def total_fraction_scattered_intensity(R=1e-6,rho=0.4,wavelength=1e-10):
    """
    calculated for sphere with constant e-density
    rho = electron density per Å³ (protein 0.43, water 0.33)
    R = radius in m
    """
    rho = rho*1e30 # convert to m3
    return 4.9*1e-25*wavelength**2*rho**2*R**4