Figure 16. The unique, once-forbidden spectrum of ⁹¹Y. The best theoretical fit is that which gives a straight line. The Fermi theory alone, a₁ =1, does not give a straight line. The correction factor a₁ = C[(W² - m_oc²) + (W_o - W)²], does give a linear plot. From Konopinski and Langer (1953)