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Neutron detection moderator embedded activation!

Posted: Fri Aug 07, 2020 8:19 pm
by Richard Hull
I debated reporting this effort in my new fusor V posting as it bears on the fusor V work and an idea I have had rolling around in my head for some time. The foregoing being said, there are good ideas and bad ideas. This, I feel, is a great idea, but in my lab and with my highly variable, high level of background radiation, the results will not allow for lower level activation detection with ease.

I will give the epitome of the process here. I am still musing over this effort and will continue the work.

I refer all to the superb moderator I used in the recent rebuild of fusor V in the Construction forum. (URL given below) I noticed that the total envelopment of the large 3He tube in HDPE far exceeded the results over the older water moderator used in Fusor IV. I also noted that indium foil placed on a plastic coupon and inserted in the new HDPE moderator. really allowed for the total replacement of the old "Neutron Oven" used with Fusor IV and was an improvement at the lower end of activation.

Both indium and silver have good activation levels for strong beta emitters which, unfortunately, have half-lives of under 25 seconds. In the past I have shut down the fusor, grabbed the activated specimen, and raced for my 2" pancake NIM based, GM counter to place the specimen smashed onto the face of the GM detector. This process took on the order of 10 seconds at best.

Due to the efficiency of my current larger HDPE moderator, I thought, "Why not take one of my many slender Russian Beta-Gamma tubes, used in my homemade GM counters, wrap it in the foil to be activated and place it in the counter moderator?"....But how to do this??

The Process

I noted the long slender 1-inch by 1-inch by 18-inch HDPE rod needed for the moderator, (see my detailed moderator posting...URL below), might provide the answer. I proceeded to mill out the section of this rod that would contain the foil covered tube and a slot for the connecting BNC cable. (see photos). The tube and BNC cable assembly within the rod were place in the central cavity just over the 3He detector.

A special counter was needed for this effort. It was made up using an Arduino Uno micro controller as its core component. It was realized that a running counter would just read the torrent of x-rays while running the fusor. Thus a "start count" push button was needed to start the counter the instant the fusor was turned of after an activation run. Also, it would be nice if after every ten seconds of counting, the count could be stored in the Arduino's EEPROM. The count is then zero'd and resumed for another 10 seconds and the EEPROM storage cell incremented by one to have the next count stored again. This process is to be repeated for 60 - 10 second periods to generate the exponential decay chain of measurements. I felt this was a fabulous opportunity. to eliminate the "running of the bulls" to the NIM counter to maximize the initial seconds of decay. A second push button was needed. ("memory dump") Once all the smoke cleared and data taken, at any point, later on, one could hook the device to a PC and dump a comma delimited string of numerically ordered counts from the Arduino's EEPROM to the Serial monitor screen on the PC and using the Control A and control C keys to paste to a notepad file. This stored file could be waltzed over to EXCEL and generate a graph! This type of automated counting and storage was right in the Arduino's wheelhouse! The counter and program were completed in a no sleep 30 hour period.

It worked!!....Kinda'...

My rather huge variable background fouled the entire process. Sure, the first three ten second countes were large and obvious to the meanest intelligence, but after that, the whole thing went to hell and back. The subsequent counts had the variable background added in and by six to ten, 10 second periods there was no semblance of a decay! I am confident that in a much lower background, this system would shine like the sun!

I submit the following "catch-up URLs for reference and the numerous photos of the steps in the process.

Fusion V build -
Moderator build -

As always click ion images to enlarge..........

Re: Neutron detection moderator embedded activation!

Posted: Sat Aug 08, 2020 4:30 pm
by Dennis P Brown
First, great pics and set up, of course. Your experiments are well thougth out.

That rolling mill is the exact one I have - my unit, I will be converting to high temperature rolling.

Re: Neutron detection moderator embedded activation!

Posted: Sat Aug 08, 2020 4:52 pm
by Frank Sanns
The larger the detection area, the more the counts. Double the area doubles the background counts as well as the signal counts.

