1. 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

2. 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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. 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

9. 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)

10. 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

12. 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

13. Percent Thoron Attenuation (Experimental & Regression Fitted) for Stated Number of Tyvek ® Membranes

14. 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

15. 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

16. 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

17. 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

18. 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