Abstract:
The radometer is a portable instrument for the measurement of the concentration of atmospheric radon/thoron in a test area. A constant velocity pump pulls the air from the outside at a constant flow rate. If the air is too moist, some or all of the sample is passed through a desiccant filter prior to encountering an electrostatic filter. The electrostatic filter prevents any charged particles from entering the sampling chamber. Once the sample has entered the chamber, the progeny of the decay of radon/thoron are collected on a detector and measured. The measured data is compiled by a computer and displayed.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has rights in this invention pursuant to the employer-employee relationship of the U.S. Department of Energy and the inventor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention, a Radometer, relates to an improved method for the measurement of environmental radon and thoron in homes and buildings. More particularly, this invention allows for the accurate measurement of radon and thoron using a portable, handheld electrical instrument and only a few minutes of time. 
     2. Description of Related Art 
     The measurement of radon and thoron in homes and buildings is important in determining the presence of a potential health hazard to persons occupying the structure. The concentration of radon is measured by counting the alpha particles resulting from the radioactive decay of radon,  222 Rn, while the level of thoron is determined by counting the alpha particles from the radioactive decay of thoron,  220 Rn. In the present invention four counting channels are used: two for radon and two for thoron. 
     An electrostatic filtering system is employed to insure that no positively charged particles enter the sampling chamber. In prior art, the filters were either in the form of a physical filter such as an open pore foam or an electrostatic filter. The use of a physical filter greatly increases the power consumption necessary to measure either the radon or the thoron. 
     Prior art often used natural air flow or manual fanning devices to move the air being sampled into a sampling chamber and then to the sensor. To provide for a more accurate measurement, the subject invention employs a constant velocity pump which provides a constant flow rate for the air moving into the sampling chamber. 
     Humidity causes the charged radioactive progeny to become neutrally charged which results in a lowering of the sensitivity of the instrument. Attempts to compensate for the humidity using a linear compensation method do not yield satisfactory results. The Radometer employs a nonlinear equation as a means of compensation together with a desiccator as is needed. 
     The Radometer measures radon and thoron by electrostatic collection of charged progeny on a solid state detector. A dual electrostatic field technique eliminates the need for a physical air filter and electric power draining pumps present in other radon measuring instruments. The Radometer allows accurate measurements to be made in 10 to 15 minutes or less. 
     To perform radon measurements in under 15 minutes requires alpha spectroscopy which the Radometer employs. This method resolves and measures the 6 MeV alpha particle that is emmitted as the radon progeny of  218 Po decay. Po- 218  has a half life of 3 minutes which permits rapid measurement provided the instrument has adequate sensitivity. Using the dual electric field concept eliminates the need for filters or pumps since it insures that no electrically charged progeny can enter the measurement volume. This allows only the  218 Po that collects on the solid state detector as a result of the decay of the radon in the collected gas sample to be measured. 
     Another problem that is commonly encountered occurs when making measurements in an area of low radon concentration following measurements taken in an area having a high concentration of radon. Generally in prior art, this change in environment causes the detector to have a high background making it difficult to distinguish low concentration values. The Radometer employs software that incorporates exponential superposition which allows for the rapid determination of low concentration values subsequent to a high concentration measurement. 
     Thus, one objective of this invention is to provide an instrument for the measurement of the concentration of radon and thoron in the environmental atmosphere in twenty minutes or less. 
     Another objective of this invention is to provide an instrument which is lightweight and provides for a long operation life using standard batteries which are readily available such as D-cell. 
     Another objective of the invention is to provide an instrument with the flexibility to operate in many different environments with little difficulty. 
     Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and other advantages, this invention is an instrument to measure the concentration of radon and thoron by electrostatic collection of charged progeny on a detector. A dual electric field configuration eliminates the need physical sampling filters while the large volume of the sampling chamber results in a fast response time. An on-board computer records the data from the detector and corrects the data as needed prior to output. The sampling chamber has a volume of 6 liters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated in the accompanying drawings where: 
     FIG. 1 is a schematic showing a cross section of the radometer. 
     FIG. 2 is a schematic of the power supply system. 