It is the signal to noise ratio that needs to be improved. Put half an inch of lead around that tube or a standard pancake probe element and you will be in heaven. Filter out most of your background gammas and let the neutrons stream right on through. Of course the moderator must still surround the lead and the probe but you will probably improve the signal to noise ratio a couple orders of magnitude.

Re: Neutron detection moderator embedded activation!

Posted: Sat Aug 08, 2020 5:29 pm
by Richard Hull
Great minds think in similar circles, Frank. I have already done that a day ago, but have yet to test it as I have spent a good deal of time on a revision to the program. The issue is the Gammas in the lab. The moderator plastic shields against 100% of the extant betas, the tube stops all alphas (thin metal shell). This means it is only a pure higher energy background of gamma. I will report on the background drop soon as I have a number of chores and honey-dos which I have ignored in the rush to get this activation effort going to my satisfaction.

Richard Hull

Re: Neutron detection moderator embedded activation!

Posted: Sun Aug 09, 2020 8:55 am
by Richard Hull
A great success this morning! Yes, I am still up at 4:33 AM. I ran the fusor for a record run this evening (see data in the new fusor V construction posting)

I had placed the lead, as noted in my post just above, on top of and to the side of the moderator on Wednesday, but only got around to testing it this AM. The 1/16" of lead seen in the attached image really cut down on the background and leveled it out a bit. Still many wild fluctuations. I'll have to massage the attached graphs a bit. Unfortunately, even though there are a lot of data points taken, 10 second counts are only going to get a few counts and any variation is going to be just terrible. We are dealing with some fast decay times so that we can enjoy some fast activation. Thus, the 60, 10 second samplings started instantly at removal of fusor HV from an activation run. Maybe only a one second delay to push the "count begin" button.....No more wasted seconds dashing to the NIM bin setup in the middle of the lab and fiddling with the pancake elevation probe prior to hitting the count start button there.

I attach an image of the lead shield which proved most helpful. Also, I attach 3 Excel plots of the data taken over 60 ten second samplings pulled from the Arduino's EEPROM following the count-and-store period after the run from activating the silver. The plots are 1. unshielded embedded GM background run. 2. Lead shield in place with new background run 3. The graph proving neutron activation of the 24.5 second isotope. At the end of 2 minutes there might be a barely detectable signature of the longer isotope. Just too much noise.

You guys be the judge. I will continue to work the numbers. The lead was not even noticed by the fast neutrons, it was the HDPE that tamed them for counting. Note: the lead positioning was based on the nearby "shed" and some material up above. It worked out OK, I think. I might try more lead, but I think I got the medium energy gammas snuffed out a bit.

Richard Hull

Re: Neutron detection moderator embedded activation!

Posted: Sun Aug 09, 2020 4:19 pm
by Frank Sanns
Looks like the limited shielding dropped your background from 15 counts down to around 12. Not a very big change but not much lead either.

No matter as your graph is showing a great activation of 82 counts (per minute?). This is a big jump over the 12 and clearly due to activation.

It appears that you are switching off the fusor once the foil is activated and measuring the decay times. While this is good to identify the half lives, it is not necessary to see how the fusor neutron output is running.

Would it not be better to measure the steady state activation numbers with the fusor running full out? After three or so half lives of run time, there will be an equilibrium. It is that equilibrium plateau that is giving you all isotope decay contributions no matter how short or long they are. Taking a background before and after the run will give the numbers you seek for the level of activation. Of course when the fusor is switched off finally after all of the measurements, you get your decay time curve like your #3 graph so nothing is lost. Only gained.

Re: Neutron detection moderator embedded activation!

Posted: Sun Aug 09, 2020 8:12 pm
by Richard Hull
The background never changes much and is irrelevant, of course, if you want to see proof of activation. In this case, with an averaged back ground of 13-15 counts, the 82 count first peak and subsequent counts are definitive and shows an over 5 times background activation level! This is what I wanted to see show up. My neutron counter is the sole arbiter of how my fusor is doing in a solid quantitative, though un-calibrated manner.

