Neutrons_Cadmium.avi

Cadmium is a strong absorber of low-energy neutrons. Initially in this video a fast neutron source (5.3-mCi AmBe) is located alongside a He-3 proportional tube detector within a plastic moderator / reflector "flux trap," and a count rate of 50 CPS is observed.

In the first experiment, a cadmium foil sleeve is pushed onto the He-3 tube and the source and plastic are replaced as before. The count rate sinks to below 10 CPS, illustrating the attenuation of low-energy neutron flux by the cadmium surrounding the detector.

In the second experiment, a cadmium foil sleeve surrounds the neutron source rather than the detector. The count rate now is 40 CPS, indicating minimal impact of the cadmium on the fast flux exiting the source. Even though the cadmium has a high capture cross-section for slow neutrons, its cross-section for fast neutrons is small.

Please see earlier two videos (Neutrons_Setup.AVI and Neutrons_Moderator.AVI) for a discussion of the detector electronics and the plastic moderator, respectively.

Category:

Science & Technology

Tags:

• neutrons
• physics
• cadmium
• NIM bin
• radiation

[vmelkon]

how does the detector head work?

[Thallium208]

@vmelkon Can you clarify what you mean by "detector head"? I'm tempted to think you mean the He-3 proportional tube itself, in which case, it works by producing charged particles from the He-3(n,p)H-3 nuclear reaction and then detecting the electrical current pulse produced when those charged particles interact with the tube gas.

[vmelkon]

@Thallium208 Yes, that's what I mean. So why is the counter device (the electronics part) so big? Is it old with vacuum tubes?

[Thallium208]

@vmelkon The electronics are all NIM-standard modules, currently sold by nuclear metrology companies like Ortec and Canberra. It's all solid-state (except for the He-3 tube, of course). The same functionality could be achieved in a more compact package if purpose-built, but NIM remains a widely-used modular standard and is convenient enough for most laboratory detector systems.

[iandegia]

nice bit of kit there...

[KarbineKyle]

AWESOME! I love your experiments! I find them quite fascinating! I hope to do the same with Cf-252, but I know that element (isotope) is very expensive. I have a small quantity of Beryllium in the shape of a sphere, but I would rather experiment with a thin X-ray tube Beryllium window too, so I can see the effects. Have you ever tried using borated paraffin wax? I heard it works very well in slowing neutrons (thermal neutrons). Plastic, or anything carbon-based works well though. Anyways, NICE!