Using Silver (or other element) Activation to Quantify Neutron Output

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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Bob Reite
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Using Silver (or other element) Activation to Quantify Neutron Output

Post by Bob Reite »

In another topic, there was a proposal to define an activation standard. The concept was that a defined mass of silver, mated with a defined moderator, using a defined pancake counter probe, would be a standard of comparison between various fusors. Is there any way to determine neutrons per second as a function of decay time or other observation? Or is the only way to calibrate against a bubble detector, calibrated REM ball or other calibrated neutron detector?
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Jim Kovalchick
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Jim Kovalchick »

Bob,
In theory activation analysis should be able to benchmark flux. There are some challenges though.

Activation time is critical. Ideally, activation should be done to saturation.

Keeping flux steady fot the entire activation period is also helpful. Excursions up or down will impact the count rate. Small excursions can probably be tolerated but will influence accuracy.

The toughest problem is the actual counting. Ideally counting should be started immediately to get the largest count rate. The bigger the count rate, the greater the accuracy. Using a method like Richard's data collection or data logging like what I am using helps to get immediate counts.

Multiple benchmark runs with a side by side bubble detector would be needed to set the constants.
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Richard Hull
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Richard Hull »

Activation and counting betas is a function of surface area and not mass. Ideally 2-5 mils thick foil is desired of a surface area that can be predetermined to match the GM detector. A thick chunk of silver or any activation metal has grave issues with self absorption that while internal activated can't have the activation radiation ever reach the counter. foils are best. Beta emitting activation metals taken to saturation are pretty much limited to silver and rhodium. Rhodium is just out of the question due to its $20,000 per ounce price. Its cross section is higher than silver. Another great thing is it is virtually fully saturated in 220 seconds (5 half-lives). As Jim notes you must absolutely be on top of the count the instant you drop power to the fusor. A 3-5 second loss in starting on Rhodium is tragic. This is why I attach the rhodium foil to the GM tube inside the moderator to count using the arduino logger within 1 second of shutdown on the fusor. I bought my tiny strip of rhodium when Rhodium was still cheap at $10,000 per ounce.

Richard Hull
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Dan Knapp
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Dan Knapp »

Would a thin layer of rhodium plated on another metal surface work as well as foil? There are rhodium plating services available online at affordable prices. Has anyone tried it?
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Richard Hull
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Richard Hull »

You would have to demand as thick a coating as they offer hundreds if not thousands of microns. It would not be cheap currently hundreds of dollars on a 2 inch diameter substrate. make sure the substrate will not activate! It would need a terrible cross section with a near infinite half life.

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: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Jim Kovalchick »

Richard,
Mass also effects activation because it sets the macroscopic cross section. It is also why a rhodium plated mirror isn't that useful for activation even though there is plenty of surface area. I do agree with you though about surface area being important for detection. The detection geometry impacts the modifying constant in the activation equations.

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Joe Gayo
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

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Richard Hull
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Re: Using Sliver (or other element) Activation to Quantify Neutron Output

Post by Richard Hull »

Regulation foils sound great. However I could not get a price from the website. Considering there is far less than $1.00 worth of silver in a box of 5 or 10 foils. I would be curious what they charge for rolling the foil, weighing and cutting them out into .5-inch circles over the $1.00 bullion charge. The ideal would be two 2 inch pancake 7311 LND mica windowed tubes with a 2.25 inch foil between them with this assembly buried within the moderator, electronically "or"ing the tubes' pulses.

I am seriously considering this.
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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Joe Gayo
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Re: Using Sliver (or other element) Activation to Quantify Neutron Output

Post by Joe Gayo »

Not cheap, but accurate and a standard. About $200 for a box of 10.
Victor Gonzalez
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Victor Gonzalez »

What about a pre-determined amount of silver (or rhodium) powder pressed in a disc with micronized polyethylene wax?

With very little amounts of metal 0.1~1g we can make a 40mm diameter disk, it's thickness and "metal density" would be chosen in the amount of wax added.

I work with precious metals and have at hand, silver powder, rhodium powder (rhodium black as well), compounds of these, nitrates, chlorides, oxides, hydroxides.

