Track detectors

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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Mark Rowley
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Re: Track detectors

Post by Mark Rowley »

Maciek,
I’d be willing to give it a go. I just reverified calibration with a bubble dosimeter a couple weeks ago.

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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

Great! My plan is to use two sets of three dosimeters irradiated in two sessions plus a reference one which will be not exposed, but otherwise identical and sent together with the rest both ways to compensate for factors like cosmic rays in the airplane, xray inspection and overall history of the CR-39 batch used. I'll prepare the drawing of the complete dosimeter and post it on the forum, so you can check how to fix it on the fusor.
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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

Regarding Lexan as the track detector. According to Solid State Nuclear Track Detection:Theory and Application by A. M. Bhagwat energy wise the detection threshold of Lexan is 80 times higher than for CR-39:


D410388A-F5B6-44AB-B87F-0153A8AD483E.jpeg

It is also worth noting that the cellulose nitrate has 4 times lower threshold than Lexan (but still 20 times higher than CR-39).
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Mark Rowley
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Re: Track detectors

Post by Mark Rowley »

Sounds good Maciek. Send me a PM when you’re ready for the specifics.

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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

Here is a quick design of the neutron detector:


BDFBC352-35D8-4B7B-995D-758CA6760106.jpeg
The detector should be placed as close as possible to the fusor to increase the count number. It would be nice if one would be positioned at the viewport, to see if the increased x-ray exposure will make any difference. If it is needed one or both screws can be longer and provided with an extra M3 nut for fixing the detector.
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Mark Rowley
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Re: Track detectors

Post by Mark Rowley »

Viewport or any distance is workable. My calibration is at 17cm distance at the same height as the grid. Not sure if that’s important for your end goal.

The brackets look good. Much better than a bare piece of plastic.

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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

Ok, so I'm ordering the aluminum flat bar and 2mm PE sheet. I should have the dosimeters ready for dispatch at the end of the next week.

In the meantime I've got some data regarding the "dead counts" of the CR-39. The four pieces of 10 years old CR-39 sheet were exposed for the alpha particles form the radioactive source for 30, 60 and 120 minutes in vacuum. After etching the samples were photographed under the microscope (6 images for each sample) and the tracks counted with the ImageJ program.

After averaging the results the track density for each sample are:

Sample 1 (30 min exposure) - 4092 mm^-2
Sample 2 (60 min exposure) - 5731 mm^-2
Sample 3 (120 min exposure) - 8932 mm^-2

As it can be seen on the following chart the 3 points are perfectly collinear:

8371894D-7F12-4E0B-BEED-F909B5640BEC.jpeg

The extrapolated count of 2450 mm^-2 for the 0 time exposure can be accounted for the accumulated damage during the storage time. As it was earlier mentioned it is best to obtain a fresh detector plastic, but nevertheless some means of determining the "dead count" for the particular batch is rather unavoidable. And after another 5 years of storage this number can't be trusted anymore. Personally I think it is best to include the "dead count" procedure for each measurement session.
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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

The work was quite delayed, but finally I was able to assemble seven fast neutron dosimeters based on CR-39 track detector.


176761CC-665F-48D0-9E15-3B6CBE702915.jpeg
Dosimeter components. 1. Front plate stamped with the detector number and three 6 mm apertures (2 mm aluminum). 2. Polyethylene radiator 2 mm thick. 3. X-ray filter (2 mm aluminum). 4. Piece of 0.5 mm CR-39 plastic with orientation notch and marked with the detector number. 5. Back plate stamped with the detector number (2 mm aluminum).



95CAF503-62B4-48FE-8660-0DE78B8DD128.jpeg
Seven assembled dosimeters. The number "0" is intended as not exposed reference for "dead count" determination.


As we have six "active" dosimeters they may be exposed in two session of three or three session of two. My idea is to place two detectors at different distances to check if the proton track count conforms to the reverse square rule. The third detector may be placed in fron of the viewport to check if the increased x-ray flux changes the recoil proton count, but I'm not sure if it is really needed. According to literature photons are not counted, but high gamma fluxes may increase overall sensitivity. As the whole experiment is intended as validation of the method it would be nice hear other's opinions (Richard?).
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Richard Hull
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Re: Track detectors

Post by Richard Hull »

These are fantastic! I would distribute them to the highest numbering fusors here for testing in an active hot neutron environment. This could be the beginning of the poor man's calibrated neutron dosimeter.

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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Mark Rowley
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Re: Track detectors

Post by Mark Rowley »

A few days ago I received the first batch of track detectors from Maciek. I was pleased to see they made the 9400 mile trip with no apparent issues.

