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 »

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 4:54 am, edited 2 times in total.
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ChristofferBraestrup
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Re: Track detectors

Post by ChristofferBraestrup »

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 »

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.


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Jim Kovalchick
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Re: Track detectors

Post by Jim Kovalchick »

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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Re: Track detectors

Post by Mark Rowley »

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.

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Re: Track detectors

Post by John Futter »

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 »

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 4:53 am, edited 1 time in total.
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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

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.
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Richard Hull
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Re: Track detectors

Post by Richard Hull »

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.

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Maciek Szymanski
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Re: Track detectors

Post by Maciek Szymanski »

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.
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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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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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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.

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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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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
Attachments
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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