FAQ - A neutron moderator for activation/detection

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Richard Hull
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FAQ - A neutron moderator for activation/detection

Post by Richard Hull » Mon May 09, 2016 6:28 am

Attached are images of a formal neutron moderator. This particular one cost about $40.00 in plastics from a local plastics firm. 100% of everything else came from my stock and junk boxes around the lab. My previous setup left an awful lot to be desired, but worked OK. This works much better.

All D-D fusion produces fast neutrons which can't do much of anything for the experimenter. Valuable neutrons are thermal, (slow), neutrons.
A moderator is a hydrogenous material that slows fast neutrons to thermal velocities. Typical moderator materials are water, paraffin, and polyethylene.

A moderator is a must for neutron detection via various electronic tubes. (boron lined tubes, boron corona tubes, BF3 tube and 3He tubes.)
A modrator is also needed for all activation experiments. Among the most common materials that are easily activated are Silver, Indium and Rhodium.

The moderator in this FAQ is made specifically for activation experiments. Usually such activation is done on metal foils of the metals noted above.
Since the moderator shown thermalizes fast neutrons, I have dubbed it my "Neutron Oven" as it cooks the material to be activated in thermal neutrons.
Out of the oven, after a suitable time, the foil of choice is removed and is radioactive for a very short time, as all the half-lives are in seconds or minutes.

This can be a valuable neutron detection scheme provided you have over 300,000 neutrons per second boiling out of your fusor and is an absolutely perfect, proof positive that fusion has been done. Add to this that a simple geiger counter is all the intrumentation needed to prove fusion. Basically, if you activate a metal and a simple GM counter reads over background radiation immediately upon being pulled from the oven, you have done fusion.

Materials list for this:

A single piece of 1" thick high density polyethylene, (HDPE), 6" wide by 36" long. (local plastic distributor- big cities)
2 - 7" long 1/4 X 20 lengths of "all thread" Hardware stores sell 3 foot lengths that you can cut to length with a hack saw.
2 - 7" long nylon or fiberglass 1/4" diameter rods
4 - Nylon 1/4 X 20 nuts with nylon washers
a few 1/4 X 20 metal nuts and washers (hardware stores)
Mounting materials to suit...Wood, G-10 fiberglass, phenolic, etc.
silver foil

Tools
drill press or hand drill with bits
milling machine if you have one or have a friend or neighbor with one.
set of taps and dies - you really just need a 1/4 X 20 die and handle.
hack saw at least...A chop saw or power hack saw would be better.

method

Saw the polyethylene into 6 equal 6" x 6" squares. I used a power hack saw for this. Note, the plastics place will be more than willing to do this for between $5.00 and $10.00 per cut. (ouch!) I wasn't going to give them my money for that.
Drill 4 accurately placed corner holes of about 5/16" in each slab of the plastic to pass the long bolts through to hold the mass together. I made a master and used it to drill the remaining slabs to match, using a drill press.

Now the hard part.....You need to make a carrier of your own design to hold your foil that can be placed within the moderator. Look at my images as a guide and use your own inventiveness.
My carrier, in the image, was made up of a 2.25" ring of metal from a junk box the ring's rim was only 3/16" wide and thick. (old lens spacer of aluminum). This ring is perfect for mounting the foil since when it is to be measured it is the perfect diameter for the 2" GM pancake detector for the beta particle counting.
Before mounting the foil, I attached a 1.5" long 4-40 hex aluminum standoff for a handle and a knurled top knob as a "stop". In my case I rolled out my foil to a thickness of .002 inches. (2 mils).

I milled a 2.3 " wide slot by 1/4" deep into the face of one of the HDPE slabs with my milling machine. Adjust the length of the slot to allow the foil covered ring to reside in the middle of the slab. The moderator is assembled as shown. using the metal all threads on the bottom and the plastic or fiberglass on the top. (note - you must thread about 3/4" long sections on the two insulating rods with a 1/4 x 20 die.) I did this due to the proximity of the high voltage terminal to the top of the neutorn oven. (see images).

