Silver Activation to estimate Fusor Performance

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Matt_Gibson
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Silver Activation to estimate Fusor Performance

Post by Matt_Gibson »

Hi everyone,

I’ve been looking around for a bit to see if there’s a way to estimate how well ones fusor is performing based on activation (silver in my case). I’m thinking it’s a bit more complicated than I imagine, but know that you need neutrons. I also know that you need to run for 3 half lives or so to reach 100% activation.

So, how do we go about evaluating whether or not 4500cpm is “good”? (picked arbitrary numbers). Do we look at how much energy was consumed to get to “x” activation (in cpm) or something else?

Could you reasonably estimate neutron TIER based off activation? Or, is it way too variable?

-Matt
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Richard Hull
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Re: Silver Activation to estimate Fusor Performance

Post by Richard Hull »

No fusor will run flawlessly steady over 3 activation periods except for maybe rhodium. 3 half-lives exposed at near you best voltage and current, even with minor short reductions in output, would give you about 88% full activation. 5 half-lives will put you in the high nineties. (not really worth the effort).

What you get is what you get, and with variable outputs over 3 periods. You will be in the big slop range in activation accuracy.

With 44 seconds of half life on Rhodium you might think it would be a snap for me, but no! I keep doing better and better on each 1 minute digital count and as such I am doing count after count as the voltage goes up and current goes up and pressure is adjusted up. So I often take 20 minutes of operation as I hit 3 runs that are close to my max voltage and pressure. Only then do I shut down the fusor and instantly hit my integral GM tube counter.

Doing math is possible, but you need to know a lot about your activation foil's thickness and square area, how much area of the counter can be impacted and counts taken as well as its efficiency in counting betas. Too thick and you lose counts by absorption.

Gammas are a bit easier, but you lose counts in transfer time on short lived gamma activation. all of this can even be worked out by math.

Activation is far more of an indication related to run-to-run changes in operation especially after a supposed improvement in the fusor setup and operation. Beta counting from isotopes that are rich beta emitters with integral GM tube and foils buried in a moderator are the best instrumented activation counters. Even then, you are only at a better crap shoot, in that you have zero interval after activation to count your finished new isotope while still in the "oven"

True calibration is best linked to a flawlessly stable, un-moved activation setup that is linked to a calibrated electronic neutron counting system.
With the calibrated electronic system, a lot of unknowns in the activation scenario are obviated.

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
Matt_Gibson
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Re: Silver Activation to estimate Fusor Performance

Post by Matt_Gibson »

Definitely sounds more complicated than I thought…

I guess this would require some standard for the material to be activated (30gauge 999 silver disk for example), neutron oven (Hdpe 3in all around the disk), and distance to the cathode center.

It would seem that we could still estimate some neutron TIER vs 999 silver activation in CPM though. 1E5 neutron TIER shouldn’t come anywhere close to 1E6 neutron TIER for a 10 min run, right?
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Richard Hull
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Re: Silver Activation to estimate Fusor Performance

Post by Richard Hull »

Yes, the TIER is always much more than any reading of any sort taken at any one point or section of the fusor.

Fusors do have hot spots. Some physical constructions have more or less hotspots than others.

Let us say that at some 10cm from a spherical fusor you activate metal or via a mR/hr reading on a rem ball and then, back figure to a flux of 15 n/sq.cm/sec and both intercept detection areas are 40 sq cm. you find your total spherical fusor has a surface area of 3,000 sq cm area. That is
3000 X 15 = 45,000 TIER. Move your moderator/activator or your remball to a different part of the fusor and you get 29 n/sq/cm/sec. and a new TIER of 29 X 3000 = 87,000 TIER. Which is correct? Both and neither... as they are what they are for only that small area of the sphere's surface.
If activation is your thing then you want to keep you moderator at that hot spot to double the activation per unit exposure time during operation.

This points out the fact that There is no isotropic emission in any fusor! The sphere with a multi opening true geodesic grid cathode would be far more isotropic in nature that a cube with a cylinder cathode where the emission would be more highly directed as in a B.O.T. device.

Activation would tend to be best done in a cube or small cylinder fusor with a cylinder grid. but this arrangement bears zero resemblance to the classic recirculating Farnsworth fusor device.

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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Jim Kovalchick
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Re: Silver Activation to estimate Fusor Performance

Post by Jim Kovalchick »

Richard just posted some real truth regarding the accuracy and actually inaccuracy of trying to measure neutron TIER. Here are some other things to consider.
1. Point source calculations break down when the source isn't really a point. This is what Richard spoke about.
2. Point source calculations break down when the detector is not a point.
3. When a detector has a size that is big compared to the size of the source or vice versa, the point source calculation gets even worse.
4. Moving the detector further away helps the point source model.
5. Moving the detector further away reduces counts which increases the significance of countrate error.
6. Counting thermal neutrons instead of fast neutrons is easier to do but harder to understand. Materials moderate and reflect neutrons. Changing the geometry of the moderator will change refection too. Surrounding the moderator with a thin thermal neutron absorber like a layer of Cd helps to ensure only fast neutrons enter your detector/moderator arrangement. Once you calibrate the detector, counting only fast neutrons will be more consistent even if you move your detector.
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Richard Hull
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Re: Silver Activation to estimate Fusor Performance

Post by Richard Hull »

Thanks for amplifying my points and adding some of your own, Jim. Yes the real issue is a big neutron source near a big detector creates real issues but does warrant lots of counts to lock in statistical certainty, but fouls the effort at some sort of true reduction due to all the other points you make. In spite of all of this, what we get out of our effort is always better than a poke in the eye with a sharp stick.

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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