[shannon623]

1) Is it theoretically possible for U-238 to decay into Fe (in a vacuum)? [even if it has never been observed]


2) Really dumb question here...

Since U is atomic number 92, is it possible for U to be converted to:
(3) Fe (atomic number 26) and (1) Si (atomic number 14)?


Thank you very much.

[Plutonium]

I don't see how.  I think uranium, just like most heavy, radioactive elements, decays into a stable form of lead.

[gippgig]

²³⁸U very rarely decays by spontaneous fission. It would be theoretically possible, but probably incredibly rare, for iron to be a fission product.
U can be artificially converted into Fe, Si, and every other lighter element by smashing it with very energetic particles.

[Borek]

There is a fundamental reason why it can't decay past Fe (to Si). Google binding energy of iron.

[snorkack]

There is a fundamental reason why it can't decay past Fe (to Si). Google binding energy of iron.

Um, uranium decays far past Fe, to He actually. Losing a lot of binding energy compared to emitting Si, or O, or C.

All light isotopes of uranium actually undergo cluster decay. So do U-232 (Ne-24, branch ratio 9e-12), U-233 (also Ne-24, branch ratio 7e-13), U-234 (also Ne-24, branch ratio 1e-13, as well as Mg-28, branch ratio 1,4e-13), U-235 (also Ne-24, branch ratio 8e-12) and U-236 (Mg-28, branch ratio 2e-13).

Whereas for some reason U-238 does not undergo cluster decay. The minor fates are spontaneous fission (branch ratio 5e-5), long range alphas (region e-8), other branches of "ternary fission" (region e-9, most widespread third product being triton) and double beta (branch ratio 2e-10).

Why does U-238 not undergo cluster decay?

[Borek]

There is a fundamental reason why it can't decay past Fe (to Si). Google binding energy of iron.

Um, uranium decays far past Fe, to He actually.

For me lighter elements are byproducts or side products, and the decay product is the heaviest stable nucleus (say, 208Pb for Thorium series).

[Enthalpy]

Why does U-238 not undergo cluster decay? [But lighter isotopes do]

Apparently it's not the propotion of neutrons in the resulting fission or cluster fragments: ²⁴Ne, ²⁸Mg have around 1.4 neutron per proton, similar to ⁹⁰Sr or ¹³¹I.

I could imagine that more neutrons help the future fission fragments to get some distance within the ²³⁸U nucleus, but this would not prevent an additional branch with cluster decay for ²³⁸U, which isn't mentioned.

Do I remember that these heavy nuclei can be ellipsoidal at rest? Maybe ²³⁸U is but ²³²U is spherical.
http://ithes.riken.jp/Site/nakatsukasa.pdf

More neutrons also reduce the electric field at the surface by increasing the radius, making cluster emission less likely. That would be simple.

I've seen no mention of spontaneous fission for ²³²U, ²³³U, ²³⁴U. Only ²³⁵U shows both spontaneous fission and cluster emission.