Now open both stopcocks, carefully suck on the tube until liquid fills the cloud chamber about halfway, then close both stopcocks. Position a bright light to one side of your viewing port. Drop in a bag of baking soda and monitor the fluid level in the cloud chamber, bleeding the pressure with the lower stopcock when the compression ratio rises above about 1.33. Wait a minute or so, then rapidly open that same stopcock completely. Particle tracks can appear only during the next brief instant, so the odds of seeing a vagabond cosmic ray are not at all good.
Alpha particles produce short tracks, whereas beta particles leave long ones.
You can obtain both an alpha and a beta source suitable for this project from the Society for Amateur Scientists. Epoxy your radioactive source to the tip of a nail that has a large, flat head. Lift this assembly through the hole in the window screening and into the cloud chamber using a drinking straw and stick the head of the nail to the magnet at the top of the glass. This arrangement will hold the source securely in place until you want to swap it for another one. Though easy to build, my cloud chamber has its limitations.