Neutrons_Setup.avi

A helium-3 proportional tube and NIM electronics are used to detect neutrons from a weak homemade AmBe neutron source. The video illustrates my preferred method for setting up the high-voltage bias, the amplifier gain, and the SCA lower-level discriminator in order to achieve sensitivity and selectivity for neutrons. I finish with a comment about the neutron energy response of the detector (it is sensitive to slow neutrons) versus the source spectrum (fast neutrons), and the need for a moderator to make this kind of detector sensitive to fast neutrons.

Basically, the procedure is as follows.

A. Begin with high voltage off, the amplifier gain at a minimum, the shaping time at 2-6 microseconds (for most neutron-detecting proportional tubes), the SCA's ULD at full scale, the LLD at zero.

B. Set high voltage to within the detector's proportional range, but not above the rated voltage of the cabling, connectors, or preamplifier. Always change the voltage slowly to avoid damage to the preamp.

C. In order to establish signals of an appropriate amplitude for the rest of the electronics chain, the amplifier gain will be increased until a strong gamma radiation response is observed. I place a gamma source (uranium ore) next to the tube to make this process faster and more definitive.

D. Pulse-height discrimination again non-neutron radiation is established by increasing the LLD on the SCA until gamma counts effectively cease. Now the detector is sensitive and selective for neutrons.

Category:

Science & Technology

Tags:

• neutrons
• nuclear
• radiation
• NIM bin

[gg459]

where did you get this? I'm building a fusion reactor but have had some trouble finding a decent neutron detector

[Thallium208]

@gg459 The He-3 detector was purchased many years ago from Don Orie at OE Technologies. Unfortunately, He-3 gas is in short supply and so the detectors are difficult to obtain now. You still probably have a number of other good options for what you're doing, however.

[2ndAsstJizzMopper]

thanks! been reading about early particle physics, and watching that moderator in action was a blast.

[TheChefEzra]

I think they are not the same smoke detect i find on ebay! i well impossible!!! now a nice quest starts!

[Thallium208]

Thanks all for your interest.

The total Am-241 activity in this source is 5.3 millicuries.

The source is composed of many sealed sources from antique smoke detectors aligned on ~40-mil beryllium metal foil. The individual sources remain intact and undamaged.

I don't share the exact brand and model of smoke detector that is preferential for this use, in order to protect my supply. However, the "secret" is well-known in the nuke hobby. They are very old technology, but still in use.

-Carl

[petehug3gemma]

I think you've misheard a little - the source is known as "ambe" which is a mixture of Am-241 and Be - the Am241 is an alpha emitter and these alpha particles when they interact with Be or Li produce neutrons so the AmBe source is much more than a smoke detector which uses only Am241. I've access to a 1Ci AmBe but won't be mentioning on here where this is!!!! But the difference is the alphas emitted from Am-241 and the neutrons emitted from an AmBe source.

Hope this helps!

[TheChefEzra]

yep how did you get 5mCi????