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In the process of radioactive transformation into lead, the uranium-238 nucleus emits a series of particles. The alpha particle consists of two protons and two neutrons (a helium nucleus). Removing this particle from the uranium nucleus reduces the atomic number by two and the atomic weight by four, resulting in thorium-234. This nucleus is in turn unstable, and emits a beta particle (a high energy electron), increasing the atomic number by one and leaving the atomic weight the same, resulting in protactinium-234. Each time an alpha particle is emitted, the nuclide moves two columns to the left on the periodic table; each beta emission moves it one column to the right. This back and forth process continues through a series of elements, each with its own half-life and chemical characteristics (the uranium decay series), ending in lead-206, which is stable.
Similar series exist for uranium-235 and thorium-232, each ending in a stable isotope of lead. Each series includes isotopes of most of the same elements, but with differing half-lives. Midway along in each of these three series is an isotope of radium, followed by an isotope of radon.
Some of these transformations include emission of gamma radiation. Bismuth-214, a descendant of radon-222 in the uranium-238 decay series, accounts for most of the gamma radiation measured by Geiger counters, scintillometers, and spectrometers used in uranium exploration.
It should be clear from the above -- chemistry ranging from heavy metals to noble gases and half-lives from nanoseconds to billions of years -- that this is a pretty interesting field. This is how the age of the earth and solar system were determined.
Some years ago I met Willy Dyck, Arthur Y. Smith, E. M. Cameron and Robert Boyle at the Geological Survey of Canada. I had just finished a masters degree in isotope geochemistry at Columbia University and Lamont, and enrolled at Queen's to pursue mineral exploration rather than moon rocks. I worked up a Ph.D. thesis topic on radon and radium and got support from the GSC and Professors W. D. McCartney, A. W. Jolliffe and J. Nielson at Queen's. Since then, for many years and for many different clients, I studied the geochemistry of these decay series and explored for uranium.
Radioactive substances do not decay; it's the rate of transformation which decays. This rate is exponential: the rate (dN/dt) is proportional to the amount or number of atoms (N) present.
dN/dt = -λN, where λ is the radioactive decay constant for the particular nuclide.
As the amount (N) decreases, the rate dN/dt decreases (decays). The solution of this differential equation is
N = Noe-λt, where No is the amount present initially.
Like all exponential processes, this rate of transformation has a half-life equal to
ln2/λ
The daughter nuclide forms at the rate at which the parent nuclide transforms,
λ1N1
If the daughter nuclide is radioactive, it transforms at the rate of -λ2N2. Thus:
dN2/dt = λ1N1-λ2N2 where the subscripts refer to parent and daughter respectively.
The solution of this differential equation is
N2 = [λ1/(λ2 - λ1)]No(e-λ1t - e-λ2t) , assuming no daughter present initially.
If the parent is so long lived that it doesn't change appreciably during the experiment, e-λ1t is close to 1 and, furthermore, if the daughter is very short lived relative to the parent, λ1 << λ2 and the last equation reduces to
N2 = [λ1/λ2]No (1 - e-λ2t), or
Noλ1 = N2λ2/ (1 - e-λ2t)
After several half lives of the daughter nuclide, e-λ2t becomes close to zero, so that
N2λ2 = Noλ1
In other words, the rate of transformation of the parent is equal to the rate of transformation of the daughter. This situation is known as radioactive equilibrium.
Radioactive equilibrium is approached with the half life of the daughter.
Starting with pure parent, daughter radioactivity is equal to one half
of the parent radioactivity after one daughter half life, three quarters
after two half lives etc. Radon-222 approaches equilibrium with its parent
radium-226 with the half life of radon-222, 3.8 days, reaching essentially
total equilibrium in a few weeks.
Click here for graph of growth and decay.
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The Uranium-238 Decay Series:
Most of the naturally occurring radioactive nuclides are members of one of three radioactive decay series, uranium-238, uranium-235 and thorium-232. Natural uranium is 99.27% uranium-238.
