Radon measuring device

A radon measuring device having a housing formed by at least two mating portions. The housing includes a plurality of apertures along at least one of its surfaces to permit entry of ambient atmosphere into said housing, a membrane or filter positioned proximate an interior surface of the housing covering the apertures for filtering the ambient atmosphere which enters the housing, and a radon detector within the housing positioned in a plane substantially normal to the surface of the housing containing the apertures. The housing can also include a filter holder within its interior for positioning both the filter and the detector in their respective positions as well as alignment tracks for alignment of the filter holder with respect to the housing.

BACKGROUND OF THE INVENTION 
This invention relates generally to dosimeters for measuring radon and more 
particularly, to a passive radon measuring device having a two-piece 
housing or diffusion chamber and a mounting insert for positioning both a 
membrane or filter and a radon track etch or energy sensitive detector 
within the diffusion chamber 
Radon is a colorless, tasteless, and odorless radioactive gas that results 
from the natural breakdown or radioactive decay of radium. Radon typically 
is found in high concentrations in soils and rocks containing uranium. It 
is believed that human exposure to elevated levels of radon can lead to an 
increased risk of developing lung cancer, depending upon the concentration 
of radon and the length of exposure. 
Recent studies have indicated that radon can accumulate in dangerous 
concentrations in residences and other structures, and particularly in the 
lower levels of buildings which typically have poor ventilation and into 
which radon enters from the surrounding soil. Radon can enter a structure 
in numerous ways including through the water supply, dirt floors, cracks 
in masonry floors and walls, floor drains, sumps and similar openings in 
the foundation of the structure. 
Various methods and devices have been employed for detecting radon. The two 
least expensive and most readily available devices are the charcoal 
canister and the alpha track detector. Both of these devices are passive 
devices that are exposed to the air in a home or other structure for a 
prescribed period of time and are then sent to a laboratory for analysis. 
Although charcoal canisters are usable for a test period of one to seven 
days, they are somewhat less reliable than alpha track detectors. 
The most basic alpha track detector is constructed as a housing or 
diffusion chamber in the form of a small cup having a strip of alpha track 
registration material affixed to the inside of the housing. When the track 
registration material is exposed to radon or its progeny (radioactive 
decay products of radon) the alpha particles produced by the radioactive 
decay of the radon or progency cause minute damage tracks to occur on the 
material. Such tracks can subsequently be enlarged and made visible by 
chemical or electrochemical etching, for instance, and the concentration 
of radon present in a particular test area can be measured. 
Although an isolated piece of track registration material can be placed in 
an environment for detection of radon, it is preferable that some type of 
housing or diffusion chamber be used, such as the cup housing referred to 
above. A housing and membrane or filter isolates the registration material 
from the ambient air, which may contain concentrations of radon progeny, 
and enables the material to be exposed to radon entering the housing from 
the environment and the daughters produced in the housing. Additionally, 
it is desirable to provide a membrane or filter over the entrance to the 
housing to prevent contamination from other sources. The sensitivity of 
these types of detectors depends upon the size, shape and material used in 
their construction. More importantly, the position of the detector strip 
within the device can also affect the performance and reliability of these 
detectors. Care must also be taken to prevent tampering with the detector 
before analysis in the laboratory, as well as maintaining the detector in 
an optimum position for receiving alpha particles from radon and its 
progeny within its interior during the test period. 
One example of a radon detector having a housing and a filter is disclosed 
in U.S. Pat. No. 4,518,860. That patent discloses a track registration 
detector for radon and radon progeny products having a housing with a 
removable circular apertured closure cap for retaining a strip of track 
registration material within its interior. The strip is retained within 
the housing by integrally formed upstanding ribs which form both a 
pedestal support and a transverse support for the strip and position the 
strip in juxtaposition with the apertures of the cap with a circular 
filter sandwiched therebetween. The presence of radon is measured on the 
side of the strip opposite the filter and apertured cap. The cap includes 
a solid circular portion in its center to provide a radiation absorber 
shield for the top surface of the strip. The entrance area of this 
detector is capable of being blocked by an item positioned adjacent to the 
apertures, and, since the detector is typically placed with its apertured 
cap facing upward, can become clogged by dust and dirt particles falling 
naturally within the ambient air. 
