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fluidistic
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Homework Statement
Mathews and Walker problem 8-2 (page 253):
Assume that the neutron density n inside [itex]U_{235}[/itex] obeys the differential equation [itex]\nabla ^2 n+\lambda n =\frac{1}{\kappa } \frac{\partial n }{\partial t}[/itex] (n=0 on surface).
a)Find the critical radius [itex]R_0[/itex] such that the neutron density insdie a [itex]U_{235}[/itex] sphere of radius [itex]R_0[/itex] or greater is unstable and increases exponentially with time.
b)Suppose two hemispheres, each just barely stable, are brought together to form a sphere. This sphere is unstable, worth n ~ [itex]e^t/\tau[/itex]. Find the "time-constant" tau of the resulting explosion.
Homework Equations
Already given.
The Attempt at a Solution
I tried to solve this PDE via a Laplace transform, but now I realize I do not think it is well appropriated due to the nabla operator.
Applying Laplace transform gave me [itex]\nabla ^2 F(n)+\lambda F(n)=\frac{1}{\kappa } [n(t=0)+sF(n)][/itex]. Also I do not know the initial condition n(t=0).
I guess I'll have to try another method and that it might involve taking the Laplacian in spherical coordinates, I'm open to use any method to solve this PDE. I'd like to know whether I'm right in thinking that it's not solvable using a Laplace transform and what other method could I use.
Thank you very much in advance.