One doesn't need a tempting metallic object close to 45,000 volts. These insulating rods are a safety factor. You could design this to go under the fusor, leaving room for it, where there is no high voltage and use 4 much simpler metal "all thread rods".

The insertion and rapid removal of the foil and holder are an absolute must as the hottest activation of silver lasts only about 80 seconds and,then, it is dead (24 sec half-life). The other radioactive isotope of silver has a nice long 2+ minute half life, but to fully activate it you must do top level fusion in a continuous 10 minute run. Only a few amateur fusors can hold 500,000++ neutrons/sec for this long.

The length of thermalization in HDPE is about 1.8 inches. Thus, I built my oven so that the foil is at this distance. The other slabs are behind this...Why?...once thermalized the neutrons ricochet all through the moderator changing direction and can also activate from behind as well.

Before the questions fly on how I obtained the foil....I have a jewelers rolling mill and rolled my own foil from a 1 troy ounce round 9999 silver Canadian maple leaf I bought years ago for about 6 bucks. It is a tedious process of annealing and rolling and re-annealing and rolling, etc., etc. You burn off lotsa' time and elbow grease. Again, my foil was .002 inches, (2 mils), thick.

Good luck with the project and use your imagination and modify what is shown to suit your skills and tools at hand.

Click on each image to enlarge.

Richard Hull
Attachments
neutoven3.JPG
This is a closeup that shows the insulated fiberglass threaded rod on the top near the high voltage terminal
Note the foil ring and holder are in the slot and the knurled knob acts as a stop and removal knob.
neutoven2.JPG
This is just a close up of the oven. It is mounted on a 1/2- inch thick phenolic plank bolted to the fusor.
neutoven1.JPG
This is my carrier ring and handle with a disk of 2 mil thick 9999 pure silver
neutoven.JPG
Here is a veiw of the the entire oven bolted to the fusor setup.
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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Re: FAQ - A neutron moderator for activation/detection

Post by Richard Hull » Thu May 12, 2016 6:14 am

Update to above.........
Data taken for "as built" moderator above. A tedious 6 hour test run on fusor IV was done to show what might be expected from this neutron oven using silver.

At what level of isotropic emission would statistically significant activation of silver take place to show fusion is being done? In short, how many neutrons per second does a fusor have to produce to activate silver in this neutron oven to show that fusion is taking place?

Look at the images above in the original post to see how close I have the moderator to the 6" diameter fusor body. Make the moderator as it is shown. Locate the silver foil exactly as I show it at 4.25CM from the face of the moderator next to the fusor.

I ran the fusor at 3 carefully regulated isotropic emission rates, (150k, 400k and 800k n/sec) for 5 minutes based on my calibrated 3He detector. I immediately removed the foil at power off and took, on average, 5.5 seconds to race to my GM instrument and place the foil under a 2" LND pancake GM tube hooked to a NIM bin with ratemeter and counted for 1 minute in each of the three tests.

Background: I was serious about an accurate background count and ran a single 20 minute background count this count was 4004 counts. (200 cpm)
note: it is important to run your background count with the unactivated foil in its normal count position under the pancake detector. In my case, this was about 7mm from the mica window with the normal protective sceen between the foil and the mica window.

All data was worked statistically as single count statistics which are treated in Knolls classic text "Radiation Detection and Measurement", 1989 edition. Single pass measurements are famous for large standard deviations and are specially treated in this work pp. 84-89.

A quick rinse example:

A single pass measurement of 128 cpm would make a standard deviation of sq.rt. 128 = sigma 11.3 or 128 counts +/-11.3 so the real mean count might be expected to lie somewhere between 117 and 139! Very sloppy, indeed.....Now for the really bad news! If we want to know the real 99% confidence range on this 128 cpm reading, a 2.6 sigma would be +/- 29.4!! So we can be absolutely sure to a 99% confidence level that the real emission rate is between 99 and 157 CPM...Why did we even bother? Well, even this is very useful. now follow the math........

