Abstract:
A radon monitor includes a housing defining a housing cavity and having an opening in an exterior wall that is in fluid communication with the housing cavity to allow air to diffuse into and out of the housing cavity. The monitor also includes input and output units and a circuit board that is positioned in the housing cavity and supported by the housing. A passive, non-electrically powered sampling chamber defines a chamber cavity and is coupled to the circuit board. The circuit board defines a plurality of apertures that allow air to diffuse between the housing cavity and the chamber cavity. A detector for detecting radon is supported by the circuit board and positioned in the chamber cavity.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/025,986, filed Feb. 4, 2008, the entire contents of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to radon monitors and methods and, more particularly, to low voltage radon monitors including a passive detection chamber, and methods for monitoring or detecting radon with the same. 
     BACKGROUND 
     Radon is a radioactive gas that is colorless, odorless, and tasteless, and is formed by the natural radioactive decay of uranium in soil, rock, and water. More particularly, Uranium 234 decays into Radium, which then decays into Radon gas, which then decays into daughter particles of Polonium 218, Lead 214, Bismuth 214, Polonium 214, and Lead 210. Upon decay of Radon gas, an alpha particle is produced that has an energy level of about 4.5 MeV to 5.5 MeV (million electron volts). Alpha particles can travel in air up to approximately 3.8 centimeters and can be easily stopped by as little as a piece of paper. When alpha particles contact a surface, they transfer their energy into the surface. 
     Radon gas is prevalent in buildings having basements or other portions of buildings on and below the surface of the ground. Studies have shown that exposure to radon gas at sufficient concentrations can cause heath problems, including among other things, lung cancer. Radon daughter particles can plate onto dust or smoke, which, when inhaled into the lungs, can stick to a surface of the lungs. When radon and its daughter particles are in intimate contact with the lung cells, the alpha, beta, and gamma particles emitted by radon and the daughter particles can cause mutation of the lung cells and initiate cancer. Accordingly, having the capability to monitor and determine radon gas levels in buildings is important for the health of its occupants. 
     The Environmental Protection Agency (“EPA”) has established an action level threshold of radon gas which is 4 picocuries (pCi) per hour per liter of air. Countries other than the United States may have hazard thresholds different than the threshold established by the EPA. A picocurie is a unit of radiation that indicates the number of radioactive decays. A picocurie is one million millionth, or a trillionth, of a curie, and represents about 2.2 radioactive particle disintegrations (decays) per minute per liter of air. Therefore, 4 picocuries would be 8.8 disintegrations (decays) per minute per liter of air. 
     Radon gas monitoring has become an integral component of real estate transactions in some states. Prior to closure of a real estate transaction in certain states, an inspector conducts a radon gas test on the premises to determine radon gas levels. This radon gas test can be conducted in a few manners. A first manner for testing radon gas levels includes using kits purchasable by consumers. Such kits include a short-term radon gas charcoal test kit and a long-term radon alpha track test kit. The short-term radon gas charcoal test kit uses a container that contains a quantity of granular activated charcoal, which absorbs the radon gas entering the canister from the surrounding air. At the end of the radon gas test period, the canister is sealed and sent to a laboratory for analysis. The long-term radon alpha track test kit includes a vessel with an internal piece of film that records the impacts of alpha particles produced by the decay of radon and its decay by-product, polonium. At the end of the radon gas test period (approximately 90 days), the radon testing kit is sent to a laboratory where the alpha tracks on the film are counted, radon concentration is computed, and analysis is reported. Such kits are relatively inexpensive, but are often times extremely inaccurate and inconsistent. 
     A second manner of detecting radon gas levels includes an AC powered electronic device such as that disclosed in U.S. Pat. No. 4,871,914. These types of radon monitors are relatively expensive and are generally only economical to purchase by professional radon gas inspectors who continually utilize the monitors to generate a steady flow of revenue. Such radon gas monitors are generally not economical for purchase by a typical home owner. AC power is necessary for such radon detectors because they include a powered sampling unit, in which samples are taken, that requires constant and significant quantities of power. Such a powered sampling unit is energized and draws, attracts, or otherwise influences radon gas or alpha particles into the sampling unit for sampling. Typical powered sampling units require greater than 250 volts of electrical power, which can present a shock hazard if the testing unit housing is broken. Because monitors with powered sampling units require AC power provided by a household outlet, it is often difficult to position such monitors in crawl-spaces or similar spaces that are rarely wired for AC power. 