I hope to begin testing sometime next week.

999ECB86-3E38-4766-9C5D-B63CF88D2E75.jpeg
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Chris Seyfert
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Re: Track detectors

Post by Chris Seyfert »

This is neat work! I would comment that when dosimetry companies use CR-39 detectors for neutron exposure, they also cover a section of CR-39 with boron to measure thermal neutrons via the 10-B(n,a) reaction, so that may be an area of interest to explore as well. Small pieces of boron nitride show up on eBay regularly and it is an easy material to work with.
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Re: Track detectors

Post by Jon Rosenstiel »

Hi Maciek,

Fusor upgrades have been completed (Ti coated copper endcaps and a recirculating chiller) and I’m finally ready to run your track detectors.

You mentioned placing your track detectors at different distances to confirm that the track count follows the inverse square rule. For some reason the inv-sq rule does not want to work well with my fusor’s neutrons, results I’ve gotten vary quite a bit.

Here’s my idea. Over the past couple of days I’ve run two inv-sq tests, one using a fast detector (5.1 cm Hornyak button, 45 kV, 7 mA, ~4.8E+06 n/s) and the other using a thermal detector. (SNK 32/200 corona tube in a 4.1 cm thick UHMW moderator, 30 kV, 5 mA, ~7.0E+05 n/s). Data from the Hornyak was taken at 15 and 30 cm from the anode’s surface. Dividing the 15 cm count-rate by the 30 cm count-rate gave a ratio of 3.2:1. Data from the corona detector was taken at 20 and 40 cm from the anode’s surface. Dividing the 20 cm count-rate by the 40 cm count-rate gave a ratio of 2.9:1. (Both should be around 4:1)

What I propose is running your track detectors at those same distances (15/30 cm and 20/40 cm to the anode’s surface) and see how your results compare to what I’m getting. I’d also like to tape one of your track detectors to the fusor’s endcap, I’m curious as to what the pattern would look like. And I’m not sure what to do about exposing the track detector to x-rays as I’ve replaced the viewport with a KF-40 blank.

Not sure about exposure times and neutron flux for the track detectors, what do you think? And what I proposed above is just a suggestion, the attached images and what I’ve written may give you an even better idea of how to go about this.

Jon Rosenstiel
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22.5 C is the endcap's temperature.
22.5 C is the endcap's temperature.
Thermal at 40 cm.jpg
Never thought I'd see condensation on my fusor!
Never thought I'd see condensation on my fusor!
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Dennis P Brown
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Re: Track detectors

Post by Dennis P Brown »

Neutrons can fill a room with cool neutrons; that is, many substances in and around a room will thermalize many neutrons and scatter them around the room. This can create issues with detectors.
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Jim Kovalchick
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Re: Track detectors

Post by Jim Kovalchick »

Dennis is correct about neutron detection. To get the best understanding of fusion neutrons, it is best to eliminate thermal neutrons entering the detection system and measure only fast neutrons incident from the fusor. Some detectors, such as my PNC 1, will do this for you by encasing the moderator with a thermal absorber like cadmium.

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Re: Track detectors

Post by Jon Rosenstiel »

Found the problem, I had failed to take into account the neutrons from the opposing endcap.

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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

That are the grat news Jon! Let us stick to the loacations for which you have the best knowlege about the flux. Regarding the exposure: For the limited knowledge we have now the efficiency may be assumed to be 30%. For the reliable counting it is good to have minimum 500 tracks/mm2 and not more than 5000 tracks/mm2. So the exposure sould be between 1500 and 15000 N/mm2. It would be nice to have each set of the detectors exposed at the different level in this range.
As for the x-rays - if you can specify the approximate dose I can do a separate test using the x-ray tube I have at work.
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Re: Track detectors

Post by Jon Rosenstiel »

Maciek,
Recommended exposure at both near and far positions works well at 10 and 20 cm. Calculations show that at those positions both minimum (150000 N/cm^2) and maximum (1500000 N/cm^2) exposures can easily be met. I should be able to knock this out within the next couple of days.

As to my inverse square conundrum…
For longer than I care to admit I’ve been puzzled by unusual and inconsistent inverse square measurements of the cube’s neutrons. Just recently it finally occurred to me that I had not been taking into account the neutrons being emitted from the opposite anode.
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Inverse square model
Inverse square model
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Richard Hull
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Re: Track detectors

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Just my thoughts gathered from what others have been posting. I once held that the spherical fusor produced the classic perfectly isotropic emission of neutrons. I was disabused of this thought pattern around 2006 with reports from the Washington group doing good experiment. The matter quickly was dropped as we looked for a hot spot to position our moderators, while making them larger using larger detectors so as to seek some moderate, average TIER readings. In only recent years have a lot of tilting as windmills occurred via experiment, spurred on by the total abandonment of the spherical devices in favor of a supposedly BOT approach to favor a more directed neutron beaming.

