Radon/thoron ratios find the valuable anomalies.
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Radon-222, from the uranium-238 decay series, has a half-life of 3.8 days. Thoron (radon-220),
from the thorium decay series, has a half-life of 55 seconds.
Our lucas-cell system can resolve these two radon isotopes in the field, just by the drop
in count rate over the first few minutes. See also
www.finderschoice.com/radon/measure_thoron.php>.
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The ratio of radon to thoron can be used to weed out false anomalies.
The ratio accounts for variations due to soil conditions (moisture, permeability etc.) and
weather
(wind and barometer), because whatever local soil and meteorological conditions cause anomalous
radon fluxes would also cause anomalous fluxes of other soil gases, including thoron.
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Another way of looking at it is that, with its shorter half-life, thoron won't travel as far as radon.
We aren't interested in what happens a few inches or feet from the sample hole.
We are interested in what's farther down, i.e. radon, and orebodies.
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So the significant radon anomaly is one that is not accompanied by thoron, hence high
radon-thoron ratios.
Field-proven
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In 1972 I ran an east-west radon-in-soil-gas line across and perpendicular to
the boulder train just south of Midwest Lake in Saskatchewan.
As expected, I noted high radon readings at the boulder train.
The ground here is low, flat and damp. The boulders give a strong gamma-ray signal as well.
At the west end of this soil line is a north-south ridge of dry sand, about two meters high and
parallel to the nearby esker. This sand ridge is removed from any uranium mineralization.
On the top this sand ridge I was surprised to find high radon readings.
However, the count rate here dropped off rapidly in the first few minutes, indicating thoron.
This thoron anomaly fits the theoretical considerations above. The dry, permeable, sandy soil
gives up its gases
readily, all its gases, methane, carbon dioxide, nitrogen, oxygen etc., as well and radon and
thoron. So, it's not really a radon or thoron anomaly indicating mineralization at depth,
but a permeability anomaly due to local soil conditions right at the sample point.
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In 1984 we ran an
orientation survey
in the Karoo of Madagascar.
By calculating radon/thoron ratios and radon, we were able to recognize anomalies due to
uranium mineralization buried as deep as 18 meters.
Simply stripping thoron out and plotting pure radon is not enough:
you have to use the radon/thoron ratios as well.
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In 2008, one of our clients reported very good results in glacial soils.
They made an important drill intersection at least three meters under anomalous surface radon
and high radon/thoron ratios.
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To our knowledge, radon/thoron ratios have not been applied to
oil and gas exploration;
they could prove useful.
False radon anomaly in sand (not ours) was drilled with negative results
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I learned of a recent survey using a different system (not ours) which can
measure radon only, not thoron. This survey was in a glaciated terrain similar to Midwest
Lake. They found a strong radon anomaly on a small sand hill, which was drilled and
turned out to be a false anomaly, not associated with uranium mineralization. Most likely this
anomaly is due to the ease of gas migration out of this dry, sandy, permeable soil. These
mobile gases would include thoron, as well as radon, and we expect that our
instrument would have identified high thoron levels (low radon/thoron ratios), and flagged
it immediately as a false amomaly.
The gamma-ray spectrometer is not sensitive to these thoron values
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These "high" thoron levels in soil gas are only trace amounts and cannot be detected by
a gamma spectrometer or scintillometer.
Our Lucas cell measures alpha radioactivity in the gas
phase, and the only likely source of alpha radiation in gas is radon and thoron.
Background in our system is low (below 2 counts per minute),
because alpha rays are weaker than beta and gamma, and cannot penetrate the walls of the
Lucas cell.
To achieve such low backgrounds with beta rays would require a massive lead shield, and for
gamma rays would be totally impractical.
The half-life of thoron is only 55 seconds, and the amount that
can get into the gas phase in this short time (tens of counts per minute)
is too low relative to the thoron gamma background (hundreds of counts per minute) for
it to be detectable with a gamma system.
High readings on the thorium channel of the gamma spectrometer indicate thorium minerals in
the rock or soil, and not thoron in a mobile gas phase.
With our Lucas cell, however, we count alpha rays.
With the low background and thoron's short half-life we need only a few dozen thoron atoms
and can count most of these atoms in the first three minutes.
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INSTRUMENTS
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We measure: radon - radium - thoron - radon daughters - alpha radiation.
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The
Lucas cell
is recognized as the most sensitive and reliable method for these elements.
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Our instruments are used around the world in exploration for uranium, oil & gas, groundwater and hydrothermal,
and in environmental protection, health physics, earthquake prediction, and evaluation of hydrocarbon and NAPL contamination.
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In the radon business since 1968, our latest major instrument update was 2015.
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Modern, low-power, field-rugged electronics. Some earlier versions still working after 35 years.
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Continuous real-time monitoring and data recording.
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Winter and summer, from the Sahara Desert to the Canadian Shield, our instruments have faced up to severe field conditions.
- Intrinsically safe functions.
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Sensitive to geochemical trace levels necessary for radon in lake water and for radon-thoron isotope ratios.
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Can work in a tent without electricity or be carried from point to point in the field.
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50 readings per day. Results available immediately.
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Rechargeable battery pack good for a long day in the field and recharges in a few hours.
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Can be operated by junior personnel if carefully supervised.
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Same instruments used for radon and radium in soil, sediment,
plant parts, rocks, water, soil gas, air, and snow, and for radon daughters in air.
- EPA compliant.
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Click here for more details of instruments.
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Click here for or other instruments, components and
accessories we provide
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Technical specification sheets and pictures of our instruments provided on request.
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Multilingual consulting and training (if required).
For instruments contact
R.H. Morse & Associates Ltd.
1-416-269-9979
morse@finderschoice.com
skype: robert.morse.toronto |
Robert H. Morse, Ph.D., P.Eng.
November 28, 2009
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