     Accordingly, a need exists for an accurate, inexpensive radon gas monitor that can be powered for extended periods of time without a direct connection to an AC power source. 
     SUMMARY 
     In some embodiments, the invention provides an apparatus for detecting radon in air that includes a housing defining a cavity, a passive, non-electrically powered sampling chamber supported by the housing, and a divider that is supported by the housing and that defines a boundary between the cavity and the chamber. The divider includes a plurality of apertures to allow air flow between the cavity and the chamber. The apparatus also includes a detector for detecting radon in the chamber. 
     In some embodiments, the invention provides a radon monitor that includes a housing defining a housing cavity and having an opening in an exterior wall that is in fluid communication with the housing cavity. The opening allows air to diffuse into and out of the housing cavity. The radon monitor also includes an output unit supported by the housing and an input unit supported by the housing. The input unit is operable to activate and deactivate the radon monitor. The radon monitor also includes a circuit board that is positioned in the housing cavity and supported by the housing, and a passive, non-electrically powered sampling chamber defining a chamber cavity. The circuit board defines a plurality of apertures, and the sampling chamber is positioned in the housing cavity for fluid communication between the chamber cavity and the housing cavity by way of the apertures in the circuit board, which allows air to diffuse between the housing cavity and the chamber cavity. A detector that is supported by the circuit board and at least partially positioned in the chamber cavity detects radon. 
     In some embodiments, the invention provides an apparatus for detecting radon in air that includes a housing defining a cavity, a sampling chamber supported by the housing, and a detector for detecting radon in the chamber. The apparatus also includes a self-contained power supply that is the exclusive source of electrical power for the apparatus. The power supply is supported by the housing. 
     Independent features and independent advantages of the present invention will become apparent to those skilled in the art upon review of the detailed description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system block diagram of a radon monitor. 
         FIG. 2A  is a first exemplary schematic of a detection module for use with the radon monitor of  FIG. 1 . 
         FIG. 2B  is a second exemplary schematic of a detection module for use with the radon monitor of  FIG. 1 . 
         FIG. 3  is a system block diagram of a tamper proofing system of the radon monitor of  FIG. 1 . 
         FIG. 4  is a front perspective view of an exemplary application of the radon monitor represented in the diagram shown in  FIG. 1 . 
         FIG. 5  is a rear perspective view of the radon monitor shown in  FIG. 4 . 
         FIG. 6  is a front view of the radon monitor shown in  FIG. 4 . 
         FIG. 7  is a rear view of the radon monitor shown in  FIG. 4 . 
         FIG. 8  is a right side view of the radon monitor shown in  FIG. 4 . 
         FIG. 9  is a cross-sectional view taken along line  9 - 9  in  FIG. 6 , shown with a second portion of a housing removed. 
         FIG. 10  is a front view of a portion of an exemplary circuit board of the radon monitor shown in  FIG. 4 . 
     
    
    
     Before any independent features and embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     As should also be apparent to one of ordinary skill in the art, the systems shown in the figures are models of what actual systems might be like. As noted, many of the modules, units, and/or logical structures described are capable of being implemented in software executed by a microprocessor or a similar device or of being implemented in hardware using a variety of components including, for example, application specific integrated circuits (“ASICs”). Terms like “processing module” may include or refer to both hardware and/or software. Thus, the claims should not be limited to the specific examples or terminology or to any specific hardware or software implementation or combination of software or hardware. 
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a system block diagram of a radon monitor  700  is illustrated. The radon monitor  700  includes a detection module  704  to detect an amount of radon gas present in air in a surrounding environment. In the illustrated embodiment, the detection module  704  includes a plurality of diffusion units  708  through which radon gas diffuses into a sampling unit  712 . In some embodiments, the sampling unit  712  includes a fixed volume metal chamber, detailed hereinafter. In the illustrated embodiment, the diffusion units  708  include membranes and apertures populated adjacent a sensing unit  716 , such as, for example, a positive-intrinsic-negative (“PIN”) junction photodiode to detect a presence of energy, such as, for example, from an alpha particle from decayed radon gas. Other exemplary sensing units  716  may include cascade photodiodes, charged surface semiconductors, CCD photo detectors, CMOS photo detectors, and the like. Once the sensing unit  716  has detected the presence of energy, the sensing unit  716  generates a signal for further processing. 
