beta decay, gamma rays are high-energy electromagnetic radiation with no mass or charge, and neutron particles refer to particles that are simply neutrons without the additional protons that characterize an alpha particle.