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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Maciek Szymanski
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Track detectors

Post by Maciek Szymanski » Thu May 13, 2021 8:14 pm

Recently I'm involved in measurements with the particle track detectors (CR-39). As I've browsed the forum there is very little information on them and mostly negative. From my experience they have quite a lot advantages:

- very cheap (Track Analysis Systems sells 10x10 mm pieces for les than £2),
- immune to x-rays and electromagnetic interference,
- etching is done with cheap, widely available NaOH and the whole processing protocol is dead simple.
- readout is done with a very basic optical microscope (200x magnification is enough),
- it is able to record 100% of moderate energy charged particle events.

The only drawbacks for me are the etching time and laborious readout (no real time measurement possible).

As one should not expect any charged particles outside the fusor, the CR-39 stacked with a thin PE radiator and a lead mask will be for me a very affordable and easy method to confirm the fusion. And there is quite a lot of good papers on this topic, including absolute calibration for fast neutrons:

https://sci-hub.se/10.1016/j.radmeas.2012.09.007

As the 100% of recoil protons are recorded, the calibration should depend only on the reflector proton recoil cross section not the properties of the detector itself.
Last edited by Maciek Szymanski on Sat May 15, 2021 8:54 am, edited 2 times in total.
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ChristofferBraestrup
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Re: Track detectors

Post by ChristofferBraestrup » Thu May 13, 2021 11:28 pm

Interesting detector design!

I think some of the aversion is to the fact that having to develop the detector slide oneself puts a gigantic dampener on data throughput.

But then, CR-39 sheets seems to be available dirt cheap as welding goggle sputter shields: https://www.ebay.com/itm/291575210863?h ... Sw0HVWCBos

I wonder if they could be used for low energy electron diffraction data recording!

Edit: Dang! Hole diameter is even proportional to energy! Here's a very old text from Brookhaven for reference: https://www.osti.gov/servlets/purl/5331288

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Mark Rowley
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Re: Track detectors

Post by Mark Rowley » Thu May 13, 2021 11:45 pm

Maciek,
I was quite interested in this method for pulse type fusion. It’s a cheaper alternative to BTI dosimeters but has some drawbacks.

Here’s a FB post I made about CR39 neutron detection back in 2019:

-Solid State Neutron Detection-

After some prompting by Robert Tubbs, I have slowly started exploring CR39 neutron detection.

CR39 is a monomer type plastic that is commonly used for eyeglass and welding helmet lenses. It’s extremely cheap and abundant.

Similar to a BTI bubble detector, it’ll react to alpha and neutron radiation by creating a small track or bubble. The bubbles for each type of particle have unique characteristics therefore allowing for easy quantification.

Unlike a bubble detector which provides immediate results, CR39 has to undergo a very simple etching process to enlarge the tracks for easy viewing under a common microscope.

Etching consists of a 6hr bath in heated sodium hydroxide solution. Afterwards, the small plastic “dosimeters” are washed and viewed under a microscope.

CR39 neutron dosimeters have been around for a few years but have yet to take on any substantial interest in the amateur science realm. Hopefully that will change. Especially since a CR39 arrangement will cost less than $20 as opposed to $400 for a couple bubble detectors.

Attached is a decent video depicting the general process. There is some discrepancy about how long the etching process should take, but unlike the video most tend to last about 6 hours.

If anyone decides to try this, I’d highly recommend getting the newest plastic available. The older it is the more likely it’ll have significant radon, NORM, and cosmic ray damage. Also, the small unseen micro-abrasions from normal wear and tear will be amplified during the etching process. So buy recently manufactured / new plastic, preferably with the protective plastic adhesive still on it.

Over the next week I’ll run some alpha particle tests and share my results with the group.

https://youtu.be/OyocGy8tV-4

——————————————————

So back to now….
After experimenting with several CR39 sources (in 2019) I’ve noticed they were all susceptible to alphas and to a lesser degree, gammas. The key to determining neutrons is the size of the bubble. Neutrons generally make larger bubs than alphas. It also appears one would need a good amount of neuts to create a reading as the weak static source I used provided negligible or inconclusive results. So it seems small output fusors or pulse systems need not apply.

Here’s a pic of alpha influence on a CR39 detector.
A250DD25-F857-4F05-958E-5A5407AD3C1F.jpeg

At the time of this experimentation, I did not have an operational fusor so everything was done using the low output static neutron source. My CR39 setup is still ready to go so I may try again using the current fusor.


