Question on Activation - Moderators

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Dennis P Brown
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Question on Activation - Moderators

Post by Dennis P Brown »

Since I am moving toward a specialized neutron fusor to try activation, I was wondering - "How to best make a moderator for a detector, in general?" Certainly, the excellent FAQ by Richard covers pretty much every question on this topic:

viewtopic.php?t=14107

While this does address all the issue like geometry, or issues of detector/fusor distance, and moderator thickness at that location I did wonder about how best to optimize such an arrangement for say Indium (In) or silver (Ag). I was wondering if using a moderator thicker for Ag and In between the material and fusor compared to the value Richard used for Rb might work better for these smaller cross-sectioned activator materials? Do these materials require a lead shield as well?

Also, and a more engineering issue - I obtained a pancake detector. I noted that it had a 3 kV connector which seems way over kill. I hooked it up to an Eberline unit and set a uranium sample over the detector head. Then I adjusted the voltage so as to find the proper operation level; the Eberline really kicked in suddenly at 700 volts. Is this the normal voltage for a pancake detector?

I'd think any coaxial connector could handle that level - confused why it has a 3 kV connector or did Eberline just go overboard?

Also, my Eberline was reading 3 mRem/hr which strikes me as reasonable and likely a good operating voltage. Or am I too low?
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Jim Kovalchick
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Re: Question on Activation - Moderators

Post by Jim Kovalchick »

Typical 2" pancake tube like that in the Ludlum 44-9 probe is 900 V.

As far as moderator thickness, you shouldn't need to vary the thickness for different cross sections because most elements you have a real chance of activating all have their highest cross section at thermal energies.
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Dennis P Brown
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Re: Question on Activation - Moderators

Post by Dennis P Brown »

Thanks for that information (both) - that does explain a lot. Good to have the correct voltage so the instrument's response is correct.
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Richard Hull
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Re: Question on Activation - Moderators

Post by Richard Hull »

Pancakes in the 2" operate over a decent range. They all have a plateau. The stock pancake by LND is indeed a 900 volt operating point. Russian and other pancakes might be different.

Sticking with the 4 digit most common 2-inch LND pancake tubes.
The older probes tended to use the 7311 tube due to its geometry and this is a 900 volt tube.
They make a nice low voltage 2-inch pancake 7317 that only needs 500 volts.

https://www.lndinc.com/product-category ... dow-tubes/

I was stunned that your tube is somewhere in the middle at 700 volts. Is your tube an LND tube? The very common 7311 might just start working at the low end of its plateau at 700 volts but I doubt it.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Richard Hull
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Re: Question on Activation - Moderators

Post by Richard Hull »

I found that I would appreciate two moderators. One for my large 3He neutron detector and another for activation with a GM tube in it intimate with the material to be activated. The activation moderator can be very small and jammed right up against the fusor for maximum flux. In general, a 5"cube is ideal and can be constructed from block HDPE around the GM tube and activation material.

You need to study what I did the the rhodium and the Russian beta detector CTC or STS-5 tube. Electronic good mine sells this GM tube for $20.00 and it is all you need for activation. It only needs 350-420 volts!

viewtopic.php?t=13568

Now all materials with a high cross section also tend to have very short half-lives (seconds!!) The key is to activate as best and for as long as you can around 2 or more half-lives, then, immediately turn off the fusor and at the same instant in time start your moderator GM count in segmented time periods. I use 5 second count periods, and record the result in an Arduino Eprom to generate the decay curve to prove activation. Running the moderator GM while the fusor is running is stupid as you will count x-rays at 10,000 cpm or more! Begin counting for activation the instant the fusor is off. A 1 second time lapse is almost too long to begin counting.

Richard Hull
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Rich Gorski
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Re: Question on Activation - Moderators

Post by Rich Gorski »

The above advice is right on. Here’s how I would proceed. Use a GM tube and wrap the material for activation (silver, indium…) around the GM tube as Richard suggests. Then stick it in the center of the blocks of moderator, get it as close to the fusor as possible and irradiate for at least 3 half-lives of the isotope you expect. Then monitor the GM response ASAP! If your using a GM in tube form then the moderator could be cylinder or a block but should surrounded the entire tube.

The best moderator thickness is difficult to determine. Many have used Monti Carlo simulations to try to calculate an optimum. It is known that fast neutrons at 2.3MeV requires a few dozen interactions in the moderator material to thermalize down to 0.05 eV. So from the GM tube's point of view thermal neutrons are coming in from all directions.

One additional thought I have is that the moderator can also be HDPE pellets (available on Amazon). Mount the GM tube along the axis of a cylindrical container and fill with pellets. The resultant moderator density may will be somewhat less than a solid block of HDPE but this seems like an easy way to experiment with the moderator thickness and see what diameter gives you the best activation results (or neutron count).

Rich G.
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