How is it possible to calculate the energy liberated by a given fission process? For example, in the fission of a $^{235}$U induced by capturing a neutron?


When 235U captures a neutron, it forms the compound state 236U*, the excitation energy is given by

E_ex = [m(236U*)-m(236U)]c^2

To find the energy of the compound state, the mass energies of 235U and n can be used - assuming that the neutron will be thermalised for this reaction to occur and thus it's kinetic energy is negligible.

m(236U*) = m(235U)+m(n) = (235.043924u + 1.008665u) = 236.052589u


E_ex = (236.052589u - 236.045563u)931.502MeV/u = 6.5MeV

This causes a fission event to occur, which can produce over twenty different products, one example is:


The energy liberated is defined by the Q value: the initial mass energy minus the final mass energy

Q = (m_initial - m_final)c^2

Using the masses of 93Rb (92.92172u) and 141Cs (140.91949u)

Q = (236.052589u-(92.92172u+140.91949u+(2*1.008665u)))*931.502MeV/u = 180.76 MeV
| cite | improve this answer | |
  • $\begingroup$ But then 236U* will gamma decay to 236U which will then actually fission and liberate energy... $\endgroup$ – Cedric H. Nov 3 '10 at 0:42
  • $\begingroup$ Ahh yes. Updated the answer to reflect this. $\endgroup$ – Geodesic Nov 3 '10 at 0:55
  • $\begingroup$ nicely done ! $\endgroup$ – Cedric H. Nov 3 '10 at 1:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.