Increasing the useful life of bubble detector tubes
Posted: Thu Oct 26, 2017 9:17 pm
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?
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?