Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 DESIGN AND PERFORMANCE OF A PASSIVE RADON DECAY PRODUCTS MONITOR P.Kotrappa and F.Stieff Frederick, MD 2 1704 ABSTRACT Electret-based passive air samplers have been used in United Kingdom and elsewhere for quantitative sampling for airborne dust. Alpha electret ion chambers (EIC) have been used for quantitative measurement of deposited alpha emitting isotopes. These two well documented principles are combined to create a passive radon progeny monitor. Large area (50 cm2) electret charged to 500 to 2000 volts collect airborne radon decay products and the collected sample is "viewed" and measured by an alpha EIC. Such collection and measurement continues for the entire period of sampling, providing an integrated signal to the electret in alpha EIC. The present work is of exploratory nature and provides the responses of three different sizes of collection electrets. Results are also compared with a simple passive device with no collecting electret. The study provides data for optimization of the design depending upon the requirement. Study is limited to a typical home with equilibrium ratios from 40 to 60%. This method can be used for both short term and long term monitoring of RDP in working level units. INTRODUCTION It is known that radon decay product (RDP) measurement is more accurate method of characterization of radon risk compared to measuring radon gas. However, most measurements carried out to date are radon gas, because of simplicity and lower cost. RDP measurements are needed for more precise correlation with health effects. There are several instruments available for measuring RDP concentration in air. In these instruments, air is sampled through a filter paper and alpha activity collected on the filter is counted to determine the RDP concentration in WL units. These require a pump and electrical power to operate. It is not practical to use these devices for extended periods. This is because of several reasons such as dust loading and the cost of running the unit for extended time. Hence these are used for short term (ST) measurements extending from 2 to 7 days only. There has been a need for a device that can be used without a pump or power and usable for extended periods. ELECTRET PASSIVE SAMPLERS Recently electret-based passive air samplers have been used in United Kingdom and elsewhere for quantitative sampling for airborne dust. This works like a pump (1). This work proved the feasibility and demonstrated the integrating capability of the passive Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc. www-aarst.ore 38 Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 electret sampler. The original design is not widely used because of the limitations of stability of electrets used in those devices. It is now practical to manufacture very stable electrets which can be used for extended period. PASSIVE SAMPLERS FOR RADON DECAY PRODUCTS Alpha electret ion chambers (EIC) have been used for quantitative measurement of deposited alpha emitting isotopes (2,3). These two well documented principles are combined to create a passive radon progeny monitor. Electrets charged to 500 to 2000 volts collect airborne radon decay products by the principle similar to the published version (1) and the collected sample is "viewed" and measured by an alpha EIC. Such collection and measurement continues for the entire period of sampling, providing an integrated signal to the electret in alpha EIC. Figure 1 gives an exploded as well as assembled view of this device. The parameters used in the study are similar to those used in published version (1). The diffusion cell is made up of 8 cm diameter Al disk with1 cm separation. The cell is positioned on the top of Aluminized Mylar foil (also 8 cm diameter) of alpha EIC (2,3),with three separation pedestals. Under normal circumstances, the concentration of RDP inside the cell is the same as that in the room. However these see surfaces available to deposit and get deposited. There is equal probability of collection on top Aluminum disk and on to the aluminized Mylar on top of the alpha EIC. More enter the cell from room air for further deposition. This is the well know principle of passive diffusion deposition. It is well known that the aerosols get collected preferentially on charged surfaces. If the bottom side of the top aluminum disk is lined with an electret, the passive diffusion deposition gets amplified. This works like an aerosol pump (1) and hence RDP collector. The alpha particles emitted by the collected RDP penetrate through the Aluminized Mylar and cause ionization inside the alpha EIC and get registered by an electret of the alpha EIC. The process continues providing integrated signal just as in EIC used for radon monitoring. The rate of discharge of electret in the alpha EIC is caused by two sources, the progeny collected in the cell and the ionization caused by radon and gamma radiation inside alpha EIC. A background alpha EIC (Aluminized Mylar window covered) provides the signal from radon and gamma radiation. The difference between the combined signal and that due to radon and gamma provides the signal uniquely relatable to progeny concentration. The signal from alpha EIC is measured in terms of volts per day, obtained by measuring total discharge and dividing it by the period of collection in days. Total discharge divided by the exposure period provides the discharge rate in units of volts per day. Copyright @ 2007 by the American Association of Radon Scientists and Technologists, Inc. www.aarst .orq 39 Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 CALIBRATION Calibration of these new passive RDP monitors is simple and straight forward. A typical home basement was chosen that has radon