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Radon-222 is a radioactive element having a half-life of 3.82 days. Radon-222, when found in atmospheric air, can present a health hazard. Safety measures should be taken when the activity of radon-222 exceeds 
per cubic metre of air.
Define radioactive decay constant.
Show that the decay constant of radon-222 is 
.
A volume of 
of atmospheric air contains 
molecules. Calculate the ratio
for the minimum activity of radon-222 at which safety measures should be taken.
.
