RADON MONITOR AND CONTROL SYSTEM BASED UPON ALPHA PARTICLE DETECTION

Abstract: A system (10) for monitoring or controlling the level of radon in indoor air, based upon measuring alpha particles due to the decay of radon or its daughter atoms. In one embodiment, the alpha particle decay of radon itself is detected and analyzed to control a vent (74) in the heating and air conditioning system (12) to automatically keep the radon level below a preselected level. In another embodiment, the daughter atoms <218>Po and <214>Po are collected from the indoor air and their alpha particle decays are analyzed to provide a sensitive monitor of radon levels or the control vents (74) in the HVAC system (12) to reduce radon concentrations to permissible levels. In addition, the system (10) provides information on the quality of the air filter (16) and indicates when it needs servicing.

Description: Description RADON MONITOR AND CONTROL SYSTEM BASED UPON ALPHA PARTICLE DETECTION Technical Field This invention relates to a radon monitor and control system which analyzes the parent atom and two daughter atoms, 2i8 Po and 214 Po, to determine if the radon level in an enclosed environment has exceeded a preselected threshold level, to activate a vent to reduce radon levels, and to monitor the quality of the air filter in a conventional heating, ventilating and air conditioning (HVAC) system. Background Art Modern energy conservation practices in residential and commercial environments have created a high level of radon gas to which building occupants are exposed. There are widely held beliefs that the levels of radon are such that the radioactive daughter atoms [polonium-218 (2i β p 0 ) an< j pol nium-214 ( 214 Po)] are producing lung and other cancers. Some studies claim that residents in homes having elevated radon levels will be subjected to an increase in the incidence of lung cancer. It has long been known that radon gas can escape from building materials or the environs of buildings to create health risks to the occupants. As early as 1955, it was pointed out by Gabrysh and Davis (Nucleonics, January 1955, p. 50) that Chattanooga black shale is relatively rich in radium and that in that region of the country radon found its way into residential and commercial buildings. In recent years, the problem of radon in indoor air has become severe due to the practice of energy conservation in modern homes. Further, high levels of radon can be found in homes located throughout the United States, as shown on a map by Fortmann (ASTM Standardization News, December 1988, pp. 50-53). The magnitude of radon levels found in these homes was analyzed by statistical sampling methods and presented in an article by Nero, et al. [Nero, Schgwehr, Nazaroff, and Revzan, Science 234, 992-997 (1986)]. These authors report that the average level of radon-222 ( 22 Rn) is 1.5 pCi/L of indoor air and that the lifetime risk of lung cancer corresponding to this level of exposure is about 0.3 percent. Between one percent and three percent of the U. S. homes have a radon level exceeding 8 pCi/L. The Environmental Protection Agency recommends that the radon level not exceed 4 pCi/L. The radon problem has been reviewed recently in such widely read journals as Physics Today (Anthony Nero, April 1989, p.32) and National Geographic (April 1989, p. 403). When radon gas is present in a building, a series of complex processes is involved in placing the occupants at risk. It is believed that the most serious threat to human health is that of lung cancer due to the depositing of alpha-emitting daughter atoms of radon.- These are 218 Po emitting 6.0-MeV alphas with a half-life of three minutes and 214 Po emitting 7.7-MeV alphas with a half-life of 150 microseconds. In this radium decay chain there are intervening beta decay processes of somewhat longer half-lives so that the effective half-life in going from 218 Po to 210 Pb is about 40 minutes. The product lead-210 ( 210 Pb) is a beta emitter with a half-life of 22 years in decaying to bismuth-210 ( 210 Bi) which terminates the chain. In a static room, the time required for the daughter atoms to diffuse to the walls would be much longer than the decay time of 40 minutes. If the daughter atom is attached to atmospheric gases or dust particles, this diffusion time is even longer. However, if the heating and air conditioning system is operating, the circulation time of air through the system is typically about 20 minutes; thus, the daughter atoms would be partially filtered. Such filtering, however, removes little of the radon gas ( 2Z2 