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Register a new userUltra-low radioactivity Kapton and copper-Kapton laminates
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Polyimide-based materials, like Kapton, are widely used in flexible cables and circuitry due to their unique electrical and mechanical characteristics. This study is aimed at investigating the radiopurity of Kapton for use in ultralow background, rare-event physics applications by measuring the $^{238}$U, $^{232}$Th, and $^{nat}$K levels using inductively coupled plasma mass spectrometry. Commercial-off-the-shelf Kapton varieties, measured at approximately 950 and 120 pg/g $^{238}$U and $^{232}$Th (1.2$times$10$^4$ and 490 $mu$Bq/kg), respectively, can be a significant background source for many current and next-generation ultralow background detectors. This study has found that the dominant contamination is due to the use of dicalcium phosphate (DCP), a nonessential slip additive added during manufacturing. Alternative Kapton materials were obtained that did not contain DCP and were determined to be significantly more radiopure than the commercially-available options with 12 and 19 pg/g $^{238}$U and $^{232}$Th (150 and 77 $mu$Bq/kg), respectively. The lowest radioactivity version produced (Kapton ELJ, which contains an adhesive) was found to contain single digit pg/g levels of $^{238}$U and $^{232}$Th, two-to-three orders of magnitude cleaner than commercial-off-the-shelf options. Moreover, copper-clad polyimide laminates employing Kapton ELJ as the insulator were obtained and shown to be very radiopure at 8.6 and 22 pg/g $^{238}$U and $^{232}$Th (110 and 89 $mu$Bq/kg), respectively.
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Noble gas permeabilities and diffusivities of Kapton, butyl, nylon, and Silver Shield are measured at temperatures between 22C and 115C. The breakthrough times and solubilities at 22C are also determined. The relationship of the room temperature permeabilities to the noble gas atomic radii is used to estimate radon permeability for each material studied. For the noble gases tested, Kapton and Silver Shield have the lowest permeabilities and diffusivities, followed by nylon and butyl, respectively.
The search for neutrinoless double-beta decay (0{ u}{beta}{beta}) requires extremely low background and a good understanding of their sources and their influence on the rate in the region of parameter space relevant to the 0{ u}{beta}{beta} signal. We report on studies of various {beta}- and {gamma}-backgrounds in the liquid- xenon-based EXO-200 0{ u}{beta}{beta} experiment. With this work we try to better understand the location and strength of specific background sources and compare the conclusions to radioassay results taken before and during detector construction. Finally, we discuss the implications of these studies for EXO-200 as well as for the next-generation, tonne-scale nEXO detector.
Simple broadband microwave interconnects are needed for increasing the size of focal plane heterodyne radiometer arrays. We have measured loss and cross-talk for arrays of microstrip transmission lines in flex circuit technology at 297 and 77 K, finding good performance to at least 20 GHz. The dielectric constant of Kapton substrates changes very little from 297 to 77 K, and the electrical loss drops. The small cross-sectional area of metal in a printed circuit structure yields overall thermal conductivities similar to stainless steel coaxial cable. Operationally, the main performance tradeoffs are between crosstalk and thermal conductivity. We tested a patterned ground plane to reduce heat flux.
Scintillating NaI(Tl) crystals are widely used in a large variety of experimental applications. However, for the use as Dark Matter (DM) detectors, such crystals demand a high level of radio-purity, not achievable by means of standard industrial techniques. One of the main difficulties comes from the presence of potassium that always accompanies sodium in alkali halides. On the other hand, the arguable DM detection by DAMA experiment using NaI(Tl) scintillating crystals requires a reliable verification able to either confirm the existence of DM or rule out the DAMA claim. Ultra-low radioactivity NaI(Tl) crystals, particularly with very low potassium content, are therefore indispensable to overcome the current stalemate in Dark Matter searches. Nonetheless, apart from DAMA-LIBRA experiments, to date, no other experiment has succeeded in building a detector from NaI(Tl) crystals with potassium content of ppb level. This work describes recent results in the preparation of ultra-radio-pure NaI(Tl) crystals using a modified Bridgman method. A double-walled platinum crucible technique has been designed and reliability tests show that 5 ppb of potassium in the NaI(Tl) crystals of 2 and 3 inches in diameter can be achieved starting from NaI powder with potassium content of the order of 10 ppb. The potassium excess is segregated in the tail-side of the as grown ingot where measured potassium concentration is above 20 ppb. The purifying effect of Bridgman growth for larger NaI(Tl) crystals is currently being tested. The work also reports on scintillation parameters of our NaI(Tl) crystals measured in a dedicated setup conceived for naked, hygroscopic crystals. The reproducible and reliable production of ultra-low radioactivity NaI(Tl) crystals reported in this work will hopefully spur the construction of new DM search experiments and, anyway, clarify the controversial DAMA-LIBRA results.
Inductively coupled plasma mass spectroscopy is a powerful technique for measuring trace levels of radioactive contaminants, specifically Th and U, in materials for use in construction of low-background rare-event detectors such as double beta decay and dark matter detectors. I describe here a technique for measuring Th and U contamination in copper using direct acid digestion and dilution, without further chemical processing, achieving results comparable to previous work which utilized more complex chemical pre-concentration techniques. A convenient research-oriented analysis environment is described as well. Results are presented for measurements of three samples from the production line of electrolytically-purified, LME (London Metal Exchange) grade A, NA-ESN Aurubis copper. Purified samples showed levels consistent with zero contamination for both elements, while weak but inconclusive indications of contamination were present for the un-purified anode copper. The best limits achieved are near $1cdot 10^{-12}$~g/g (95% CL) for both Th and U measured for copper from the cathode of the purification process.
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