why doesnt 238U decay spontaneously by emitting a proton?
The principal modes of nuclear decay, particularly for heavy elements are gamma (EM), beta (electron) and alpha (2p,2n = 4He nucleus).
One could do a binding energy calculation to see which reaction, p or alpha emission, dumps the most energy, i.e. which one is more 'spontaneous'. One more readily observes photoneutron emission rather than photoproton emission.
The absence of proton emission implies that the proton is more tightly bound in nuclei.
Even spontaneous fission in some transuranic nuclei is preferred to proton emission.
Photoproton emission requires fairly high thresholds.
The reaction doesn't happen because it is prevented by energy concerns. In short, a proton and a Pa237 atom weigh more than a U238 atom:
[Edit: Thanks to Astronuc for pointing out that Pa237 should be used instead of U237.]
Ha, actually, it was the checking of neutron and proton driplines some years ago the thing that got me off from my mathematical cushions into pesky phenomenology. Empirically it can be seen that the stability line, where usual isotopes live, is exactly middle way between the proton and neutron driplines. This is, the number of protons you must remove from a stable isotope to make it neutron unstable is about the same that the number of protons you remove to drive it neutron unstable. Or something so. The moral is that long living atoms (as existing U atoms are) live far from the driplines.
Ah, and obviously, the more neutrons you add, the more you escape from the proton dripline. So if U235 does not like proton decay, bet that U238 will not be better.
My favorite reaction, as for energy concerns it concerns, is Hg201 to alpha + Pt197 and then Pt197 decays beta. Someone should use it in a film script. Lot better than Davinci code stuff.
EDITED: Ok, Uranium fission gives us about 200 MeV, while here we get slightly a bit more than 1 MeV, and adding both reactions. But it is enough for a heater to work.
what r proton and neutron driplines?
The proton dripline is where a proton heavy nucleus has too many protons to prevent another proton from binding to the nucleus. It's the limit to potential isotopes on the proton heavy side. The neutron dripline is similar to proton dripline, forming a limit to potential neutron heavy isotopes. I'm not really sure what limits neutrons from 'glomming' onto an already neutron heavy nucleus.
Hopefully someone can expand on my limited knowledge of the subject.
Check the chart of nuclides, and look at which isotopes are stable. Too many or too few neutrons produces nuclei which decay to a more stable configuration.
In particular, look at http://wwwndc.tokai-sc.jaea.go.jp/CN04/CN003.html
Ca-40 is the last stable nuclide for which P=N, where P = number of protons and N = number of neutrons.
Separate names with a comma.