RE: Practical demonstration of a half-life

["Edwards, Richard W" <Richard.Edwards@PSS.Boeing.com>]

Take two clear containers (large glasses work) and fill them to the brim with a colored liquid. Something with a fairly dense color such as water with a fair amount of blue food coloring. Put the cylinders in a large flat pan (the reason for doing so will become obvious) with a white backdrop behind them. Now there are a couple of ways to proceed:

1) Pour out half of the liquid from one glass and refill with clear water, explaining that this is half of the radioactive (i.e. blue) atoms changing into non-radioactive (i.e. clear) atoms. Compare to the original glass. Repeat several times. 

2) Start pouring clear water into one glass in a continuous dilution process, explaining that the blue "radioactive" atoms are being changed into the "clear" non-radioactive ones. You may want to stop periodically to show the difference in color between the two glasses.

You can use these approaches to show that, eventually, a "radioactive" material becomes indistinguishable to measuring instruments, (i.e. eyes) from a "nonradioactive" material. You can also demonstrate that final activity is dependent upon initial activity by altering the amount of dye you put in the two glasses. You can even demonstrate short versus long half-lives by changing how rapidly you perform the water replacement.

Another approach that might work is to use chemoluminescence such as the agents in various cold lights (such as the Cyalume sticks). These degrade fairly quickly when exposed to air, although not so fast that you can insert the demonstration in the middle of a lecture. It's more suitable for a demonstration that lasts throughout the class.

Like most such demonstrations, I recommend a fair amount of practice to get it down.

Rick Edwards, Analyst
The Boeing Company
richard.w.edwards@boeing.com

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