Water as Neutron Shield

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.
Dan Knapp
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Re: Water as Neutron Shield

Post by Dan Knapp » Fri Jul 29, 2016 1:43 pm

I regret that this discussion seems to have turned into an argument. I'll make two last points and then let the issue rest from my end.
The title of this thread refers to neutron shielding, not x-ray shielding. The gammas in question are the capture gammas that result from hydrogen (or boron) capture of the thermalized neutrons whether in water, paraffin, or plastic. My question was how effective slate is in shielding those energetic gammas.
With regard to paraffin being cheap. I just checked, and McMaster charges $53 for a 1/2" x 12" x 19" sheet. Richard's 3" thick shadow shield would cost $300. Water is of negligible cost, borax is about $3 for a four pound box, and a good scrounger can find (heavy) plastic containers at no or neglible cost.
The bottom line, as Richard pointed out earlier, is that all of this is academic. For the home fusor, distance is the only shield really needed. But I continue to maintain that if one feels a need for shielding, water is the simplest and cheapest.

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Dennis P Brown
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Re: Water as Neutron Shield

Post by Dennis P Brown » Fri Jul 29, 2016 3:54 pm

Borax powder is a good idea and I think that is a win; as for gamm'a from the paraffin, again, read my post - the flux is far, far too small to even consider that point so why make it at all and yet once again? Requiring three inches of paraffin isn't necessary for such a shield in a fusor but to each their own in which case, paraffin is getting up in cost as you say for three inches. I too find this arguing by you and I pointless. Facts are facts and they speak for themselves.

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Richard Hull
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Re: Water as Neutron Shield

Post by Richard Hull » Fri Jul 29, 2016 4:53 pm

Never, ever order from McMaster unless you want to pay the maximum allowed by law or any other source. I would buy from McMaster only if they were the only source on planet earth for what I absolutely had to have. I bought 2- 12X24X 1" thick slabs from "missies candles" on the internet for about $48 for both! Go to the highly competetive candle makers web sites, not the rip-off, list price plus 200% markup suppliers. Again forget McMaster even exists and you are far better off, economically. Note petroleum based products vary with the market and paraffin is one of those items.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Jason C Wells
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Re: Water as Neutron Shield

Post by Jason C Wells » Sat Jul 30, 2016 5:38 pm

This is not arguing. It's instruction. I'm glad for it.

Dan Knapp
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Re: Water as Neutron Shield

Post by Dan Knapp » Tue Dec 06, 2016 2:14 am

At risk of restarting a debate, I wanted to share an inexpensive approach to neutron shielding for a small fusion device. I recognize that the need for neutron shielding for a fusor (outside a regulated environment) is questionable, but I put together a shield in an abundance of caution (or perhaps wishful thinking?) because the Penning trap reactor we’re exploring is uncharted territory and might ultimately produce enough neutrons to be of concern (see http://iec2015.es.titech.ac.jp/presentation/O-8.pdf for details).
During a recent visit to Phoenix Nuclear, I noted that they were using stackable 3.5 gallon HDPE water storage containers filled with borated water for neutron shielding. The container they use is called a “WaterBrick,” a product sold to survivalists for emergency water storage. These 9 inch wide containers are very sturdy and stack nicely, but they have two sizeable holes through them for strength, which produce a significant void in the shielding volume. They are also rather expensive ($15-20 each depending upon quantity purchased). I found a less expensive container in the form of stackable 2.5 gallon heavy wall HDPE containers available online from United States Plastic in small quantities for $6.45 each (usplastic.com; Item #75176). They also sell 5 gallon containers, but the 2.5 gallon are easier to handle and stack (they weigh about 22 pounds each filled). I filled them with saturated boric acid solution (purchased online from Florida laboratories, 5 pounds for $15.19). Borax is available in the grocery store and cheaper, but saturated boric acid gives a higher boron concentration (9.7 g/l vs. 7 g/l for saturated borax). I stacked these on a Harbor Freight hydraulic lift cart which makes the shield easily moveable and height adjustable. The short dimension on these containers gives about 8 inches of water thickness (more than the thickness of WaterBrick considering the holes through the center). Staggering them slightly avoids any beam leakage between the containers. I have not been able to find numbers for the attenuation half length of neutrons by saturated boric acid solution, but plan to actually measure this at some point. One would not expect this much water to be of great help in shielding x-ray or gamma radiation, but the half length for 100 keV gammas is only about 4 cm; so this thickness of water would give about 97% attenuation of 100 KeV radiation. The x-rays from most fusors would be of lower energy, so this shield would give significant attenuation of x-rays; but one would not want to depend upon it alone for x-ray shielding. I will add a layer of sheet lead in front of the water containers. I just wanted to share this in case anyone was looking for relatively cheap neutron shielding. Sorry about the sideways picture; I couldn't figure out how to rotate it. The pdf will open with the correct orientation.
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Dennis P Brown
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Re: Water as Neutron Shield

Post by Dennis P Brown » Tue Dec 06, 2016 11:53 am

Not a fan of water around such high voltage systems - one is exchanging a small risk for another small but now a needless risk; especially since most fusors either don't work (most built are abandoned before ever running) or those that do, are run for short periods of time. The day a fusor achieves a real neutron threat will mean that the person is in very advanced work and shielding using standard materials (dry shielding) would be a given and an irrelevant cost compared to their advanced support equipment/power supply costs.

As for x-ray shielding, I use slate as needed - very cheap, easy to cut/drill/mount and very safe compared to water or lead.

Dan Knapp
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Re: Water as Neutron Shield

Post by Dan Knapp » Tue Dec 06, 2016 2:41 pm

Phoenix is a multimillion dollar operation using 300kV sources and they use water filled HDPE containers. I rest my case.

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Richard Hull
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Re: Water as Neutron Shield

Post by Richard Hull » Tue Dec 06, 2016 3:20 pm

Shielding, the type and amount required, is an applied voltage issue in any fusor or electrostatic accelerator. Common sense and an understanding of radiation physics is a "must have" as well. Beyond this, as I noted in an earlier post in this thread, any radiation emitting device, in a formal professional or institutional research setting, will always be overdone to satisfy the resident CHP. This is usually tied to the ALARA mantra and specific guidelines, but also for fear of future litigation.

We, as fusioneers, do not labor under such draconian restraints. As noted, also in this posting thread, the number of folks who do fusion is near zero related to the number of those excited about doing it and arriving here. Couple this with only a tiny fraction of those "winners" actually going on to do research with a functioning, finished fusor, which might run for anything like an extended period of maybe an hour or two, over days, weeks or months, and you wind up with only knowledgable, capable people, who already know and understand the need for shielding, if any, in their effort.

Shielding is always a great topic of discussion, it seems. For those going "all the way", it is necessary, but for 99.95% of those discussing it, ad nauseam, it is a case of putting the cart way out in front of the horse. We must note that the vast majority of winners are losers when it comes to continuing to use their fusors. The bulk are here for the win. Few hang around after getting their name on the listing.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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