State what is meant by the decay constant of a radioactive isotope.
Show that the decay constant 
is related to the half-life 
by the expression
Cobalt-60 is a radioactive isotope with a half-life of 5.26 years (
). A cobalt-60 source for use in a school laboratory has an activity of 
. Calculate the mass of cobalt-60 in the source.
.
