A weather balloon is the poor man's satellite... A lot of new ideas popped up during the preparations for our first weather balloon experiment. One idea is to use an expansion cloud chamber to visualize the paths taken by ionizing cosmic radiation and sending the images and videos back to earth. The most common type of cloud chamber found is the diffusion cloud chamber. The diffusion cloud chamber operates continuously and is very easy to build, but has the disadvantage of requiring dry-ice. Instead of dry-ice peltier elements could be used, but they have high weight and power consumption. By contrast an expansion cloud chamber could be operated by a CO₂ -cartridge and two micro solenoid valves. Contrary to popular belief it is not very difficult to build an expansion cloud chamber and could be a fascinating school or college project as well. The picture below shows an expansion cloud chamber design with common parts: The alpha particle source is Am241 from a dismantled ionization smoke detector. Water, methyl alcohol or ethyl alcohol vapor work too, but isopropyl alcohol is the best choice. The operating voltage for the electrostatic clearing field between the platform and the grounded wire loop at the top of the chamber is not critical, can be between 200 and 1000 V, but it's essential to the operation of the chamber. The best results we achieved not in abruptly withdraw the syringe plunger to drop the pressure, but compressing the air in the chamber first by the syringe, putting the squeezed rubber hose from the syringe and open then the hose suddenly to release the overpressure. The following two photographs show typical alpha particle tracks. The alpha particle source is on the right. The photographs are individual frames, converted from the video. The tracks are displaced from the original paths by the air flow during expansion. As the tracks only last for a fraction of a second, we slowed down the video sequence 4 times. TBC