AU624349B2 - Device for monitoring radon exposure - Google Patents

Description

PcI: OPI DATE 01/06/89 AOJP DATE 06/07/89 APPLN. ID 29239 89 PCT NUMBER PCT/US88/03952 INTERNATIONAL APPLICATION PUBLISHED UNDER THE PAT'ENT COOPERAT'ION TREATY (PCT) (51) International Patent Classification 4 (11) rn nal R bli Nuber WO 89/ 04499 G01T 1/24, H01L 31/00 AI lo cnf g l 4 r G01T 1/24, H01L 31/00 A (43) rn nal lic n 8 May 1989 (18.05.89) (21) International Application Number: PCT/US88/03952 (81) Designated States: AT (European patent), AU, BE (European patent), BR, CH (European patent), DE (Eu- (22) International Filing Date: 4 November 1988 (04.11.88) ropean patent), FR (European patent), GB (European patent), IT (European patent), JP, KR, LU (European patent), NL (European patent), SE (European (31) Priority Application Number: 117,059 patent).

(32) Priority Date: 5 November 1987 (05.11.87) Published (33) Priority Country: US With international search report.

Before the expiration of the time limit for amending the claims and to be republished in the event of the receipt (71X72) Applicant .nd Inventor: GJERDRUM, David, M. of amendments.

[US/US]; 559 Barron Avenue, Palo Alto, CA 94306

(US).

(72) Inventor: DECUIR, Joseph 1002 Ventura Avenue, Albany, CA 94706 (US).

(74) Agents HAMRICK, Claude, S. et al.; Rosenblum, Parish Bacigalupi, 55 Almaden Blvd.. 5th Floor, San Jose, CA 95113 (US).

(54) Title: DEVICE FOR MONITORING RADON EXPOSURE

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(57) Abstract A device for monitoring radon exposure,. ding a cube-shaped, light-tight detection chamber (40) having approximately one-inch square sides, five sides of which each contain an integrated circuit alpha particle detector with a sixth side having fluid intake (42) and exhaust ports (46) and temperature (48) and humidity (50) sensors. Each integrated circuit device is responsive to detect alpha particles emitted by any decaying radon-222 atoms or daughter isotopes within the detection chamber. The detected results of each integrated circuit device are communicated to an external logic system which calculates the relative level of radon concentration within the chamber and stores the information in memory, or reports the information to a variety of peripheral devices.

4 WO 89/04499 PCT/US88/03952 Specification Device for Honitoring Radon Exposure' -i |r BACKGROUND OF TH INVENON Field of the Invention The present invention relates generally to radiation detection systems, and more particularly, to a system for monitoring radon-222 exposure within a number of different sample environments under a variety of conditions.

Discussion of the Prior Art The uranium Mining industry has long known that radon-222 gas will collect in various concentration levels at or near the surface of the earth above naturally occurring uranium 238 deposits.

Accordingly, a large number of radiation detectors have been developed over the last decade to measure the concentration of radon-222 gas in the atmosphere above the earth or in the ground just below the earth's surface.

Radon-222 is a radioactive gas with a balf-life of 3.825 days and is generated during the radioactive transformation of uranium 238. During the initial decay of radon-222 to polonium-218, radon atoms will emit alpha particles having approximate voltages of 1eV (megaelectron volts). The quantity of these alpha particles is directly proportional to the level of radon concentration within the imediate environment. Thus, measuring the quantity of alpha particles within a certain area has long been an accepted method of 4k WO 89/04499 PCT/US88/03952 -2- I determining the level of radon concentration, (see U.S. Patent No.

2 3,665,194).

3 One prior art method of detecting alpha particles emitted by 4 decaying radon-222 'ithin a certain environment consists of positioning an alpha particle detector within a housing and 6 selectively passing air or gas across the surface of the detector, 7 thereby allowing the detector to interact with the alpha particles 8 Patent Nos. 4,342,913 and 4,607,166). Other prior art 9 devices use filters to capture radon-222 atoms and/or daughter isotopes from the surrounding environment so that the alpha 11 particle emitting matter can be disposed in close proximity to the 12 radiation detector. Related prior art devices are disclosed by 13 U.S. Patent Nos. 3,968,37i and 3,558,884, as well as U.S. Patent 14 No. 4,618,860. (See also U.S. Patent Nos. 4,417,142; 4,426,575; and 4,468,558, all issued to Kristiansson et al, for similarly 16 related devices). The type of radiation detection devices which 17 have been used in prior art devices has varied greatly. A number 18 of these devices, such as those using a chamber inserted in the 19 ground, have used a scintillation counter Patent No.

4,362,014), a nuclear track detector foil Patent No.

21 4,385.236). a phosphor screen in combination with a photo 22 multiplier Patent No. 3,056,886), or a semiconductor detector S23 Patent No. 4,104,523, issued August 1, 1978, to Volfert).

S24 When charged particles move through a semiconductor of a 25 semiconductor device they lose kinetic energy, primarily through S26 ionization processes, just as they do when they move through the WO 89/04499 PCT/US88/03952 -3- 1 gas of a gas detector. By developing electric fields across a 2 depletion region of an n-type/p-type semiconductor, a sensitive 3 region can be developed. When an ionization particle traverses the 4 sensitive region it produces electron-hole pairs that are swept away by the electric field and thereby produce an electric-current 6 pulse which can be detected and measured. Although such devices 7 are known, their detection accuracy has not been high and the cost 8 of adequately sized and accurate detectors has been prohibitive.

9 Although prior art radon-222 detectors have proven capable of detecting levels of radon concentration above or in the ground, 11 such devices are restricted as to the type of different 12 environments or different conditions in which they can be used. In 13 addition, such devices have had little utility outside of the 14 scientific environment because the devices reported levels of radon concentration in terms which carried little or no meaning for most 16 persons. A need has arisen for a radon-222 detector which is 17 capable of operating in a number of different environments under a 18 variety of different conditions and reporting the levels of radon 19 concentration in easily understandable terms.

