Note: Descriptions are shown in the official language in which they were submitted.
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SPECIE'ICATION
This invention relates to electroscopes, condenser ion chambers
and devices which store free electric charge, and to their employ
for the detection and measurement of radiation, radioactive
gases, and radioactive aerosols in the ambient environment.
Electroscopes and condenser ion chambers are well known in the
prior art as means for measuring radioactivity. Briefly
described, they comprise tubular structures having a centrally
located insulated electrically charged electrode. Radiation
ionizes the resident gas therein, and the ions and charges thus
formed are attracted to the charged center electrode. Some of the
charge located on said central electrode is neutralized, thereby
changing the angular deflection of a leaf or quartz fiber
indicator, the change in angular deflection being related to the
amount of radioactive material present, and to the period of
measurement. The electroscope and condenser ion chamber may be
separate devices or may be integral in nature and many
arrangements are known in the prior art.
Weak radiation sources e.g. environmental, are difficult to
measure with an electroscope unless electrical leakage from the
center electrode is reduced to a small value by some means. In
those cases where the sensitive volume is not sealed from the
ambient environment, and where said ambient environment can
penetrate into the sensitive volume, or where said ambient
environment is being directly sampled without prior treatment to
remove moisture, a drying material may be present to minimize
leakage. Said drying material is often expended quickly,
particularly if humid air is being sampled, and measurement
periods are limited in duration and not prolonged. In some
sampling situations the drying material may absorb the material
being measured, thereby interfering with the measurement in
progress, and there is a danger that the said material being
measured will be rereleased at a later time or in a manner which
will interfere with subsequent measurements. There are no
examples in the prior art, however, where electroscopes and
condenser ion chambers have been employed for the sampling and
the measurement of radioactive gases and aerosols present in the
ambient environment, particularly at the low levels often
encountered therein.
However, in accordance with the present invention, there is
provided a device for detecting and measuring radiation,
radioactive gases and radioactive aerosols in the ambient
environment, said radiation being alpha particle, beta particle,
X-ray, neutron, and gamma ray, said radioactive gases being
radon, thoron, tritium and beta particle emitting, and said
radioactive aerosols being radon progeny, said measurements being
of variable length and sometimes of prolonged duration, said
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measurements being made directly in the ambient air or on
surfaces without pretreatment of the sample atmosphere or the
radioactive surface being measured.
The invention comprises an electroscope and condenser ion
chamber, a means for sampling the species to be monitored, a
means for charging an insulated electrode, a means for reducing
or eliminating charge leakage across said insulated electrode,
and a means for reading out changes in the charge level on said
electrode.
The general objective of the present invention is to provide an
improved, simple, weather independent and inexpensive device of
the aforedescribed character having means for positively
excluding humidity and sources of error arising, thereby ensuring
accurate readings and enabling radiation measurements to be
carried out over extended periods and/or a considerable time
after charging.
More particular, objects within the framework of the foregoing
include:
To provide a means for guarding the insulated electrode over
prolonged periods against moisture and other agents capable
of falsifying the reading.
To provide a means for sampling and detecting alpha
particle, beta particle, gamma ray, X-ray, or neutron
radiation in the ambient environment, said means being
suitable for measuring radiation from low to high levels,
and said means of sampling and detecting being of variable
length and often of prolonged duration.
To provide a means for sampling and detecting radioactive
gases such as radon, thoron, tritium and beta emitting gases
in the ambient atmosphere, said means being suitable at
concentrations ranging from low to high levels, and said
means of sampling and detecting being of variable length and
often of prolonged duration.
To provide a means for sampling and detecting radioactive
solids such as radon daughters in the ambient atmosphere,
said means being suitable at concentrations ranging from low
to high levels, and said means of sampling being of variable
length and often of prolonged duration.
To provide a means for charging the device and a means for
adjusting the charge levels.
To provide a charge storing condenser, normally disconnected
from the indicator means, thereby insuring a source of
charge for the indicator and multiple chargings before
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recharge, the condenser comprising air spaced plates.
To provide a simple and accurate means for reading out the
state of charge.
The invention will now be further described in conjunction with
the accompanying drawings, in which Figure 1 is a sketch of a
condenser ion chamber and electroscope reader, and Figure 2 is a
sketch of an integrated electroscope and condenser ion chamber
according to the prior art. Figure 3 is a sketch of an integrated
electroscope and condenser ion chamber according to the present
invention.
