Proton and neutrons behave in totally different ways though they are almost have same mass ( Neutron slightly heavier than proton). In nucleus Neutron decays into proton ,electron and electron antineutrino and is called beta negative decay.This involves a weak nuclear force and hence uses the W- boson for the decay process.The other type beta positive decay involves a proton converting into neutron ,a positron and an electron neutrino. Here also the decay is carried out by the weak nuclear force but uses W+ boson instead of W- .
These process happen inside a nucleus . Outside the nucleus a single neutron decay within 15 minutes but proton doesn't decay and there is no experimental proof so far and it estimated that the life time of proton is extremely high 1034 years. Since a proton converts into a neutron which is slightly heavier than proton some energy has to be converted into mass which is not possible outside the nucleus.
Super-Kamiokande experiment to find a proton decay is still elusive and this experiment is very crucial for GUT (Grand unification theory). In fact Japan is building even a bigger hyper-Kamiokande detector for finding a proton decay.