Events occurring in the past decade have lead to greater 21 concerns regarding the potential environmental human health hazards 22 presented by human produced and naturally occurring radioactive 23 sources in the vicinity of the home and work place. The high- 24 energy levels of alpha particles, which are much higher than the othei by-products of atomic break-down, such as beta particles S26 (electrons) and gamna radiation (photons), can cause chemical i i WO 89/04499 PCT/US88/03952 -4- 1 reactions to occur in virtually any object, when that object is 2 struck by an alpha particle. In some objects the displacement of 3 electrons may not prove to be consequential since atoms are simply 4 displaced with no resulting change in atomic structure. In the human body, however, the displacement of a single electron or atom 6 can cause a local chemical reaction which may eventually result in 7 cancer formations. In view of the above, the Environmental 8 Protection Agency has recently issued a warning concerning the 9 cancerous effect of certain levels of radon-222 concentration in the home and work place.

11 12 SUMMARY OF THE PRESENT INVENTION 13 It is therefore an object of the present invention to provide 14 a system for accurately monitoring the presence of radon-222 in the home and work place and for reporting the levels of radon 16 concentration in easily understandable terms.

17 Another object of the present invention is to provide a 18 system for monitoring the presence of radon-222 that is adaptable 19 to use within a gaseous or liquid environment.

Still another object of the present invention is to provide a 21 system for monitoring the presence of radon-222 that is adaptable 22 to use with a variety of different types of alpha particle 23 detection devices.

24 A further object of the present invention is to provide a system for monitoring the presence of radon-222 which is capable of i

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WO 89/04499 PCT/US88/03952 1 restricting the flow of the fluid of the sampled environment so as 2 to maintain the greatest likelihood of alpha particle detection.

3 A still further object of the present invention is to provide 4 a system for monitoring the presence of radon-222 in which an integrated circuit device may be used for the accurate and 6 controllable detection of alpha particles.

7 A still further object of the present invention is to provide 8 a system for monitoring the presence of radon-222 which can be 9 affixed within a building or used as a mobile, personal dosimeter.

Another further object of the present invention is to provide 11 a system for monitoring the presence of radon-222 capable of 12 providing updated results indicating changing levels of radon-222 13 concentration, in dosimetric terms, for observation, storage, or 14 use by other systems.

yst 16 includes a cube-shaped, light-tight detection chamber having 17 approximately one-inch sa re sides, five sides of which each 18 contain an integrated circu alpha particle detector, with a sixth 19 side having fluid intake and e haust ports and temperature and humidity sensors. Each integrat d circuit device is responsive to 21 detect alpha particles emitted by ay decaying radon-222 atoms or 22 daughter isotopes within the detection chamber. The detected 23 results of each integrated circuit devc is communicated to an 24 external logic system which calculates the elative level of radon conctntration within the chaber and stores the inforation in-

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1 According to the present invention there is provided a device for monitoring radon exposure, comprising: means forming a photon impenetrable detection chamber of a predetermined volume for continuously receiving a sampling flow of fluid from an environment being monitored; at least one alpha particle detector being an integrated circuit disposed within said chamber, said detector having a plurality of radiation sensitive capacitance means positioned to be exposed to alpha particles within said sampling flow, said alpha particle detector being responsive to said alpha particles contacting said capacitance means and operative to develop a detection signal; and a monitoring system for receiving a plurality of input signals including said. detection signal, calculating the level of radon concentration within said environment, and generating an output signal commensurate therewith.

A preferred embodiment of the monitoring system includes a cube-shaped, light-tight detection chamber having approximately one-inch square sides five, sides of which each contain an integrated circuit alpha particle detector, with a sixth side having fluid intake and exhaust ports and temperature and humidity sensors. Each integrated circuit device is responsive to detect alpha particles emitted by any decaying radon-222 atoms or daughter isotopes within the detection chamber. The detected resu]ts of each integrated circuit device is communicated to an external logic system which calculates the relative level of radon concentration within the chamber and stores the information in WO 89/04499 PCT/US88/03952- -6- 1 memory, or reports the information to a variety of peripheral 2 devices.

3 These and other objects of the present invention will no 4 doubt become apparent to those skilled in the art, after having read the following detailed disclosure of a preferred embodiment 6 which is illustrated in the several figures of the drawing.

7 8 IN THE DRAVING 9 Fig. i is a perspective view of an integrated circuit device which may be used in accordance with a preferred embodiment of the 11 present invention; 12 Fig. 2 is a diagram schematically illustrating the memory 13 cells and the cell output store of the internal electronic 14 structure of the integrated circuit device of Fig. 1; Fig. 3 is a diagram further schematically illustrating one 16 memory cell of the plurality of cells shown in the integrated 17 circuit of Fig. 2; 18 Fig. 4 is an exploded, partially broken, partially cut-away, 19 perspective view of a cube-shaped detection chamber in accordance with a preferred embodiment of the present invention; 21 Fig. 5 is a perspective view of the integrated circuit device 22 of Fig. i, having an attached chamber over the detection element, 23 in accordance with a first alternative embodiment of the present 24 invention; I 25 Figs. 6A and 68 illustrate some of the physical constraints 26 on the shape of the detection chamber in accordance with both a r~ -i t i: WO 89/04499 PCT/US88/03952 -7- 1 preferred and first alternative embodiment of the present 2 invention; 3 Fig. 7 illustrates some of the physical constraints on the 4 shape of a detection chamber in accordance with a second alternative embodiment of the present invention; 6 Fig. 8 is a block diagram illustrating a monitoring system in 7 accordance with a preferred embodiment of the present invention; 8 Fig. 9 is a block diagram further illustrating the system 9 control of the monitoring system depicted in Fig. 8; Fig. 10 is a diagram schematically illustrating the 11 electronic structure of a solar cell alpha particle detector in 12 accordance with a third alternative embodiment of the present 13 invention; 14 Fig. 11 is an exploded, partially broken, partially cut-away, perspective view of a cube-shaped detection chamber in accordance 16 with a fourth alternative embodiment of the present invention; 17 Fig. 12a is a cross-section view of an alpha particle 18 detector in accordance with a fifth alternative embodiment of the 19 present invention; Fig. 12b is a perspective view of a scintillation counter 21 alpha particle detector in accordance with a fifth alternative 22 embodiment of the present invention; and 23 Fig. 13 is a perspective view of a Geiger counter alpha 24 particle detector in accordance with a sixth alternative embodiment of the present invention.

