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Attentuation of Thoron (Rn 220 ) in Tyvek ® Membranes Paul Kotrappa, Lorin Stieff and Frederick Stieff Rad Elec, Inc. 5716-A Industry Lane Frederick, MD 21704 PKotrappa@radelec.com
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Purpose of the work To find an appropriate barrier to stop thoron entry, without stopping radon entry Why do we need such a barrier? Thoron (Rn 220 ) is an isotope of radon, is similar to radon, & often accompanies radon entry Thoron is chemically similar to radon but has different radioactive & radiobiological properties
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Radioactive & Radiobiological Properties Radon gas has a half-life of 3.8 days and its’ decay products have a half life of 30 minutes Thoron gas has a half life of 55.6 seconds and its’ decay products have a half life of 10 hours Inhaled thoron & its’ decay products are shown to be less harmful by a factor of 3 compared to radon & radon decay products
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Applications where a barrier is important to stop thoron interference Measuring radon Thoron can lead to uncertainty in measured radon levels Monitoring for radon during uranium exploration work Thoron can cause wrong conclusions Measuring radon flux from surface of granite Thoron can lead to uncertain flux for radon
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Other methods to minimize thoron interference when measuring radon CRMs using a small volume sampling pump Introduce delay loop that delays the sampling gas by ~5 mins before entry into sensitive volume Diffusion devices sampling through large area filters Introduce sufficient delay in diffusion time (~5 minutes, 5 times the half life of thoron)
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Methods to minimize thoron interference when measuring radon (cont.) Use limiting orifices By controlling the ratio of diffusion area to sensitive volume it’s possible to introduce effective delay time For example: S Chamber E-PERM® has a diffusion inlet of 0.3 cm 2 & sensitive volume of 210 cm 3 This provides an area to volume ratio of 0.0014 cm -1 Response of these chambers is >3% thoron
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Methods to minimize thoron interference when measuring radon (cont.) Current Work Showed: 1mm thick Tyvek ® decayed thoron by 50% 4mm thick Tyvek ® decayed thoron by 95% Both without decaying radon
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Why Tyvek ® Membrane Attenuates Thoron, but not Radon Thoron takes a finite time to diffuse through the membrane Such finite time is comparable with half life of thoron (56 sec) Thoron partially decays during this transmitting time This leads to attenuation Radon takes the same finite time to diffuse through the membrane Such time is negligibly small compared to the half life of radon (3.8 days) hence radon does not attenuate during this transmitting time
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Why Dupont Tyvek ® Membrane was used Tyvek ® is a material made by DuPont Popular membrane used as “building wrap” and as shipping envelopes Unique property makes it transparent to water vapor, but not water droplets This unique property also makes it transparent to radon Tyvek ® membrane is tear resistant, antistatic, inexpensive, & easy to handle Tyvek ® has been studied for transmission to radon& results published in 2012 AARST meeting (Stieff, 2012)
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Physical Properties of Tyvek ® used in current work Commercially available #14A (antistatic) Tyvek ® membrane was used Tyvek ® is made by pressing very fine spun bonded Olefin fibers (a form of polyethylene) Thickness of each membrane: 0.1296 mm (0.01296 cm or 5.1 mil or 125 µm) Equivalent to 0.006704 gm/cm 2 Density of each membrane 0.517 gm/cm 3
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Calculation & Attenuation of Thoron for Different Thicknesses of Tyvek ® Membranes # of Membranes IJKLDaysCF (I, J)CF (K,L)kBq/m3 Exp. % Transmitted 16434814314120.12590.473686.8627512.57473100 27666044314120.12593.634786.8627512.0911496.1542 37666434314120.12594.1358586.862758.70309169.21093 46935824314120.12592.4139586.862757.85905262.49875 54253244314120.12585.6548586.862757.68332161.10125 67786764314120.12594.714186.862756.86551454.59769 72511704314120.12581.4400586.862756.20688649.35998 86765934314120.12592.3368586.862755.44117543.27069 93242514314120.12583.4189586.862755.2509241.7577 105815114314120.12590.062486.862754.46802435.53176 114814154314120.12587.543886.862754.2813834.04747 125925204314120.12590.319486.862754.6274836.79982 136045424314120.12590.756386.862753.71529829.54574 145204584314120.12588.597586.862753.84846530.60474 155925314314120.12590.4607586.862753.64471828.98445 163883344314120.12585.307986.862753.31412226.3554 173342834314120.12583.9586586.862753.10964824.72933 184774304314120.12587.6851586.862752.53818220.18478 245815434314120.12590.743686.862751.61020912.80511 314964694314120.12588.4304586.862750.6927115.508748
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Percent Thoron Attenuation (Experimental & Regression Fitted) for Stated Number of Tyvek ® Membranes
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Radon Concentration for Tyvek ® Membranes Number of Membranes Thickness of Tyvek ® Radon Conc. (Bq/m 3 ) 10.1296503 50.648519 101.296492 151.944514 243.24503 314.01519 Average thickness of each #14 Tyvek ® membrane is 0.1296 mm
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Thoron Concentration for Tyvek ® Membranes Number of Membranes Thickness of Tyvek ® Thoron Conc. k(Bq/m 3 ) % Transmitted 10.129612.57100 50.6487.6861.1 101.2964.4735.53 151.9443.6428.98 243.241.6112.80 314.010.695.50 Average thickness of each #14 Tyvek ® membrane is 0.1296 mm
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Conclusions Attenuation of thoron gas is studied using calibrated 960 ml E-PERM ® thoron monitor for different thicknesses of Tyvek ® membrane 1mm thick Tyvek ® decayed thoron by 50% 4mm thick Tyvek ® decayed thoron by 95% Both shown to respond to radon w/o attenuation Diffusion sampling instruments Entry area covered with 4 mm thick Tyvek ® stack Expected to respond >5% to thoron
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Conclusions Cont. A stack of 30 membranes provides 4 mm thick membrane stack for practical use 4 mm thick Tyvek ® stacks can be used with radon flux monitors for measurements on granite Can also be used during uranium explorations to minimize interferences from thoron Tyvek ® membranes are tear resistant, antistatic and inexpensive
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Conclusions Cont. Equation represents the percent attenuation (P) accurately to calculate the number (M) membranes needed to attenuate thoron by a required factor: P = 109.3092 – 30.6801 × Ln(M) 0.9881 is the multiple regression coefficient This equation provides a controlled attenuation of thoron when needed
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