Increasing the useful life of bubble detector tubes

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Dan Knapp
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Increasing the useful life of bubble detector tubes

Post by Dan Knapp » Fri Oct 27, 2017 1:17 am

My investigation of the literature on the tensioned metastable fluid detector (TMFD) led me into literature on the bubble detector tubes (which are another form of TMFD). The bubble tubes have been used extensively by fusor builders, but even in a group purchase, they are expensive; and they have a relatively short useful life (typically months). The expense could be effectively reduced if there was a way to extend their useful life. Upon inquiry to Bubble Technology Incorporated (BTI) about what is known of the failure mode when tubes cease to work, I was given a paper titled “The Life Span of the BD-PND Bubble Detector” (copy attached). This paper describes studies done at a Belgian MOX plant where the bubble detectors are used as personal dosimeters. To understand the lifetime and failure mode information, it is helpful to know a bit about what the tubes are made of and how they are made.

From this paper and several others (e.g. Applied Radiation and isotopes Vol 69, pp.1453-58, 2011), I have gleaned the following information about the tubes. I could find no patent literature on the tubes. Apparently BTI protects their effective monopoly on these tubes through trade secrets of the materials and production processes rather than by patents. The detector tubes contain a polymerized polyacrylamide gel with a dispersion of microscopic droplets of a proprietary Freon mixture. Tube sensitivity is varied by varying the number density of the droplets (more sensitive tubes have more droplets). The contents of the tube are compressed with a screw-on piston cap. The tube is activated by unscrewing the piston to relieve pressure on the droplets to change them to a superheated state. Upon being hit by an energetic neutron (> 100keV), the fluorine atoms in the Freon recoil and transfer energy to the droplets causing vaporization and bubble growth. The bubbles persist until the tube is repressurized by replacing the piston cap. Upon repressurization, the bubble vapor condenses back to liquid droplets, and the tubes can be reused. The tubes also contain a proprietary temperature compensation liquid, which increases the pressure inside the tube at higher temperature maintaining a more constant superheated state for the droplets. The tubes are shipped sealed in a metal container tube, and BTI recommends that they be stored in the metal tube.

While the BD-PND tubes are relatively small (19 x 145 mm), the detectors can be made much larger. The same type of contents are used in 1 liter detectors being used in the PICASSO dark matter search experiment. Some details have been published on how these detectors are made in collaboration with BTI (Nuclear Instruments and Methods in Physics Research A, Vol. 555, pp. 184-204, 2005), but key details are still omitted. With the information that is available in the open literature, the amateur could conceivably homebrew detector tubes. One would need a Freon that boils just below ambient temperature and the ingredients for the gel. Polyacrylamide gels are widely used in biochemistry labs, so a biochemist friend could get you the ingredients. The necessary acrylamide and methylene bis-acrylamide can also be purchased on Amazon.

But back to the topic of the title of this post, how to extend the life of the BTI tubes. BTI only guarantees the tubes for three months, but the attached paper gives recommendations on how the life can be extended to one to two years. There are three failure mechanisms:
1. Water diffusing out of the detector tube.
2. Water diffusing from the gel to the Freon droplets and Freon diffusing from the droplets to the gel.
3. Inelastic deformation of the polymer around the bubbles.
Storing the tubes in their sealed storage container retards mechanism 1, so one should keep the tubes in the storage container when not in use. The paper reported that such storage extends life by ten months. There is not a lot that can be done to retard mechanisms 2 and 3 other than avoiding overheating the tubes. The key to longer life seems to be using the tubes regularly, but not too often. It appears that regular cycling of the pressure increases the useful life, but too often is bad. The paper reports that tubes used “regularly,” but not daily, could be used for one to two years. Tubes used daily lasted only four months. The bottom line is that it appears you can maximize tube life by pressure cycling regularly, but not too often. Andrew Seltzman has reported here that he gets much longer useful life than what is usually reported. Perhaps Andrew uses his tubes more regularly than others?
Attachments
PND Lifetime Vanhavere.pdf
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Andrew Seltzman
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Re: Increasing the useful life of bubble detector tubes

Post by Andrew Seltzman » Fri Oct 27, 2017 4:13 am

I wouldn't say that I use may tubes very regularly, I'd estimate 10-20 times a year. I do however, keep my detectors stored within the aluminum tube at a relatively constant room temperature. I store them in a horizontal position (fairly sure this is irreverent). The failure mode that I've almost always seen is failure of the epoxy seal between the plastic "bulb" and aluminum compression chamber part causing the tube to leak(once a tare in the rubber membrane after using a pencil eraser as a compression shim on an old dosimeter). In one case this happened shortly after a large temperature swing to cold temperatures(probably thermal expansion).
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Richard Hull
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Re: Increasing the useful life of bubble detector tubes

Post by Richard Hull » Sat Oct 28, 2017 6:18 am

I have purchased 3 BTI dosimeters. I followed BTI's storage suggestions to the letter.
The first BTI was to prove to myself I was doing fusion in fusor III. I used it rather infrequently and it lasted about 1 year.
The second BTI was used to prove fusor IV was working to myself and others. I also used it to verify and do the math to get a handle on the average flux. It was used a lot and lasted about 7 months according to my lab notes.
The third and last BTI was used to calibrate and set a standard K or constant factor with my, now stable, 3He, electronic neutron detection setup used in fusor IV data reduction. This last BTI was used daily for almost a month and re-pressurized about 4 times a day as I gathered a huge data set with multiple runs daily.

