On Mon, Nov 21, 2011 at 12:22 PM, Daniel Rocha <[email protected]>wrote:
> I did recognize, but even so, I am not sure what you mean by energy needed > for capture. For example, in large nuclei, the required energy is 0, since > k-capture doesn't need to be induced or stimulated. > > It's all about the difference in mass between the reactants and the products. A neutron's mass is equal to the proton mass plus the electron mass plus 780 keV/c^2. That energy has to be supplied. And that's why isolated neutrons decay spontaneously to a proton and electron with a half-life of about 15 minutes. The excess energy is taken up mostly in kinetic energy of the electron. Electron capture that occurs spontaneously in larger nuclei release energy on electron capture, because the product nucleus has a lower mass than the parent + electron. 7Be for example captures an electron to become 7Li, releasing about 800 keV energy.