If anyone would be interested, for this forum, I can sell the wax disc with silver for just the price of shipping (from Brazil).
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Pablo Llaguno »

Nothing to do with activation, but there is also the possibility of indirect measuring of neutron output by tracking damage (tracks) registration of charged particles produced by the interaction of neutrons with carbon and oxygen atoms of some dielectric materials. For example, https://www.sciencedirect.com/science/a ... 0207010595, used polycarbonate and showed some interesting results.
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Rich Feldman
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Rich Feldman »

There's another thread, within past couple of weeks, about developing tracks in solid detector media.
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Jim Kovalchick
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Jim Kovalchick »

Track etching will have less variables and less equipment requirements. Not a bad idea for standards. Does anyone have enough experience that they can comment on repeatability?
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Bob Reite
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Bob Reite »

While not the precision we would like, Richard gave me an insight for at least determining the order of magnitude of neutron output using the results of silver activation.

The test set up would be silver foil, inside a moderator which also has a geiger tube that is sensitive to beta radiation. A CDV-700 with the beta window open on the "hot dog" tube in theory would suffice, but a mica windowed pancake probe wold be even better. The silver in the moderator would be exposed for 5 minutes while attempting to keep the neutron output at a high constant output. Counting must begin as soon as possible after HV shutdown.

That said, and corrections and comments are invited, this is what I believe could be determined by subjective listening to counter clicks, although actually counting and recording pulses vs time would be even better.
So from low to high

Note: All table entries have been revised from original post based on Richard's input

Under 150,000 n/s: At this low level, it would be impossible to discern the silver decay from background.

150,000 to 500,000 n/s: The initial decay of AG-110 wold be audible for a minute or so, but unless you can start monitoring the sample within seconds of shutdown you'll likely miss it at the lower end of this range.

500,000 to 1,000,000 n/s: The counter should be clicking as fast as it does when testing the counter with a beta check source.

1,00,000 - 2,000,000 n/s Faster clicks, but still recognizable as distinct pulses.

Above 2,000, 000 n/s: Counter audible output now sounds like bacon frying! Pulses have merged into a steady hiss.
The more reactive the materials, the more spectacular the failures.
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Richard Hull
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Richard Hull »

No! Silver foil or thin sheeting @ 400k n/s TIER exposed for 4-5 minutes would have the counter really clicking away to the ear. No counting needed. (assumes very close neutron oven of HDPE to the fusor)

Image below is silver in my fusor last year in early August at 250,000 n/s TIER the peak is at 80 counts in 5 seconds or 12X80 = 960cpm rate on a GM counter if you are on it the instant the fusor is turned off.
If it takes you 1 minute after the fusor is turned off the 5 sec count is ~30 X12 or 360 CPM. Well above a normal 50cpm background to the ear. This was one of my first runs with the imbedded GM and silver, before I wrapped the tube with a cylinder of silver. (crummy results) I got much better by HEAS 2020.

You just need an honest average rate of even less than 200,000 n/s TIER to make silver sing.

Richard Hull
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NeutronActivationAG 8 Aug 20  TIER.jpg
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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Joe Gayo
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Joe Gayo »

Here is the equation I use (A is in Becquerel, make sure you correct for the 4pi efficiency of the detector and foil thickness [~85% for 0.1mm])
fluence.PNG
No need for subjectivity. Personally, Mn is my first choice, because of isotopic purity, and single gamma energy in the decay which makes absolute counting easier.
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Bob Reite
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Bob Reite »

OK, I have revised the table entries based on Richard's input.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
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Richard Hull
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Re: Using Silver (or other element) Activation to Quantify Neutron Output

Post by Richard Hull »

Oops! I forgot. Regarding the above chart....Way back, this time last summer, I set my homemade GM Arduino data logger for 10 second count intervals. Once I started with Rhodium, I changed one line of code and made it count and log every 5 seconds. As such, The 80 X 12 should be 80 X 6 or 480 cpm rate at the instant of shutdown and not 960 cpm. Seems I did not read my own legend at the time axis on this old chart. Still, it was activation well above background. At that time I was using one single little strip of silver under the GM tube.

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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