Helium Isotopes

It takes two protons to make a helium atom. The difference between isotopes is the number of neutrons. Helium has seven known isotopes, ranging from He-3 to He-9. Most of these isotopes have multiple decay schemes where the decay type depends on the overall energy of the nucleus and its total angular momentum quantum number. This table lists helium isotopes, half-life and type of decay.

Reference: International Atomic Energy Agency ENSDF database (Oct 2010)

Isotope	Half-Life	Decay
He-3	Stable	N/A
He-4	Stable ≈ 0.5 x 10-21 sec - 1 x 10-21 sec	N/A p or n
He-5	1 x 10-21 sec	n
He-6	0.8 sec 5 x 10-23 sec - 5 x 10-21 sec	β- n
He-7	3 x 10-22 sec - 4 x 10-21 sec	n
He-8	0.1 sec 0.5 x 10-21 sec - 1 x 10-21 sec	β- n/α
He-9	unknown	unknown
p n α β-	proton emission neutron emission alpha decay beta- decay