If the parent of the series is separated from the daughters and then the series is left undisturbed, then, after a time, the whole series approaches equilibrium. In other words, each member of the series transforms at the same rate. Equilibrium is attained in the uranium-238 decay series in about 1 million years, in the uranium-235 series in about 100,000 years, and in the thorium series in about 100 years.
The amount of a particular nuclide present at any given time after the parent is separated from the rest of the series can be calculated by solving a set of equations derived by Bateman (Kaplan, 1962). The following table shows the solutions for these equations for the uranium-238 series down to radon-222. The units used express the amount of nuclide present as a proportion of that present at equilibrium.
| Initial | 1,000y | 10,000y | 100,000y | 1,000,000y | |
|---|---|---|---|---|---|
| uranium-238 | 1 | 1 | 1 | 1 | 1 |
| thorium-234 | 0 | 1 | 1 | 1 | 1 |
| protactinium-234 | 0 | 1 | 1 | 1 | 1 |
| uranium-234 | 0 | .003 | .026 | .24 | .938 |
| thorium-230 | 0 | <3X10-5 | <.002 | .08 | .907 |
| radium-226 | 0 | <10-5 | <.0016 | .08 | .907 |
| radon-222 | 0 | <10-5 | <.016 | .08 | .907 |
It is clear from this table that any significant correlation between uranium-238 and its daughters below protactinium-234 in post-Pleistocene material indicates similar geochemical behaviour, not radioactive production. For example, radon-222 in fresh water got there by some means other than the transformation of uranium in solution.
It should be noted, however, that even in orebodies with gross disequilibrium, large amounts of the decay products are normally present.
Gamma radiation is measured by the scintillometer, spectrometer and geiger counter. These surveys were responsible for the discovery of much of the world's uranium resources.
Uranium itself, or yellowcake, is not very radioactive. Most of the gamma radiation in uranium ores comes from bismuth-214, which comes after radon in the uranium decay series.
In theory, there could be a uranium orebody so recently formed that there was no gamma-ray anomaly. Another theoretical possibility is that the surface exposure of a uranium ore body could have most of the uranium leached out, leaving the less soluble radium in place. This would give a gamma-ray anomaly but low uranium assays.
Because gamma rays penetrate a few inches of rock, gamma measurements reflect the radioactivity of a considerable volume of rock, perhaps a ton or more. The advantage of evaluating such a large volume is particularly important in the case of heterogeneous ores with a nugget distribution of uranium minerals. Results are immediate. Any departure from equilibrium in the ore will introduce an error into the assay.
When an alpha particle leaves the uranium nucleus it soon picks up two electrons to become atomic helium. This is an inert and non-radioactive gas and, as therefore can migrate long distances.
Radon and radium are two very different elements with different chemical and physical behaviour.
If you look at the periodic table of the elements, you will see that radon is a noble gas
like helium, neon, argon, krypton and xenon, while radium is an alkaline earth like calcium, strontium and barium.
Both radium and radon are radioactive, emitting an alpha particle.
Both are midway in the uranium decay series with radium coming before radon.
The alpha particle contains two protons and so when the radium nucleus emits a alpha particle the element moves
two columns to the left in the periodic table, becoming radon.
The half-life of radium is 1622 years and radon is 3.8 days;
therefore, all the radium and radon in the earth come ultimately from decay of uranium.
The immediate source of all radon is the decay of radium.
Because radon is a noble gas it is easy to isolate it from other material and count the scintillations in a Lucas cell.
To measure radium we remove all the radon, seal the sample in a jar,
and then let new radon grow in again in solution in water. All this new radon is due to radium in the jar.
We use the same instruments and technique to analyse this new radon, and perform a simple calculation to derive
the radium content.
There are other methods of determining both radon and radium, but our method, based on the Lucas cell,
has been used by scientists since the Lucas cell was invented 60 years ago.
See http://www.finderschoice.com/radon/radium.php.
I mentioned above that radon and radium have different chemical and physical behaviour and we see this in geochemistry.
