"""

```
sphereradiusfromshellvolume(volume,step)
```

Computes the radius that corresponds to a spherical shell of
a given volume.

"""
function sphereradiusfromshellvolume(volume,step)

  fourthirdsofpi = 4*pi/3
  if 3*step*volume - pi*step^4 <= 0.
    return 0.
  end
  rmin = (sqrt(3*pi)*sqrt(3*step*volume-pi*step^4)-3*pi*step^2)/(6*pi*step)
  return ( 0.5*( volume/fourthirdsofpi + 2*rmin^3 ) )^(1/3)

end