Rn) with a half-life of 3.8 days; thus, the daughter atom 218 Po, which has a half-life of only three minutes, comes quickly back into equilibrium with the radon. While the home filtering system could have some value in reducing the level of 214 Po, it will be of little value for filtering the 21β Po. There are a number of factors which can determine the level of radon in a building. The rate of diffusion of radon into a building depends on the building location, type of soil, weather and seasonal variations, the design of the building, and its maintenance. For instance, small cracks developing in a basement wall as the building ages could drastically change the rate of radon accumulation. Worse, even at a constant rate of entry, there are a number of housekeeping factors such as the amount of ventilation, the number of times the doors are opened, and the type of heating system and its schedule of use which can alter the level of radon. When these factors which control the level of radon itself are considered with other factors, such as room humidity and dust, which control the fate of the daughter atoms produced by the radon, it can be seen that the health risk cannot reliably be tracked by an occasional canister sample sent to a laboratory. Accordingly, it is an object of the present invention to provide a simple, accurate, and low-cost device for monitoring and/or controlling the level of radon in homes and commercial establishments. Moreover, it is an object of the present invention to provide such a radon detector and/or control system that keeps the average radon level below a guideline or preselected threshold level in a manner which is consistent with reasonable energy conservation practices. The device of the present invention is based upon the detection and/or control of radon by monitoring 222 Rn or radon products — namely, 214 Po and 218 Po, which are commonly referred to as daughter atoms. By monitoring the detection rate or count of these three atoms in separate channels in the preferred embodiment, data can be gathered concerning the need for HVAC filter replacement and/or steps to be taken to accomplish venting the air within the enclosed environment to the outside, thereby reducing the health hazard due to the daughter atoms of radon. Disclosure of the Invention Other objects and advantages will be accomplished by the present invention which provides a radon monitor and control system for detecting and controlling the level of radon and its products, or daughter atoms, in an enclosed environment such as a home or commercial building. The level of each of these three types of atoms ( 222 Rn, 218 Po, and 214 Po) can be determined with detectors such as pulse ionization chambers or solid state particle detectors. One such satisfactory alpha particle detector is the DIAD II, manufactured by EG&G ORTEC, 100 Midland Road, Oak Ridge, Tennessee 37830. Further, these devices can be used to identify separately each type of atom since the alpha particle energies differ and are, respectively, 5.5 MeV, 6.0 MeV, and 7.7 MeV. In this mode of operation, the detectors are usually referred to as alpha-particle spectrometers. Several embodiments of the invention are based on the separate use of the data obtainable with the alpha spectrometer. In one embodiment, the data obtained on the level of the radioactive gas itself is used to activate a vent in the HVAC system to control the level of radon in the building. In another embodiment, the level of 21β Po is determined by collecting daughter atoms from the room onto a collector for a more sensitive monitor of the radon level and to provide indication of these levels. In still another, and preferred embodiment, the levels of 2 * °Po and 214 Po are determined and are used to control the HVAC system. One such control feature, is the reduction of the radon level by activating a vent based on the information on the 18 Po level. Another significant control feature is the monitoring of the efficacy of the HVAC filter for removing dust particles from the air, based on the level of 214 Po as affected by the HVAC fan. In this filter testing mode, the fan should be left on for 30 to 45 minutes as will become more evident in the illustrations below. Still another use of information derived from the alpha-particle spectrometer is that of testing the performance of the collector for the polonium atoms by monitoring the ratio 2 1 B Po/ 222 Rn in the detector port. In summary, the invention utilizes an alpha-particle spectrometer that can distinguish the species 222 Rn, 218 Po, and 214 Po, and