The Atomic Clock

Age determination by nuclear methods has been well described by Dr. Edward J. Olson of the Field Museum in the following quotation, which is used by permission. He wrote in the museum's Bulletin:

Suppose we had a large box with 6,400 green marbles in it. Then imagine that by some process in exactly one year half of the marbles had turned red. This leaves 3,200 green ones and 3,200 red ones. Suppose that in one year half of the remaining green ones become red, leaving 1,600 green and a total of 4,800 red. If the process continues in this manner we may then construct a table:

Passage of Time


0 years


1 year


2 years


3 years


4 years


5 years


6 years


7 years

Red Red Divided by Green

3,200 1

4,800 3

5,600 7

6,000 15

6,200 31

6,300 63

6,350 127

If we know that this process goes on with regular precision, we could look at such a box, count the reds and the greens and then say how long the marbles had been sitting there. For example, if we found 6,200 red ones and 200 green ones we could say that the process had been going on for five years. In fact, we need not necessarily go through the trouble of counting all the marbles. The right-hand column in the table show the quotient of reds divided by greens. Thus, we need only take out a random sample of a few hundred marbles and count the reds and greens, divide the former by the latter and, if our sample is average, we should obtain a value close to 31 — a time of five years. This process goes on until the last green marble has shifted to a red color. At that time the clock may be considered to have run down. Whal we have just described, in a fairly simplified form, is the so-called atomic clock upon which the much publicized methods of radioactive dating are based.

Rather than by marbles changing color, the actual atomic clock operates by atoms changing to other atoms. The time required for half the population of atoms of one kind to change to another kind is called the half-life.

Before going on let's look once more at the box of marbles to clear up another definition. Let us imagine that every time a green marble converts to a red one it gives off a loud clicking sound. During the first year we would observe 3,20 0 clicks, or an average of around 62 per week. This is moderately noisy. During the second year, however, there would be only 1, 60 0 with 31 per week on the average. During the third year there would be only 8 0 0 clicks, or about 15 per week; and so on. Thus the rate of noise-making would drop off year by year until it finally stopped. At any time during the life of this clock we would have a definite noise level. This we call the level of activity. In the case of atoms this is called the level of radioactivity. So far then we have two methods to measure time. We might, as mentioned before, count a sample of red and green marbles and figure the time from that; or we might simply count the number of clicks per week, or per day, etc., and figure the time from the rate at which they are being produced. In the first method we need not necessarily know how many green marbles were present in the beginning since we are only measuring the quotient of reds divided by greens, which will be the same no matter how many greens were there originally (if you don't believe me you might give it a try, starting with, say, 1 0, 0 0 0 green ones). We need to know only the half-life, which in this example is one year. In the second method, however, we have to know the original population of greens in order to correlate the level of activity with the age of the system. If the halflife is only a year, or an hour, or, as in the case of some atoms, only a few seconds, it is obvious that such clocks will "run down" in a short time and be of little value. To use such weak-springed clocks we have to have an extremely delicate chemical method to analyze exactly the number of green atoms and red atoms. Once the number of green ones has fallen below our ability to separate them in the laboratory, the clock is, for all purposes, dead even though there might be some few green atoms still present. The same is true if our ability to detect the clicks per unit of time is limited by our laboratory devices.

Minerals from igneous rocks are generally the most satisfactory for age determination, although an earthy mineral, glauconite, associated with sedimentary rocks, is also used. The specimen to be tested should be a single mineral, unaltered since it was formed, and it should contain a measurable amount of both the parent and daughter elements.

Potassium changes into argon, a gas, with a half life of 1.32 billion years, which makes measurements of these two elements a suitable means of dating rocks as old as 4.5 billion years, the estimated age of the oldest material in the crust of the earth. In this test, typical of several frequently used, the rock sample, mica or a feldspar, is fused to free the argon gas, and the amount of gas is measured by a mass spectrometer, which by magnetic means isolates the element to be measured. Other methods measure the ratio of rubidium and strontium, which have a similar relationship, or of uranium and thorium.

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