     The detection module  704  also includes a signal conditioning unit  720  to condition the signal received from the sensing unit  716 . In addition to the alpha particles that are detected to produce radon measurements, decaying daughter particles also emit beta and gamma particles, which may also be detected by the detection module  704 . The signal conditioning unit  720  is therefore provided to filter the signals provided by the sensing unit  716  to reduce or eliminate the signatures of the beta and gamma particles. In the illustrated embodiment, the signal conditioning unit  720  includes an optional signal translator  724  that translates the signal received from the sensing unit  716  from one format into another format acceptable for further processing, a pulse generator  728 , and a signal filtering unit  732 . In some embodiments, the signal translator  724  includes a transimpedance circuit that translates the signal, such as a current signal, into another format, such as a voltage signal. In cases where the sensing unit  716  generates a signal with a format that is acceptable for further processing, the signal translator  724  becomes an optional component for the signal conditioning unit  720 . The pulse generator  728  converts the signal from either the signal translator  724  or the sensing unit  716 , which is an analog signal into a digital signal for further digital processing, detailed hereinafter. The filtering unit  732  filters any direct-current (“DC”) signals that can exist, for example, between components such as the signal translator  724  and the pulse generator  728 , limits frequency bandwidth of the signal, adjusts impedance, and shapes the signal to accentuate the alpha particle signature while attenuating the beta and gamma particle signatures. 
     The radon monitor  700  also includes a processing module  736  to process the digital signals received from the detection module  704 . In some embodiments, the processing module  736  is capable of operating in an awake mode (or high power mode), and a sleep mode (or low power mode). Based on options programmed into the processing module  736 , or received from an interface module  740 , the processing module  736  also processes the digital signals received from the detection module  704  to determine a radiation level, and outputs signals indicative of the determined radiation level and the options selected, for example, for display purposes. In some embodiments, the processing module  736  is preprogrammed to process the radiation level minimally to conserve electrical power. In some embodiments, the processing module  736  also stores and retrieves data indicative of the determined radiation level internally with an internal memory and/or externally with an external memory, generically and collectively referred to as a memory module  742 . In the illustrated embodiment, the processing module  736  includes a microcontroller, such as, for example, PIC16F689 and an external memory, such as, for example, an EEPROM. Other microcontrollers and other external memory can also be used in other embodiments. In some embodiments, the radon monitor  700  may include a removable memory to provide a portion of the external memory of the memory module  742  in order to archive or facilitate transport of the stored data without transport of the entire radon monitor. Exemplary removable memories include, for example, a secure digital card, thumb drive, portable hard drive, memory stick, and the like. In such cases, the radon monitor  700  includes a removable memory receptacle  742  to receive a removable memory device and/or a port for interfacing with the removable memory device. Also, after removal of the removable memory from the removable memory receptacle or port  742 , another removable memory device may be inserted into the removable memory receptacle  742  or connected to the port  742  to replace the removed removable memory device. With the new removable memory inserted into or connected to the radon monitor  700 , data sampled by the radon monitor  700  can continue to be stored. 
     The interface module  740  includes an input unit  744  to receive input selections from a user, and an output unit  748  to output signals in response to the signals generated by the processing module  736 . In the illustrated embodiment, the input unit  744  includes an actuator  752  to receive input selections from a user. Other embodiments may include as an input unit  744  one or more keypads, switches, mechanical keys, remote control devices, smart cards, RF identification tags, and the like. 