26 A l WO 89/04499 PCT/US88/03952 -8- 1 DETAILED DESCRIPTION OF A PREFERRED EMBODIENT 2 Radioactive particles of sufficient energy can cause 3 considerable disruption to any material which they encounter. As 4 previously stated, if alpha particles strike living matter, they can break apart chemical bonds in the intricate macro molecules 6 resulting in local injury or even genetic damage. Similarly, in 7 complex artificial crystal structures, such as common commercial 8 integrated circuits, alpha radiation can knock electrons or entire 9 atoms out of place, resulting in either temporary erroneous operation or permanent damage.

11 The known vulnerability of integrated circuits to radiation 12 has been a major problem in the art of digital memory devices.

13 especially with regard to dynamic random access memory (DRA). If 14 some of the steps taken to avoid radiation problems with conventional integrated circuit devices are reversed, the radiation 16 detection ability of the integrated circuit device can be greatly 17 enhanced. The later type of integrated circuit device would be 18 ideal for use in accordance with a preferred embodiment of the 19 present invention, as is illustrated in Fig. 1. in place of less accurate and more expensive prior art semiconductor detectors.

21 In Fig. i, the integrated circuit 2 is shown in a 22 conventional chip carrier package, having an interactive detection 23 area 4, which contains a vast amount of electronic components 24 internally connected to the signal pins 6. The signal pins 6 can I 25 have either extended pins (PLCC) or embedded pins (LCCC) which 26 provide means for communicating information to and from the _i I i i iii i WO 89/04499 PCT/US88/03952 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 integrated circuit 2, to matching interface pins within socket 8.

The integrated circuit 2 can be inserted or removed from the socket 8 as desired without need for soldering the signal pins 6 to the interface pins. A registration notch 9 is also provided in the chip carrier package for correct orientating of the chip during use.

Although the preferred embodiment of the present invention utilizes a dynamic memory integrated circuit, a DRAh, almost any type of alpha particle sensitive device may be utilized in accordance with the present invention, as is further discussed below. To fully understand how the integrated circuits of the present invention differ from prior art circuits, it is first necessary to understand the internal characteristics of integrated circuits. Dynamic memories are generally comprised of a large quantity of small capacitors (memory cells), each of which is capable of retaining a specific charge representing particular information. A change in the charge or voltage, retained by one of these tiny capacitors can be representative of a state change in the memory, such as a change from a 0 to a 1, or a 1 to a 0.

While a state change can be electrically caused to occur at any time, state changes can also occur because of the capacitor's natural tendency to-leak voltage, or for other reasons. To prevent the loss of or change of information, each capacitor is periodically read to ascertain its current voltage level, and then rewritten with the same charge to assure the maintenance of the particular information retained by the capacitor; a process

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WO 89/04499 PCT/US88/03952 1 referred to as refreshing the memory. Rewriting will normally be 2 effective to correct both soft errors and firm errors, which may 3 occur in different integrated circuit devices. A firm error is 4 defined as a change in data occurring as the result of a transfer of charge, from the floating gate of a floating gate memory, by 6 ionizing radiation. Another type of error, a hard error, cannot be 7 corrected. Hard errors are a result of permanent damage to the 8 charge detection characteristics of the device. Hence, a device 9 must not only be able to recognize soft errors or firm errors, but also bard errors, and be able to remove the affects of the latter 11 from the system.

12 The lower the level of charge or voltage retained by a 13 capacitor, the greater the likelihood of that capacitor being 14 affected by radiation. If a capacitor already has a very small charge, the leakage of any of that charge will make the capacitor 16 even more vulnerable to radiation. Thus, a high energy particle, 17 such as an alpha particle, interacting with one or more of these 18 capacitors, or memory cells, may be sufficient to knock the charge 19 across a capacitor and cause a change in the data. The subsequent refresh cycle would simply read the new data and rewrite that same 21 information rather than the previous information, thereby merely 22 amplifying and preserving the error.

23 Prior art methods of compensating for the vulnerability of 24 DRAM circuits to radiation have included some of the following I 25 steps: j i 1 t

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WO 89/04499 PCT/US88/03952 -11- 1 improving the circuit manufacturing process to reduce 2 the leakage rate of the capacitors; 3 increasing the capacitor voltage to lessen the 4 likelihood, of radiation particles displacing charges stored in the capacitors; 6 refreshing the capacitors with greater regularity in 7 order to retain a higher average charge within the capacitor; and 8 reducing the level of radioactive material in the DIP 9 or chip carrier package in order to decrease the exposure of the capacitors to radiation sources. By reversing all but the last of 11 the above compensation methods, a DRAM circuit can be converted 12 into a highly efficient radiation detector.

13 Removing the plastic or ceramic protective coating over the 14 detection area 4, with which DRAM's are normally outfitted, will allow radioactive particles to come into direct contact with the 16 surface of the detection area 4. The DRAM can still be protected 17 from the environment by covering the exposed surface with a mylar 18 coating, or something similar, that will permit the passage of -19-.alpha particles (and their daughter isotopes). Operating the memory cells at lower overall voltages and reducing the refresh 21 rate will also increase the effect of any radiation coming into S22 contact with the detection area. It should be noted however, that 23 although the voltage level and refresh rate can be lowered, both 24 must still be sufficient enough to compensate for the thermal noise 25 level present in the memory cells. If the preceding steps are t:* WO 89/04499 PCT/US88/03952 -12- 1 taken, there will be a greatly enhanced likelihood that an alpha 2 particle 10 striking the detection area 4 will be detected.