Background = 200 cpm sigma of +/-14 (from real data shown above.)

Using Knolls methods on my three runs:

150,000 n/s GM reading 261 cpm sigma of +/-16 minus background 200cpm = real count 61cpm. But wait, we must take the sums of the counts for background and activation count and find its square root to find the new sigma for our 61cpm reading above the background. (sq.rt. of 200+261 = 22)
The final result is our silver is activated to 61cpm above background with a sigma of +/- 22 cpm. At 2.6 sigma for 99% confidence = +/- 57
The result for 99% confidence 61cpm +/-57
Is this pass statistically viable to show fusion to a 99% confidence level? You bet! Just barely, but 99% for sure..... You have done fusion at 150,000 n/s

400,000n/s GM reading 381cpm This is obviously a big win without statistics, but let's do it for grins and googlers. Count above background is 181 and the sigma is
(sq.rt of 200 +381) = +/-24 2.6 sigma = 62 for 99% confidence 181-62 (worst case)=119 super big win Final result for 99% confidence 181cpm +/-62

800,000n/s GM reading 538cpm actual count over back ground = 338 sigma of +/-27
99% confidence of 338 cpm +/-70

The take away here is that this neutron oven, as made, will deliver 99% confidence fusion proof using just a 2" pancake GM counter at a fusor operating level of 150,000n/s!

Notes: It is tough to hit 150,000 n/s with fusor voltages under 28kv @ 12ma with 9 microns of D2 pressure.
400,000 n/s were obtained with 37 kv applied @ 11ma and 11.9 microns of D2 pressure.
800,000 n/s were obtained with 41kv applied A12.5 ma and 14.1microns of D2 pressure.
At one sigma acceptable levels, this oven might work to prove fusion at 80,000 n/s.

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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Re: FAQ - A neutron moderator for activation/detection

Post by Rex Allers » Thu May 12, 2016 8:12 am

I found another source of silver foil that I bought from a couple months ago.
www.riogrande.com -- a jewellery materials supplier

I bought a piece 2" x 6" x .010" .999 silver. It was about $24 shipped. I then cut it into 3 pieces 2" square. I'm not sure if it is an option for buyers outside of the US, but I thought it was a decent deal.

I'm not in a position to try activation yet, but I think this should be fine for it when I'm ready. I also have some similar sized pieces of Indium foil that I found on ebay a while back.

Richard, I don't think you mentioned how thick (thin) your hand-rolled foil was. Do you think that matters much in your analysis?
Rex Allers

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Re: FAQ - A neutron moderator for activation/detection

Post by Richard Hull » Thu May 12, 2016 5:18 pm

Rex, a very astute observation. I did forget to mention my foil thickness. It is .002 inches, (2 mils) thick. Long ago, in a distant past, (1970's), I made custom jewelry to make good side bucks. I did lost wax casting, (still have the gear) and used Rio Grande for many findings and metals. I highly recommend them. Note: I have corrected my original posting and placed the thickness of my foil within it. Thanks again.

Foil thickness:

I felt that a particular foil thickness might allow for reverse side "shine through" of the beta particles which might utilize all of the active silver volume and boost activation detection numbers. This means that, within the oven, counts would be generated from isotopes created through the entire volume of the foil, including those activating the foil from the back side of the foil. Of course, in a good "ideal" moderator there is, effectively, no back side.

Using the density of silver in mg/sqcm and some tables, I calculated that the foil would have to be a bit thinner than my .002 inches to allow for significant reverse side "shine through" of the mean beta energies of both silver isotopes created. However, the most energetic betas are from the 24 second isotope and might see some shine through. .0005inch or .5mil foil would be more desirable for shine through counting. However the .002 inch that I rolled is a good bit more sturdy, mechanically, and gave satisfactory results.

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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