     In response to the input selected on the input unit  744 , the input unit  744  produces a selection signal which wakes the processing module  736  to generate one or more output signals that drive the output unit  748 . For example, in the illustrated embodiment, the output unit  748  includes a display  756  that displays information such as the determined radiation level in response to the output signals. Examples of display  756  include a liquid crystal display (“LCD”), an array of light emitting diodes (“LED”), and the like. In some embodiments, the display  756  also optionally includes a serial interface or a wireless interface for connecting the radon monitor  700  to a computer for external display, control, monitoring, or other purposes. Exemplary serial interfaces include, for example, RS-232, USB, and the like. Exemplary wireless interfaces employing protocols include, for example, wireless local area networks (“WLAN”) such as WiFi, personal area networks (“PANs”) including Bluetooth, and radio frequency (“RF”) links such as XBee radio links, and the like. A wired Internet interface connection may also be included to provide remote access to the radiation readings. The output unit  748  also includes a sound circuit  760  that receives the output signals from the processing module  736 , and produces a sound or tone as an audio indicator. In some embodiments, an actuated audio indicator indicates a radiation level above a predetermined or programmable threshold, such as, for example, 4 pCi. Audio indication can occur immediately after the radiation level exceeds the predetermined or programmable threshold, or after the radiation level exceeds the threshold for a predetermined or programmable period of time, such as, for example, between 2 to 30 days. In other embodiments, an actuated audio indicator indicates a startup and/or an initialization of a diagnostic run, low battery, other diagnostics or actuator feedback, and the like. Furthermore, in some embodiments, the output unit  748  is programmed or structured to be actuated for a predetermined amount of time, such as, for example, 15 seconds, to conserve electrical power. In some cases, the output signals that drive the sound circuit  760  has a predetermined duty cycle. To conserve electrical power, the duty cycle of the output signal is generally minimized or reduced, and/or the predetermined amount of time can also be preset to have millisecond-long, second-long, minute-long, or hour-long intervals. 
     The threshold at which the audio indicator activates to emit audio can be predetermined or programmable. In instances where the threshold is predetermined, the radon monitor  700  is manufactured and distributed with a single predetermined threshold that cannot be changed by a consumer or other entity. For example, in the United States, the EPA has established a hazard threshold level of radon gas that is 4 picocuries (pCi) per hour per liter of air. Accordingly, radon monitors  700  distributed in the United States that have predetermined thresholds are sold with a predetermined threshold of 4 pCi per hour per liter of air. Radon monitors  700  having a predetermined threshold can be distributed in countries other than the United States. However, these countries other than the United States may have hazard threshold levels of radon gas different than that of the United States. Accordingly, the radon monitors for the countries other than the United States can be set with a predetermined threshold in accordance with their particular country&#39;s hazard threshold level of radon gas. In instances where the threshold is programmable, the radon monitor can be manufactured and distributed with a factory threshold and the consumer or other entity can re-program the radon monitor to have a different threshold. In some embodiments, the radon monitor may be re-programmed an infinite number of times. In other embodiments, the radon monitor may be re-programmed a definite number of times. 
     A power supply module  764 , such as, for example, a battery, generally powers the radon monitor  700 . In the illustrated embodiment, the power supply module  764  supplies an amount of power to components of the radon monitor  700  at a variety of power levels, such as, for example, high and low levels. For example, when the processing module  736  is operating in an awake mode (or a high power mode), the processing module  736  draws a relatively high amount of electrical power or current, such as, for example, milliamps, from the power supply module  764 . When the processing module  736  is operating in a sleep mode (or a low power mode), the processing module  736  draws a relatively low amount of electrical power or current, such as, for example, microamps, from the power supply module  764 . Operation of the processing module  736  in the sleep mode facilitates operation of the monitor  700  on battery power for extended periods of time. 
     The radon monitor  700  also includes a low voltage detection module  768  to detect an amount of voltage supplied by the power supply module  764 . When the amount of voltage supplied by the power supply module  764  falls below a predetermined level, such as, for example, 4.6 V, the low power detection module  768  sends a low power detected signal to the processing module  736 . When the processing module  736  receives the low power detected signal, the processing module  736  generates an output signal indicative of a low level of electrical power supplied by the power supply module  764 . In turn, the output unit  748  is actuated to produce corresponding outputs. 
     The radon monitor  700  also includes a timing module  772  to produce a pulse signal to trigger the processing module  736  or other components, such as the detection module  704 , of the radon monitor  700 . The timing module  772  can generate the pulse signal based on a regular or irregular predetermined or programmed time. The timing module  772  can also generate the pulse signal in response to signals generated by the actuator  752 , which is typically actuated by a user. Although the timing module  772  is shown as an external circuit with respect to the processing module  736 , the timing module  772  can also be intrinsic to the processing module  736  in other embodiments. In some embodiments, the timing module  772  is configured to run at a low voltage level, such as, for example, 3.3 V nominal, to conserve electrical power. 