Referring to Figure 1, are indicated a prior art electroscope and
a condenser ion chamber sensing part at 51, and a readout part at
50.
The sensing part at 51 is constructed from a cylinder 25, a
central electrode 23 mounted in an insulator 22, a means for
charging the central electrode 23 at 14, a means for reading out
the charge 21, and means for entry of radiation and radioactive
materials at 8, 9, 27, 35, 26 and 28.
The center electrode 23 is mounted in an insulator 22 and is
electrically connected to a readout pin 21. The pin 21 and
insulator 22 are sometimes protected from humidity and factors
which affect electrical leakage by means of a cap 20 and a drying
material 10. The cap 20 is removable for readout and/or charging
purposes. The volume between the center electrode 23 and the ion
chamber wall 25, is the sensitive volume 24, which is the region
where electric charge, originating from radioactive
disintegrations occurring therein, are attracted to the center
electrode 23. The sensitive volume 24 is usually defined as the
volume within a grounded metal cylinder 25 which may be described
by a diversity of volumes, shapes and dimensions. The cylinder 25
is usually sealed to prevent entry of materials and moisture. A
drying material 10 is sometimes positioned within the sensitive
volume 24 to protect the insulator 22 from ambient moisture. A
filler cap 11 is provided to replace expended drying material 10.
The center electrode 23 is usually constructed of a conducting
material such as stainless steel, brass, or a glass fiber coated
with a conductor such as gold. The insulator 22 must be clean,
free from contaminants and moisture, and have a resistance to
electric current such that the charge is retained on the center
electrode 23 for considerable periods e.g. months. TEFLON*,
ceramics, sulfur, ceresin waxes, ebonite*, amber, quartz,
polystyrene, have been employed with success.
The center electrode 23 is charged by means of an electrostatic
or electronic charger 14, or other means for charging, and during
the charging procedure, a charging arm 13 momentarily contacts
211625 2
the center electrode 23 to apply the charge. Alternatively, the
charger 14 may be part of the electroscope readout device at 50,
in which case, the electroscope readout part and condenser ion
chamber sensing part at 51, are joined electrically by means of
pins 18 and 21, before a charge is applied to the center
electrode 23. The amount of charge applied and/or the angle of
deflection of the indicator 3, can be adjusted by means of the
indicator angular adjustment 6, often comprised of a radiation
source or a leakage resistor. The electroscope readout part and
condenser ion chamber are then disconnected before the condenser
ion chamber is deployed to make a measurement.
The sensitivity of the condenser ion chamber is related to
factors such as the size of the sensitive volume 24, to the
strength of the electric field around the central electrode 23,
and to features which change the electrical capacitance of the
device e.g. capacitors. The condenser ion chamber may have a
means for adjusting the size of said sensitive volume 24, said
capacitance and said electric field, and by changing any of said
measurements that affect the sensitivity, the monitoring period
can be changed or adapted to suit individual monitoring
objectives e.g. short period of 24 hours or a longer period of
several months. A typical sensitivity for a one liter sensitive
volume 24 is several degrees per day deflection of the indicator
3, in a gamma radiation field of 15 microroentgens.
Although drying materials 10 have been occasionally employed to
reduce humidity and thereby to minimize electrical leakage,
electroscopes and condenser ion chambers have not been previously
employed to measure radioactive gases and particulates present in
ambient air. DRIERITE*, silica gel, magnesium perchlorate, and
phosphorous pentoxide are frequently employed drying materials.
To make a measurement, the radioactive material to be gauged is
positioned in the sensitive volume 24 by various means, and a
charge is applied to the center element 23. Ions are generated in
the sensitive volume in proportion to the concentration of the
radioactive species present, or to the frequency of the
radiation, said charge being attracted to the central electrode
23 where it neutralizes some of the applied charge located
thereon. As a consequence, the angle of deflection of the leaf or
quartz fiber indicator 3, is reduced. The applied charge is
measured before and after a suitable monitoring period, and the
residual charge or charge lost, which is a measure of the amount
of radioactivity present, is determined.
