# I0 I Probability Neutron Attenuation X Recall t = N t

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I0 I Probability Neutron Attenuation X Recall t = N t
mfp for scattering ls = 1/Ss mfp for absorption la = 1/Sa …………. total mfp lt = 1/St Probability per unit path length. Probability Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Neutron Moderation (revisited)
Show that, after elastic scattering the ratio between the final neutron energy E\ and its initial energy E is given by: For a head-on collision: After n s-wave collisions: where the average change in lethargy is HW 6 1H ? Collision Parameter Reference Average decrease in ln(E) after one collision. Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Most probable and average energies?
Neutron Moderation HW 6 (continued) Reproduce the plot. Discuss the effect of the thermal motion of the moderator atoms. On 12C. Most probable and average energies? Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Neutron Moderation HW 6 (continued) Neutron scattering by light nuclei
then the average energy loss and the average fractional energy loss How many collisions are needed to thermalize a 2 MeV neutron if the moderator was: 1H 2H 4He graphite 238U ? What is special about 1H? Why we considered elastic scattering? When does inelastic scattering become important? Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Nuclear Fission Surface effect Coulomb effect ~200 MeV  Fission
Fusion  Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Nuclear Fission B.E. per nucleon for 238U (BEU) and 119Pd (BEPd) ? 2x119xBEPd – 238xBEU = ??  K.E. of the fragments   1011 J/g Burning coal  105 J/g Why not spontaneous? Two 119Pd fragments just touching  The Coulomb “barrier” is: Crude …! What if 79Zn and 159Sm? Large neutron excess, released neutrons, sharp potential edge, spherical U…! Crude Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Nuclear Fission 238U (t½ = 4.5x109 y) for -decay.
238U (t½  1016 y) for spontaneous fission. Heavier nuclei?? Energy absorption from a neutron (for example) could form an intermediate state  probably above barrier  induced fission. Height of barrier is called activation energy. Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Nuclear Fission Liquid Drop Shell Activation Energy (MeV)
Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Nuclear Fission =  fission  Volume Term (the same)
Surface Term Bs = - as A⅔ Coulomb Term BC = - aC Z(Z-1) / A⅓  fission Crude: QM and original shape could be different from spherical. Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

Nuclear Fission Consistent with activation energy curve for A = 300.
Extrapolation to 47   s. Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

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