3 In addition to the previously mentioned enhancement 4 techniques, other steps can be taken to produce an even better radiation detector. These additional steps may be better 6 understood by reference to Fig. 2, which schematically illustrates 7 the internal structure of the detection area 4. As stated, the 8 detection area 4 is comprised of a large quantity of tiny 9 capacitors, or memory cells 12, which are generally aligned in an orthogonal array of rows and columns, but which may alko be aligned 11 in any o.her suitable manner, such as in concentric circles. When 12 DRAM arrays are used for their conventional function, such as data 13 storage, a large portion of the area 4 is devoted to "word" and 14 "bit" access lines, rather than for bit storage capacitors. The ratio between storage capacitors and access lines in conventional 16 DRAM arrays may be as high as 1:1. By configuring the area 4 to 17 have a greater quantity of memory cells, or larger memory cells, 18 and thereby increasing the ratio of memory cell space to access 19 lines, the likelihood of detecting radiation can be increased by a proportional amount.

21 It should also be noted that if the direction in which a 22 charge is moved in response to the detected alpha particle is in 23 the same direction as the existing data charge in the capacitor, no 24 state change will occur and no radiation will be detected. In other words, if the state of a particular memory cell corresponds 26 to a 1, which ay for example be represented by some positive WO 89/04499 PCT/US88/0395 2 -13- 1 voltage level within the capacitor, and the alpha particle induces 2 a greater voltage level to be retained by the capacitor, the state 3 of the memory cell would still be 1, rather than moving from I to 4 0. Since there is no recognizable higher state of 1 in binary logic, there can be no net data charge change in such a situation 6 and thereflore no radiation detection.

7 If the distribution of the data in the memory cells 12 of the 8 detection area 4 is random, or the direction of induced charge in 9 the capacitor of the memory cell 12 has an equal opportunity for being in either direction, then no more than one half of all 11 radiation exposed to the detection area 4 will be detected. To 12 increase the radiation detection ability of the detection area 4.

13 each capacitor or cell must be designed to detect charge 14 differences in either direction, an effect which will essentially double the probability of radiation detection. Such a design may 16 be accomplished by designating each detector to have a quiescent 17 charge difference of zero (meaning the memory cell is inactive, or 18 held at neither the phase stats of 1 nor 0, whereby all charge 19 changes of sufficient.,size may be detected. The only limitations to such a design would be a result of the residual thermal noise 21 level within the detector and the sensitivity of the detector 22 circuit itself.

23 The quiescent charge is developed and maintained within the 24 cells 12 through means of the three voltage inputs 16. 17 and 18, 25 and are reported to a cell output store 20 through communication 26 line 22. It should be noted, however, that the cell output store ii:1 *i 1 i4 WO 89/04499 PCT/US88/03952 -14- 1 20 provided for in the preferred embodiment is not absolutely 2 necessary in all situations, provided some means is available for 3 sending out the state changes of the cells 12. In fact, the cell 4 output store 20 may actually be undesirable, because it requires space that would be otherwise available for detection cells. In 6 addition, because each cell in the cell array is read and refreshed 7 periodically, to a quiescent charge difference of zero, it may also 8 be necessary to clear the cell output store 20 of any stored memory 9 during each refresh cycle. If such is the case, the read and subsequent output of information simply eliminates the actual need 11 for a cell output store.

12 A detailed schematic illustration of a memory cell 12 from 13 detection area 4 is provided in Fig. 3. Each cell is comprised of 14 an integrated capacitor 30 for radiation detection, having a sense amplifier 32 wired to clamp a capacitor 34 to a certain voltage 16 level. Any charged induced across the capacitor 30 would be 17 detected and balanced by the sense amplifier 32. The output of the 18 sense amplifier 32 is input to a well known dual comparator circuit 19 36. which detects the level of activity in the sense amplifier 32 and reports the detected result to the cell output store 20 for 21 transmission to external circuitry over the signal pins 6 and 22 interface pins. Because soft errors represent detected radiation 23 in the memory cells 12, there is no need for soft error recovery 24 circuitry',thereby rtmoving a large quantity of the circuitry normally included within the detection area 4.

1 r 1 11 1 1 WO 89/04499 PCT/US88/03952 1 The usage of larger cells in the detection area 4 and the 2 elimination of much of what is currently utilized in such devices, 3 means inherently simpler cells and larger scale mask works. As a 4 consequence, the physical size of the detection area 4 in the preferred embodiment (1.2 Ch/side) may be much greater than the 6 size of the storage area in the conventional DRAM cir.uit 7 (generally only 0.3 CM/side). A considerable portion of the cost 8 associated with producing modern DRAM circuitry is a direct result 9 of the reduced size of the circuit, hence, larger and simpler circuits may actually be less expensive to produce than smaller 11 more complex circuits, provided sufficient quantities are produced 12 to pay for their development.

13 Once a more effective radiation detector has been developed, 14 it is necessary to design a detection chamber which fully utilizes the new detector. In designing an appropriate detection chamber, a 16 number of factors are of primary consideration. First, radiation 17 detection results must be reportable in dosimetric terms, such as 18 directly or derivatively from a base measure of curies/liter, based 19 on the fluid of the test environment, which may.be either air, water, or some other substance.

21 In order to produce results of alpha particle detection in 22 dosimetric terms, it is necessary to be able to calculate the 23 volume of the environment tested over a certain period of time.

S24 Given a normal concentration of radon-222 in air, an alpha particle S1 25 is emitted directly, or indirectly as a daughter isotope, from the 26 radon-222 decay at least once every 300 minutes/mili-liter of 1 1 1 1 1 1 WO 89/04499 PCT/US88/03952 -16- 1 sample environment. Thus, if the volume capacity of the detection 2 chamber and environment flow rate are known, and a time base is 3 available, such as a CPU clock, the detected results may be easily 4 reported in dosimetric terms.

Some of the additional factors which also need to be 6 considered when attempting to accurately determine the 7 concentration of radon-222 are as follows: increased radon-222 8 levels accruing during the nocturnal lowering of the troposphere; 9 increased radon-222 levels caused by inversion layers; (3) increased radon-222 particles caused by the trapping of radon-222 11 levels within buildings; and the creation of higher 12 concentration levels of radon-222 in some areas of the building 13 because of air flow patterns within the building. Hence, in 14 addition to simply calculating the level of radon-222, the detection device must be able to systematically adjust any reported 4 16 results in accordance with the natural environmental changes that 17 occur within the same test period.

18 An additional consideration in the design of the detection 19 chamber concerns the visibility range of alpha particles emitted during the decay of radon-222 and its daughter isotopes.

21 Visibility range means the distance which an alpha particle will 22 travel before transforming into a different state or energy level.

23 In air, alpha particles have a visibility range of approximately 24 one inch, while in water the visibility range for an alpha particle i 25 is only approximately 1000th of the visibility range in air. or 26 around thirty micro-meters. The difference in visibility range is 1 'Y 1 1 1 1 1 1 i' i WO 89/04499 PCT/US88/03952 -17- 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 primarily due to the difference between the density of the two environments. Thus, a properly designed or tuned detection chamber will take into consideration the small visibility range of the alpha particles to be detected. An elongated tube, with a detector at the bottom, placed on or embedded in the ground, is not tuned for the particles being detected and cannot properly direct the flow of fluid past the detection area and therefore cannot achieve optimum results. Accordingly, it also may be desirable to equip the detection chamber with an environment density detector, the output of which can be used to correct errors in reported concentration levels caused by fluctuations in the density of the environment.

Fig. 4 illustrates an exploded, partially broken, partially cut-away, perspective view of a cube-shaped detector chamber 40 in which an integrated circuit 2 is centered in the sockets 8 in five of the six faces of the chamber 40. It should be noted, that only one DRAM, or detector 2, is shown in its complete chip carrier package form, while the remainder of the detectors are simply shown as being comprised of the detection area 4, in order to illustrate that a DRAM type detector 2 or any other type of surface radiation detector may be used in place of the DRAM detector 2. The sixth face of the chamber 40 is provided with an intake port 42, which is connected to a supply tube 44, and an exhaust port 46, which may be connected to an exhaust tube similar to the supply tube 44, for transporting fluid to and from the sample environment. In addition, the sixth side of the chamber 40 is provided with aI; i

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WO 89/04499 PCT/US88/03952 -18- I temperature sensor 48 and humidity sensor 50, which communicate 2 temperature and humidity changes to an external monitoring system 3 over lines 51 and 52, respectively. A humidity sensor is not 4 required for monitoring a liquid environment. Although the chamber 40 can have sides of any length or width desired (or shape), the 6 highest probability of detection will only be maintained if certain 7 rigid geometric principles are followed, which will be further 8 described below.

9 A first alternative embodiment of the detector chamber depicted in Fig. 4 is shown in Fig. 5, in which a domed, or 11 hemispherically shaped chamber 60 is placed directly over the 12 detection area 4 of the DRA 2. as indicated by the dashed lines 4.

13 Chamber 60 is affixed to the integrated circuit 2 to form a light- 14 tight chamber. Many integrated circuit packages are equipped with connectors for heat sinks to which the chamber 60 can be readily 16 connected, or the chamber 60 may be affixed in some suitable 17 manner. As in the chamber 40, the domed chamber 60 has an intake 18 port 62 connected to a supply line 64, and an exhaust port 66 19 connected to an exhaust line 67. Likewise temperature sensor 68 and humidity sensor 70 are also provided for within the chamber S 21 for communication to an external monitoring system over the 22 communication line 72.

23 Given that the alpha particles have a particular visibility 24 range and each detector surface has a distinct shape, the highest detection level can be achieved only when the detection chamber is 26 conformed to take advantage of these characteristics. Determination R 1 WO 89/04499 PCT/US88/03952 -l9- 1 @f the erret ehape of either the domed chamber 60 or the cube- 2 shaped chamber 40 may be calculated by reference to Figs. 6a and 3 6b. which illustrate some of the physical limitations on the 4 detection chambers. Fig. 6a illustrates a cross-section of detection area 4, which has a length, and a visibility range d of 6 alpha particles. If a radon-222 atom emits an alpha particle 7 within the range d and in the direction of detection area 4, then 8 the detection area 4 will be within the visibility range of the 9 alpha particle, and therefore capable of detecting the alpha particle's presence. Hence, the operable range of the detection 11 area 4 can be represented by the elliptically-shaped dashed-line 12 78, which is a distance d from at least one point on the surface of 13 the detection area 4 at all times.

14 A top view of the detection area 4, as shown in Fig. 6a, can be depicted as shown in Fig. 6b. where the detection area 4 has 16 sides x. and is bounded by an outline of the cube-shaped chamber 17 shown by the elliptically-shaped, dashed-line 80. The 18 .rcumference of the ellipse 80 is a distance d from at least one 19 point on the surface of the detection area 4 at all times. To understand the enhanced detection characteristics of the cube- 21 shaped chamber shown in Fig. 4, four of the sides of the cube, each 22 having a side length S and characterized by the long and short S23 dashed line are depicted in Fig. 6b. As can be seen from Fig.

i 24 6b, points within the chamber (shown by line 82) are within the range of the detection area 4, and further, the combination of the iI WO 89/04499 PCT/US88/03952 1 five detectors create overlapping ranges which enhances the radon- 2 222 detection capabilities of the detection chamber 3 As previously stated, the detection area 4. may be an 4 orthogonal array of rows and columns, or any other shape as may be appropriate. Hence. Fig. 7 illustrates a second alternative 6 embodiment of the detection chamber in which the detection area 4 7 is circular and the circumference of the range d from points on the 8 surface of the detection area 4 would form a spherically shaped 9 chamber centered about the detection area 4.

Due to the difference in visibility ranges for alpha 11 particles in air versus water, an acceptable detection chamber for 12 use in a water environment would be required to be much smaller 13 than a similar detection chamber used in an air environment.

14 However, the same geometric constraints applicable to the air environment chamber must be followed; only on a much smaller scale.

16 Likewise, it is to be understood that the integrated circuit, or 17 detector, as designed for this particular detection chamber would 18 also be sensitive to other forms of radiation, such as beta and 19 gamma (photons) radiation. To avoid problems caused by other forms of radiation, the chamber must be a light-tight chamber and 1 21 impervious to other forms of radiation. If the chamber is 22 constructed out of a non-radiation emitting and radiation blocking 23 substance, such as aluminum or an aluminum alloy, the detection 24 chamber can be used to effectively restrict alpha particles from being admitted. while not emitting alpha particles itself.

