not just described, Explained


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Shaking the protactinium bottle

An organic layer floats on top of an aqueous layer

In the watery layer there is a mixture of isotopes, including the protactinium we want to watch decay.  If we shake the bottle then we can isolate the protactinium in the oily layer and detect the beta particles it gives off.

You change where the protactinium is, not how it's decaying

This is a simplified explanation of what happens.

Try Why Do Astronauts Float by Julian Hamm

Protactinium-234m is part of the uranium-238 decay chain (don't worry about what the 'm' means).  The protactinium is continuously produced by its parent and continuously decays into its daughter.

Because everything (apart from the uranium) is in equilibrium (it decays as fast as it’s produced) the measured radioactivity would be constant.  When the bottle is shaken the protactinium moves out of the watery ('aqueous') layer and into the oily ('organic') layer.

We start timing when the oil layer settles out after shaking.  Over time the protactinium decays into its daughter and gives off beta particles.  These are what we detect with our Geiger counter.

After five or ten minutes most of the protactinium has decayed.

While the protactinium decays in the organic layer, more is being produced from the parent in the aqueous layer.  So if we want to restart our experiment we have to wait for the protactinium to be sufficiently concentrated in the aqueous layer.

Any beta particles produced in the aqueous layer that make it through the plastic bottle hit the sides of the GM tube so aren’t detected.

back to Lesson 15: Half-life part 2