In the end, averages were taken based on all parameters of value to the statisitcs. I used the data to reduce to a K multiplication factor of 21.6. Thus, what ever the 3He counter records as a CPM reading, I should multiply that number by 21.5 to arrive at an isotropic neutron emission per second. I decided to round down in a conservative manner to a simple k=20. I use this factor to this day.

Used as mentioned above, this last BTI went south after about 4 months. I did not record in my notes when it went dead, but the heavy use and multiple recompressions in that flurry of activity probably hastened its demise.

Note: In past posts on the BTI, I have used and recommended the 33 bubble/mrem BTI for fusioneers. I have also recommended a standoff distance from a working fusor to prevent overheating the dosimeter during data collection runs. The dosimeter is best placed at the standoff distance near the bottom of the fusor so that air convection currents can't heat it. Many have taken my advice.

I am rather sure I'll never need to purchase another BTI, but appreciate this post's thread on the best manner in which to use the BTI to prolong its useful period of operation. This should be good data for future users of this very nice neutron dosimeter.
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Bob Reite
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Re: Increasing the useful life of bubble detector tubes

Post by Bob Reite » Sun Oct 29, 2017 5:44 am

My first tube lasted about 9 months. The failure mode was that the bubbles would no longer compress. It could still be used by counting bubbles before and after and taking the difference, until it was so full of bubbles that you could not keep track of them, but the accuracy would be questionable. My use tended to be feast or famine. During active research I would be taking 10-12 measurements per day over a period of two or three days, then when the paying jobs got real busy it would sit in storage in the 50-60 degree F basement (in the container) for several months.

I bought another pair of tubes six months ago to research the accuracy between samples, and to do a calibration of my REM ball. So far both are fine. They measure within 10% of each other. Sort of like the old days of 10% resistors. I got the REM ball calibration confirmed, so when these die, I can rely on the REM ball.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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Richard Hull
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Re: Increasing the useful life of bubble detector tubes

Post by Richard Hull » Sun Oct 29, 2017 9:18 pm

Good Work Bob! I assume your data will require the rem-ball to be at a fixed distance from the source. This is the only way I could warrant calibration with my fixed, digital counting, 3He setup.

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
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Bob Reite
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Re: Increasing the useful life of bubble detector tubes

Post by Bob Reite » Mon Oct 30, 2017 12:35 am

For convenience, I plan to keep the REM ball at the same distance, but since I know the mREM/h reading is valid if for some reason I have to double distance the reading will be 1/4 what it was at the old location for the same system output.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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Richard Hull
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Re: Increasing the useful life of bubble detector tubes

Post by Richard Hull » Mon Oct 30, 2017 6:22 am

I have two rem balls and used them in the 2000-2004 time frame. The small tubes in them were not sensitive enough for my tastes. With the construction of fusor IV I wanted super efficient counting and went with the large 20-inch P4 type Reuters and Stokes 3He tube. I think the bulk of the rem balls I have encountered used the 3" Nancy Woods BF3 tubes.

These rem-balls work extremely well and are designed to be equal to what a person's dose would be once a correction factor is figured in for the energy of the primary neutron source. A table was included with each Rem-ball. I have seen general correction tables for the normal 6" diameter ball published in a number of books on radiation detection.

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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Frank Sanns
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Re: Increasing the useful life of bubble detector tubes

Post by Frank Sanns » Wed Nov 01, 2017 2:03 am

The two that I owned both failed within a year. The first one had a good workout including doing some multiple day long background measurements. Some bubbles did form with no neutron sources operating but they were few and far between.

Both of my detectors failed by your mechanism #1 evident from actual gel leaking out around the seal. The second detector that lasted only 4 months showed this with polymer actually coming out of the seal and hardening. It happened within a few weeks of not being used and surprised me as I had a run all set up to use it. I pulled out my trusty He3 pair and did it the old fashioned way never to buy another bubble detector again at least for now.

For the neophyte or even intermediate neutron producing fusor operator, they are fairly full proof as long as they do not get warm from the fusor. I think it is fair to say that no other detector can give confirmable and reliable results so they are a good tool for many. No more of my money will go to one though as I think it is poorly constructed to have the seal actually leak water and polymer.

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Bob Reite
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Re: Increasing the useful life of bubble detector tubes

Post by Bob Reite » Thu Nov 02, 2017 12:07 am

My failed bubble detector never showed signs of leaking. Just that the bubbles would not compress anymore.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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Re: Increasing the useful life of bubble detector tubes

Post by David Kunkle » Thu Nov 02, 2017 3:55 pm

"1. Water diffusing out of the detector tube.
Storing the tubes in their sealed storage container retards mechanism 1, so one should keep the tubes in the storage container when not in use. "

If this is the case, putting a drop of water in the aluminum tube would keep the relative humidity at 100% in storage. If water is diffusing out of the detector, this should *theoretically* eliminate #1 as an issue.

Mine is more than 1.5 yrs. old now, but does still register a bubble from background if I leave it decompressed for several days, and the bubbles still re-compress. I'm going to go ahead and add the water to mine. Don't see any down side- there's nothing to corrode.
If your experiment needs statistics, you ought to have done a better experiment.

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