The electroscope readout part at 50 comprises an enclosure 15, a
central insulated electrode 5, a leaf or fiber indicator 3, scale
4, and a readout pin 18. The central electrode 5 is mounted in an
insulator 17 and is connected to a readout pin 18. The readout
pin is covered by a cap 19 to protect the insulator 17 from
moisture and gases, which are present in the ambient environment,
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and which may influence electrical leakage. The cap 19 may
contain a drying material 10.
The state of charge on the condenser ion chamber sensing part at
51 is determined by electrically connecting it to the
electroscope readout part at 50 by means of metal pins 18 and 21.
The pins do not make contact during the readout procedure, since
the condenser ion chamber may be partially discharged by so
doing. An accurate readout is possible by placing pins 18 and 21
in close proximity to one another. The light weight foil leaf or
metal coated quartz fiber indicator 3, located in the
electroscope readout part at 50, is directly connected to the
center electrode 5, or is hinged 2, and is free to deflect in
proportion to the charge present on the central electrode 5. The
deflection of the leaf or metal coated fiber indicator 3, is read
by means of an angular scale 4, the deflection being read with
the naked eye, by means of a microscope, by a projection method
which involves a light source, or electronically, and a diversity
of readout methods is possible. The deflection may be presented
in angular units or units of radioactivity. The readout method
may be mechanical, electrical, or optical, may vary in the nature
of the information provided, and the readout method is not
limited to any specific scale 4, units of measurement, or form of
communication.
The center electrode 5 is contained within an electrically
grounded element 1 and an electrostatic shield 16 which prevents
charge buildup, reduces outside electrical interference, and
ensures a symmetrical field around the leaf or fiber indicator 3.
A drying material 10 is sometimes present in the electroscope
readout part at 50, to keep the electroscope insulator 17 dry. A
means for replacing the drying material is provided 11. The
readout pins 21 and 18 are normally capped since exposure to the
ambient environment e.g. moisture or pollution, may increase
electrical leakage across insulators 17 and 22. The caps 19 and
20 are removed during the readout or charging procedure.
The device of Figure 1 was employed in the prior art for the
measurement of radioactive sources such as radium, and
radioisotopes, said radioactive sources often being moved into
and out of the sensitive volume 24 by means or a drawer 28,
designed to minimize entry of the ambient atmosphere. Weak
sources can be measured, provided that the discharge rate due to
radiation from the source, is large compared to the discharge
rate due to electrical leakage. Electroscopes and condenser ion
chambers, where the sensitive volume 24 is sealed from the
ambient environment, have been successfully employed as
dosimeters for the measurement of radiation fields e.g. X-rays,
gamma rays, located external to the dosimeters. Similarly, sealed
electroscopes, with windows 26 transparent to the radiation being
monitored, have been successfully employed for sampling alpha,
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beta, X-ray and gamma ray radiation sources, located external to
the device.
Many methods of sampling for airborne materials are known in the
prior art and may be active, passive, continuous or discontinuous
in nature. Active methods employ an air pump, tubing, and may
have a means for pretreating the air sample to remove moisture
and interfering gases. Passive methods may employ a diffuser 27
e.g. membrane, through which the sample moves by a diffusion
mechanism. In continuous methods, the electroscope device is
continuously exposed to the ambient air sample, whereas in spot
methods of sampling, a sample is taken, then the sampling process
is stopped, to make a measurement.
No examples in the prior art, where radioactive gases in the
ambient air, such as radon, thoron, xenon, or krypton, have been
sampled and measured, and where a diffusion process has been
employed as a means for sampling these gases, have been
identified. In a typical sampling process based on diffusion, the
ambient air sample, containing the radioactive materials to be
measured, moves passively from the external environment through
the diffuser 27, and into the sensitive volume of the condenser
ion chamber at 51, where said radioactive materials will be
detected and measured. Said diffusers 27, however, often permit
the entry of moisture and other interfering materials, and the
air being sampled must be either pretreated, or the sensitive
volume 24 must be kept dry and free from interfering materials,
by means of drying materials 10 or other materials located in or
near the sensitive volume 24, particularly when measurement
periods are long or low levels of radioactive materials are being
measured.
