Attentuation of Thoron (Rn 220 ) in Tyvek ® Membranes Paul Kotrappa, Lorin Stieff and Frederick Stieff Rad Elec, Inc A Industry Lane Frederick, MD

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

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

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

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)

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 cm -1 Response of these chambers is >3% thoron

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

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

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)

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:  mm ( cm or 5.1 mil or 125 µm)  Equivalent to gm/cm 2 Density of each membrane  gm/cm 3

Calculation & Attenuation of Thoron for Different Thicknesses of Tyvek ® Membranes # of Membranes IJKLDaysCF (I, J)CF (K,L)kBq/m3 Exp. % Transmitted

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

Radon Concentration for Tyvek ® Membranes Number of Membranes Thickness of Tyvek ® Radon Conc. (Bq/m 3 ) Average thickness of each #14 Tyvek ® membrane is mm

Thoron Concentration for Tyvek ® Membranes Number of Membranes Thickness of Tyvek ® Thoron Conc. k(Bq/m 3 ) % Transmitted Average thickness of each #14 Tyvek ® membrane is mm

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

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

Conclusions Cont. Equation represents the percent attenuation (P) accurately to calculate the number (M) membranes needed to attenuate thoron by a required factor: P = – × Ln(M) is the multiple regression coefficient This equation provides a controlled attenuation of thoron when needed