In analogy with combustion,
"Heat generated by molecule burning
to more stable end products."
and with Einstein's insight --E = mc2 -- energy generated by less stable nucleus (atom) "burning" into more stable ones.
What then is the binding energy of nucleus? That is, what are the masses of the nuclei?
Definitions -- what is
Prominent features. Th-232 1010; U-234 106/4; U-235 0.7 109; U-238 4.5 109yr.
Magnitude of Binding energy of nucleus is difference between the mass of the nucleus and that of constituent nucleons -- just like chemistry.
Binding energy per nucleon, most bound lie lowest.
(Binding energy is positive; plot emphasizes stability.)
4He = α + 2 e-
56Fe: most bound
To left: nuclei
To right: nuclei
Units: u (amu or atomic mass unit) = 1.6606 10-27 kg.
Conspiracy. Mass of 168O = 16 u.
Conversion: u = 931 MeV = 0.149 nJ=149.0 pJ;
neutron = 1.008669 u, proton = 1.007276 u.
Ref: Nulear Mass Table
On a mass basis (235 1.66 10-27 = 0.39 10-24
235U fission produces 72 TJ/kg.
Methane combustion yields 50 MJ/kg. (TNT: 4 MJ/kg)
Fission is million times more energetic!
23592U plus slow neutron has other fission reactions. See text for example and distribution. But the effectiveness of fission persists.
Most neutrons are prompt; 0.7% delayed ms→min.
Average number of neutron released is 2.4. That there is more than one neutron permits a chair reaction.
1. Stable energy requires neutrons cooled & most absorbed.|
2. Reaction products in excited states that decay by emitting α (He-4 nucleus), β (both + & - electrons) and γ (MeV xrays).
⇑ Relative penetration of α, β & γ.
⇐ Distribution 235U fission fragments. Most radioactive; some long lifetimes.
To get higher Z, we can use repeated
neutron capture until eventually one of the neutron decays and Z increase
(due to new proton.) That can only happen if neutron capture is much
slower than beta decay (neutron -> proton + electron). With successive
neutron capture, rate increase until it exceeds beta decay rate.
Fe(56) -> Fe(57) -> Fe(58) -> Fe(59) -> Co(59)
This slow process works until Pb(208) is reached when no higher elements are stable enough for slow capture to work. Nuclear physics has worked hard to calculate these processes and in the effort found interesting aspects, the most relevant here is that the process work best is old, mostly burnt-out stars carbon-oxygen cores (not H-He ones). They are supported by helium burning in a shell around the core that serves as good neutron source. Moreover the these stars convects newly formed higher Z elements to the surface where they may released by stellar wind or supernova explosion.
This process is still being worked on and I keep seeing new papers refining the process. Each, of course, claims it is the solution. All involve intense neutron sources so a target element "absorbs" neutrons until it reaches some special value such as "magic number" of neutrons and then coverts to higher Z element. Stay tuned.