The beaming seems apparent but is not absolute as fusion at our level is not to be in the true thermal sense, but more in the quantum probabilistic manner where directionality itself seems skewed with some often significant scattering. I feel we must admit to not one semblance of purity of effort in our amateur efforts. We just are not in control of the parameters. No two fusors are alike or are operated alike.

I applaud the bold and great efforts to get a handle on the question, "Where are the neutron most prevalent?" "How much scatter is there?" These are very doable, of course. It is a mere job of multipoint measurement done well over, ideally, a 360 degree sphere about the device at a fixed distance. But the results apply to that one device only and to perhaps any absolute carbon copy device made to the original exact specs and materials.

I am especially interested in the results of the track detection efforts underway expressed in this thread.

The discussion of lab scattered neutrons, cold neutrons, and such meaningless twaddle only figures in on low output devices or to strong devices where measurements are taken at unreasonably long device to detector ranges. It is to be remembered that at the strongest point in the best of fusors, there is little to discuss related to a true laboratory, reactor grade neutron "flux" or "fluence". The same goes for any discussion of scattering affecting results if near zero detector to detector distances are in play.

I fear our old spherical devices were "real" fusors, for they had in their original design, geodesic grids with multiple BOT points within the sphere in pseudo-isotropic emission. The modern BOT fusor devices, (bi-directional) would be better served as being assembled into single point, true beam on target, linear ion accelerators if more of less intense direct neutron beam sources are desired. It is all real D-D fusion, after all.

Again not wet-blanketing all the recent work, it is refreshing and exhilarating to see it being done and well reported on. My skin in this game is neutrons and not fusion at all, it never was. I now fully realize the spherical fusor is the most artistically beautiful expression of the amateur fusion effort with the most beautiful symmetrical imagery. Yet it will forever remain the weakest of all amateur neutron sources when employing small moderators using small detection or activation efforts where the main object of experiment is concentrated neutron output.

Richard
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: Track detectors

Post by Jim Kovalchick »

I agree with Richard's discussion. The concept of TIER is a lot more complex than we can understand with our moderated tube detectors hitched up close to our fusors. Calibrating then with fast neutron bubble detectors or CR 39 plastic helps, but unless one maps the fast neutron flux with detail 360 around the fusor, discussing TIER is almost silly. Even after a map is made, Monte Carlo may be needed to tell the tale. It is apparent that point source calculations for TIER are only telling us what the true TIER is less than.

The advent of the cube fusor should be seen as our effective liberation from the futility of comparing each other's fusors by tier. We should really be looking at only the simple concept of flux. Fusors are NOT energy devices. They are neutron flux sources, and getting lots of neutrons as quickly as we can through a cross sectional area we have access to is the name of the game. A big TIER means nothing if by the point you can first put an activation target in the neutrons, the spread is too thin to do anything. We should instead care about flux and fluence. This game should be as much about engineering a useful neutron source as it is about pretending to master the physics of IEC.

It probably looks like I drifted from this thread's topic, but it is my belief that the CR 39 studies will support my point. There is no TIER to be found here, only some good information about flux that we should have been talking about all along.

Jim K
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Richard Hull
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Re: Track detectors

Post by Richard Hull »

Jim supported my thoughts. I have considered a linear accelerator, but the x-ray blast would be significant and one would lose certain aspects of IEC, (kinetic velocity space effects), in such an effort. The track results just might tell us a lot, yet.

It is to be remembered that fusion in velocity space is, by nature isotropic while surmised spherical shell BOT fusion is more directional. All fusion is still ruled by quantum probabilities.

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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Mark Rowley
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Re: Track detectors

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Maciek,
I just received the detectors back from Jon (thanks Jon!) and will have them shipped over to you later this week.

Many thanks to Jon R for helping out with the neuts! My system has been totally disassembled due to an upcoming home sale and when I get back in the game it will be beam-on-target from this point forward. Jon's willingness to step in for me is highly appreciated.

Looking forward to the detector results!

Mark Rowley
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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

The package with the detectors just arrived back in Warsaw. Seems tha everything is in perfect condition. I am very keen to do de etching and see the results, but unfortunately on Sunday I'm again traveling to Prague for experiment. I will be back on October 20th and will do the etching as soon as I possible.
I would like to thank Mark and Jon for cooperation.
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