### Proton Emission & Capture

I. Decay of a Free Proton

The decay of a free proton has been predicted, but never observed. The half-life of this proposed decay is at least 1034 years. This is considerably longer than the current estimates for the age of the universe (a bit less than 1.4 x 1010 years).

If you want to delve more into this topic, I recommend you start with the Wikipedia entry on proton decay.

II. Proton Emission

Proton Emission is rare. Somewhere around 25 isotopes are known to engage in proton emission.

Example #1:

--->    +

Nitrogen-11 also undergoes proton emission:

--->    +

Carbon-9 has an interesting decay scheme. 61.6% of the time it decays by positron, then another proton emission to produce Be-8 (which then decays) and 38.4% of the time, it decays by positron, then alpha to Li-5, which then decays by proton emission.

Example #2:

--->    +   <--- He-4 is stable

--->    +   <--- He-3 is stable

--->    +   <--- He-2 is not stable

Example #3:

--->    +

He-2 (two protons, zero neutrons) undergoes proton emission. Or, you could characterize the decay as the He-2 simply splitting apart (called spontaneous fission).

Example #4:

--->    +

Proton emission is not confined to just the lighter elements. From the Wiki article:

--->    +   <--- the first proton emission discovered, this decay accounts for 1.5% of the decays by Co-53m

From the Wiki article, 69-Tm-147 and 71-Lu-151 also decay by proton emission.

Example #5:

The Wiki article also identifies two isotopes that decay by (simultaneous) emission of two protons:

--->    +  2

--->    +  2

III. Proton Capture

Here's the Wikipedia article on proton capture. It's rather short. Be sure to also look at the images related to proton capture.

Because it's hard for a positively-charged proton to enter a positively-charged nucleus, proton capture only occurs at high temperature. High as in the temperatures in the Sun.

In the Sun (as well as other stars) there is a process called the CNO cycle. This chain of reactions involves several proton captures and is one of the sources of energy that the Sun (and other stars) generates.

If you pursue more on this topic, be sure to look up Hans Bethe and Fred Hoyle.