Presentation on theme: "Measurement Overview Units of Measure Sampling Methods –Measurement Duration What the Lab Sees."— Presentation transcript:
Measurement Overview Units of Measure Sampling Methods –Measurement Duration What the Lab Sees
Units of Measurement Radon is measured in pCi/L. Radon Decay Products are measured in working levels (WL). 100 pCi/L of radon gas, in equilibrium with its decay products, produces 1 WL of radon decay products.
pCi/L or Working Level? Depends on Sampling Mechanics Insert Slide #4
What does the Lab look When measuring radon gas?
Radon Radon (with all RDPs from room filtered out) –Gamma radiation from RDPs of radon collected. –Alphas from radon and/or RDPs from radon collected. –Ionization from radon and RDPs from radon collected. –Damage to material from alphas from radon, and RDPs from radon collected. –Alphas and betas from radon, and RDPs from radon collected.
Radon Decay Products (WL) Radon decay products are collected on a filter using a carefully calibrated air pump. –The lab looks at alpha particles released from radon decay products trapped on filter.
Thoron (Radon 220) Gas Measurement Interference Can be incorrectly measured along with radon 222. This can occur with some radon measurement devices. The device may interpret the energy released from the short half-life of thoron with the energy released from radon gas. Thorons short half-life means that sniffing measurements can be affected if measured immediately on site. Passive devices usually not affected. Thoron interference can be corrected for.
Activated Charcoal Devices (AC) Measure radon. Different devices have different optimal deployment periods. Used by both professionals & homeowners. Require no power to operate. Cannot be read in field must be read at laboratory.
How Lab Analyzes Activated Charcoal Devices Device emits gamma from RDPs from radon. Detected by scintillation detector coupled with photo-multiplier tube.
Practical Concerns About Activated Charcoal Devices Open faced canisters are biased towards last 12 to 24 hours of measurement. Since the radon is adsorbed onto the charcoal it is possible for radon to escape before the device is sealed to be sent to the lab. Diffusion barrier devices biased to last 2 to 3 days. Reduce moisture in-take (diffusion barrier) or compensate for moisture in calibration. Sensitive to air flow. Sensitive to temperature extremes.
Advantages of Activated Charcoal Devices Convenient and economical Can be used for 48 hour test Can be easily mailed to lab for analysis Unobtrusive and make no noise Passive, does not require power Results can be provided very quickly
Disadvantages of Activated Charcoal Devices Limited to short-term sampling. Because of bias towards latter portion of sampling period, they are not true integrating devices if radon peaks and valleys are dramatic. Provide no indication of changes in radon during measurement. Therefore, tampering detection by measurement alone is difficult.
Types of Alpha Track Devices Filtered (AT) –RDPs from room are filtered out. Only radon can enter chamber. Results in pCi/L. Unfiltered (UT) This device is not commonly used in the United States. –Radon and RDPs from room enter chamber. Results are in pCi/L after E.R. is factored into calculation (usually E.R. of 0.5 is assumed.)
Alpha Track Devices Measure Radon, Results in pCi/L AT device: RDPs in room are filtered out. UT device: RDPs in room are allowed in –Tracks are chemically enhanced and counted under microscope (manually or by computer). Normal integration period: 3 months to 1 year. Designed for long-term measurements.
Characteristics of Alpha Track Detectors Relatively low sensitivity. Will over-respond if exposed in turbulent air or breeze (because it is a passive device). »Alpha Tracks create a permanent record and are true integrating devices.
Advantages of Alpha Track Devices Relatively low cost Convenient Distributed by mail Unobtrusive Needs no external power Can measure long-term characteristics
Disadvantages of Alpha Track Detectors Long measurement period necessary Precision errors, especially at low concentrations, if small area of chip is counted
Electret Ion Chamber (EC) Radon in device causes ionization. Ions cause electret to lose voltage.
Theory of EC Operation Radon decays into RDPs, releasing alpha and gamma radiation. RDPs from radon inside chamber continue to decay, releasing alpha, beta, and gamma radiation. All alpha, beta, and majority of gamma radiation cause some ionization of air in chamber.
Theory of EC Operation (continued…) Electrons released during ionization process collect on electret surface, thereby reducing its positive voltage. Resultant change in voltage is calibrated to average radon concentration for the duration of exposure.
Measuring Electret Voltages Electret voltages are measured before and after deployment. Reported radon is a function of: –Voltage drop. –Duration of deployment. –Calibration curves used to calculate (Approx. 2 volts drop per day in room at 1 pCi/L). Same orientation, temperature & at controlled humidity.