No examples have been identified in the prior art where
electroscopes or condenser ion chambers have been employed for
the detection and measurement of tritium. The measurement is
complicated by the fact that the beta particle radiation from
tritium is very short range and a windowless 35 e.g. wire grid,
or a flow through 8, 9 sampling method must be used. If a drying
material 10 is present to remove moisture, thereby reducing
electrical leakage from the central electrode 23, then said
sample of tritium may be absorbed and changed in character. In
addition the drying material must be frequently replaced and
monitoring periods must be of short duration.
Thoron gas has been detected and measured by means of an
electroscope and condenser ion chamber. A flow through
arrangement, comprised of sampling ports 8 and 9, tubing, and a
means for pumping the sample into and/or through the sensitive
volume 24 was used. In prior art investigations, the thoron gas
sample originated from a solid thorium oxide source, samples were
pretreated to remove moisture and interfering substances, and the
samples investigated had much higher levels of radioactivity than
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is traditionally found in the ambient environment.
Electroscopes and condenser ion chambers have been routinely
employed for the detection and measurement of gamma or X-ray
radiation and are in common use as dosimeters. The condenser ion
chamber is usually sealed for this application and the
sensitivity of the device can be changed by varying the size of
the sensitive volume 24, or by adjusting the electrical capacity
e.g. add1ng a capacitor to the central electrode 23.
Referring to Figure 2, an alternate design is presented for the
electroscope and condenser ion chamber, in accordance with the
prlor art. It functions in a similar manner as the arrangement of
Figure 1, and includes some elements in common therewith,
identified by the same reference numerals.
The device of Figure 2 combines the functions and features of an
electroscope readout device and condenser ion chamber in a single
unit. Said device of Figure 2 utilizes a single center electrode
5 for both the electroscope readout part and the condenser ion
chamber sensing part, and does not require readout pins 18 and
21, caps 19 and 20, and drying material 10, described in Figure
1. The combined electroscope and condenser ion chamber of Figure
2, may be more complex and costly than that of Figure 1, but said
device of Figure 2, being more compact and convenient to use, is
probably more suited to use by the non professional. In the
design of Figure 1, since only one electroscope readout part is
required for any number of condenser ion chamber sensing parts,
said sensing parts being simple and inexpensive to construct, the
cost of measuring a number of sources simultaneously should be
less than the device of Figure 2. The condenser ion chamber
sensing part of Figure 1 is readily shipped through the mail to
the monitoring location, and is probably less fragile for this
application than the device of Figure 2.
Referring to Figure 3, there is shown an electroscope and
condenser ion chamber in accordance with the present invention.
It funct1ons in a similar manner as the arrangements of Figures 1
and 2, and includes some elements in common therewith, identified
by the same reference numerals. The invention embodies the
concepts of the separate electroscope reader and condenser ion
chamber of Figure 1, and the combined electroscope of Figure 2.
The invention embodies a means for reducing charge leakage across
the insulator 30, said means including a diffusion limiting tube
32, drying agent 10, and diffusion ports 31, organized in a
manner to protect the insulator 30 from moisture, and since the
influx of moisture into this area is severely retarded by the
diffusion limited design, the lifetime of the drying agent and
subsequently the measurement times possible are extended to very
long periods. The exact physical arrangement of the elements, the
number of elements employed, the dimensions and material
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composition of the elements of said method for limiting diffusion
and reducing charge leakage across the insulator, may vary from
design to design and the invention is not limited in this manner.
The insulator 30 and diffusion tube 32 are constructed from
teflon material, and the central electrode 47 is constructed of
stainless steel of 6.35 millimeters diameter, said electrode
being centered in the diffusion tube 32 of 9.8 millimeters
internal diameter. The diffusion tube 32 extends below the bottom
surface of the enclosure for the drying material 10, and
protrudes into the sensitive volume 24. The center electrode 47
may contain a diffusion deflector 34, which wraps around the
diffusion tube 32, to further limit the rate of diffusion of
moisture and ambient air towards the insulator 30, and many
designs and arrangements of the elements for guarding the
insulator 30 against ambient moisture are possible.
The diffusion tube 32 is about 38 millimeters long, and is
surrounded by a layer of drying material 10, enclosed in the
bottom section of the electroscope. Channels 31, of 1.6
millimeters diameter, between the drying material 10 and the
diffusion tube 32 act as moisture traps. An additional channel 33
connects the drying material 10 to the electroscope sensitive
volume 24 to keep the top part of the insulator 30 dry. A cap 11
for changing the drying material 10 is present.