1 1 1 1 1 1 iI r-w WO 89/04499 PCT/US88/03952 -21- 1 As was previously mentioned, the detection area 4, as well as 2 other elements of the detection chamber and detectors, must be 3 protected from the corrosive effect of the environment in which 4 they are utilized. Thus. alpha particle passive filters, such as mylar films or covers, which were previously described, should be 6 used to protect the detection area from the test environment. It 7 should also be noted that use of a common chamber, that is, a 8 detection chamber which also has temperature and/or humidity 9 sensors, provides data of greater relevance than do systems utilizing sensors located outside of the test chamber.

11 Fig. 8 depicts a block diagram illustrating an external 12 monitoring system 100 in accordance with the preferred embodiment 13 of the present invention. Monitoring system 100 communicates over 14 bus 101 with detectors 102, which include radon detectors 2, and may also include temperature and humidity sensors or any other type 16 of I)ensing device. The monitoring system 100 includes a system 17 control 104 that is supplied with power through the "POWER IN" line 18 114 and, in turn, supplies power and timing signals throughout the 19 monitoring system 100 and to detectors 102. The system control 104 also directs the sampling of detectors 102, controls the operation 21 of memory 116, and the interaction with ventilation system 117 or S22 any peripheral devices 120 connected thereto. Ventilation system 23 117 receives fluid from the environment through input 118, cools, 24 heats, or filters the fluid and either recirculates it within the environment or exchanges it with some other source through exhaust 2i 6 19.

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WO 89/04499 PCT/US88/03952 -22- 1 Data collected by the detectors 102 is periodically sampled 2 from the detectors 102 by system control 104. The data is 3 subsequently channeled to the system memory 116 after the data has exy+ern<aoy fnpoor 4 been processed for storage. Additional configuration data, such as the time or constants to be used in determining the level of radon 6 concentration, can be entered from the peripheral devices 120 or 7 ventilation system 117 or storage by system memory 116.

8 The system control 104 can be better understood by referring 9 to Fig. 9, which further illustrates additional components of the system control 104 of Fig. 8. As stated above, data collected by 11 detectors 102 is periodically sampled by sampling processor 106 12 then communicated to data processor 108. where the processed data 13 is input to memory 116. If additional data is to be stored in 14 memory 116, it is input from either ventilation system 117 or peripheral devices 120 to I/O (Input/Output) processor 110 and then 16 communicated to memory 116 through data processor 108. In 17 accordance with the timing characteristics of system control 104, 18 data stored in memory 116 is periodically read from the memory 116 19 by data processor 108. Data processor 108 evaluates all of the data according to pre-programed algorithms in order to determine the 21 level of radon concentration in dosiitric terms. After 22 cai. ting the level of radon concentration, data processor 108 p 23 communicates the calculated data to I/O processor 110 which outputs S24 the data to ventilation system 117 or peripheral devices 120.

U 25 Under normal operating conditions. the calculated data would be automatically output from I/O processor 110 to peripheral yI t k WO 89/04499 PCT/US88/03952 1 devices 120, where the level of radon concentration can be reported 2 in readily (human) understandable terms, or without further 3 extrapolation. This process may be readily changed by entering 4 configuration data changes, such as timing or control changes, through peripheral devices 120 or ventilation system 117 to data 6 processor 108. Control changes would be modifications in the 7 configuration data or in the algorithms used by data processor 108, 8 or some similar change. Similarly, data processor 108 may be 9 programmed to ignore some of the data stored and transferred from memory 116, such as may be necessary when one sensor or detector is 11 determined not to be functioning correctly.

12 As described, the above system can be utilized as a radon 13 detector in either liquid or gaseous environments, such as wells 14 and water supplies, or buildings and mine shafts. If linked to the ventilation control system 117 of a building, or other type of 16 control system, the results of the radon detector can be used to 17 regulate the flow of fluid through an area in such a way as to 18 reduce the level of radon concentration, or sound an alarm if 19 dangerous levels have been achieved. Likewise, the above device can be made portable, so that a person, such as an uranium miner, 21 can be provided with accurate, easily understandable detection r 22 results.

23 Also described above, there are a number of different types 24 of alpha particle sensitive detection devices which can be utilized in accordance with the present invention. Semiconductor related 26 devices, such as solar cells and charge coupled devices (CCD), are WO 89/04499 PCT/US88/03952' -24- 1 particularly adaptable to use as alpha particle detectors. For 2 example, solar cells, which may be viewed as being comprised of 3 thin layers of n-on-p type semiconductors bracketed by ohmic 4 contacts, may be easily substituted for the DRAM integrated circuit described in the preferred embodiment. When a photon interacts 6 with the semiconductor surface of a solar cell, outer shell 7 electrons are excited by the photon, leaving energy holes which 8 when subsequently filled generate electron propagations along the 9 lattice to eventually produce a potential at the ohmic contacts.

Alpha particle interacting on the other hand, causes the electrons 11 to be temporarily stripped from the lattice, thereby inducing 12 short-lived 5-15 nanosecond), strong current pulses 13 (typically 20 microamperes) in the region of the interaction 14 (within 30 micrometers of silicon), which have a polarity opposite that induced by photons.

16 For maximum interaction with the sampled environment, the 17 surface of the solar cell should be directly exposed to the 18 environment. However, this unprotected semiconductor surface would 19 be highly vulnerable to the possibility of electromechanical changes which might result in permanent lattice damage. To avoid IJ 21 this possibility, solar cells can be coated with silicon dioxide, 22 but the coating should be as thin as possible, such as is found on 23 some of the solar cells utilized in space applications. Another S24 area ot concern is that interference created by thermal energy and electrostatic discharges from dust particles may result in some S26 noise that cannot be blocked out by the glass covering the solar l- 11 6 WO 89/04499 WO 8904499PCT/US88/03952 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 23 24 26 cell. However, signal discrimination of detected thermal energy is possible by electrically biasing the solar cells and carefully tracking the environment chamber's temperature. Likewise, the detection of simultaneous interactions amongst several solar cells in the same chamber can be used to discriminate signals caused by electrostatic discharge It should be noted th,--t since it is necessary to provide a constant electrical bias to the solar cells, if for no other re*jon then to overc~ome loadinig requirements at the ohmic contacts, power consumption management of the detector is required. However, this level of power consumption should not be detrimental to the utilization of this embodiment in most applications.

