First off, yes my oscilloscope screen is dying. It's very old. It's a portable unit I carry around for testing, the good scope stays in my home lab.
Anyway, the particle detected here is almost certainly a β⁻ from background radiation or isotopes part of the detector itself. I would expect a muon to have more energy, the detector is not very sensitive to gamma rays at all, and in it's current configuration cannot see alpha particles at all.
The case is aluminium, technically 26Al is a β⁺ emitter with a long half live (~700,000 years). However, 26Al generally doesn't form on Earth, that half life is short compared to the age of the Earth, and I didn't buy the case offworld. So while it technically could instead be a positron, it's quite unlikely.
