Elastic neutron energy transfer and bounded moderation
A nonrelativistic neutron of mass m and kinetic energy E scattered elastically from a free stationary nucleus of mass M retains E′/E=(η²+1+2ηcosθ_cm)/(η+1)², η=M/m.
Key knowledge
A nonrelativistic neutron of mass m and kinetic energy E scattered elastically from a free stationary nucleus of mass M retains E′/E=(η²+1+2ηcosθ_cm)/(η+1)², η=M/m. Thus αE≤E′≤E with α=[(η−1)/(η+1)]²; light targets can transfer a larger fraction of energy. If scattering is isotropic in the center-of-momentum frame, E′ is uniform on that interval and the mean logarithmic decrement is ξ=1+αlnα/(1−α), with ξ=1 at η=1.
Values and equations
η=M/m; E′/E=(η²+1+2ηcosθ_cm)/(η+1)²; α=[(η−1)/(η+1)]²; ξ=1+αlnα/(1−α); ⟨ln(E_g/E₀)⟩=−gξ
- M,m
- Target and neutron masses kg
- E,E′,E_g,E₀
- Initial/scattered and generation energies J
- θ_cm
- Center-of-momentum neutron angle rad
- η,α,ξ
- Mass ratio, minimum energy fraction and mean logarithmic decrement
- g
- Nonnegative collision count
Conditions and scope
- m,M,E>0; θ_cm is the neutron deflection in the center-of-momentum frame. Use the actual mass ratio rather than assuming integer nuclear A exactly; isotropy is an additional model, not a consequence of energy conservation.
- Independent repeated collisions with stationary identical free targets and unchanged angular law give ⟨ln(E_g/E₀)⟩=−gξ. This does not give the exact first-passage collision count or the arithmetic mean energy. Bound-atom/thermal motion, anisotropy, resonances, absorption and inelastic channels need transport data and different models.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Introduction to Applied Nuclear Physics: chapter 8 (2012): Section 8.1.2, printed pages 107–109: center-of-momentum velocity, exact mass-ratio energy transfer, conditional isotropic energy distribution and logarithmic decrement. η denotes actual M/m before the source's approximate A substitution. The source's reversed-sign collision-count expression, claims of universal angular independence and statement that scattered particles never attenuate a narrow beam are excluded.
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