FAQ - Activation Materials - Silver

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Richard Hull
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FAQ - Activation Materials - Silver

Post by Richard Hull »

For those successful fusioneers making over 100k n/s, activation work is possible. for those working at the lower end (100k-300k) armed with only a GM counter, the best activation material is Silver. It has a very high cross section and a broad resonance band so that even 1kev neutrons can activate the silver. In many ways silver is the ideal first activation material for the amateur fusioneer.

Silver consists of two natural isotopes of nearly equal proportions. Ag107 and Ag109.
These activate in a resonance and thermal neutron field to make Ag 108 and Ag 110, respectively.

See cross section plot below The plot is color coded, Red=Ag 107, Green=Ag 109. One can readily see that the cross section is high even for relatively fast neutrons.

The highest thermal cross section is exhibited by Ag109 (~89 barns), but unfortunately, its activation product, Ag 110, has a half life is only about 24 seconds and is not suitable for quick removal and counting, for the most part. (By the time you can effectively transfer the material, almost half of its activation is gone.) Full activation of this isotope can be accomplished in just under three minutes. Weak fusors that can't be run full blast for over a few minutes can certainly use this isotope for counting if one is fleet of foot.

We are most interested in Ag 107, (~39 Barns), which still has a high cross section, but to our advantage, its activation product, Ag108, has a half life of 2.5 minutes.

Silver 107 activates to its maximum level of Ag 108 in a fixed flux in about 10 minutes of fusor operation. As the Ag108 is a beta emitter of ~2.5 minutes half life, a handy GM counter with a digital counter connected will herald activation success at very low cost.

Silver is easily obtained and worked. Any old US 10-25-50 cent coins made in 1964 or earlier are 90% silver. Sterling silver items contain 92.5% silver. The best source is a Canadian silver "Maple Leaf" or a US mint "Silver Eagle". These can be had form a local coin dealer or a dealer that buys and sells silver and gold bullion. These ingot coins or any standard 1 troy ounce silver, collector ingots are "fine silver" and at least 99.9% pure silver. They are readily hammered or rolled into sheet with a bit of annealing (re-softening) in between.

For beta counting purposes, there is little need in trying to activate a piece of silver larger than the detector's sensitve area. Likewise, there is little need of activating a thick piece of silver as the betas from internal atoms will not make it out of the piece into the GM detector. Thus, thin sheets or foils are the item of choice in most silver activation work.

Procedure for Silver activation:

A moderator of about 6-8 inch thickness is needed. This can be assembled out of high density polyethylene or parafin blocks or slabs. Plain distilled water in a suitable container is also a good moderator.

The silver sheet to be activated is placed at least 1.5-2 inches deep into the moderator, but should be handy for rapid removal following a run as it will be most active at the instant you turn your fusor high voltage off and, then, rapidly decrease in radiation output.

The size, (volume), of the moderator and the placement of the silver within it will have a significant effect on the activation process, regardless of flux level. In general, the larger the volume of the moderator, the better. This is offset by the need to place the moderator assembly as close to the fusor shell as is physically possible. Most well designed moderator are "rear and side heavy". More moderator material volume is contained in the rear 180 degree fan or area of the activation product. This allows for more back scattered neutrons.

Expose the nearby moderator and silver for at least 10 minutes to your highest level of operation. Make sure all is prepared for proper count GM counting. (Instruments all on and reset to zero.) You might practice the removal and run to the counter a few times.
At the end of the exposure period Turn off the high voltage and immediately remove the silver sheet.

Rapid removal of the activated sheet or foil from the moderator and placing it under the GM counter is critical. Run a full 1 minute count, and then another, and so on, for about 15 minutes. Leave the silver in place and count after 30 minutes to obtain a fair back ground. A good, low end, working fusor should almost double the count between minute #1 and the 30 minute count.

The beta rays represent virtually the entire torrent of radiation from activated Ag108. These are very potent and have a peak energy of 1.6 to 1.8 mev.

Activated Silver contains some metastable states that are of no interest or significance to the amateur fusioneer. The gamma rays produced are sparse and AG 108 produces, intrinsically, only a .633 mev gamma ray, (1.8%). A good Gamma ray spectrometer is required to capture and verify the energy of these rays. Most amateur fusioneers do not have such a luxury item, though several have these instruments and have posted their results here.

Interestingly, one might make a kludged up neutron counter by taking a thin walled, all aluminum GM tube like the Victoreen 1B85 and rolling it up in Ag foil and then placing the entire tube in a moderator!

