Note: Descriptions are shown in the official language in which they were submitted.
10~ 117
One method and apparatus by which the presence and
concentration of radon and its alpha-emitting daughters can
be effectively detected and monitored is disclosed in
U.S. Patent No. 3,665,194 - Alter et al., dated May 23, 1972
(Canadian Patent 911,622 - October 3, 1972). A sheet of
solid state track-registration material (the alpha particle
detector) is disposed in a protective environment at a
location to be checked for the emission of radon. If the
track-registration material becomes irradiated by alpha
particles, minute damage "tracks" are created therein, which
tracks can be enlarged and made visible by contact with a
reagent to which the tracks display preferential chemical
reactivity.
Ordinarily this uranium exploration is carried out by
burying in the earth inverted cup-shaped housings containing
the track-registration material. The housings are set forth
in a predetermined arrangement (e.g. a grid system) and
permitted to remain for a preselected period of time (e.g.
.~ .
four weeks). Thereafter the housings and detectors are re-
~ 20 moved, the detectors are subjected to a chemical etching
;~ solution and the number of tracks etched on each detector
is counted by microscopic inspection. Correlation of the
results from the various housing are made in order to deter~
mine whether subsurface uranium ore is present and where.
Another method of uranium exploration substitutes an
electronic solid state detector of alpha particles for the
track registration material of the method described herein-
above. These electronic detectors may be used repetitively
either in the same or new locations.
;~ 30 A method and apparatus are described for reducing or ;~
removing the background noise caused by thoron gas (220Rn)
'~ in uranium exploration conducted by the detection
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of radon gas ( 22Rn) emanating from the ground~ This is
accomplished by the use of a number of alpha particle de-
tectors, each of which is disposed in a protective enclosure.
A barrier which permits, but deliberately retards, the
passage therethrough of gases is disposed in the path to be
traversed before such gases can reach each alpha particle if~
detector. The increase in the transit time made necessary
by requiring soil gases to move through the barrier should
be sufficiently long to allow the decay of most of the thoron,
thereby eliminating its contribution to the total signal ~ ~
indicated at the detector. ~ ~ ;
The barrier should not in itself be a source of sub-
stantial quantities of either Rn or Rn. Thé material
of which the barrier is made should not be soluble in water
whereby it can remain stable when in contact with soil. The
enclosure, e.g. cup, should have imperforate side and (in
the inverted position) top areas.
The instant invention conslsts of radon detection
apparatus for ura~ium-ore prospecting, comprlsing in
~ combination: an imperforate protective housing defining an ~;
enclosed volume and having an opening therein, said housing ~
being adapted for burial in the earth with said opening ~;
disposed at the underside of said housing; a body of alpha
par~icle detection material disposed within said housing and ~;
secured thereto for exposure to irradiation by alpha particles
from soil gases entering said enclosed volume through said
opening; and a quantity of a porous medium so disposed be- ~ -
tween said body and the soil that soil gases leaving the -~
soil and entering said housing must traverse the thickness
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.~
o~ said quantity of said porous medium whereby said porous
medium functions as a diffusion barrier that retards the
passage of gases, said porous medium being substantially
free of content emitting either ORn or Rn.
The invention also provides in a method of prospecting
for alpha-emitting ore bodies involving burying a series
of protective housings in the earth in a predetermined
series of locations, each housing defining an enclosed
volume and containing a body of alpha particle detection
material mounted therein spaced from said opening, each
housing being buried with said opening disposed at the
underside thereof, and after a predetermined exposure period
making a determination of the extent of exposure of said
alpha particle detection material to alpha particles during
said exposure period, the improvement comprising the step
of: disposing at each housing a retarding barrier interposed
between the earth and said alpha particle detection material,
said barrier delaying passage of soil gases therethrough a
- sufficiently long period of time to allow decay of sub-
stantially all of any Rn content in said soil gases. ~ -
The description below sets forth the manner and process
of making and using an embodiment of the invention and the
accompanying drawing forms part of the description for
schematically illustrating the invention and the best mode.
The view shown in the drawing schematically illustrates
utilization of the instant invention in uranium exploration.
In order to facilitate the description of this invention
reference will be made to the application thereof in the
method disclosed in the aforementioned U.S. 3,665,194 in
which a solid state track-detector is
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~, ;r
~ 117 RD 9059
used as the alpha particle detector. This invention is
equally applicable when other forms of alpha particle
detector are employed.
The uranium exploration by the method descrbied
in U. S. 3,665,194 cup lO would be placed in excavation ll
so as to rest on the soil 12 at the bottom thereo.
