The sun generates its energy through nuclear fusion, making helium from hydrogen. The main process by which this happens, called the proton-proton chain, goes as follows:

• Two protons come together to create deuterium, a positron, a neutrino, and energy: p + p D + e⁺ + + energy
• The deuterium fuses with another proton to make ³He: D + p ³He + energy
• Fianlly, ⁴He is produced by fusing two ³He nuclei together: ³He + ³He ⁴He + p + p + energy

What is the total energy released during the creation of one ⁴He nucleus? Give your answer in MeV, and remember that you need to make two ³He nuclei in the process. Be sure to use at least five significant figures for your masses, but your final answer should have three or four sig figs.

Question 2

Nuclear medicine often uses radioactive samples as a tracer to monitor processes inside the body. Suppose a nuclear reactor makes a certain isotope sample with an initial activity of 146 mCi, and the sample gets delviered to a hospital 16.0 hours later, when its activity has become 69.0 mCi. Luckily, the sample remains usable until its activity drops below 18.5 mCi. How long after the delivery is this sample usable?