In addition to the diffusion means for keeping the insulator 30
dry described above, other means for reducing electrical leakage
are possible e.g heating the insulator, and the invention is not
limited to the use of diffusion and drying materials for this
purpose.
The electroscope sensitive volume 24 is enclosed in a metal
cylinder, with transparent viewing faces 12, covered internally
with a conducting material 16, to prevent charge buildup. A means
6 for adjusting the charge 6 on the center electrode 47 may be
present. The angular position of the leaf or quartz fiber
indicator 3 is easily read through the transparent faces 12 by
means of the naked eye, a microscope, or other sensitive means,
and the position of the indicator 3 readout from a scale 4. A
duplicity of electroscope charge measurement and readout methods
is possible, and the invention is not limited in this manner.
The invention may have a charge storage condenser at 52, which is
separate from the center electrode 47, said condenser having a
much larger charge storage capacity than that of said center
electrode 47. The storage capacitor at 52 consists of a condenser
plate 42, condenser wall 43, condenser charger arm 44, and
condenser insulator 45. The storage condenser at 52 is charged by
means of the charging arm 44 and the electroscope charger 14.
Once charged, the storage condenser at 52 will be capable of
charging the center electrode 47 several times. Contact between
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the condenser at 52 and the center electrode 47 for charging
purposes can be made by a variety of methods e.g. swivel arm,
button, and pin 44. The condenser at 52 was charged successfully
by electrostatic, electronic and piezoelectric methods.
The sensitive volume 24 of the electroscope and condenser ion
chambers in the prior art were often cylindrical in shape and
enclosed by a solid wall 46. In the present invention, the wall
46 may be porous or open, and constructed from a wire mesh or
gauze. A variety of construction materials may be employed
provided that they are electrically conducting. The dimensions
and shape of the sensitive volume 24, and the construction
materials employed, can be chosen to achieve specific monitoring
objectives e.g. increasing the size of the sensitive volume 24
will normally increase sensitivity. The sensitivity may also be
changed by other means, such as by connecting a capacitor to the
central electrode 47 i.e. by this means more charge can be placed
on the central electrode 47, and as a result, more charge must be
collected from radiation events per angular change in the leaf or
quartz fiber indicator 3.
As mentioned above, the ion chamber wall 46 is not necessarily a
solid surface impermeable to air and radioactive substances, but
may be a relatively open structure, through which radioactivity
present in the ambient air surrounding the device, is free to
diffuse without pretreatment e.g drying. In the prior art, a
relatively open wall 46 would mean that drying materials 10 must
be frequently changed, and the prior art device would require
frequent maintenance and attention to ensure that said drying
materials 10 are not exhausted. In addition, performance may be
erratic and unreliable, and the material which serves to pretreat
the ambient air may absorb or chemically react with the substance
being measured e.g tritium may be absorbed by drying materials,
thereby affecting the measurement being made.
The invention can be employed for measuring radioactive gases
such as radon, thoron, tritium, beta emitting gases and aerosols
such as radon daughters, by simply placing said invention into
the atmosphere to be monitored, or by other sampling means,
without any treatment of said atmosphere to remove moisture.
Measurement periods can be long and levels of radioactivity can
be low. The invention simplifies the measurement of short range
radiation from surfaces e.g. tritium, since said short range
radiation e.g. range of several millimeters, can easily penetrate
to the sensitive volume 24, through an open mesh wire grid 35 or
chamber wall 46 described above. Said radiation from tritium,
does not have to penetrate fragile ultrathin windows 26 within
which much of the radiation would be absorbed. Since the
invention can be inserted directly into the atmosphere to be
monitored, radioactive substances whose nature or concentration
may be altered by a treatment process, can as a result be
monitored directly without treatment. The invention is not
21162S2
limited to the measurement of any particular radioactive material
or to the location of the material being measured.
Radon, thoron, beta emitting gases, and tritium diffuse readily
from the ambient air surrounding the device through membranes,
thin plastics, and porous materials 27, into the sensitive volume
24 by a passive means. The prior art teaches that it may be
possible to selectively measure individual radioactive gases
which differ in radioactive decay rates or rates of diffusion, by
altering characteristics of the diffusion materials in a manner
which affects the time of diffusion e.g radon from thoron gas. In
other methods of sampling, ambient air is pumped into 8,9 and
through the sensitive volume 24 of the invention. Electroscopes
have not been previously employed for the measurement of radon,
thoron, tritium and beta emitting gases present in the ambient
air surrounding said electroscope, nor has a diffusive method of
sampling been previously employed with said electroscope.
