How Are Radioisotopes Used in Research?
They May Be Used as “Tracers”
Man’s attempts to describe the universe consist of finding answers to the questions he puts to Nature:
How deep is a well? Toss in a rock.
Where is the cat? Hang a bell on him.
How far does a wild duck fly? Put a marker on his leg.
Where are the fireflies? Just watch at dusk.
Is our satellite still up? Listen for the radio signal.
Other questions arise in agricultural research:
How fast do roots grow? How deep? How soon does water get to them after a rain?
When does a mouthful of hay reach a cow’s stomach?
How long until nutrients get into her blood? Her milk?
How far will pine pollen travel on the wind?
How deep does an earthworm burrow?
To answer these questions, scientists need some kind of miniature genie, one who will shout at the proper moment, “I’m here!” When the root has reached the fertilizer or the water has reached the root; when the hay becomes transmuted to milk, or the earthworm arrives at a particular spot—then this invisible little servant who has made the trip could announce, “I’m here!”
Such a helpful genie exists as the radioactive atom: he is invisibly small, obedient, transportable, digestible, immune to fire, flood, or famine, able to travel under his invisible cloak to the secret hiding places of Nature’s creatures and announce to waiting Geiger tubes, “I’m here!”
The physically unstable radioactive atom behaves chemically exactly like its stable counterpart until the instant it emits its radiation and becomes stable. For example, radioactive phosphorus behaves, biologically and chemically, like stable phosphorus until it emits a beta particle and becomes stable sulfur. If the beta particle enters a gas-filled Geiger tube, it produces a tiny burst of electrical energy which is registered by the counter.
Like fireflies which reveal themselves at dusk by flashes of light, radioisotopes announce their numbers and locations to sensitive Geiger tubes by flashes of invisible “light.”
How Effective Are Radioactive Tracers?
One way to see how valuable radioactive tracers are is to compare them to standard chemical techniques. A sensitive chemical test can perceive molecules as dilute as 10⁻⁷; that is, it can detect a molecule surrounded by 10 million molecules of another kind. A good radioactive tracer technique, by comparison, can distinguish concentrations of 10⁻¹¹; that is, it can trace one in 100 billion.
In other words by the chemical test you could find a person in metropolitan New York with a secret tattoo on the roof of his mouth. By the tracer method you could find this same person anywhere in the world, even if the world population were multiplied fiftyfold.
In the chemical test you could distinguish the equivalent of one kernel of corn in one-tenth of a boxcar load; in the tracer, one kernel in 850 boxcars.