Activation is for "grins and googlers" to demonstrate how you can, indeed, make things radioactive with neutrons, (neutrons being the ultimate proof of D-D fusion)....A cool demo thingy, but it has to be super convincing to a critical audience. As a long time shower of my fusors, the audience is the thing, here. I have run my fusors for reporters, idiots, the idle curious, as well as the fully capable and fusion savvy people. Activation, for most, is just another "show and tell" during fusor operation, but for the adroit and trained in the art, it is the real proof of the pudding, regardless of some counter roaring in the background that the operator is "claiming" are real neutron counts.

I realize that...
1. With enough lead, I could run the activation counting system constantly and take readings. I can piddle with that at any future time. I would also have to alter the software to store thousands of ten second count intervals. (Easily done in a single line of code on the Arduino). However, by shutting off any source of radiation, (shutting down the fusor), all the more impressive that you have made silver radioactive....
2. I also realize that the full activation time for the 24.5 second half-life, Ag activation product at any given, fixed flux level, is only 2.45 minutes. This is why I shut the system down and took the count after finally obtaining 68k CPM on the neutron counter for about 3 minutes. I had been running the system for 20 minutes at 40k CPM, then inching it to 50k CPM, and then finally bumped over 60k CPM, ( a record on fusor V, to date), thus, I realized this was a peak and took several one minute counts on the neutron counter well over 60k CPM. At the point of 68k CPM, I shut the system down and instantly started the counting.

The 40 k CPM for some minutes was not going to do it for me, nor was the 50k CPM for subsequent minutes. All they were doing is lightly activating the long lived product over this long period of varying, yet ever increasing flux, and ever peaking the short lived one, but only to those levels of fluxing. It was the short lived product coupled with the over 60 k CPM, for the required 3 minutes, that I was after and I got it.

I will look into more lead, but only from the perspective of killing more background noise levels. All familiar with PMT castles, know that more lead can mean a limit to background levels where the lead becomes a contributor to background, due to the following points.

I also wonder if some of the noise is related to gamma, Compton scattering in the silver and the even the fusor, for that matter, due to the harder gammas. Remember, some of the gammas are up to 2.8 MeV. I doubt there is much pair production though as photoelectric electron scattering and Compton scattering would predominate for the far more prevalent under 1 MeV gammas.

Nice to talk these issues out.

Richard hull

Re: Neutron detection moderator embedded activation!

Posted: Sun Aug 09, 2020 11:42 pm
by Richard Hull
I just came in from the lab running the badckground count with 10 times the original thickness of lead. 5/8-inch thick plates of lead were used. This might have knocked the background down a point or two, if that. However, I noticed a lot of higher peaks. In lead this thick, Compton scattering and pair production and photo electrons are possible, Bumping the counts up in the extreme count range. make of this what you will.
Attached image. You just can't get this thing quiet in my background of gammas.

Richard Hull

Re: Neutron detection moderator embedded activation!

Posted: Mon Aug 10, 2020 3:37 am
by Frank Sanns
Something is not right Richard. Think about how much the tungsten backed geiger probe blocks spurious background radiation. You counts should have been cut down at least by a factor of 10. Are you sure you are not getting noise in your tube or electronics? Even new lead would not be contributing hardly any extra counts compared to your lab background.

Re: Neutron detection moderator embedded activation!

Posted: Mon Aug 10, 2020 3:50 am
by Frank Sanns
I just took my Ludlum low energy gamma probe and tested a hot radium source at 1 inch. The counts were cut by a factor of 10 with just 0.25" of lead.

Also, there is an unusual amount of variability for in your background data for that length of count times. look at your higher counts. They are making a straight line with good correlation one data point to the next. But when it get down closer to the noise floor, it becomes extremely variable.