The design of a monitoring system utilizing solar cell alpha particle detectors does not significantly differ from the design utilized in the preferred embodiment. The sample chamber geometry for a solar cell based alpha particle detector would generally be the same as that for the preferred embodiment. However, attenuations in alpha particle visibility caused by the silicon dioxide coating and the large rectangular surface areas characteristic of solar cells do not necessarily favor a cubical sample chamber as the optimum use of sample volume space.

Although the chamber geometry may have to be modified, the electronic structure ot the alpha particle detector would be quite similar to-.,that called for in the preferred embodiment. As will be shown below, solar cells, when used in, this application, f unction with circuitry similar to the individual cells within the DRMI 4

P

WO 89/04499 PCT/US88/03952 -26- 1 array. Basically, the solar cells in this embodiment are 2 representative of an instance where discrete components are 3 substituted for integrated circuitry. In particular, the circuitry 4 of a solar cell detector would be basically the same as that depicted in Fig. 3 above except that integrated capacitor 30 has 6 been replaced by a solar cell circuit 130.

7 Cell circuit 130 is comprised of a solar cell 132 having a 8 surface area which is sensitive to alpha particles 10. In addition 9 to the solar cell 132, there is an equivalent circuit 134 comprised of diode 136. resister 138 and capacitor 140. which corresponds to 11 the electrical characteristics of solar cell 132 but which lacks 12 the capacity to have a current charge due to energy being imparted 13 upon its surface. Equivalent circuit 134 and solar cell 132 14 provide the inputs to sense amplifier 142 having an output which forms one input to the comparator circuit 36 of Fig. 3. Comparator 16 circuit 36 detects the level of activity in the sense amplifier 32 17 and reports the detected result. It should be noted that sense 18 amplifier 142 must be able to amplify the short duration current 19 pulses (5-16 nanoseconds) caused by alpha particle interactions.

Similar to the DRAi and solar cell is the charge coupled y 21 device (CCD), which is comprised of a pattern of closely spaced 22 light (or alpha particle) sensitive collective elements or 23 electrodes disposed on and electrically isolated from the surface 24 of a semiconductor material. sufficiently large voltages are applied to the electrodes, potential wells capable of holding and 26 transferring separate charges are formed under the electrodes.

%i R! WO 89/04499 PCT/US88/03952 -27- 1 When the semiconductor is exposed to alpha particles near where 2 such a potential well is formed, charge carriers generated by the 3 electrodes are collected and held at the potential well. These 4 stored charges may then be output via clock signals from the CCD as rows and columns of information relating to the quantity of 6 detected interactions.

7 As is depicted in Fig. ii. CCD's of the proper dimensions may 8 be utilized in exactly the same mechanical configuration as the 9 DRAM integrated circuits of the preferred embodiment. For example, Fig. 11 is substantially the same as Fig. 4 of the preferred 11 embodiment, except that the integrated circuits 4 of Fig. 4 are 12 replaced by CCD's 202 and chamber 40 is replaced by chamber 240, 13 which internally contains the electrical connections for the CCD's 14 202. Although CCD's are relatively expensive, they are particularly adaptable to signal discrimination techniques 16 necessary for isolating detected thermal energy, electrostatic 1 7 discharge and isolated high energy interactions photons, beta 18 particles, etc.).

19 A fifth alternative embodiment of the present invention is depicted in Figs. 12a and 12b. in which the scintillation effect is S21 utilized to detect alpha particles. Some scintillation materials, 22 such as zinc sulfide, emit light at wavelengths requiring photo 23 multiplier tubes for effective detection. An alpha particle 24 detector in accordance with the present invention which utilizes zinc sulfide scintillation materials is depicted in Fig. 12a..

26 Chambr 360 supplies a tuned sampled environment through its input J:4 WO 89/04499 PCT/US88/03952 -28- 1 and output to a scintillator or fluor 362. The total light output 2 of the fluor 362 is functionally related to the total energy lost 3 by a moving charged particle in the fluor, such as the alpha 4 particle and any secondary electron produced by an alpha particle interaction. The light produced by the fluor 362 liberates 6 electrons photoelectronically from the photosensitive surface on 7 the face of the photomultiplier tube 364. These electrons are then 8 accelerated by the photomultiplier to produce additional secondary 9 electrons in an avalanche effect until a final output signal 366 of sufficient magnitude is produced.

11 Other scintillation materials, such as calcium tungstate, 12 emit light at longer wavelengths than other scintillation material.

13 By coupling a scintillation coated surface exposed to alpha 14 particles with a solid state photodiode capable of detecting light emitted by the scintillation coating, an effective alpha particle 16 detector can be developed. Accordingly, Fig. 12b depicts a cross- 17 section view of an alpha particle detector comprised of a housing 18 450 having a chablier 460 tuned to the visibility range of the 19 sampled environment, an object lens 462, a focusing lens 464 and a photovoltaic cell or photodiode 466 having an electronic output 21 467. Chamber 460 has an intake port 470 and an output port 472 22 through which the sampled environment flows. On the outermost 23 inwardly facing wall of the chamber 460 is a coating of calcium 24 tungstate 474 or similar scintillation material.

Since the coating 474 is within the visibility range of 26 emitted alpha particles passing through the chamber 460, light will 3 Y ^I r I-1PI eWO 89/04499 PCT/US88/03952 -29- 1 be admitted by the coating 474 when alpha particles are detected 2 and imaged onto the photodiode 466 through object lens 462, which 3 forms the opposing wall of light tight chamber 460, and focusing 4 lens 464. Through this embodiment, an inexpensive alpha particle detector can be developed for use in the consumer market. However, 6 since many scintillation materials scintillate at varying rates 7 throughout their life. the coating 474 must either be made from a 8 very stable, long-life material or be readily replaceable.

