Helium Isotopes, Radioactive Decay and Half-Life

This is a periodic table tile for the element helium.
This is a periodic table tile for the element helium. Todd Helmenstine
By
Anne Marie Helmenstine, Ph.D.
Chemistry Expert
  • Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
  • B.A., Physics and Mathematics, Hastings College
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
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Anne Marie Helmenstine, Ph.D.
Updated February 28, 2020

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:

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