Today I got access to a neutron source, so I wanted to do a small experiment.
I buried my CsI(Tl) in around 2 kg of borax and exposed that, which showed a wonderful prompt capture peak!
But it clearly shows a strong peak at 477 keV!
The interesting thing about this is that it is not an activation peak!
It only appears during irradiation, and instantly stops when the neutrons stop.
It is well known, that ¹⁰B has a giant cross section for thermal neutron capture (around 1 kBarn), which results in a ¹¹B(m), that decays into an alpha particle and ⁷Li, which comes out as metastable around 94% of the time.
That ⁷Li(m) then decays into ⁷Li, giving off a 477 keV gamma, which is exactly what we can se here! The reaction is called "prompt", because it is happening within less than 10 ns, or one shake, between neutron capture and gamma release.
While many hobbyists have built fusors, AmBe or PoBe neutrons sources and activated lots of stuff with them I can't recall ever seeing someone capture the prompt gammas.
Maybe it's a first?
Lukas