concentration between that ranges from 3 to 10 pCi/L. Reference device used was a calibrated E-RPISU. The E-RPISU and the passive devices with associated blanks (for radon and gamma signal measurement) are deployed as per EPA protocols. These were operated for the desired sampling period. The calibration factor (CF) is defined as the net signal (volts per day, VPD) divided by the RDP concentration in units of mWL. The discharge rate is normalized to the mid point voltage of 400 volts by using the published equations for alpha EIC (2,3). The calibration factors were determined for electrets of different areas (large, medium and small area) and for the device that did not have collection electrets. RESULTS Preliminary experiments proved that there is no significant change in response when electret voltages are in the range of 500 to 2000 volts. Because of the small gap width, the electret field is large enough to collect the particles. Table-1 gives the calibration factors for large area electrets. Table-2 gives the results for medium area electrets and Table-3 gives the results for small area electrets. Table-4 gives results for the collection cell when no electret was present. Table-5 gives the summary of all the results, including collection areas, calibration factors, and relative calibration factors, relative to large area collection electret and relative areas, relative to large area electret. DISCUSSION OF RESULTS There is some collection of RDP in Aluminum surfaces as seen by the response of 0.25 VPD per mWL (Table-4). This may be a right choice for long term sampling. Expected volts drop for 180 day sampling is 45 volts for 1 mWL or 900 volts for 20 mWL (.02 WL). When expected levels are in excess of 10 mWL, long term electret has to be used. Wider standard deviations in the calibration factors may be due to several factors. These include the expected non uniformity of RDP levels in the room, finer dependence of responses on the electret voltages. Large area electret samples at a rate which is nearly 14 times that of passive sampling with out a collecting electret. These samplers are a right choice for short term sampling. Medium and small area collection electrets behave in between. It is expected, that larger the area of the electret, higher is the collection and higher is the response as seen in Table-5. Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc. www.aarst.orf 40 Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 In these studies, negative electrets were used because of the ease of making and handling negative electrets of high surface potential. Current studies are limited to equilibrium ratios in the range of 40 to 60 %, usually found in typical homes. It is worth noting that the standard deviation in the responses of simple diffusion sampler with no collecting electret. Future studies are needed to cover wider range of equilibrium ratios. These studies are planned in the next phase of the study. USEFUL CONCLUSIONS Simple diffusion sampler with no electret collector promises to be a practical device for both short and long term monitoring of RDP. ACKNOWLEDGEMENTS Authors are grateful for the useful discussions with Dr. Dan Steck and Mr.Doug Kladder. References: 1. M.A. Hemingway, I. Strudley, R.C.Brown, S.Froude, and M.M. Smith "An electret based passive sampler used for sampling airborne pigment dust, rubber fume, and flour dust" Ann.0ccup.Hyg Vol: I , pp 653-65 8, 1997 2. Ken Kasper "Passive Electret Ion Chambers-Technology Monitor Column" Health Physics 76:475-476 (1 999) 3. S.K.Dua, S.K.Biswas, P.Szerszen, J.Boudreax, and M.A. Ebadian: "Measurement of Surface Alpha Contamination Using Electret Ion Chambers" Health Physics 76:664674 (1 999) Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc. www.aarst.orq 41 Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 Tablet Large Area Electret Collector Calibration Data Days Net NVPD 2.083 2.083 72.03 59.76 2.083 86.03 2 2 2 2 2 2 4 4 4 4 4 4 78.70 58.43 62.57 72.84 56.68 81.75 84.16 66.56 71.O5 96.78 80.89 78.48 Grand Avg. Grand SD Table-2 Medium Area Electret Collector Calibration Data Net NVPD 63.55 51.88 54.50 49.03 55.25 63.77 72.23 70.09 95.25 60.57 Days 1.0625 1.0625 1.0625 1.0625 1.0625 0.8958 0.8958 0.8958 0.8958 0.8958 rnWL 43.7 43.7 43.7 43.7 43.7 39.3 39.3 39.3 39.3 39.3 Average STDEV % STDEV Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc. www.aarst.orq 42 Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 Table-3 Small Area Electret Calibration Data Days Net NVPD mWL 2.04 2.04 2.04 2.04 2.04 4 4 4 4 4 1 .O625 1 .O6X 1.0625 1.0625 1 .(I625 13.80 15.88 17.78 17.84 12.54 11.56 19.79 14.76 22.96 12.73 14.32 15.37 14.01 18.95 1 1.77 20.4 20.4 20.4 20.4 20.4 21.1 21.1 21.1 21.1 21.1 19.25 19.25 19.25 19.25 19.25 Average STDEV % Table4 No Electret Calibration Data Days 4 1.75 0.8958 1.1 04 H 465 297 462 434 NVPD(P) NVPD(B) Net NVPD 9.336696 12.94985 21.81082 17.02559 4.542461 5.726597 11.31568 8.403693 4.794235 7.223257 10.49513 8.621902 Average STDEV % STDEV 0.250224 0.0155 6.210255 Table5 Relative resonses of Large, Medium and Small Area Collecting electrets Re1 Rel Area (cm2) Response Resp. Area 59.5 3.6121 1.00 1.00 Large E Medium E 38.5 1 5484 0.43 0.65 Small E 21.2 0.7706 0.21 0.36 No E 59.5 0.2502 0.16 1.00 Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc. www.aarst.orq 43 Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 Aluminum Disk Collecting Electret Spacer and Holder Aluminized Mylar Body of Electret Ion Chamber (EIC) Electret and Electret Holder o f EIC Fig 1A Aluminum Disk ^@ Collecting Electret Spacer and Holder Body o f Electret Ion Chamber (EIC) Electret and Electret Holder of EIC Passive Radon Progeny Monitor (Passive R-RPISU) Fig 1A Exploded View and Fig 1B Assembled View Aluminized Mylar