Dryer lint will give you a nice decay curve if you meter it over an hour or two.
Back in the late 1950s and early 1960s, my dad taught college biology, and he always had a fan running pulling outside air through a piece of good quality filter paper, which his grad students took down every day and checked.
I remember several winters when he told us kids, as we got dressed to go play in a fresh snowfall, that it was our job making sure to tell all the neighborhood kids not to eat the snow --- that was when the background count went up noticeably; that was plumes from the surface nuclear tests taken down by snowstorms as they passed over our area.
They're still findable in snow cores:
http://www.ccrc.sr.unh.edu/~cpw/ArcticRes/Eclipse02_NSFreport_2003.pdf
Figure 5.
137 Cs profile from core 3. The 1963 and 1954 peaks represent fallout from increased
atmospheric nuclear weapons testing during those years.
Handy tools, geiger counters. Every home ought to have the basic equipment for understanding how the world's working these days. The Yahoo group, and the Usenet newsgroups, are good places to learn about them.