     In the illustrated embodiment, the sensing unit  716  is designed to use a very low or substantially minimum amount of electrical power while operating. Some of the components of the radon monitor  700 , such as, for example, the timing module  772 , are also designed to consume a relatively low amount of electrical power while operating. Other components, such as, for example, the processing module  736  and the output unit  748 , draw a relatively high amount of electrical power. To conserve electrical power, the timing module  772  is structured to power the high electrical power consuming components on a time-limited basis. That is, time periods are limited at which the power supply module  764  powers the high electrical power consuming components. Similarly, the timing module  772  is also structured to continuously power the low electrical power consuming components at a very low or substantially minimum level, such that the radon monitor  700  can repetitively detect the radiation level. 
       FIGS. 2A and 2B  are exemplary schematics of two suitable detector circuits  800  for use with the detection module  704  of the radon monitor  700 , wherein like numerals refer to like parts. As a portion of the sensing unit  716 , each detector circuit  800  includes a detector  804 , such as, for example, a PIN photodiode to detect a presence of ionizing radiation energy. For example, when an atom of radon gas decays in the sampling unit  712 , the decaying atom emits an alpha particle with an amount of energy. When the alpha particle strikes the photo detector  804 , the energy is transferred from the alpha particle to the photo detector  804 . Particularly, the energy released by the particle creates a current perturbation in the junction of the photo detector  804 , thereby creating a current fluctuation in the photo detector  804  and a current output signal in response to the current fluctuation. 
     In the illustrated embodiments, the signal translator  724  includes a transimpedance amplifier stage  808  and a voltage normalization amplifier stage  812 . The transimpedance amplifier stage  808  translates the current output signal into a relatively low analog voltage signal, typically, in microvolts or nanovolts. The voltage normalization amplifier stage  812  then translates, amplifies, and/or normalizes the low analog voltage signal from, for example, a microvolt or nanovolt signal to a normalized analog voltage signal, e.g., 1 V peak, each time energy from an ionizing radiation is detected. The pulse generator  728  includes a comparator  816  which translates the normalized analog signal into a level acceptable by the processing module  736 . 
     To provide accurate, unbiased test results from the radon monitor  700 , it is desirable to detect any unusual influences that occur during testing. Unusual influences may include human interaction, otherwise known as tampering. Tampering with the operation of the radon monitor can greatly affect the test results. Consequently, the radon monitor  700  also includes a tamper proofing module  776  ( FIG. 1 ) to detect if aspects of the radon monitor  700  have been tampered with during radon monitoring processes, detailed hereinafter. Tampering may include, for example, opening a window to allow fresh air into the environment, blowing a fan on the radon monitor or otherwise actively venting the environment in which the radon monitor  700  is located, turning off the radon monitor, placing an item over the radon monitor or otherwise inhibiting air flow to the radon monitor, moving the radon monitor to a different location within the environment or building, etc. 
       FIG. 3  shows an exemplary tamper proofing system  300  for use with the tamper proofing module  776  of  FIG. 1  in a block diagram format, and wherein like numerals refer to like parts. Changes in temperature and relative humidity are indicators that the radon monitor has been tampered with by, for example, moving the radon monitor, affecting the conditions of the environment in which the radon monitor is located, etc. Both temperature and relative humidity can be measured to determine if tampering has occurred. To detect a change of temperature surrounding the radon monitor  700 , the tamper proofing system  300  includes a temperature sensor  304  to measure a surrounding temperature of the radon monitor  700 . For example, when the radon monitor  700  is activated to monitor radon levels in the monitoring process, the temperature sensor  304  measures an initial temperature of the environment in which the radon monitor  700  is located. The radon monitor  700  then stores the initial temperature in the memory module  742 . The temperature sensor  304  then repetitively measures subsequent surrounding temperatures at various predetermined times. The tamper proofing system  300  sends the measured temperatures to the processing module  736 , which may store the measured temperature in the memory module  742 . Furthermore, the processing module  736  determines a difference between the initial temperature with subsequently measured temperatures. In some embodiments, if the difference between the initial temperature reading and the subsequent temperature readings is sufficiently great, the tamper proofing system  300  sends a tampering signal to the processing module  736 . The tamper proofing system  300  can send a tampering signal to the processing module  736  immediately upon the difference becoming significantly great or after a predetermined or programmable period of time. The processing module  736  receives the tampering signal and logs a tampering time and a tampering type in the memory module  742 . Relative humidity and changes in relative humidity can be measured and interpreted in the same manner as temperature described herein, except sensors capable of measuring relative humidity are used instead of temperature sensors. 