9 A sixth alternative embodiment of the present invention is depicted in Fig. 13, which is a perspective view of a Geiger 11 counter alpha particle detector. Embedded within a sealed casing 12 540 is a Geiger-Muller tube 544, which when operating in the Geiger 13 region, produces an output voltage pulse of approximately constant 14 magnitude for each ionizing event that takes place within its cylindrical electrode 546. This output voltage is input to a 16 Geiger counter control circuit 550. which produces an output signal 17 552 corresponding to the level of radon concentration within the 18 sampled environment of chamber 560. Although Geiger counters are 19 particularly prone to indiscriminate sensitivity, use of the sealed casing 540 and the tuned chamber 560 with light tight intake port 21 562 and output port 564 makes it possible to effectively restrict 22 exposure of the electrode 546 to only alpha and beta particles in 23 the sampled environment.

24 It should also be noted that in some embodiments, i 25 temperature. humidity and environment density detectors may need to 26 be maunted on the lateral surface of the sample environment chamber C' 1 1 I 1 WO 89/04499 PCT/US88/03952 1 in order to calibrate for oclusion with the alpha particle 2 detection surface.

3 Although the present invention has been disclosed above in 4 terms of a preferred embodiment, it is contemplated that numerous alterations and modifications of the invention will be apparent to 6 those skilled in the art after having read the above disclosure.

7 It is therefore intended that the following claims be interpreted 8 as covering all such alterations and modifications as fall within 9 the true spirit and scope of the invention.

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Claims (10)

1. 3. A device for monitoring radon exposure as claimed in claim I or 2, wherein said detection chamber includes a temperature sensor for monitoring the temperature of said sampling flow and Sdeveloping a temperature signal for input to said monitoring system.
2. 4. A device for monitoring radon exposure as claimed in claim 1, 2 or 3, wherein said detection chamber includes a humidity 1 sensor for monitoring the relative humidity of said sampling 31 V A tr,^ r 1 11 1 1 1 I' !i flow and developing a humidity signal for input to said monitoring system. A device for monitoring radon exposure as claimed in any one of the preceding claims, wherein said detection chamber includes means for receiving and electrically interfacing said alpha particle detector with said monitoring system, and wherein said detection chamber is formed by a dome shaped cover wherein all interior surface points of said cover are located a predetermined distance d from at least one of said capacitance means, where d is a positive integer proportional to the visibility range of alpha particles within said environment. S 6. A device for monitoring radon exposure as claimed in any one of claims 1 to 4, wherein means forming said chamber is a cube having at least one alpha particle detector disposed within each of five faces of said cube and means disposed in said five *i faces for receiving and electrically interfacing said alpha particle detectors with said monitoring system, and wherein all interior surface points of said cube are within a distance d of at least one of said capacitance means, where d is a positive integer proportional to the visibility of alpha particles within sai.d environment.
3. 7. A device for monitoring radon exposure as claimed in any one of the preceding claims, wherein said alpha particle detector is an integrated circuit having a plurality of radiation sensitive memory cells arranged in an orthogonal array of rows and columns.
4. 8. A device for monitoring radon exposure as claimed in claim 7, wherein said memory cells are charged in either a positive or i 32 i Irar i' I t negative direction in response to interaction with said alpha particles.
5. 9. A device for monitoring radon exposure as claimed in claim 7 or 8, wherein said orthogonal array encompasses an area of at least 1.0 sq. cm. A device for monitoring radon exposure as claimed in claim 7, 8 or 9, wherein said memory cells operate at a voltage charge level and refresh rate approaching the thermal noise level of said memory cells.
6. 11. A device for monitoring radon exposure as claimed in any one of claims 1 to 6, wherein said alpha particle detector is an S* integrated circuit having a plurality of radiation sensitive memory cells arranged in concentric circles.
7. 12. A device for monitoring radon exposure as claimed in claim 1, further comprising: peripheral devices for entering externally input data and for S: displaying a calculated level of radon concentration, and S. wherein said monitoring system includes memory means for receiving and storing data signals including said detection 20 signal and outputting selected data upon command; and system control means for receiving detector data signals input from a plurality of said detectors, receiving said externally input data input from one or more of said peripheral devices, processing said detector data signals for input'to said memory means within said data signals, processing said externally input data for input to said memory means within said data signals, receiving selected data output from said memory means upon command of said system control means, processing said selected 33 f Vc i i: z j r r:: ii L i data in a predetermined manner to generate a calculated level of radon concentration signal corresponding to the level of radon concentration within said environment, and outputting said calaulated level of radon concentration signal to one or more of said peripheral devices.
8. 13. A device for monitoring radon exposure as claimed in claim 12, wherein said system control means includes: sampling processor means for receiving said detector data signals input from said plurality of detectors and developing and outputting a processed detector data signal; input/output processor means for receiving said externally input data input from one or more of said peripheral devices, generating an externally input data signal, receiving said calculated level of radon concentration signal and outputting said calculated level of radon concentration signal to one or S more of said peripheral devices; and data processing means for receiving said processed detector data signal and said externally input data signal, outputting said processed detector data signal and said externally input data signal to said memory means, commanding and receiving said S* selected data from said memory means, processing said selected data in a predetermined manner to generate said calculated level of radon concentration signal corresponding to the level of radon concentration within said environment, and outputting said ca.lculated level of radon concentration signal to said input/output processor means for communication to one or more of said peripheral devices.
9. 14. A device for monitoring radon exposure as claimed in claim 12, wherein said peripheral devices include a ventilation 34
10. 74- I 1 r: Li r II system which develops and outputs said externally input data signals to said system control means, receives said calculated level of radon concentration signal input from said system control means, and modifies the flow of fluid within said sampled environment in order to compensate for the level of radon concentration within said sampled environment. A device for monitoring radon exposure substantially as herein before described with particular reference to the accompanying drawings. 4 4 4 4b44 4 2Q 4 Dated this 11th day of March, 1992 PATENT ATTORNEY SERVICES Attorneys for DAVID M GJERDRUM r.