Why does a slow neutron cause fission in uranium-235 more readily than a fast neutron?

[Leronira]

Here's a question.
Why does slow neutron is most likely to cause a nuclear fission in Uranium 235, compare to a fast neutron?

 

[mgb_phys]

In simple terms because a fast neutron doesn't spend enough time near the nucleus to have an effect

 

[Leronira]

Hmm...How about the reason for slow neutrons not being able to be absorb by uranium 238?

 

[Astronuc]

Here is the plot of cross-section of (n, gamma) reaction for U-238 as a function of neutron energy.

http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=4554&mf=3&mt=102&nsub=10

Note that the absorption cross-section is much higher for thermal (slow) neutrons than fast.

U-238 does capture neutrons and becomes U-239, which decays to Np-239, which decays to Pu-239.

Fission cross-section for U-238 as a function of neutron energy.
http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=4554&mf=3&mt=18&nsub=10

U-238 is a much more stable nuclide than U-235.

(n,gamma) for U-235
http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=4551&mf=3&mt=102&nsub=10

(n, fission) for U-235
http://www.nndc.bnl.gov/sigma/getPlot.jsp?evalid=4551&mf=3&mt=18&nsub=10