code: def alpha_abs(self, R, amp, sigma):
          
          sigma_x, sigma_y = sigma, sigma
          amp_density = self._amp2d_to_3d(amp, sigma_x, sigma_y)
          alpha = self.mass_2d(R, amp_density, sigma) / np.pi / R
          return alpha
docs: absolute value of the deflection
          :param R:
          :param amp:
          :param sigma_x:
          :param sigma_y:
          :return: