Radon control system

A detector of ambient radon concentrations in real time comprising a housing, at least three conductive elements one of which being grounded, another being biased with respect to the other conductive elements, and at least one other conductive element being connected in series with an amplifier circuit and a counter circuit, respectively. In operation, when an energized alpha particle enters the housing, it ionizes air molecules, thus producing ions which are collected by the conductive element by virtue of the potential between that element and the other elements. The voltage pulse is amplified by the amplifier circuit and then counted.

FIELD OF THE INVENTION 
The present invention relates generally to the technology of radon 
detectors and particularly to radon detectors that continually monitor 
radon levels in ambient air. 
BACKGROUND OF THE INVENTION 
It is well known that radon, a radioactive gas formed by the radioactive 
decay of radium, actinium, or thorium, poses a significant health hazard 
to humans if they are exposed to radon gas in quantity. Recently, it has 
been discovered that radon gas seeps into homes and other structures from 
radioactive element sources found in the earth. Once in the home, the 
radon attaches to air-borne particles which may be inhaled. Continual 
exposure to radon over four picoCuries per liter of ambient air breathed 
is believed to significantly increase the incidence of lung cancer. 
It is well known that adequate ventilation of lower floors of structures 
alleviates the harmful radon concentrations. Thus, most homes with unsafe 
radon concentrations may be made livable merely with adequate ventilation. 
Currently, there are four ways in which to detect and determine radon 
concentrations. All of these methods are derived from alpha particle 
detection methods which are: the scintillation counter, the gas counter 
including both Geiger and proportional types, the solid state junction 
counter, and the activated charcoal detector. 
The scintillation counter wa historically the first utilized in early 
experiments on radioactivity. The scintillator was viewed with a 
microscope to count the individual flashes of light produced by each alpha 
particle stopping event. In the modern scintillation type counter, the 
scintillation material is disposed on a photocathode of a photomultiplier 
tube which amplifies the signal and provides information about the energy 
of the alpha particle. An analyzer is then required for analysis of the 
analog signal from the multiplier to count the alpha particles present. 
The coating of the scintillator must be opaque to prevent the admission of 
ambient light into the system. To accomplish this, the coating must be 
very thin making it susceptible to scratches which lead to "light leaks". 
The use of a scintillator for radon detection has heretofore been 
described by Madnick et al, U.S. Pat. No. 4,984,535 issued Jan. 16, 1990. 
The Madnick device, as described, would be expensive to manufacture, would 
be susceptible to "light leaks", and would require calibration. 
Gas-filled alpha particle detectors use a specific gas as the detector 
material depending upon whether the mode of operation is a Geiger counter 
or an ionization/proportional counter. In either case, the working gas for 
alpha or radon detection is hermetically sealed. Entrance to the 
ionization zone by the incoming alpha particle is through a thin, fragile 
plastic or metal window. The output signal pulse is constant in the Geiger 
counter operation but is related to the energy of the alpha particle in 
the ionization and proportional counter operations. The existence of 
delicate windows for the entrance apertures for the incoming alpha 
particles make the ga filled counters unsatisfactory for continual use 
because the window may be easily damaged. 
The junction counter is a solid state p-n junction with a reverse bias 
which collects ionization charges from passage of an alpha particle 
through the depletion layer. It can be made compact and portable. The 
limitation of the junction counter lies in the stringent requirements for 
avoiding scratching and abrasion of the metallic electrode surface of the 
detector. This electrode is light sensitive and the coating serves to 
block ambient light; thus it can be easily scratched resulting in a "light 
leak". Equally important, the active surface must be free from moisture 
and dust. 
Another means for detection of radon concentrations is the activated 
charcoal detector. This method, however, is not adaptable for continual 
monitoring of radon concentrations in real time. 
From this it is evident that the radon detection methods, currently 
available, have intrinsic deficiencies which make them impractical to 
monitor ambient radon concentrations in real time. The present invention, 
however, remedies these deficiencies by providing a rugged radon detector 
that monitors ambient radon concentrations in real time. Further, the 
present invention needs no calibration means to distinguish between alpha 
particles, beta particles or gamma rays as the present invention is only 
capable of monitoring alpha particle disintegrations from radon as well as 
from other radioactive sources. 
SUMMARY OF THE INVENTION 
It is the primary objective of the present invention to provide a low cost 
radon detector capable of continually monitoring ambient radon 
concentrations in real time. Another objective of the present invention is 
to warn the occupant of a monitored area of unsafe radon concentrations or 
to alleviate an unsafe radon concentration by ventilating the area 
automatically. 
