Fermi Gamma-Ray Space Telescope

You know, back when I was a boy, they weren't puttin' up a new satellite every 15 minutes. We put up one at a time and waited until it died before we put even thought about puttin' up another one. Now they are putting up so many you can't keep track of them.     Last April I stole a post from E.B. Misfit about dark lightning. This satellite is how we found out about it.      So this is all very cool, but how are they detecting these gamma rays? I mean gamma rays are pretty much undetectable, that's what makes them so dangerous. You don't know you've been exposed until you're dead. Basically what they do is put up some radiation shielding and wait for a gamma ray to impact an atom in the shield. Any such impact will cause a certain amount of excitement, like generating a positron and an electron which are detectable by ordinary stuff. Problem is you need a lot of shield material to be sure of stopping/detecting the gamma ray. Wikipedia puts it succinctly: Space-based pair-conversion detectors tend to make for rather expensive missions, since they unavoidably contain several hundred kilograms of lead or tungsten.