An alpha particle has two protons and two neutrons, so it has a positive charge.
(Since it has two protons it is a helium nucleus.)
Beta emission is when a high speed electron (negative charge) leaves the
nucleus. Beta emission occurs in elements with more neutrons than protons, so a
neutron splits into a proton and an electron. The proton stays in the nucleus
and the electron is emitted.
A nucleus which is in an excited state may emit one or more photons (packets of
electromagnetic radiation) of discrete energies. The emission of gamma rays does
not alter the number of protons or neutrons in the nucleus but instead has the
effect of moving the nucleus from a higher to a lower energy state (unstable to
stable). Gamma ray emission frequently follows beta decay, alpha decay, and
other nuclear decay processes.
Positron emission involves a particle that has the same mass as an electron but
a positive charge. The particle is released from the nucleus when a proton
splits into a neutron and positive electron.