Other detectors are of a size most suitable for use as a personal dosimeter 
or for exploration purposes. 
The present invention provides a track etch radon detector which is of 
relatively uncomplicated construction and yet achieves the desired 
advantages of permitting free flow of radon within its interior without 
exposing the track registration material to ambient air, reduces the 
chance of becoming blocked with foreign material or adjacent items, and 
which provides a seal of the component parts to resist and/or prevent 
tampering other than by an authorized testing facility. The construction 
of the invention has a relatively large sensitive volume and detector area 
and therefore can be used for the same time period as charcoal canisters 
but without the inadequacies of such charcoal canisters. 
SUMMARY OF THE INVENTION 
The invention is characterized by a radon measuring device having a housing 
formed by at least two mating portions secured together, in which at least 
one of the mating portions includes a plurality of apertures for the 
passage of radon from the ambient atmosphere into the interior of the 
housing. A membrane or filter is positioned within the housing proximate 
the apertures to filter the ambient atmosphere passing into the housing. A 
radon detecting element is positioned within the housing interior for 
measuring radon in the ambient atmosphere passing into the housing. The 
detecting element is positioned along a plane which is normal to the plane 
upon which the apertures of the housing are positioned. 
The device also can include a filter holder for positioning both the filter 
and/or the radon detecting element at desired positions within the 
housing. The insert can include outwardly extending tab projections for 
registry with corresponding alignment tracks formed on the inside surface 
of the housing for alignment of the insert with respect to the housing. 
Various objects and advantages of the invention will become apparent in 
accordance with the disclosure herein in which the preferred embodiment is 
described in detail in the specification and illustrated in the 
accompanying drawings. It is contemplated that minor variations may occur 
to the skilled artisan without departing from the scope or sacrificing any 
of the advantages of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1 and 4, the radon measuring device embodying the 
invention is designated generally by the reference numeral 10. The device 
10 includes a housing 12 having a first housing portion 14 and a second 
housing portion 16, a filter holder 18, a filter 20 and a radon detector 
or foil 22. 
The housing 12 is generally in the form of a right circular cylinder and 
includes a cylindrical side wall 24 having a predetermined height that is 
enclosed on its ends by end walls 26 and 28. Typically, the housing 12 is 
positioned during use as illustrated in FIG. 1, with its longitudinal axis 
substantially parallel to the vertical plane. It may also be used with the 
longitudinal axis in the horizontal or other plane. 
The housing 12 is preferably formed by joining the two housing mating 
portions or mating halves 14 and 16 which are substantially identical in 
size and dimension and are semicylindrical in shape. Separation of the 
housing 12 is thereby accomplished along a diameter line passing through 
the circular end walls 26 and 28. It is to be understood, however, that 
the particular shape of the housing portions 14 and 16 as well as the 
position of their parting line can vary and the housing portions 14 and 16 
need not be identical mating halves. 
The housing portions 14 and 16 include semi-cylindrical side walls 24a and 
24b respectively, as well as respective end walls 26a, 26b, 28a and 28b. 
If desired, the corner between the side wall 24 and the end walls 26 and 
28 can be beveled. As FIG. 4 illustrates, in order to provide an overlap 
between the two housing portions 14 and 16 to form a more effective seal 
therebetween, the second housing portion 16 can include an outwardly 
extending flange 29 around the perimeter of its open face for cooperation 
with a reduced portion or recess 29a formed around the perimeter of the 
open face of the first housing portion 14. The housing 12 can be 
constructed of a plastic material having an electrostatic property similar 
to that of the detector foil 22 or of conductive material so that the 
housing 12 does not attract, repel or otherwise affect the free movement 
of alpha particles emitted therein. If the housing forming material has 
electrostatic properties the same as the foil, radon progeny will deposit 
throughout the housing. More preferably, the housing is constructed of 
electrically conductive static dissipative material with the result that 
radon progeny will tend to deposit uniformly on the foil, rather than 
unevenly and on the housing walls because of static charge build-up. 
