I found that factor online.It said to get intensity to < 1% but by my calculations, for 200 KeV beam using formula:
HVL = 0.693/u
need 7 HVL layers to get to 0.8% which is 4.48 mm.
where u = 10.15 cm^-1
equation for radius of curvature of relativistic electron in magnetic field?
is this correct:
R = gamma * m * v / e * B
where gamma is lorentz factor, m is electron mass, v is velocity, e is electron charge and B is magnetic field strength.