Typically a board would be used to cover the hole and this
in turn would be covered with dirt from the excavation
thereby burying cup 10. This procedure would be repeated -
with a number of such cups in some desired pattern, the
cups remaining buried for the test period. Mounted
within each cup is a piece, or sheet, 13 of alpha particle
track detector material, preferably cellulose nitrate, to
measure the emanation from the ground below of the
gaseous radon isotopes 220Rn and 222Rn. The 222Rn gas is
a decay product of uranium and, therefore, the detection
of such emanations would be an indication of the presence
of uranium in the earth. The other alpha particle-
emitting gas, 222Rn, is a decay product of thorium and,
hence, tracks induced thereby in track detector 13
cons~tu~e an unwanted background caused by alpha particle
emissions from thoron gas entering the mouth of cup lO,
instead of permitting gases leaving the soil through sur~
face 12 to enter directly into the internal volume of cup ;
10, a layer 14 of a porous medium is disposed between
surface 12 and sheet 13 (or other alpha particle detector).
The sides and top of cup 10 are imperforate in order to ~;
prevent the short-circuiting by soil gases around layer
14. Thus, any soil gases reaching the sheet of track
detector material 13 must first pass through layer 14.
A zone, or volume, as least 6.0 centimeters thick must
remain in the cup between the upper surface of layer 14
.. ' ~,'`` "' .
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~ 91~7 RD 9059
and the underside of the surface of sheet 13 so that in
passing through this zone of air, the alpha particles
are slowed sufficiently that they can be detected by
this form of alpha particle detector.
In the arrangement shown, the lip 16 of cup 10
is embedded in layer 14 so that the cups anchored and
the mouth thereof is closed off by the porous medium.
If a solid porous disc is to be employed as porous medium
14, it may be preferable to rest the disc on surface
12 and, in turn, to rest the lip 16 of cup 10 on the upper
surface of the disc.
The thickness of the porous medium 14 should
be appreciably greater than the mean diffusion distance
for 20Rn in the given porous material, (e.g. about ;~
2 cm. in loose soil) and appreciably less than the mean
diffusion distance for 222~n in the same porous medium
(e.g. 150 cm. in loose soil). Thus, the thickness will
depend upon the porosity of the porous material and may be
in the range of from about 0.1 cm. to about 150 cm.
The preferred thickness for the layer of porous medium
would be in the range of from about 1 cm. to about 4 cm.
The thickness of the layer of porous medium
should be sufficient to insure the decay of most of the
alpha particles emitted by any 20Rn that may be presen~
while the thoron gas is diffusing through the medium.
Some radon may also decay in transit therethrough and `
the decay products produced collect in the porous
material. By way of example, diffusion through a 4 cm.
thickness of a suitable medium having an Rn diffusion
constant D = 0.05 cm.2/sec. will reduce the alpha particle ~ -
emitting capability of the thoron component present in
the soil gases to 13.5% of its initial value. Since the
~- ~ . . . ~
RD 9059
117
typical 220Rn contribution is less than 50% of the total
signal encountered and, therefore, less than 50% of the
total to which sheet 13 would be subject, the Rn
contribution would be reduced to less than 7% of the
total signal. This amount would be comparable to the
statistical uncertainty with which radon measurements are
normally made.
Almost any porous material, solid or particulate,
insoluble in water and in which the pores are interconnecting
may be used, e.g. sand, felt or other fibrous sheet or
mat, porous plastic, fritted glass, porous silica body,
etc. Ideally in order to determine whether the porous
material to be employed is not a source of objectionable -~;
; quantities of 220Rn and/or 222Rn comparison is made with
the background to be encountered (i.e. the activity of
the soil where measurements are to be made.) As much as
59% of the background reading can be tolerated. The -
comparison can readily be made by comparing the readout (e.g. ~ ~-
, , ., ~
over a 30 day period) from the porous medium itself with
the readout from a sample of the soil. Preferably the `
activity of the porous medium will be less than 10% of the ~;~
soild background activity. ~ 7
This invention, in addition to having the
utility described hereinabove with respect to uranium -~
exploration, can also be advantageously utilized in
thorium exploration. Thus, if readings are first made
with the usual arrangement described hereinabove, and
then measurements are repeated using the intervening ~ -
porous medium according to this invention, the differences, -- ~
,:. . .
if any, recorded between the first and second sets of
readings will supply the requisite information on the -~
Rn levels and, therefore, indicate the presence or
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RD 9059
absence of thorium.
A hole about 2 feet deep is prepared in the
soil. The bottom of the hole is covered with a layer
about 4 cm. thick of high silica content sand. The
content of uranium and/or thorium should be sufficiently
low so that the emission of 220Rn and/or 2 2Rn will be
less than 5% of the background activity for these gases.
An imperforate plastic cup is placed in the
inverted position with the lip thereof embedded in the
sand. Mounted in the cup (as shown in the drawing) is
a sheet of cellulose nitrate as the alpha particle
detector. The cup dimensions are such that the distance
from the top layer of sand to the surface of cellulose
nitrate is at least 6.0 cm. The cup with the layer of
sand in place is buried as described above and the method
comprising waiting about 4 weeks and then recovering the
cups for readout applies.
" ~
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