When the ratio of neutron-to-proton dictates that a neutron must break down it will form

a helium nucleus
an alpha particle
a proton and an electron***
Two protons

<<Under the Standard Model of particle physics, because the neutron consists of three quarks, the only possible decay mode without a change of baryon number is for one of the quarks to change flavour via the weak interaction. The neutron consists of two down quarks with charge −1⁄3 e and one up quark with charge +2⁄3 e, and the decay of one of the down quarks into a lighter up quark can be achieved by the emission of a W boson. By this means the neutron decays into a proton (which contains one down and two up quarks), an electron, and an electron antineutrino.

Outside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 s (about 14 minutes, 46 seconds); therefore the half-life for this process (which differs from the mean lifetime by a factor of ln(2) = 0.693) is 613.9±0.8 s (about 10 minutes, 14 seconds).[2] Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:

n⁰ → p+ + e⁻ + νe

Neutrons in unstable nuclei can also decay in this manner. However, inside a nucleus, protons can also transform into a neutron via inverse beta decay. This transformation occurs by emission of a antielectron (also called positron) and a neutrino:

p+ → n⁰ + e+ + νe

The transformation of a proton to a neutron inside of a nucleus is also possible through electron capture:

p+ + e⁻ → n⁰ + νe

Positron capture by neutrons in nuclei that contain an excess of neutrons is also possible, but is hindered because positrons are repelled by the nucleus, and quickly annihilate when they encounter electrons.

When bound inside of a nucleus, the instability of a single neutron to beta decay is balanced against the instability that would be acquired by the nucleus as a whole if an additional proton were to participate in repulsive interactions with the other protons that are already present in the nucleus. As such, although free neutrons are unstable, bound neutrons are not necessarily so. The same reasoning explains why protons, which are stable in empty space, may transform into neutrons when bound inside of a nucleus. >>

from Wiki, well written article