Find alpha diffusion ratios from cryo oven with alpha detectors. 
            a: list of alpha detector histograms (each helicity)
            b: list of beta  detector histograms (each helicity)

def _get_asym_alpha(self,a,b):
        """
            Find alpha diffusion ratios from cryo oven with alpha detectors. 
            a: list of alpha detector histograms (each helicity)
            b: list of beta  detector histograms (each helicity)
        """
        
        # just  use AL0
        try:
            a = a[2:4]
        except IndexError:
            a = a[:2]
            
        # sum counts in alpha detectors
        asum = np.sum(a,axis=0)
        
        # sum counts in beta detectors
        bsum = np.sum(b,axis=0)
        
        # check for dividing by zero 
        asum[asum == 0] = np.nan
        bsum[bsum == 0] = np.nan
        
        # asym calcs
        asym = asum/bsum
        
        # errors
        dasym = asym*np.sqrt(1/asum + 1/bsum)
        
        return [asym,dasym]