By constructing the wall 46 from polyethylene or boron, fast or
slow neutrons can be detected and measured. The neutrons to be
measured, interact with the wall 46, to produce alpha particles
which ionize the gas resident in the sensitive volume 24. The
amount of ionization or the angular deflection of the indicator 3
is a measure of the neutron flux. Electroscopes have not been
previously employed for neutron measurements.
The present invention may have a window at 26 for the monitoring
of alpha and beta radiation, originating from sources external to
said invention e.g. a contaminated surface or source. The
invention is placed such that the radiation to be monitored is in
close proximity to the window 26, and is able to penetrate said
window 26 to ionize the air in the sensitive volume 24.
In the prior art, a drawer 28, door or other means for
introducing radioactive sources such as Radium-226 and Americium-
241 into the sensitive volume 24, was a freguently used means for
sampling radioactive sources. In the present invention,
radioactive sources which emit low energy radiation, or sources
with a low emission rate, are readily measured for long periods
and there is no need to seal the sensitive volume 24 from ambient
moisture. The diffusion insulator 30, is protected by the
diffusion tube 32 and other elements which guard the insulator 30
against moisture.
The present invention may have a means for collecting radioactive
aerosols on a filter 39 at 53, said aerosols being alpha, beta,
gamma ray, or X-ray emitters, and a window 36 through which said
radioactivity will penetrate and enter the sensitive volume 24.
By this means, alpha and beta emitting aerosols present in the
ambient air can be detected and measured. Said means for
collecting a radioactive aerosol may consist of an 0.8 micrometer
glass fiber filter 39 e.g. radon daughters, contained in a filter
21162~2
holder 40, comprising a filter backing 38 of stainless steel,
airflow ports 37, and a pump connector 41, said filter holder 40
being connected to an air pump. Monitoring by this means may be
a continuous procedure, where the radioactive aerosol is
continuously collected on a filter 39, and the radiation so
collected, gauged at regular intervals e.g. days. A relationship
between the rate of discharge of the electroscope, and the total
energy released in the sensitive volume 24 due to alpha, beta,
gamma, X-ray radiation or the working level or equivalent energy,
is employed to establish the average for the monitoring period.
Radon daughters may be analyzed by this procedure.
Windows 26 and 36, constructed of mica or aluminized MYLAR* of
about 1 milligram per square centimeter in weight, transmit alpha
particle radiation.
A glass fiber, or membrane filter 39 e.g NUCLEPORE*, MILLIPORE*
of 0.8 micrometer pore size, is suitable for the collection of
radon daughter aerosols. The filter 39 is located in a standard
filter holder 40, arranged such that the filter 39 is located
about 2 millimeters from the window 36, and the aerosol sample is
collected on the window 36 side of the filter 39. The alpha
activity builds up on the filter 39 with time, and reaches a
level which is proportional to the alpha activity due to radon
daughters in the air being sampled. For continuous monitoring
purposes, flow rates of 2 liters per minute will produce a
significant angular deflection of the electroscope indicator 3,
after a 24 hour sampling period, for a working level less than
0.02 WL.
other methods for sampling and collecting a radioactive aerosol
such as impingement or electrostatic precipitation, are well
known by those knowledgeable in the art. These methods can also
be incorporated into the invention as means for sampling and
detecting radon daughters and other aerosols.
The invention is flexible and can be optimized in performance
e.g. design, shape, arrangement of parts, size of the sensitive
volume, and materials employed, for the detection and measurement
of a multiplicity of radioactive materials such as radon, thoron,
beta emitting gases, tritium, and radon progeny at environmental
levels in the ambient air. It can also be optimized for the
detection and measurement of alpha, beta, X-ray and gamma-ray
emitting materials, on surfaces or in the ambient environment
external to the device, and for the detection and measurement of
neutrons.
Various changes and modifications may be made in the above
described measuring apparatus without departing from the spirit
or scope of the present invention.
* REGISTERED TRADEMARK
11