     In some embodiments, to prevent tampering with the radon monitor  700  by blowing air over the radon monitor  700  with a fan or by opening a window, or by inhibiting air flow to the radon monitor  700  by sealing, enclosing, or covering the radon monitor  700 , the tamper proofing system  300  includes an air flow sensor  308  that determines a flow of air near the radon monitor  700 . In some embodiments, the air flow sensor  308  can measure a quantity or speed of air flowing through or around the radon monitor  700 . As such, similar to the temperature sensor  304 , the processing module  736  detects an initial reading and subsequent readings to identify any changes of air flow in the monitoring process. When the changes exceed some predetermined thresholds or tolerances, the air flow sensor  308  sends a tampering signal to the processing module  736 . Similar to the temperature sensor  304 , the tampering signal can be sent by the air flow sensor  308  either immediately upon exceeding the threshold or after a predetermined or programmable period of time. 
     In some embodiments, the radon monitor can deactivate or turn-off upon sensing tampering. The radon monitor  700  can include a timer  312  to identify a deactivation time, among other things. 
     In other embodiments, the radon monitor  700  is preset to be activated for a predetermined amount of time, such as, for example, 72 hours. This amount of time is stored in the timer  312 . In such a case, the radon monitor  700  continues to monitor the radon level until the radon monitor  700  reaches the predetermined amount of time (e.g., 72 hours) and is deactivated. However, if the radon monitor  700  is deactivated before the expiration of the predetermined time stored in the timer  312 , the timer  312  will generate a tampering signal and send the signal to the processing module  736 . In turn, the processing module  736  receives a timer tampering signal, stores a tampering event, and sends a signal to the output unit  748 , which will produce a desired output. 
     In some embodiments, the tamper proofing system  300  includes a displacement sensor  316  to detect any displacement of the radon monitor  700  after the radon monitor  700  has been activated. Exemplary displacement sensors include, but are not limited to, a pressure sensor, mercury switch, humidity sensor, motion sensor, accelerometer, Hall effect sensor, capacitance sensor, tap or shock sensors, and the like. The displacement sensor  316  measures an appropriate parameter, such as, for example, pressure, when the radon monitor  700  has been activated. The radon monitor  700  then sets the measured parameter as a base. The processing module  736  takes subsequent measurements of the parameter (e.g., pressure) against the base. Deviations of the subsequent parameter can be indicative of a movement of the radon monitor  700 . In some embodiments, when the displacement sensor  316  senses that a subsequent parameter deviates from the base, or deviates from the base by a preset amount, the processing module  736  and/or the displacement sensor  316  generate(s) a tampering signal and stores the tampering signal. In some embodiments, the radon monitor  700  will terminate operation upon sensing a tampering signal associated with the displacement sensor  316 . 
     In some embodiments, the tamper proofing system  300  also includes a key monitoring module  320  to monitor key touches or changes after the radon monitor  700  has been activated. For example, after the monitor  700  has been activated, if there are unauthorized changes in the input unit  744 , the key monitoring module  320  can be set to generate a tampering signal. When the processing module  736  receives the tampering signal, the processing module stores the tampering signal. In some embodiments, the processing module  736  may activate the output unit  748  to indicate an appropriate output through the display  756  or the sound circuit  760 . In some embodiments, the radon monitor  700  also includes a lockout function that would require a user to enter a keycode via the input unit  744  to unlock and allow manipulation of the radon monitor  700 . Without a proper keycode, the radon monitor  700  remains locked and may record an event associated with an improperly entered keycode. 
     It should be understood that recording, analysis, and computing of the data generated by these various tamper proofing capabilities of the radon monitor  700  can be performed by internal or external software, such as, for example, software stored in a personal computer, or other computing device, rather than by on-board components of the radon monitor  700 . 