The two semi-cylindrical housing portions 14 and 16 are fastened together 
to form the cylindrical housing 12 by a fastening member 30. As FIGS. 2, 
2a, and 4 illustrate, the fastening member 30 includes flexible lock arms 
32, formed integral with the sidewall 24b on opposite sides of the second 
housing portion 16, for registry with corresponding abutments 34, 
positioned on the interior surface of the sidewall 24a on opposite sides 
of the first housing portion 14. Although FIG. 4 illustrates the lock arms 
S2 and abutments 34 as being of a tongue-and-groove arrangement, they can 
include an additional step portion 35, as illustrated in FIGS. 2 and 2a, 
which is actually part of the flange 29, in order to assist in sealing the 
housing 12 around the fasteners 30. 
Preferably, the fastening member 30 is a tamper evident type of fastener 
that is easy to engage, yet is difficult to open without fracturing the 
wall 24 to indicate tampering. Alternatively, the fastener 30 can be any 
type of fastener so long as it functions as described and the two housing 
portions 14 and 16 remain secured together during use. Moreover, the 
positioning of the lock arms 32 and the corresponding abutments 34 can be 
interchanged with respect to the sidewalls 24b and 24a without departing 
from the teachings of the present invention. 
To enable passage of ambient atmosphere into the housing 12, the first 
semi-cylindrical housing portion 14 includes, in the preferred embodiment, 
a plurality of apertures 36 positioned around the circumference of and 
extending through its side wall 24a. Although twelve apertures 36 are 
illustrated, the number and size of apertures 36 can vary so long as a 
free flow of radon from the ambient atmosphere into the housing 12 is 
provided. 
As FIGS. 1 and 3 illustrate, in addition to the filter 20, to further guard 
against foreign matter from inadvertently entering the housing 12, the 
apertures 36 can be generally rectangular in shape and can be formed with 
bottom surfaces 38 that slope outwardly with respect to the interior of 
the housing 12. Accordingly, any foreign matter such as dust, dirt, or 
water which may come in contact with the housing 12 will be deflected away 
from the apertures 36 and the interior of the housing 12 by the sloped 
bottom surfaces 38. If desired, to further aid in reducing contamination 
within the interior of the housing 12 in addition to the sloping of the 
bottom surfaces 38, top surfaces 40 and side surfaces 42 of each aperture 
36 similarly can be sloped (not illustrated) thereby providing the 
apertures 36 with a counter-sunk configuration. 
Due to the positioning of the apertures 36 in the sidewall 24 rather than 
in the end surface 26, dust and dirt particles present in the air will not 
fall directly through the apertures 36 to settle on and block the filter 
20. Moreover, the probability of blocking all of the apertures 36 by any 
item placed near the exterior of the housing 12, or by positioning the 
housing, is reduced, since such an item would be required to have a 
concave shape corresponding to the curvature of the housing 12 and be 
large enough to extend across all of the apertures 36 in order to block 
the same. 
The filter holder 18 is semi-cylindrical in configuration and is sized and 
dimensioned generally similar to the side wall 24 of the housing portion 
14 for registry therewith, but is slightly reduced in size so that it may 
fit within the interior of the housing portion 14. The filter holder 18 
includes a semi-cylindrical side wall 44 and projecting top and bottom tab 
portions 46 and 48. The side wall 44 includes an exterior surface 50 which 
is designed for registry with an interior surface 52 of the first housing 
portion 14, as FIGS. 2, 3, and 4 illustrate. For passage of ambient radon 
through the filter holder 18 from the apertures 36 in the first housing 
portion 14, the side wall 44 further includes a plurality of apertures 54 
that can be of size and dimension similar to that of apertures 36. 