The real time ambient radon detector disclosed herein, in its preferred 
embodiment, includes a housing; a plurality of wire screens interspaced 
and fitted in the housing, the outer most wire screen being grounded, at 
least one inner wire screen being connected to a power source, and a 
separate medial wire screen being connected to an amplifier circuit; and a 
counter circuit in series with the amplifier circuit. 
In operation, the voids in the wire screens permit the ambient air 
including the alpha particles emitted by radon gas to enter the housing. 
The inner wire screen is biased negatively with respect to the outer 
grounded wire screen. Therefore, when alpha particles enter the chamber 
through the voids in the wire screens, the alpha particles release energy 
within the chamber producing ions in the ambient air. The ionization of 
the air by the alpha particles, therefore, is converted to an electric 
charge by the applied voltages. Electric charge accumulates on the 
separate medial wire screen producing a voltage pulse at the input of an 
integrated circuit. After amplification, the resulting voltage pulse is 
distributed to the counter circuit which, in turn, is distributed to the 
relay circuit. Each energetic alpha particle which enters the housing and 
which is stopped produces an individual voltage pulse. Due to this one to 
one ratio, no calibration of the device is necessary. The device ignores 
low LET particles such as beta-rays and gamma-rays as has been 
experimentally varified. 
Because the present invention needs no delicate surface as found in other 
real time alpha particle and radon detectors, the detector disclosed 
herein can be ruggedly and compactly constructed, is portable, is simple 
to operate and maintain, and detects alpha particles with a hundred 
percent efficiency meaning that the present invention is capable of 
detecting every alpha particle entering the chamber.

DETAILED DESCRIPTION THE PREFERRED EMBODIMENT 
With reference to FIG. 1, energetic alpha particles enter the cylindrical 
housing 100 through wire screens 101, 102, 103, 104, 105. The outer wire 
screens 101 and 105 are grounded and the inner wire screens 102 and 104 
are biased with respect to the outer wire screens 101 and 105 and the 
medial wire screen 103 by a battery 160. The potential difference between 
the inner wire screens 102 and 104 and the medial wire screen 103 produces 
an electrical field which collects the negative ions on the medial wire 
screen with the battery polarity shown. Ions collected on the medial wire 
screen 103 produce a voltage on the medial wire screen 103 which is given 
by the formula: 
EQU Vp=E(eV)/32.times.1.6.times.10.sup.-19 .times.1/C Volts (1) 
wherein E is the energy lost by the radon particle in the detector and C is 
the capacitance of the collector. The energy E is typically 3 MeV and the 
capacitance C is approximately 2.times.10.sup.-12 F. Consequently, the 
typical pulse height is 7.5 mV. This pulse decays with a built in time 
constant .uparw.=RC, wherein R is the resistance of the integrated circuit 
130. The voltage pulse from a radon particle collection event is delivered 
to the base of the integrated circuit which is connected in an emitter 
follower configuration so that an amplified signal is delivered through 
output lead 151. 
The bias potential with respect to the chassis is imposed on the inner wire 
screen 102 and 104 to eliminate extraneous currents which may be created 
by chemical vapors and moisture and which may effect the input bias. 
Therefore, this feature makes the invention particularly applicable to 
environments with high moisture, chemical vapor or particle laden 
atmosphere such as may be found in a basement of a house. 
Once the voltage pulse is amplified it is counted by a counter circuit 170. 
The counter circuit then adds the counts of radon particle disintegrations 
for a predetermined time period. If the count of radon particle 
disintegrations exceeds a predetermined number for a predetermined time 
period, a signal is then sent to the relay 180 which activates the exhaust 
fan 190. The predetermined number may be any number; however, it is 
recommended that it be no greater than the number of radon particle 
disintegrations that would occur for a concentration of radon equal to 4 
pico Curries per liter of ambient air or 0.00222 disintegrations per 
minute per cubic centimeter. 
FIG. 2 shows the connection diagram for an electrometer integrated circuit, 
for example ICH 8500, which is well suited for incorporation into 
embodiments of the present invention. Terminal T1 is connected through 
resistors 46 and 49 to terminal T2 and the input 47. Example values of 
resistors 46 and 49 are 10.sup.10 and 5.times.10.sup.4 ohms, respectively. 
Resistor 49 is a potential dividing resistor in a zero adjust rheostat. 
Terminals T3 and T8 are connected to a ground. A shielded cable output 51 
is connected to terminals T5 and T6. 
The present invention as described above will detect and count all alpha 
particles including those from radon and from other radioactive sources as 
well as other heavy particles from sources such as cosmic radiation. 
Although these particles will effect humans in all the same manner, it may 
be advantageous to detect only radon disintegrations. FIG. 3 is a diagram 
of a radon detector embodying the present invention which exclusively 
detects radon disintegrations. As shown, there are two radon detectors 
fabricated substantially in the manner described above. One detector is 
permitted to operate in ambient air and the other is hermetically sealed 
and left inoperational for approximately two weeks. The hermetically 
sealed detector must be cleared of radon to function correctly. Because 
222 radon has a half life of 3.8 days, the detector must be inactivated 
for a duration of time in order to permit any radon trapped in the 
hermetically sealed detector to decay. Once the sealed detector is cleared 
of all radon, it will detect all disintegrations caused by heavy particles 
excluding radon. The detector exposed to ambient air, however, will count 
all the ion discharges caused by the disintegration of heavy particles 
including radon disintegrations. The difference between the counts of the 
respective detectors will then be equal to the number of radon 
disintegrations in ambient air. 
As shown in FIG. 3, the hermetically sealed detector 30 may be constructed 
such that the housing 300 is formed by any air tight material, such as 
lucite piping sealed at either end by a sealant and lucite paneling. The 
grounded outer screen 301 is then formed within the housing 300. The inner 
biased screen 302 is formed within the outer screen as shown and biased by 
a power source 306 with the polarity shown. The medial screen of FIG. 1 
may be replaced by a conductive rod 305 concentrically arranged within the 
housing 300. The conductive rod 305 is then connected to an 
integrated/emitter follower circuit 320, as described above, which in turn 
is connected to the counter circuit 330, relay circuit 340 and exhaust fan 
350. The detector exposed to ambient air 31 varies from the hermetically 
sealed detector 30 only in the respect that the housing 400 permits the 
free flow of ambient air. As illustrated in FIG. 4, this may be 
accomplished through the use of rods 410 connecting a floor 411 and 
ceiling 412. 
In operation, the hermetically sealed detector 30 will produce voltage 
pulses only from the disintegration of heavy particles excluding radon 
sources; the exposed detector 31, however, will produce voltage pulses for 
all heavy particle disintegrations. After the respective voltage pulses 
are amplified by the respective integrated circuits 320 and 321, the 
voltage pulses are counted by respective counter circuits 330 and 331. 
Thereafter, the differences of the respective counts is calculated by 
processor 360. If the difference of the counts is greater than a 
predetermined number for a predetermined time period, the relay circuit 
340 is triggered which, in turn, activates the exhaust fan 350. 
Although the alpha particle detector and/or radon detector as described 
above is susceptible to microphonic interference (due to the suggested 
integrated circuitry), it is not anticipated that this interference will 
pose any problem for the function of the preferred embodiment of the 
present invention. However, in areas such as commercial settings that 
generate microphonics it would advantageous to compensate for this 
interference. 
FIG. 5 is an alternate embodiment of the present invention that compensates 
for microphonic interference. As illustrated, the present invention may be 
altered by dividing the medial screen of FIG. 1 or the conductive rod of 
FIG. 3 into at least three different portions. Because microphonic 
interference would effect all three portions of the medial screen of FIG. 
1 or the conductive rod of FIG. 3 simultaneously and because ionization of 
alpha particles would, at most, effect only two of the portions of the 
medial screen or conductive rod, the voltage pulses caused by microphonic 
interference can be eliminated by selectively eliminating those voltage 
pulses emanating simultaneously from all the portions of the medial screen 
or the conductive rod. 
As illustrated in FIG. 5, medial wire screen 105 of FIG. 1 or conductive 
rod 305 of FIG. 3 may be divided into three different portions 36a, 36b 
and 36c. The output of all three portions 36a, 36b and 36c, then, may be 
fed through a gate circuit 700 which selectively inputs a signal to a 
switch 710 when all three portions 36a, 36b and 36c produce a voltage 
pulse simultaneously. The signal from the gate circuit 700 would 
inactivate the switch 710, thereby stopping the voltage pulses caused by 
microphonic interference from being detected by the counter circuit. 
It is anticipated that there are numerous variations of the present 
invention with regard to the various instruments that may be activated by 
the relay circuit. These variations include alarm devices, both audible 
and visual, as well as various ventilation means proven to alleviate 
unsafe radon concentrations. Moreover, it is also anticipated the present 
invention may be utilized as a counter device with either analog or 
digital readout alone or in combination with the alarm or ventilation 
means. These variations are considered to be well known in the art and 
need no further elaboration. 
Obviously, many modifications may be made without departing from the basic 
spirit of the present invention. Accordingly it is understood by those 
skilled in this art that, within the scope of the claims appended herein, 
the invention may be practiced in manners other than those specifically 
described herein.