Found in 2 comments on Hacker News
mikepurvis · 2022-05-09 · Original thread
The author of the Harper's piece also expanded it into a 2004 book:

https://www.amazon.com/Radioactive-Boy-Scout-Backyard-Nuclea...

K2h · 2012-08-07 · Original thread
The book is The Radioactive Boyscout

http://www.amazon.com/The-Radioactive-Boy-Scout-Backyard/dp/...

I read the book. from what I remember he started with smoke detectors and worked his way up from there. Just a few letters to some supply houses saying he was a researcher or something. he effectively mail ordered his radioactive sources I think.

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