The Aacapa Near Space Exploration Club is a group of high school students in Santa Barbra, California.
On May 5, 2012 they had a very successful high altitude balloon flight up to 111,814 feet (34km). It's a pleasure to say that on-board was a Geiger Kit with an STS-9 GM tube along with the Logging Shield.
The kits were built by student Julio Bernal (who had never held a soldering iron before !) and the project was under the guidance of faculty member Levi Maaia.
Below are a few photos and a great video, but there is much more on the club's Facebook page as well as TV station KEYT's page.
Of course the Geiger Kit was just one part of the great effort by the team of students that pulled off this amazing flight. However, I was interested in the background radiation data that they recorded during this flight.
Below is a plot that shows the CPM measured at altitude. (click for larger image)
Unfortunately CPM data stopped at 90,000 feet - about 2 hours into the 3 hour flight. The club was nice enough to send me their Geiger Kit and Logging Shield so I could find out what happened.
I discovered that the 555 timer had failed. Looking at the spec, I saw that the TLC555C is characterized for operation from 0°C to 70°C. From other data I learned that the temperature at the point of failure was about 16°C. However, 50 minutes before, it was down to -36°C.
So I can only guess that the cold killed the 555. For the next flight I was able locate another variant of the 555 ( TLC555IP) that is good down to -40°C.
Even with the missing data, however, the relationship between background radiation and altitude is clear. However, it does seem to be trending downward after it's peak at 75,000 feet. Thanks to some good people on the GeigerCounterEnthusiasts group, I think I may know why.
If you look up "Pfotzer curve" you learn that as you encounter less atmosphere, there is less material to create secondary emissions when hit by cosmic rays. Here is a very good white paper on it, and if you look near the bottom - Figure 12, you will see a similar curve.
With a more robust timer we hope to learn more on the the next flight.
Way to go Anacapa Near Space Exploration Club! You guys rock.