I can't help you much with theory, sorry. I'm a hardware type. The gist of the idea is that propane is good for making particle detectors[1]; and since there are propane tanks distributed fairly widely, then one ought to be able to observe lots of cosmic rays from within them. The first trouble was figuring out a way to build enough detectors, cheaply enough. I had hoped to come up with something simple that could be installed without too much trouble. Most of the good detector ideas would require the tank to be empty when the detector was installed, so retrofitting existing propane tank installations would be a super slow process. There are a lot of other problems with the idea and we had other ideas to pursue at the time so we didn't go very far with it.
[1] Bubble chambers, spark chambers, flash chambers, drift chambers / tubes, RPC's,