Spencer Attix stopping power ratios

spr.py

def stoppingPowerRatio(R50D,z):
    
    """Given the beam quality R50,D calculates the Spencer Attix stopping power ratios at depth, z, using 
    the fit by Burns et al given in the 2003 IPEM code of practice for electron dosimetry.
    
    This fit is valid for for beam qualities R50,D in the range 1-20 cm of water and for depths of 0.02 to
    1.2 times R50,D"""
    
    a = 1.075
    b = -0.5087
    c = 0.0887
    d = -0.084
    e = -0.4281
    f = 0.0646
    g = 0.00309
    h = -0.125
    x = np.log(R50D)    # np.log gives the natural log ln of a number
    y = z/R50D
    S = (a + b*x + c*x**2 + d*y)/(1 + e*x + f*x**2 + g*x**3 + h*y)
    return S