     FIG. 3 illustrates the electronics which collect, analyze and output the data. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a cross sectional schematic of the radometer  10 . Air samples enter the radometer  10  via entrance ports  12 ,  14 , or both depending on the water vapor content of the sample. If the water vapor content is not of concern, port  14  is completely closed by means of valve  13 , and the sample enters through port  12  and passes unaltered directly through duct  16  to the intake of a constant velocity pump  18 . If the water vapor content of sample is of great concern, port  12  is closed by means of valve  13 , and the sample enters through port  14  where it is routed through a desiccant filter  19  to remove most of the water vapor prior to entering duct  16  leading to the constant velocity pump  18 . The removal of the water vapor improves the sensitivity of the instrument, the Radometer. For cases in between, where only part of the water vapor needs to be removed, valve  13  is adjusted to route part of the air stream through port  12  and part through port  14 . The proportion of the total air flow which flows through each port is dependent on the amount of water vapor which needs to be removed to retain the desired instrument sensitivity. The constant velocity pump  18  is used so that the air sample travels at a constant flow rate to the sample chamber  20 . Maintaining a constant flow rate is important for measuring thoron due to its short half-life. After exiting the constant velocity pump or pump  18 , the air sample enters a diffusion duct  22  and flows to a perforated cap  24 . The perforated cap  24  is separated from the perforated base  26  of the sample chamber  20  by an air gap of approximately one inch. The sample chamber  20  including its base  26  is at a voltage potential of 3000 volts with respect to ground. The cap  24  of the diffusion duct  22  is grounded resulting in it having a potential relative to the base  26  of approximately 0 volts. The voltage differential across the air gap between the base  26  and the cap  24  creates an electronic filter  23  which prevents any charged progeny from the parent radon/thoron gas from entering the sample chamber  20 . This type filter is effective since it prevents all charged particles from entering the sampling chamber  20 . 
     Once the air sample is in the 6 liter sample chamber  20 , the radon/thoron gas undergoes a radioactive decay to form the progeny  214 Po and  218 Po for radon and  216 Po and  212 Po for thoron. A detector  28  is positioned so that its detection surface is interior to the sample chamber. The electrical potential of the detector  28  is at ground (0V) potential so that a second electrical field is setup this one interior to the sample chamber. Since the radon/ithoron progeny are 80% positively charged, a collection mechanism is established by which these charged progeny migrate from the interior volume of the sample chamber to the detector  28 . Neutral progeny that enter the sample chamber and undergo further decay will also be collected but not as Po- 216  or Po- 218 . The use of a 6 liter sampling chamber results in improved measurement sensitivity when compared to smaller chambers. Further increases in sensitivity can be attained by increasing the volume of the sample chamber. In most cases, the voltage potential of the sample chamber relative to the detector is also increased as the size of the sample chamber is increased. 
     The electronics used to record the counts as detected by the detector  28  is housed in the electronics compartment  30 . A four channel electronic system is used to count the radon/thoron progeny. These channels register alpha counts from  218 Po with an energy of 6.0 MeV and  214 Po with an energy of 7.7 MeV in the radon decay chain, and from  216 Po with an energy of 6.8 MeV and  212 Po with an energy of 8.8 MeV in the thoron decay chain. Thoron also produces  212 Bi with an alpha energy of 6.06 MeV. This cannot be resolved from the  218 Po alpha at 6.0 MeV; however,  212 Po and  212  Bi are produced in an approximate 2 to 1 ratio which permits a correction to be made. For example, if 50 counts are registered on the  212 Po channel then half or 25 must be subtracted from the  218 Po channel to correct for the contribution of  212 Bi. This correction is made by an on board computer. The power source for the system is contained in the handle  32  of the radometer. In the current embodiment, three D-cell flashlight batteries are used  92 , FIG.  2 . The use of the three batteries will power the radometer for several days. 
