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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).

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**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).

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**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).

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**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).

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**Nuclear Fission Surface effect Coulomb effect ~200 MeV Fission**

Fusion Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

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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).

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**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).

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**Nuclear Fission Liquid Drop Shell Activation Energy (MeV)**

Nuclear Reactor Theory, JU, Second Semester, (Saed Dababneh).

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**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).

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**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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