It began with an online video. Andrew Walker had watched as a collector of radioactive objects showed off the items he had procured over the years, such as antiques containing uranium. Walker thought that looking for such pieces ‘in the wild’ might make an interesting and unusual hobby – so he bought himself a Geiger counter.

He quickly realised, as scientists will tell you, that radioactivity is everywhere. The first elevated reading he got was in the parking area at a Mexican fast-food restaurant in Idaho. “I noticed that when we pulled in there, my Geiger counter was going off,” he recalls. Something nearby was flinging out the tiny subatomic particles that make up radiation in slightly higher quantities than might be expected – and though he couldn’t figure out what it was, Walker was suddenly aware of this otherwise invisible activity.

After that, he began looking for other intriguing examples.

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His pursuit of radioactive objects is still a hobby. By day, Walker is a cinema projectionist and a film-maker, not a scientist. But he enjoys sharing some of his finds on Twitter and Instagram, where he can discuss them with others who are similarly curious.