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Question 22.2: In the P1 approximation, the angular neutron flux ϕ(r, E, u)......

In the P_1 approximation, the angular neutron flux ϕ(r, E, u) can be written as the sum of two terms:

\phi(r , E, u) = \phi_o(r ,E)P_o(μ) + \phi_1(r ,E)P_1(μ)

or

\phi(r , E, u) = \phi_o + μ\phi_1

where P_o(μ) and P_1(μ) are the first two Legendre polynomials. What is the physical meaning of ϕ_o and ϕ_1 in this case?

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In the P_1 approximation, ϕ_o(r, E) is the isotropic neutron flux ϕ (with no angular dependence), and in a future section, we will learn that ϕ_1(r, E) is equivalent to the directional neutron current J(r, E).

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