     With reference to  FIGS. 4-8 , the radon monitor  700  represented in the system block diagram of  FIG. 1  is illustrated in an exemplary application that is not meant to be limiting. The monitor  700  is capable of having different configurations and applications. The monitor  700  includes a housing  900  having a first portion  902  and a second portion  904  connectable together to collectively form the housing  900  and define a cavity  906  within the housing  900 . Various components of the monitor  700  are positioned in the cavity  906 , detailed hereinafter. In the illustrated embodiment, the display  756  is supported by the housing  900  in a window defined in a front surface  908  of the housing  900 . In some embodiments, the display  756  is a liquid crystal display (“LCD”) having a two line, thirty-two character configuration. Alternatively, the display  756  could be an LCD having different configurations. Also, in other embodiments, the display  756  could be a variety of other types of displays such as, for example, an array of light emitting diodes (“LED&#39;s”) or a PC type computer connected to the radon monitor  700  via a serial or parallel interface or a wired or wireless interface such as those described above. In the illustrated embodiment, the input unit  744  is supported in the front surface  908  of the housing  900  and is a depressible actuator  752 . In other embodiments, the actuator  752  may be a slidable actuator, a rotatable actuator, a touch screen, a toggle switch, a keypad, and the like. In embodiments where the input unit  744  is a touch screen, the touch screen may substitute for the display  756  and perform all the functions and operations performed by the display  756 . The input unit (i.e., the actuator  752 ) has many operations including, for example, but not limited to, turning the radon monitor  700  on and off, establishing time periods for the timing module  772  (e.g., showing a two day radiation level average or a thirty day radiation level average), transferring the radon monitor  700  between the sleep mode and awake mode, resetting and/or re-initiating operation of the radon monitor  700  when a new test is desired or when the radon monitor  700  is moved to a new location, resetting user programmable parameters, disabling the sound alarm for low battery or radon action level exceeded permanently or for a limited period of time, entering a keycode to lock and unlock the radon monitor  700 , and the like. 
     With particular reference to  FIGS. 5 ,  7 , and  8 , the monitor  700  includes a plurality of openings in the form of louvers  910  defined in a rear surface  912  of the housing  900  for allowing air from the environment to diffuse into and out of the housing  900 . The housing  900  defines a receptacle  914  near a bottom thereof for receiving and supporting the power supply module  764 . As indicated above, the power supply module  764  in the illustrated embodiment is at least one battery  764 . A cover  916  is removably connectable to the housing  900  over the receptacle  914  to selectively cover and uncover the receptacle  914  in order to secure the power supply module  764  in the receptacle  914  or facilitate removal and replacement of the power supply module  764 . The power supply module  764  is a low-voltage power supply module capable of providing the necessary power for all operations of the monitor  700 . By having a low-voltage power supply module  764  on-board, the monitor  700  is easily portable and can be placed in environments without AC power sources available, accessible, or inconveniently located. In some constructions, the power supply module  764  includes four (4) C-type batteries for powering the monitor  700 . In such constructions, the four (4) C-type batteries each have a nominal voltage of 1.5 volts and, when connected in series, provide 6 volts to power the monitor  700 . In other constructions, the power supply module  764  may be other numbers of batteries, including one (1), and other types of batteries, fuel cells, or other self-contained sources of electrical power for powering the monitor  700 . As described above, the monitor  700  includes an awake mode and a sleep mode. In the awake mode, the monitor  700  draws typical amounts of power from the power supply module  764  to conduct normal operations. In the sleep mode, the monitor  700  draws lower amounts of power, or no power at all, and the monitor  700  typically enters into the sleep mode after a predetermined or programmable period of time. The sleep mode assists in extending the operational time of the radon monitor  700  by decreasing the power demand on the power supply module  764 . As an example, if the power supply module  764  comprises four (4) C-type batteries and the radon monitor  700  is operated under normal operation, the radon monitor  700  should operate for a minimum of about forty-five (45) days or a maximum of about one (1) year. 
