Question

Read the Operating Instructions for the Cs/Ba-137m Isotope Generator. Place a watch glass under the radio nuclide cow and elute a few drops of radioactive barium 137 onto the filter paper. Place your sample under the Geiger tube immediately after elution. Any delay will result in loss of accuracy. Why?

Answer 1

For Barium - 137, the half life is very less. So it decays very quickly to its stable ground state by the emission of a 0.662 MeV gamma ray.

For Ba - 137, (half life) t_1/2 = 2.55 minutes = 153 seconds

The radioactive decay law is given as : for any time t, where is the initial amount of sample (in moles or atoms or entities) and is the undecayed amount left after t time. At t = t_1/2,  .

So even if there is a delay of say 10 seconds, the undecayed portion left will be:

That is, just within 10 seconds, about 4.5% of the sample has already decayed. Longer the delay time, more the decay and lesser is the sample available for testing. After the sample has been acquired in the solution, it only takes about half an hour for it to practically disappear.

Ba - 130, on the other hand, is a long lived isotope of Ba with a half life of the order of 10²¹ years.