As described above, the membrane or filter 20 serves to exclude ambient 
radon progeny and other contamination within the housing 12. The membrane 
or filter preferably is formed from a material that separates radon from 
dust particles, aerosols, and radon progeny in the ambient atmosphere and 
is typically made of paper, fiberglass or other filter material or a 
semipermeable membrane material to a desired thickness and filtering 
capability. To position the membrane or filter 20 with respect to the 
apertures 36 and 54, the end portions of the exterior surface 50 of the 
filter holder 18 include outwardly extending flanges 55 which form slots 
56 to accept the top and bottom portions of the filter 20. The flanges 55 
and the slots 56 preferably span the length of the side wall 44 in the 
area of the apertures 36 but can be shorter and/or positioned at a 
plurality of different locations along the length of the side wall 44 if 
desired so long as the filter 20 is maintained in its proper position. 
When the housing 12 is assembled, the slots 56 hold the filter 20 in 
position between the interior surface 52 of the first housing portion 14 
and the exterior surface 50 of the filter holder 18 so that the filter 20 
covers the apertures 36 and 54 and abuts against the interior surface 52 
as will be explained later. Accordingly, ambient atmosphere cannot pass 
into the interior of the housing 12 without first going through the filter 
20. Alternatively, the flanges 55 can be formed on the interior surface 52 
of the first housing portion 14 to hold the filter 20 against the 
apertures 36 and the filter retaining portion of the filter holder 18 can 
be eliminated. In either case, the filter 20 of the present invention is 
preferably rectangular in shape. Since the filter material typically comes 
in rectangular shaped segments, excess waste material must be trimmed to 
form the circular shaped filters common in prior art radon detectors. Such 
trimming is not necessary with the rectangular filter 20 of the present 
invention, thereby eliminating waste material and reducing the overall 
cost of the device 10. 
To align the filter holder 18 within the first housing portion 14 and 
provide proper alignment of the filter 20 with respect to the apertures 36 
as described above, interior surfaces 58 and 60 of end walls 26a and 28a 
respectively of the first housing portion 14, each include two elongated, 
substantially parallel alignment tracks 62, 64, 62' and 64'. Each 
alignment track 62 and 64 can be slightly beveled toward each other and 
together they define a respective receiving channel 66, 66' therebetween. 
Preferably, the bottom tab 48 is of generally planar configuration formed 
with beveled edges 65, 65' to be matingly received between tracks 62, 64 
such that tab 48 is retained within channel 66. Top tab 46 is similarly 
planar in configuration with beveled edges 67, 67' for receipt within 
tracks 62', 64' of the top channel 66. It is to be understood, however, 
that similar types of alignment means may be employed without departing 
from the teachings of the present invention. For example, as seen in the 
alternate embodiment of FIG. 5, the channel between tracks 62", 64" on the 
interior surface 60 can accept bottom tab 48' which rides above tracks 
62", 64" with overlying depending portions or flanges 65", 65'". 
Alternatively, both the top and bottom tabs 46 and 48 can include flanges 
such as 65" and 65'" to accept their respective alignment tracks 62, 64, 
or the like. 
To position the filter 20 in juxtaposition with the interior surface 52 of 
the first housing portion 14 and effectively screen the apertures 36, 
interior surfaces 68 and 70 of the end walls 26b and 28b respectively of 
the second housing portion 16 can each include positioning bars 72 and 74. 
As will be explained later, when assembled, the positioning bars 72 and 74 
of the interior surface 68 can engage the top tab projection 46 directly 
in order to force the filter holder 18 in the direction of arrow "A" 
toward the interior surface 52 of the first housing portion 14. Similarly, 
the positioning bars 72 and 74 of the interior surface 70 can be formed to 
engage the foil 22 positioned between tracks 62, 64 against bottom tab 
projection 48 as described below. Accordingly, upon fastening of the 
fastening member 30, the filter 20 and the radon detecting foil 22 will be 
maintained in their desired positions. 
As FIG. 2 and alternate embodiment FIG. 5 illustrate, the radon detector 22 
preferably is positioned within the recess 66 formed on the interior 
surface 60 of the housing portion 14. In this position, the radon detector 
22 lies in a plane substantially normal to the semi-cylindrical side wall 
24 and at approximately the geometric center of its plane. The detector is 
disposed in a plane which is parallel to the oppositely disposed flat 
plane defined by end walls 26 with the result that alpha particles 
striking the foil are as uniformly distributed as possible. 