For example, radon in stream water can come from radium in the associated sediments,
from radon in spring water or from water upsteam.
The two elements provide two different geochemical prospecting techniques.
The abundance of radon is extremely low, one part in 1012 in uranium minerals. There are only 200 tons of radon in the entire earth's crust. This low abundance, combined with its moderate solubility in water, means that water never becomes saturated with radon. Bubbles of radon do not exist as do bubbles of air or the common gases.
On the other hand, where water is in contact with air or soil gas, Henry's law applies and a chemical equilibrium is approached, so that concentration in the gas phase approaches four times that in water. The concentration of radon in the atmosphere is always low due to dilution and radioactive transformation so that radon passes from water and soil gas into the atmosphere and never back again.
Similarly, when bubbles of air are moving through the water they will scrub out the radon. This is how we extract radon in the lab to put it into an alpha counter. This degassing process would also work in surface water ripples and waves, but do bubbles of air move through ground water? See the next section.
Separation of radon from its parent radium is limited by the 3.8-day half life of radon. Radon anomalies related to, but distant from, uranium mineralization are due to migration of its parent radium.
Movement of radon in soil gas is limited by the rates of transportation of soil gas, rates of diffusion of radon in soil gas, and the half life of radon (3.8 days). Tanner (1958?) explained and calculated some limitations on the distance radon might migrate in soil. I'm not sure if this is the right reference and would appreciate some enlightenment. Claims of radon migrating in the ground over distances exceeding 20 meters should be evaluated with these limitations in mind.
Below the water table radon might get into bubbles of other gases. It is unlikely that these bubbles would move very far relative to the half-life of radon.
Movement of radon in surface water is limited by the rates of transportation of the water, rates of diffusion of radon in the water, rates of loss of radon to the atmosphere, and the half life of radon (3.8 days).
To ensure accurate and precise readings, it is important to be aware of counting statistics.
It is tempting to speed up the lab by counting for a too-short time. Particularly for radon in lake water this would result in missing many important anomalies.
In airborne scintillometer surveys it is important to include the broad-band channel. The uranium-only signal is usually too low to register given the high speed of the aircraft.
Radon-222 (radon), from the uranium-238 decay series, is the principle isotope, and the one we're interested in. It has a half-life of 3.8 days. Radon-220 (thoron) is the isotope from the thorium decay series. It has a half-life of 55 seconds.
When analysing radon at the sample site, as in the case of radon in soil gas and radon in snow, thoron can make a large contribution to the reading and mask any radon anomalies. It must be factored out and radon/thoron ratios included in the interpretation.
Portable radon detectors are available from R.H. Morse & Associates Ltd. 416-269-9979 radon@finderschoice.com
Our systems have the advantages of portablility, instant results, discrimination of thoron, and sensitivity. Detection limits in water approaching 1 picocurie per liter are possible with a 15-minute counting time. Based on the lucas cell, our instruments are also used for scientific research and health physics.
Click here for pictures of instruments.
Field labs are operated for radon and radium. These labs use the same portable radon detectors that are used for radon in soil gas and snow. Electricity is not necessary. Locations included Colombia, Madagascar, Ungava, Baker Lake. Inexperienced personnel can be trained but must be closely supervised. These field labs are available from R.H. Morse & Associates Ltd. 416-269-9979 radon@finderschoice.com
Click here for pictures of instruments.
In the Karoo of Madagascar radon surveys in soil gas respond to mineralization buried under 18 meters of sandstone, but not under shale. Karoo details
Soils in the Canadian Shield are less uniform, but radon surveys are still useful. Vein type uranium deposits northwest of Yellowknife are are commonly found along faults and shear zones which are expressed at surface by gullies. Radon anomalies are observed in these gullies above drill-intersected uranium veins. details from Radiore
Radon can also be measured in snow.