information on all three types of atoms is used to monitor and improve indoor air quality. Brief Description of the Drawings Figure 1 shows a. ecay scheme of 222 Ra, the type of particles emitted, their energies, and the half-lives of daughter products in the decay to 210 Pb. Figure 2 depicts the alpha-particle spectrometer and the data stream generated by detecting 222 Rn, 218 Po, and 21 Po. Figure 3 illustrates the use of the data stream to monitor the level of indoor radon, to reduce the radon level, to monitor the efficacy of the HVAC air filter, and to self-check the collector of the daughter atoms. Several options are shown for monitoring and venting radon. Figure 4 illustrates the logic involved in maintaining the average radon level below the permissible level. Figure 5 shows how the 214 Po data is used to monitor the efficacy of the HVAC fan. Figure 6 illustrates the use of the ratio 218 Po/ 222 Rn in self-checking the performance of the collector of daughter atoms. Figure 7 is one illustration of how the entire data stream is used to monitor radon, to reduce radon levels, to monitor the HVAC filter, and to self-check the detector in an integrated HVAC system. Figure 8 is a flow chart of the system operation. Best Mode For Carrying Out The Invention Referring first to Figure 7, a radon monitor and control unit, generally indicated at 10, is constructed in accordance with various features of the present invention and provides a number of embodiments to accomplish several objectives. The invention 10 can be configured to be a stand-alone unit to monitor the ambient radon level in a building 14. In the preferred embodiment, the invention can be configured into the HVAC system 12 of a building 14 to control or reduce radon levels therein. The invention 10 also provides means for monitoring the efficacy of the HVAC filter 16, and for self-checking the radon detector 18. When used as a part of an overall control system as illustrated in Figure 7, the monitor 10 will regulate the ventilation system of a conventional HVAC system 12 to maintain the radon level below a preselected threshold. A portion of a radium 19 decay scheme is shown in Figure 1 to illustrate how 222 Rn 20 is generated in the environs of most buildings in the United States and elsewhere. This radioactive gas can enter these buildings through transport and diffusion processes to place the occupants at risk of an increased lung cancer arising primarily from the inhalation of the daughter products 218 Po 22 and 214 Po 28. Shown in Figure 1 are the decay processes in which radon 222 Rn 20 is transformed into lead 210 Pb 30. In these steps, there are three alpha-particle emitters, 22 Rn 20 which e its a 5.5-MeV alpha particle with a half-life of 3.8 days, 2 * 8 Po 22 which emits an alpha particle of 6.0 MeV with a half-life of three minutes, and 214 Po 28 which emits an alpha particle of 7.7 MeV with a half-life of 150 microseconds. As has already been described, there are intervening beta decay processes of somewhat longer half-lives so that the effective half-life in going from 218 Po 22 to 210 Pb 30 is about 40 minutes. The product lead-210 ( 210 Pb) 30 is a beta emitter with a half-life of 22 years in decaying to bismuth 210 ( 210 Bi) (not shown) which terminates the chain. The detection and resolution of these individual alpha particles is an essential feature of the present invention and is shown diagra matically in Figure 2. There are several known means for detecting alpha particles and some of these, such as pulse ionization chambers and surface barrier detectors, provide information on the energy of individual alpha particle interactions. Figure 2 illustrates the use of a surface barrier detector at 18; however, this could be some other solid state device, provided that it is a device in which the amplitude of the electrical pulses generated is uniquely related to the energy of the alpha particle striking the detector. Because these devices can resolve the energy of a particle, we will refer to them as alpha-particle spectrometers. In Figure 2, a typical alpha-particle spectrometer 18 is shown mounted inside an enclosure 32 which is open to the atmosphere for admission of radon gas. An alpha particle from the decay of 222 Rn can be resolved as belonging to the 5.5-MeV group since the dimensions of the enclosure 32 and the air pressure therein, normally one atmosphere of pressure, are such that only a small fraction of the particle's energy is lost before striking the alpha-particle- spectrometer 18. Thus, one may assign the impulses having amplitudes