You measure the radioactivity of a mineral sample. You repeat the measurement 100 days later and find only 52 percent of the radioactivity remains. What is the half-life of the mineral sample? (Hint: The equation for half-life is t ½ = (tf - ti) * log 2⁄log [Ni ⁄ Nf], where ti is the initial recording time, tf is the final recording time, Ni is the initial total number of atoms, and Nf is the final total number of atoms.) Express your answer in units of years.