Science Buddies: "Ask an Expert"

I'm planning on doing the "Particles in the Mist: See Radioactive Particles Decay with Your Own Cloud Chamber!" https://www.sciencebuddies.org/science- ... ml#summary

I know my independent variable (the materials that can stop/absorb the decay particles), I just want to know how I'll be able to measure the dependent variable. It says that the plastic lid of the petri dish is first inside the cloud chamber, then it says you can test the plastic lid by putting the radioactive source on top instead of inside?

I am pretty confused, help would be greatly appreciated.

You need to do some research (reading about) into cloud chambers to understand how they work. Part of this experiment is figuring out how to use a cloud chamber to particle emissions (decay emissions) from a radioactive source. The usual asumption is that over a short period of time, the decay average decay rate is constant. This assumption is valid for the two different radioactive sources because their half lifes exceeds 10 years. Do some research on half lives of the radioactive materials involved to understand this.

If your radioactive source is inside the cloud chamber, then all of the decay emissions should show up as tracks in the cloud.
If your radioactive source is on the other side of some material, the cloud chamber will only show the tracks that enter the cloud through the material (plastic lid).

This doesn't sound like a fair test to me because there are two things that are different:
1) placement of the radiation source with respect to the cloud, and
2) test material

I suspect you will need to adjust your test proceedure so that you eliminate the placement difference.