     Referring now to  FIG. 9 , a cross-section of the monitor  700  is shown with the second portion  904  of the housing  900  removed. The monitor  700  further includes a circuit board  918 , which is positioned in and supported by the housing  900  by upper and lower support members  920 ,  922 . The sampling unit  712  includes a sampling chamber  924  positioned in the housing  900  and connected to a rear surface of the circuit board  918 . In the illustrated construction, the sampling chamber  924  is semi-spherical in shape and has an outer surface and an internal surface, which defines a sampling cavity  926 . In other constructions, the sampling chamber  924  can be other shapes, such as, for example cubical, conical, polygonal, and the like, and can be supported within the housing  900  in other manners. In some embodiments, the sampling chamber  924  is made of highly ferrous metal. Alternatively, the sampling chamber  924  can be made of other materials that block alpha particles and are air tight. The sampling chamber  924  is passive in that it does not require power and, accordingly, is not powered by the power supply module  764 . Since the sampling chamber  924  is passive, the chamber  924  does not draw, attract, or otherwise influence radon gas or alpha particles into the sampling cavity  926  defined by the sampling chamber  924 . Thus, the monitor  700  relies on diffusion of air and radon gas into and out of the sampling cavity  926  in order to test air in the environment. The sensing unit  716  is positioned in the sampling cavity  926  and only takes readings of alpha particles present in the sampling cavity  926  by decaying radon gas. In other words, any alpha particles present outside of the sampling chamber  924  are not detected by the sensing unit  716 . The sampling chamber  924  also inhibits radio frequency (RF) from interfering with the sampling occurring in the sampling chamber  924 . 
     To efficiently determine the radon levels present in the sampling cavity  926 , given that radon levels are measured with respect to time and to volume of air, and that a relatively small radon monitor  700  enhances its portability, the sampling cavity  926  is designed to have a volume that is a binary fractional portion of fixed reference volume, such as one liter. For example, the sampling cavity  926  can be designed to have a volume that is a quarter (¼) of a liter, which is 250 cm 3 . In such a case, an actual radon reading can be obtained by multiplying the radon level present by a factor of four to translate the radon level into annihilations per liter. In the illustrated embodiment, the sampling cavity  926  has a volume of 1/64 th  of a liter and no portion of the sampling chamber  924  defining the sampling cavity  926  is spaced greater than 1.5 centimeters (cm) from the sensing unit  716 . Alternatively, the sampling cavity  926  can have different volumes and portions of the sampling chamber  924  can be spaced different distances from the sensing unit  716 . This illustrated embodiment of the sampling cavity  926  provides a volume of about 15.625 cm 3 . In such cases, the actual radon reading is obtained by multiplying the radon level present in the volume of 15.625 cm 3  by a multiplier of 64. In digital processing of binary signals, the multiplier 64 is itself a power of 2, or 64=2 6 . As such, multiplication of 64 can be done efficiently with binary shifting. That is, shifting the binary signals indicative of the radon level present in the sampling cavity  926  six times will result in an actual radon reading with respect to a liter of air. In most cases, using binary shifting to multiply is more efficient than other types of multiplications performed by a controller. In other embodiments, other binary factors can also be used. In yet other embodiments, the processing module  736  can perform other types of multiplications of the present radon level to obtain an actual radon level reading. 
     Furthermore, essentially all parts in the sampling chamber  924  are within a predetermined radius of the sensing unit  716 . In the illustrated embodiment, as described earlier, the sampling cavity  926  of the sampling chamber  924  has a shape of semi-sphere and a radius of about 1.5 cm with the sensing unit  716  being a center of the sampling cavity  926 . Alternatively, the sampling cavity  926  can have different volumes and portions of the sampling chamber  924  can be spaced different distances from the sensing unit  716 . 
     With reference to  FIGS. 9 and 10 , the diffusion units include a plurality of apertures  708 A and an optional membrane or filter  708 B. The apertures  708 A are defined in the circuit board  918  in close proximity to the sensing unit  716  and the location where the sampling chamber  924  is connected to the circuit board  918 . The apertures  708 A allow air to diffuse through the circuit board  918  and into and out of the sampling chamber  924 . The filter  708 B may optionally be attached to the circuit board  918  over the apertures  708 A to reduce entry of undesired debris, such as dust, smoke, and the like, from entering the sampling chamber  924 , while allowing air and any radon gas in the air to pass therethrough and into the sampling chamber  924 . Alternatively, the filter  708 B, if used, may be supported within the housing  900  in a different manner as long as the filter  708 B is positioned to prevent entry of undesired debris into the sampling chamber  924 . 
     Although the illustrated embodiment includes a single sampling chamber  924  and sensing unit  716 , other embodiments may include additional sampling chambers  924  and sensing units  716  positioned on the same circuit board  918  or a different circuit board. 
     The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. Although particular constructions of the present invention have been shown and described, other alternative constructions will be apparent to those skilled in the art and are within the intended scope of the present invention.