The detector 22 can be any type of desired alpha sensitive material such 
as, for example, a polycarbonate foil or similar type of detector. As 
FIGS. 4 and 5 illustrate, in order to enable proper exposure of the 
detector strip 22, the length of the tab 48, 48' of the filter holder 18 
is to be less than the full length of the channel 66. Accordingly, when 
the device 10 is assembled, the detector strip 22 is positioned within the 
channel 66 between the tabs 48, 48' and the positioning bars 72 and 74 of 
the surface 70 of the second housing portion 16 and is exposed to the 
interior of the housing 12. The tab 48 (FIG. 4) has an end wall 75, 
against which the radon detector 22 abuts so that proper positioning of 
the detector strip is achieved. Tab 48' has a depending abutment 75' which 
functions in a like manner. A spacing member (not shown) may be inserted 
between end wall 75 or abutment 75' and detector 22 so that detector 22 is 
positioned in the geometric center of its plane. In such event, 
positioning bars 72 74 would be shortened along surface 70 to terminate 
before end wall 28b. 
To assemble the device 10, the filter 20 is positioned within the slots 56 
on the exterior surface 50 of the filter holder 18 while the radon 
detector strip 22 is positioned within the channel 66 formed in the bottom 
interior surface 60 of the housing portion 14 against the end wall 75. The 
tabs 46 and 48 are then aligned with and inserted within the receiving 
channels 66 and the alignment tracks 62 and 64 on the interior surfaces 58 
and 60 of the walls 26a and 28a of the first housing portion 14 
respectively. Upon insertion of the filter holder 18, the membrane or 
filter 20 is moved into position against the interior surface 52 of the 
first housing portion 14 and the shortened bottom tab 48 enables 
positioning of the radon detector strip 22 in the channel 66 for proper 
exposure of the radon detector strip 22 within the interior of the housing 
12. Thereafter, the second housing portion 16 is aligned with and secured 
to the first housing portion 14 by fastening members 30 in an overlapping 
fashion as described hereinabove. During fastening, the top tab 46 of the 
filter holder 18 is simultaneously advanced toward the interior surface 52 
by the positioning bars 72 and 74 of the interior surface 68 of the second 
housing portion 16, thereby forcing the membrane filter 20 against the 
interior surface 52. At the same time, the positioning bars 72 and 74 of 
the interior surface 70 of the second housing portion 16 engage the radon 
detector strip 22 to hold it in position against the end wall 75 within 
the channel 66. Accordingly, the device 10 is completely assembled and the 
two housing portions 14 and 16 cannot be opened without authorization 
except by fracturing the housing wall thereby providing an indication of 
such an unauthorized opening. 
During use, the device 10 is positioned in an area where the concentration 
of radon is to be measured. As explained above, such an area is typically 
an enclosed area such as the basement of a structure or any other desired 
area. The device 10 may be positioned with its longitudinal axis parallel 
with the vertical plane and its end wall 26 facing upward. Alternatively, 
the device 10 can be provided with a hook on any convenient surface to 
suspend the device in any desired location. Thereafter, the device 10 is 
left in the area for a prescribed period of time, typically one to 52 
weeks. After such time has passed, the device 10 is then sent to a 
designated laboratory for analysis. Upon arrival at the laboratory, the 
device 10 is first inspected for any signs of tampering and is then broken 
apart into its two housing halves 14 and 16. The radon detector 22 is then 
removed and the above described etching process is performed on the radon 
detector strip 22 to provide a measure of the concentration of radon in 
the monitored area. 
Modifications and variations of the present invention are possible in light 
of the above teachings. For example, although the device 10 is illustrated 
for use as a stand alone unit, it also can be mounted to a wall or carried 
by a user. It is therefore to be understood that within the scope of the 
appended claims, the invention may be practiced otherwise than as 
specifically described.