Electret Ion Chamber- Measurement Periods Normal integration period is 2 days to 1 year depending upon configuration, type of disk used, and anticipated radon concentration. Can be used for short-term and long-term measurements. Measures radon; results are in pCi/L
Types of Electret Ion Chambers Insert slide #36 ES –Used for short-term measurements EL –Used for long-term measurements
Characteristics of ECs Sensitive to turbulence and breezes (because it is a passive device). Slight performance difference at higher elevations. Affected by external gamma radiation.
Advantages of ECs Can be used for short-term and long-term measurements. –Electret can be re-used until voltage falls below the desired operating voltage for the device used.
Disadvantages of ECs Are sensitive to external gamma radiation, which should be corrected for. Are sensitive to altitude changes, which should be corrected for. Touching surface, surface contamination or impact can damage electret. Measure pre/post voltages at same temperature.
Charcoal Liquid Scintillation (LS) Description Plastic or glass vial with a few grams of charcoal. Radon diffuses through filter in cap, is absorbed onto charcoal. Normal integration period is 2 to 7 days. Designed for short-term measurements only. »Insert slide #41 Measures radon; results in pCi/L.
Theory of LS Operation Radon is trapped on charcoal. Charcoal is removed from vial and added to a second vial, which contains a liquid scintillate. After 4 hours, alphas and betas from radon and its RDPs cause scintillate to release visible light. Visible light pulses are counted with a PMT.
Characteristics of LS Devices The relatively low sampling rate is compensated for by the high efficiency (90%) of the counting apparatus. Small possibility of over-response from turbulence or breezes.
Advantages of LS Devices Same as other activated charcoal devices, i.e., –Relatively low cost –Unobtrusive Methodology can be used to measure radon in water also.
Disadvantages of LS Devices Same as for other activated charcoal devices –Limited to short-term sampling –Biased towards latter periods of exposure –Does not provide indication of changes in radon concentrations during the measurement period
Grab and/or Sniffing Radon Devices Grab sample with a scintillation cell and PMT (GS) Continuous Radon Monitor (CR) put into sniffing mode
Sniffing Sniffing is when a grab sample is taken, but rather than waiting 4 hours for the collected radon to come to equilibrium with its short-lived RDPs, it is measured right after sampling. Used as a diagnostic tool for finding entry routes. Provides quick measurement to identify relative differences between multiple measurements.
Radon Sniffing Devices Continuous Radon Monitors (CR) can be used as sniffers. –This will be discussed in the next section.
Radon Sniffing Devices-continued It is important to note that any test lasting less than 48 hours cannot be used as a basis to determine it a mitigation system should be installed
Continuous Radon Monitors (CR) Scintillation Cell and Photomultiplier Tube (PMT) Pulsed Ion Chamber Solid State Silicon Chip
Theory of Operation of Continuous Radon Monitors (CR) Radon is collected from room by either a pump (active mode) or by diffusion (passive mode). RDPs are filtered out. Alpha particles from radon (active mode, also called sniffing) or radon and its RDPs (passive mode) are counted. Measures radon, results in pCi/L.
Generic Continuous Radon Monitor in Active or Sniffing Mode »Insert slide #53
Solid State Detector Example »Insert slide 59 Provides continuous measurements. Must measure and record hourly to be used as a stand-alone device for real estate transactions.
Practical Concerns About Continuous Radon Monitors Ramp-up time –In passive mode, it takes approximately 4 hours for the monitor to respond to changes in radon levels in the room air. –The first 4 hours of a measurement, therefore, should not be used in a 48 hour exposure. –44 hours of contiguous data sufficient for short-term.
Practical Concerns About Continuous Radon Monitors (continued…) When in active or sniffer mode (pump on): –The air is in the chamber for a short period of time. –Short lived thoron is counted along with radon. –As a result, the readings should be used qualitatively only, unless the thoron is compensated for.
Advantages of Continuous Radon Monitors Exposure duration is variable, 48 hours to many months. Relatively good precision (most models). Can track hourly variations (or more often depending upon model). Can be down-loaded or printed out on site. Can indicate when tampering or ventilation occurs.
Disadvantages of Continuous Radon Monitors Relatively costly Requires trained operator Some units do not have good precision around 4.0 pCi/L (±2.0 pCi/L)
Continuous Working Level Meter Pump collects RDPs on filter. Alpha from RDPs strike chip. »Insert slide #65 & 66
Continuous Working Level Meter Sampler programmed. Detector taken to location. After deployment data downloaded.
Practical Considerations for Continuous Working Level Devices Avoid use in dusty, smoky, and high humidity environments. Air flow must be well maintained and well known (0.1 L/min.). If results are converted to pCi/L, the equilibrium ratio used to do so must be disclosed in report. »Measurement Unit Activity »Measurement Unit Course Exam