corresponding to about 5.5 MeV to the decay of a 222 Rn atom. Now it happens that the level of concern presently recommended by the Environmental Protection Agency, EPA, is about 4 pCi of radon per liter of indoor air. In an enclosure volume of about 10 cubic centimeters, such as that illustrated in Figure 2, the disintegration rate is only about 4 alpha particles per hour due to the decay of 22 Rn. Further, the disintegration rate and hence the rate of alpha-particle emission due to the daughter atoms * 8 Po and 214 Po is, at equilibrium, of like magnitude. Thus, the data stream, 36, as an output of the data processor 38, illustrated as a part of Figure 2, will consist of resolved energy peaks 40 which can be associated with the infrequent decay of 222 Rn 40A, 218 Po 40B, and 214 Po 40C, respectively. Note, however, that in Figures 2 and 7, the enclosure 32 contains a filter 34 which serves to collect the daughter atoms since these are usually in the form of small dust particles in the indoor air. Forced flow of air 42B through the detector assembly 32 will cause a large increase in the 2 * 8 Po 40B and the 14 Po 40C count as shown in Figures 2 and 3, but will not materially increase the count rate due to 222 Rn 40A. The various options for the beneficial use of the information from the alpha-particle spectrometer 18 is illustrated by means of Figure 3. For a stand-alone unit to monitor the radon level, there are three options: Option 1, direct detection of 222 Rn; Option 2, the detection of 218 Po; or, Option 3, the detection of 214 Po. The use of the daughter atoms will be recognized as indirect methods for detecting radon but they are quite reliable and can be used for more sensitive detection of radon by forcing air 42B through the detector enclosure 32. As shown in Figure 7, each of the monitoring options just described for detecting indoor radon can be used to activate the exhaust vent 74 in the HVAC unit 12 to control the radon level, thereby keeping its average level below that of concern. Of these, Option 2, based on detection of 18 Po, is preferred because it provides a much faster response to changing radon levels. The use of 214 Po is very well suited to monitoring the efficacy of the air filter 16 in the HVAC system 12. Finally, the ratio 218 Po/ 222 Rn 78 is used to monitor the air flow through the daughter atom filter or collector 34, serving as a self-check of this vital feature of the spectrometer. Several methods of data acquisition and processing are available to achieve the objectives of this invention. Analogue methods are not as desirable, especially for the analysis of the low count rate of the 222 Rn data, as digital methods are readily available in microprocessors. For illustration of further features of this invention, the use of microprocessors will be understood, but it is not the intention to limit the invention to their use. The use of the data stream 36 as illustrated in Figures 2 and 3 can be further clarified by referring to additional illustrations in Figures 4, 5, and 6. Thus, the manner in which the sensors are used to keep the radon concentration below an acceptable or permissible level is shown in Figure 4. In the example shown, vent 74 is turned on (opened) when the radon level approaches 6 pCi/L 44; and after a short time, when venting has decreased the radon level to about 2 pCi/L, the vent is closed 46, for energy conservation purposes, until after the passage of a much longer time, when the radon level again approaches the trip level 48. Conversion of count rate to picocuries per liter is achieved through a calibration process which is well known to those skilled in the art of radiation physics. Likewise, Figure 5 illustrates how the HVAC air filter 16 is monitored using the 2 x 4 Po data. If the monitor assembly 10 is operated with its auxiliary fan 50 operating constantly, then the response of the monitor 10 to the main air fan 58 (see Figure 7) will be as shown in Figure 5 for three filter conditions — namely, no filter 52, poor filter 54, and good filter 56. It should be noted that the typical exchange time for air in a residence is about 20 minutes. For this reason, it will sometimes be desirable to program the fan 50 especially for the filter test, so that the fan 50 is kept on long enough for a valid test. This could be done routinely, for example once per week. When it is found that the monitor's output does not change sufficiently during this test period, an indicator 62 appears on the panel 60 of Figure 7 to indicate that the main filter 16 needs attention. In the case of