We list some radioactive isotopes and their associated half-lives. Assume that
each decays according to the formula \(A(t)=A_{0} e^{k t}\) where \(A_{0}\) is the
initial amount of the material and \(k\) is the decay constant. For each
isotope:
\- Find the decay constant \(k\). Round your answer to four decimal places.
\- Find a function which gives the amount of isotope \(A\) which remains after
time \(t\). (Keep the units of \(A\) and \(t\) the same as the given data.)
\- Determine how long it takes for \(90 \%\) of the material to decay. Round
your answer to two decimal places. (HINT: If \(90 \%\) of the material decays,
how much is left?)
Americium 241, used in smoke detectors, initial amount 0.29 micrograms, half-
life 432.7 years.