Mark Rowley

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Jim Kovalchick
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Re: Track detectors

Post by Jim Kovalchick » Thu May 13, 2021 11:51 pm

I have held on to a pile of new CR-39 welder mask plates for a while but have never got around to using them. I read somewhere that the welder mask plates may have a coating on them that needs stripped off first. I was going to try it without stripping first.

Jake Hecla told me that when he was in high school working with the Carl Greninger fusor, they used CR 39 plastic to map the neutron flux. It seemed to me like a great application of the plastic.

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Mark Rowley
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Re: Track detectors

Post by Mark Rowley » Fri May 14, 2021 12:00 am

Jake had told me the same thing. Specific to the two brands of CR39 I tested, removing layers didn’t seem to make any difference. But again, I didn’t have the neutron source that currently available.

One issue about removing the layers… once complete very special attention to repolishing the surface has to me made. No doing so wreaks havoc during the etching process. It’ll essentially become a giant cloudy piece of plastic.

Mark Rowley

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Re: Track detectors

Post by John Futter » Fri May 14, 2021 6:59 am

Here we go
many many years ago i did this with new polycarbonate sheet developed in hot ethanol
I used our accelerator @ work when it had deuteron beams on and put my setup near the slits / bending magnet.
polycarbonate sandwiched between 2"wax blocks to moderate the fast neuts
I took pics with our optical microscope and yes there was noticable damage

might be back around 2008 2010 somewhere on this site

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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski » Fri May 14, 2021 9:28 am

Yes, CR-39 is sensitive to α particles (and other charged particles too). This is why I like the idea of using the thin polyethylene radiator in front of the CR39. 1,5 mm PE will stop all charged particles, so only recoil protons form PE will be recorded. Plus some protons recoiled in the CR39 itself by fast neutrons passing the radiator, but recoil cross section in CR39 will be much smaller compared to PE due to smaller percentage of hydrogen. Even this effect can be taken in account by covering the detector partially by PE, partially by - let's say 0,5 mm Al and partially leaving it uncovered. Than the uncovered part will record all charged particles including the neutron generated recoil protons in CR39, the Al covered part only recoil protons in CR39 and the PE covered part the recoil protons from PE and recoil protons in CR39. By comparing track densities at each area the fluxes can be deducted.
Unfortunately in general case it is not possible to distinguish recoil proton tracks from other particles as the recoil protons have wide energy spectrum, so the tracks will be of many sizes. For a specific case the clever etching and counting protocol may be developed.

Here is an example of α particle tracks from monoenergetic source passing through different aluminum filters, after 2 hour etching in 6,25N solution at 70ºC. Photographed in the incident light with 50x planhromatic objective:

montaz-14-05-2021.jpg
Last edited by Maciek Szymanski on Sat May 15, 2021 8:53 am, edited 1 time in total.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski » Fri May 14, 2021 2:45 pm

I've made a quick etimation based on the Castillo paper. The sandwich detector seems to register 30% of the neutrons. So for 1e6 N/s TIER the detector placed 15 cm form the fusor center should record 190 tracks per square milimeter after 3 minute exposure. Not much, but detectable.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Richard Hull
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Re: Track detectors

Post by Richard Hull » Fri May 14, 2021 3:52 pm

While posted on before in a limited fashion, this is another method in the neutron detection kit.

Unfortunately, I am loath to allow it as proof of fusion and neutrons at present. Once a rigid methodology is established as to precise elimination of charged particles in a manner that assures only proton recoils are seen then I would imagine we might all agree on allowing the CR-39 plastic to be used.

The pity is, as Maciek notes, It detects all charged particles. As Mark points out, development is critical. Microscopic imaging is demanded for proper presentation. (Maciek did a great job at it above).

If a strict procedure is advanced and accepted, that would be great if all applicants in future follow it to the letter. This is the case with all of our detection rules for acceptance of fusion. This is a good thread as it struggles to work out such a good methodology as would be truly useful.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski » Sat May 15, 2021 8:53 am

If someone with working neutron producing fusor and good neutron measurement setup wants to cooperate I can prepare set of CR-39 dosimeters with holders, PE radiators and filters. The irradiated dosimeters can be sent back to me for etching and readout. All results and protocols will be published on the forum.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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