The radon/thoron ratio is very important, not only to separate uranium from thorium mineralization, but, also to account for different soil types. Just south of Midwest Lake, near the radioactive boulder train, is a sand ridge with very different soil conditions from the surrounding clay till. This ridge is a meter or two high and trends north-south, parallel to the nearby esker. Readings on this ridge show high radon, high thoron, and low radon/thoron ratios. There is no gamma-ray anomaly here. This situation reflects the higher rate of migration of the two radon isotopes (radon and thoron) from this porous and permeable sandy soil. It is unrelated to the orebody, the radioactive boulder train, or uranium or thorium mineralization in the ground. Similarly, the radon anomalies in the Karoo of Madagascar were not detectable until the false high values with low radon/thoron ratios were filtered out. Karoo details
These surveys should supplement rather than replace the scintillometer.
A radon anomaly in the water of Midwest Lake, near the uranium deposit and the radioactive boulder train, was confirmed by three different operators in three different summers, all before the mineralization was intersected by the drill. map and details
Scintillometers are very useful in prospecting. Radioactive boulders and boulder trains can be readily detected. Excavation can be guided by scintillometer readings. Follow up drilling of a radioactive boulder train discovered in this manner led to the discovery of the Midwest Lake deposit in Saskatchewan.
Portable spectrometers are also very useful in this process, even the McPhar model TV-1A, a threshold spectrometer with a crystal size of only 1.5 inches by 1.5 inches. This instrument has been very useful in quickly sorting out pitchblende boulders from pegmatite.
We made the Kiggavik uranium discovery at Baker Lake in 1974 by flying a
gamma-ray scintillation spectrometer set on broad band.
Due to counting statistics, the uranium channel
wouldn't have detected the anomaly.
Eric Onasick (geophysics) and Ko Griep (geology) made the discovery
in the evening of of August 7.
map and details
Radon can be determined in air in uranium exploration drill holes, and give a wider anomaly than gamma probes. details
Radium and radon in groundwater (wells or exploration holes) can be a strong indicator of nearby uranium mineralization. map and details
Radium and uranium have found to be useful indicators in stream sediments. There is some evidence for stream sediments that radium anomalies are closer to the bedrock uranium source than are uranium anomalies (radium is less mobile). If water is available it is anlaysed for radon and uranium. Locations include Canada, Madagascar, Colombia and Australia. radium details from Bancroft
Radon in the atmosphere can be determined by collecting particles on a filter and counting the alpha rays from the solid radon daughters. At night, the air at high elevations cools sooner and, being heavier, flows down the valleys into the low-lying areas where the warmer air is rising. Measuring the radon in these drainage winds gives an indication of radon emanating from uranium in the ground upstream.
Track Etch and Alpha nuclear are systems which accumulate the radon count over a period of days or weeks. alphameter anomaly at McClean Lake North
Tanner, A. B., 1958: Increasing the efficiency of uranium exploration drilling for uranium by measurement of radon in drill holes. Proceedings of the 2nd U.N. International Conference on the Peaceful Uses of Atomic Energy, vol III, pp 42-45.
Tanner, Allan B., 1964: Radon migration in the ground: A review, Chap. 9 in Adams, J.A.S., and Lowder, W.M., eds., The Natural Radiation Environment: Chicago Univ. Press, p. 161-190.
Tanner, Allan B., 1980: Radon migration in the ground: A supplementary review, in Gesell, T.F., and Lowder, W.M., eds., Natural Radiation Environment III: U.S. Dept. Energy Rept. CONF-780422, v. 1, p. 5-56.
Kaplan, Irving, 1962: Nuclear Physics, 2nd edition.
Morse, Robert H. and Lewis M. Cook, 1979: The role of radon and radium in uranium drilling, Mining Engineering, June.
Syahrir, 2004: Transport of radon in still water, Ph.D. Thesis, University of Cincinnati.
Dyck, Willy and A. Y. Smith, 1968: Use of radon-222 in surface waters for uranium geochemical prospecting, Can. Mining J., April, pp. 100-103.
Robert H. Morse, Ph.D., P.Eng.
R.H. Morse & Associates Ltd.
Toronto, Ontario
tel. 416-269-9979
email:
morse@finderschoice.com
skype: robert.morse.toronto
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