     As noted, with the decay of the radon/thoron gas, the charged progeny are attracted to the detector  28  by the electric field set up by the 3 Kv voltage source  50 , FIG. 2, and the grounded detector  28 . Once on the detector, the progeny further decay by alpha emission. The alpha emission causes a small electric pulse in the detector which is coupled to an amplifier  40 . The pulse from the amplifier is routed to four comparators  42 , 44 , 46 ,and  48 . This enables the two alpha energies from the decay of the radon gas (Rn- 222 ),  218  Po and  214 Po to be separated and counted. It, also, allows the two energies from the decay of thoron gas (Rn- 220 ),  216 Po and  212 Po to be separated and counted. The output of each of the comparators is as follows: comparator  42  measures  212 Po, comparator  44  measures  212 Po + 214 Po, comparator  46  measures  212 Po + 214 Po + 216 Po, and comparator  48  measures  212 Po + 214 Po + 216 Po + 218 Po. Individual counts are obtained by subtracting the count from one comparator from another to obtain the desired result. For example, the count for  216 Po is obtained by subtracting the reading of comparator  46  from  44 . The pulses coming out of each of the four comparators are counted by counters which are paired one on one with each comparator. Counters  52 ,  54 ,  56 , and  58  are paired with comparators  42 ,  44 ,  46 , and  48  respectively. Once a second, the information in each of the counters is transferred to shift registers  62 ,  64 ,  66 , and  68  where it is shifted as one long  32  bit binary number to the computer  70 . The computer  70  then separates the  32  bits into four counts, analyzes the data and displays the information on the display  72  in pCi/l. The quad digital analog converter  50  combines with the computer  70  to automatically set all adjustments to the Radometer  10 . A radioactive source  210 Po is used to calibrate the Radometer. The  210 Po is deposited on and remains on the detector  28  and produces alpha particles having an energy of 5.3 Mev. This energy level is well below the 6.0 Mev energy level of  218 Po which has the lowest energy level of the isotopes of interest. In normal operation, controlled by the depressing and releasing push-button  74 , the  210 Po isotope is not registered, but when the operator desires to calibrate the instrument the push-button is held in the depressed position until a signal appears on the display  72  indicating that the instrument, Radometer, is in the calibration mode. Instructions are then transferred from the computer to the DAC  50  to set Comparator  48  to count the  210 Po source in addition to the other four isotopes. The calibration operation completely checks all of the electronics in the instrument from the detector  28  to the display  72 . 
     A humidity/temperature sensor  76  is coupled to the computer  70  and is used to determine the moisture content of the air sample. The instrument was tested over a range to temperatures/humidity, to develop a data base. This data base was used with a nonlinear fitting technique to develop a set of equations to determine the sensitivity of the instrument at various atmospheric conditions of temperature and humidity. These equations were programmed into the instrument&#39;s computer  70 . Using these equations, the instrument displays its sensitivity on the display panel  72 . When high humidity lowers the sensitivity of the instrument below the acceptable level, the operator can raise the sensitivity by letting the air sample flow through the desiccant cartridge  19  to dry the sample. The constant velocity pump  18  insures the increase pressure drop due to the cartridge  19  has no effect on flow rate. 
     To perform a radon concentration measurement in 10 to 15 minutes requires alpha spectroscopy which resolves and measures the 6 MeV alpha that is emitted as the radon progeny  218 Po decays. Po- 218  has a half life of 3 minutes which permits a rapid measurement provided the instrument has adequate sensitivity. The sensitivity of the Radometer is typically 3 cpm/pCi/l. Radon has a higher energy alpha that results from the decay of  214 Po which has an energy of 7.8 MeV, but is half life is almost an hour making its measurement unsuitable for “sniffer” application. As noted above, the instrument measures both Po- 218  and Po- 214  but only uses the  218 Po to calculate the radon concentration. Since a dual electric field is used, the electrostatic filter  23  and the field internal to the sample chamber, this insures that no electrically charged progeny can enter the sample chamber  20 ; thus, the only  218 Po that can be collected on the solid state detector  28  must originate in the sample chamber  20 . 
     In cases where measurements are taken first in an area of high radon/thoron concentration and then in areas of low radon/thoron concentration a software program in the computer  70  employs an exponential superposition technique to correct for the high background brought on by the high concentration of radon in the initial sampling. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical applications and should enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.