A point source isotropically emitting 10^8 fast nuetrons

[bel10]

Homework Statement

A point source isotropically emitting 10^8 fast nuetrons per second falls out of its shield onto a railroad platform 3 m horizontally from the track. A train goes by at 60 mph. Ignoring scattering and attenuation, what is the fluence of nuetrons that would strike a passenger at the same height above the track as the source?

2. Relevant equation
Fluence=dN/da

The Attempt at a Solution

I know you need to write the distance to the train as a function of velocity and time. This can be done by having r^2=9+v^2*t^2. If the person in exactly across from the point source at t=0. I know I need to integrate this equation over some dt from -infinity to infinity but I'm confused as to what integral this goes into.

 

[andrevdh]

Maybe you can consider the passenger moving radially away from the point source?