In analogy with combustion,
"Heat generated by molecule burning to more stable molecules.
and with Einstein's insight --E = mc2 --
Energy generated by less stable nucleus "burning" into more stable ones.
What then is "burning" and
In chemistry, O2 sets zero for stability.
In nuclear physics, the atomic mass unit u is set by 168O = 16 u,
Definitions -- what is
|Stable nucleons more neutrons than protons.|
|Last stable isotope is Bi-209 (see 208Pb).|
|U half lives.234U: ~½ 106 y; 235U: ~109 y; 238U: 4 109 y.|
|Above stability line:||Z -> → Z-1 + β + + &gamma|
|Below stability line:||Z -> → Z+1 + β - + &gamma|
|Large Z elements||Z -> → Z-2 + α + γ|
|Difference between nucleus mass & constituent nucleons|
|5626Fe: most bound|
|⇒ merge: fusion|
|⇐ split: fission|
Leftmost dip: |
4He = α + 2 e-
Units: Standard: 168O ≡ 16 u.
u (amu or atomic mass unit) = 1.6606 10-27 kg
Convert: u = 930 MeV = 0.149 nJ; neutron = 1.0087 u, proton = 1.0073 u. Ref: Nuclei Mass Table
|2 n (1.0078)||-2.0156||u|
On a mass basis (235 1.66 10-27 = 0.39 10-24 kg); 235U fission produces 71.7 TJ/kg.
Methane: 50 MJ/kg. (TNT: 4 MJ/kg).
Fission is million times more efficient!
23592U plus slow neutron has other fission reactions; see text.
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).
Most neutrons are prompt; 0.7% delayed ms→min. This will be important for controlling nuclear fission reactors for safety.
⇑ Relative penetration of α, β & γ.
⇐ Distribution 235U fission fragments. Most radioactive; some long lifetimes.