4. Consider the following graph which shows the radioactivity due to strontium-90, or 90Sr, in the Ukraine countryside near the Chernobyl nuclear reactor after the reactor burned and exploded on 26 April 1986. a. Determine the half-life of 90Sr. Show your work or explain your logic. b. Approximately how intense is the radiation from 90Sr near Chernobyl now, compared to immediately after the disaster