# How can negative beta-decay energy be negative?

The International Atomic Energy Authority (IAEA)'s Nuclear Data Services list tables of nuclear data, including a table of atomic masses and beta decay energies, data taken from Huang et al., Chin Phys C, volume 45, 2021.

The decay energy for most $$\beta$$- decays is positive, as one would expect from the event which must be exothermic as the ejected electron has kinetic energy. However, some of the decay energies in the table linked above (and the original paper from where the data was taken) is negative.

For example, we can see that the beta-decay energy of $$^{40}_{17}\textrm{Cl}$$ is 7480 keV, whereas the beta-decay energy of $$^{40}_{18}\textrm{Ar}$$ is -1504 keV. Does this mean that the $$\beta$$- of $$^{40}_{18}\textrm{Ar}$$ is endothermic? It seems unbelievable to me that a mass of $$^{40}_{18}\textrm{Ar}$$ would cool its environment due to decay.

• Better question: would it decay through that channel? May 30 at 12:38
• Ar-40 is a stable isotope of Argon. May 30 at 15:42