Two issues....One is x-rays from the fusor. A thin lead sheet might do to shield the moderator from x-rays. Thus, the tube would not count fusor x-rays.

The second issue is one of electronic noise. The GM tube works off gas amplifcation and should be virtually noise immune in a normal, stable, non-pulsed fusor environment. Such a counter would be slow to react or to report ups and downs in fusion due to the two beta emitters formed. However, it would make for a pretty cool, inexpensive and somewhat noise immune electronic neutron counter.

Richard Hull

References:

The Table of Isotopes - 6th edition, Lederer, Hollander, Perlman, 1968, John Wiley & Sons
ENDF VI Library (online - URL given above)


EDIT May 2010 *************************************

When considering a moderator, the plus ultra data and now to be considered the final word, is given in a fabulous exposition by Carl Willis. All here should consult
viewtopic.php?f=13&t=6104#p40456

This being an ancillary to the above FAQ

R.H.
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The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Carl Willis
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Re: FAQ - Activation Materials - Silver

Post by Carl Willis »

Hi Richard,

Commendations for starting a FAQ on activation materials. Silver is a strong contender for a beginning activation project, especially so if all the equipment one has is a Geiger counter. Silver + Geiger tube is purportedly the technology used in the LANL "albatross" area monitor for pulsed neutron fields.

Activated silver is actually a pretty poor gamma emitter relative to its beta yield. A single gamma energy of 633 keV is emitted in only 1.8% of decays from Ag-108, and the shorter-lived Ag-110 emits gammas only very rarely.

Look forward to hearing more from your efforts with the souped-up Fusor IV.

-Carl
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Richard Hull
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Re: FAQ - Activation Materials - Silver

Post by Richard Hull »

Carl,

I started this posting yesterday at work and did not have all my books and data, I have ammended my post accordingly this morning with the reference given in hand. (Images attached to original post)

Indium is the good Gamma emitter and I confused the two at the time of assembly.

Thanks for the corrective action.

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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Re: FAQ - Activation Materials - Silver

Post by abhaylett »

"The highest thermal cross section is exhibited by Ag109 (~89 barns), but unfortunately, its half life is only about two seconds and is not suitable for quick removal and counting, for the most part."

-Now I'm confused! Isn't the half-life of Ag109 about 24.6 seconds?

Ben
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Richard Hull
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Re: FAQ - Activation Materials - Silver

Post by Richard Hull »

Ag 109 is stable! Its activation product Ag 110 and has a half life of 24.4 seconds. I have ammended my posting to reflect the 24 second Ag 110 half life. I misread the decimal point. I also noted in new text that one must be very rapid to remove the silver from the moderator and place it under a counter to effectively count this fleeting isotope.

Sorry for any confusion. Keeping one's wits while composing this kind of thing is tough and doesn't get easier with age.

Hopefully, all is well now.

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
Dan Tibbets
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Re: FAQ - Activation Materials - Silver

Post by Dan Tibbets »

Question concerning counting intervals with a Geiger counter with activated isotopes. I've seem recomended counting durations of 5 sec for Ag 110 (1/2 life ~25 sec) and 1 minute for Ag 108 (1/2 life of ~ 2.5 min.). Can I take this to mean that counting durations of ~ 1/4th to 1/2 of the target half life are good compromises to achieve detectable count differences while still being able to plot out the 1/2 life decay patern.
Would longer counts be acceptable for demonstrating minimal activation? For example- background count, activation of Ag, followed by one or two 10 minute counts, followed by one or two more back ground counts, all hopefully to show a small excess count above the noise/ background count.

Dsn Tibbets
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Re: FAQ - Activation Materials - Silver

Post by Richard Hull »

Dan,

You have the right idea and good questions. There is little sense in counting beyond 3 half lives of any fast isotope. On very short lived stuff you will never see a decay curve plot out without a PHA mode count from a decent MCA system.

To just show you have done it, expose the silver or indium or whatever and, as quickly as possible, transfer to a counter and count for a period of 3 half lives. Record the count. Without touching or moving the foils, wait ten more half lives and count again for 3 half lives. If you have activated, there will be a difference that can't be explained by chance unless the delta count numbers are real close. With silver in my system I counted about 300 more counts on the first run than the second as performed above.

As long as you do not fiddle with or move the foil between counts, background data is unimportant as that was included in both counts, the bulk of the delta certainly being due to a created isotope.

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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