Why is U-235 fissionable by thermal neutrons but U-238 not

[saleem]

I have this question

Why U-235 is fissionable by thermal neutrons whereas U-238
requires E>1.4MeV
Thank you:

 

[neu]

235 is fissle as it's activation energy/fissile barrier, the energy required to cause irreversible deformation of the nucleus is very low.

If you look at the semi empirical mass formula the coulomb and surface area terms reduce and increase respectivly (like a harmonic oscillator) when a neutron is absorbed by the nucleus. If the energy is great enough the coulomb term dominates and causes an irreversible deformation which leads to break up of the nucleus.

the activation energy is the energy required to do this

Im studying this currently myself so apologies if I am saying what you already know

 

[malawi_glenn]

235 is fissle as it's activation energy/fissile barrier, the energy required to cause irreversible deformation of the nucleus is very low.

If you look at the semi empirical mass formula the coulomb and surface area terms reduce and increase respectivly (like a harmonic oscillator) when a neutron is absorbed by the nucleus. If the energy is great enough the coulomb term dominates and causes an irreversible deformation which leads to break up of the nucleus.

the activation energy is the energy required to do this

Im studying this currently myself so apologies if I am saying what you already know

if he knewed he wouldn´t asked ;)