a disposable filter, replacement would be indicated; while in the case of an electrostatic type filter, service would be suggested. Another use of the data stream is illustrated in Figure 6 where the ratio 218 Po/ 222 Rn 78 is plotted as a function of time. This ratio, determined automatically by the microprocessor, rises to a value substantially greater than one within a few minutes after the stream of air 42B starts through the detector assembly 32 (see Figure 7) , comes to an equilibrium value ReQ 80 in a few minutes because of the three minute half-life of 218 Po, and remains constant thereafter unless the collector 32 of the daughter atoms becomes blocked by dust or other airborne material. Whenever the ratio 78 drops substantially below the nominal value, an indicator 64 is given on the panel 60 of Figure 7 that the collector 34 (Figures 2 and 3) needs attention. Alternatively, the data could be used to automatically advance another collector 34 into the detector assembly 32 (Figures 2 and 3) . Figure 8 diagrammatically.illustrates the functions of the three system options in flow chart form. Option 1 illustrates the use of the radon gas atoms, 2 2 Rn, to operate the exhaust/control vent 74. Option 2 uses the daughter atom pollonium 218 and Option 3 uses daughter atom pollonium 214 to accomplish the same end objective, i.e., open the exhaust/control vent 74 to vent contaminated air to the outside. All three options base their operation on the atom count obtained from the alpha particle detector 18 coupled through the data stream 36. While a preferred embodiment of a radon monitor and control system based on the detection of 222 Rn, 218 Po, and 214 Po atoms has been described and illustrated, it will be understood that there is no intent to limit the invention to such disclosure, but rather it is intended to cover all modifications and alternate constructions falling within the spirit and scope of the invention as defined in the appended claims and equivalents thereof.

Claims: Clai s 1. A radon monitor comprising: a collector of radon decay products; a detector for determining the number of alpha particles in a selected energy range; means for acquiring and processing data to convert alpha counts or count rates to radon concentration; and means for displaying radon concentrations and the condition that a threshold level has been exceeded. 2. A radon detector comprising: a collector of radon and its daughter atoms; a detector for determining the number of alpha particles in selected energy ranges; means for acquiring and processing data to convert the alpha particle counts in selected energy ranges Lo the number of atoms of elements selected from the group consisting of 222 Rn, 218 Po, and 214 Po. 3. A radon monitor and control system based upon alpha particle detection of radon or its progeny atoms within an enclosed environment comprising: a collector of radon and its daughter atoms; a detector for determining the number of alpha particles in selected energy ranges; means for acquiring and processing data to convert the number of alpha particles in selected energy ranges to the number of atoms of elements selected from the group consisting of 222 Rn, 218 Po, and 214 Po; and means for venting atmosphere within a closed environment until the level of radon or its progeny atoms detected is reduced to a desired level. 4. A monitor of the efficacy of a dust filter in a conventional HVAC system comprising: a collector of dust containing radon daughter atoms; a detector for determining the number of alpha particles emitted by 214 Po; means for acquiring data from said alpha particle detector and signals from the HVAC fan to determine the ability of the filter for collecting dust; and means for communicating to the attendant of the HVAC system the efficacy of the filter. 5. A monitor of the efficacy of a collector of alpha particle emitters comprising: a collector of dust or other materials containing atoms that emit alpha particles, mounted proximate an enclosure containing a trace of radon in an atmosphere of other gases; a detector of alpha particles for determining the number of alpha particles in specified energy ranges; means for forcing the said atmosphere through the said collector or filter; data processing means for determining the ratio 218 Po/ 222 Rn as a function of time; and means for communicating to an operator the efficacy of the collector.

Inventors: HURST GEORGE S, HURST, GEORGE, S.
Applicants: CONSULTEC SCIENT INC, CONSULTEC SCIENTIFIC, INC.
Application year: 1990
Main classification: G01T1
IPC codes: G01T1