Thank you! That was driving me crazy. And now I see why I got confused: it's another phonograph-related dangerous-element fact that I learned from reading Theodore Gray's book, The Elements [0] [1].

Now if I could only remember why I thought neutron beams were involved...

[0] https://theodoregray.com/PeriodicTable/Elements/084/index.s7...

[1] https://www.amazon.com/dp/B011IZNWZU