You Can't Escape Cosmic Rays
July 1948 Popular Science

July 1948 Popular Science [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Popular Science, published 1872-2021. All copyrights hereby acknowledged.

I thought beginning sentences with the word "So" was a relatively recent (and annoying) thing, but the author of this 1948 Popular Science magazine article on cosmic rays (which are actually particles) uses it at least six times. For some reason those kinds of grammatical peculiarities stand out to me when reading or listening. Other than that, Mr. Mann does a nice job presenting the basics of cosmic rays, which were a fairly newly discovered phenomenon in the day. You can tell by the large number of named PhD's researchers who were delving into the physics of cosmic rays that it was a hot topic, probably highlighted in importance because of the discovery that cosmic rays were (are) constantly bombarding every plant, animal, and mineral on Earth. Surprisingly, no mention is made of the role Einstein's special relativity theory plays with measurements made on the muons created by cosmic rays. The Muon Paradox is explained by time dilation. Concern over whether these highly energetic particles could cause cancerous mutation in living tissue was a great motivator. Fortunately, our atmosphere protects us from the majority of effects produced by cosmic rays. Astronauts outside the atmosphere, on the other hand, need to worry.

You Can't Escape Cosmic Rays

By Martin Mann

So let's take an imaginary flight and meet more of these tiny bullets that constantly rain down on us.

You are sitting in the rain right now - a rain of atomic particles. You cannot see, smell, or feel them. But they are showering you and everyone else day and night, winter and summer.

They are cosmic rays. Although they were discovered about 35 years ago, scientists have only recently learned just what they are and a little about what they do. No one knows yet exactly where they come from.

Although these powerful cosmic particles pass through your body every second of your life - and they are much like the particles that come from an exploding atomic bomb - they apparently do not hurt you. The medical men reason that the human race, during many centuries of exposure to this much radiation, has become adapted to it.

Cosmic rays, while very powerful individually, are spread out more thinly than the radiation from an X-ray machine or the horrible new bomb. They are comparable, in fact, to the drops of rain that fall at the start of a summer shower - heavy, but widely spaced.

Recent experiments with mice indicate, however, that cosmic rays may not be quite as harmless as most of the experts have thought. At the University of Maryland Medical School, Dr. Frank H. J. Figge arranged eight cages of mice so that they received varying amounts of cosmic radiation. Some of the cages were covered with lead plates, because lead intensifies these rays. Other cages were not covered, so that the mice in them would receive only the normal amount of radiation. Dr. Figge then injected all the mice with methylcholanthrene, a chemical that induces cancer. In the cage receiving the most concentrated dose of cosmic rays, 21 of the 23 mice developed cancer in 10 weeks. In the worst of the uncovered cages, only 10 mice had developed cancer during the same period.

Dr. Figge is careful to point out that this one experiment only indicates something worth further investigation. It by no means proves that cosmic rays cause cancer.

There are ways in which the atomic particles from out in space certainly affect you. When they enter the upper part of the atmosphere, they transmute some of the nitrogen there into a radioactive form of carbon called carbon 14. It is just like the carbon 14 that is manufactured now in atomic piles. But this carbon 14 is present in the air you breathe (as part of the carbon dioxide), and gets into your body and stays there. Nobody knows whether this hurts you.

Such recent discoveries as this may turn out to be merely interesting tidbits of knowledge, with no practical value. But other things learned from cosmic rays already have proved tremendously important. Some of the knowledge that enabled physicists to produce the atomic bomb was acquired by studying cosmic rays. So they are being studied a lot more now, and there's no telling what will come of this work.

Suppose we take an imaginary trip - a very imaginary trip - into space to observe this mystery with the scientists. Since we cannot see cosmic rays, we will take a cloud chamber with us. It's a device with which the particles can be tracked and identified. We'll also take a Geiger counter to count the rays. Our guide on this strange tour will be Dr. Walter Heitler, cosmic ray expert from the Institute of Advanced Studies in Dublin, Ireland, who is now a visiting professor at Columbia University.

Dr. Heitler suggests that we follow the rays down through the atmosphere as they approach the earth. All right, let's keep our imaginary space ship going until it leaves the atmosphere, about 200 miles up. That is higher, of course, than anyone has actually traveled or even sent instruments. But scientists are pretty sure that out here, where the wide open spaces really begin, the cloud chamber will catch the tracks of protons (the positively charged particles found in atomic nuclei) and probably little else. Proton tracks can be distinguished from others in the cloud chamber because a proton is about 1,845 times as heavy as an electron and its track curves in the direction of positive charge.

These protons are the things that come from far away. Physicists call them the "primary" radiation. They come fantastically fast, with energies as high as a million billion electron volts. Even the new atom-smashing machines now being built (PS, Jan. '48, p. 113) will reach only one billion electron volts.

Bombardment Increases Near Earth

Now let's follow these high-powered protons down to see what happens when they smash into the earth's atmosphere. The air does not become thick enough to make much difference to the protons until they're down to within about 34 miles of the earth. Near that point, however, our Geiger counter begins to click faster, recording an increasing number of rays. And the tracks of different kinds of particles appear in the cloud chamber. Our space ship is now being bombarded not only by protons but also by electrons and neutrons - two other components of ordinary atoms-and by several mysterious particles called mesons or mesotrons.

Dr. Heitler explains that in this part of the atmosphere there are enough atoms of air - oxygen and nitrogen - to get in the way of the high-speed protons from outer space. As protons bang into these atoms, they knock loose their outer clusters of electrons. And some of the protons may penetrate to the nuclei of the air's atoms and smash them open, turning loose the protons and neutrons in their interiors.

How Mesons Are Born

The mesons, however, are actually created rather than blasted out of something already existing. A high-speed proton passing close, to a neutron, loses some of its speed energy. This speed energy is converted into mass (it's as though an atomic bomb worked backwards), and a negative charge from the neutron hooks onto the new mass. Where originally there were two fast particles, a proton and a neutron, there are now three: a slower proton, a proton made from the old neutron, and a meson.

Mesons may also be created by collisions between two protons, yielding a meson, a proton, and a neutron. And scientists have found, furthermore, that there are several different kinds of mesons. The first type discovered weighed about 200 times as much as an electron. In the last few months, however, mesons that weigh 350 and 800 times as much as an electron have been found, and the physicists expect to turn up still other varieties, These mesons can have a single negative charge (like an electron), a single positive charge (like a proton), or be neutral (like a neutron).

As we ride clown through the atmosphere in our imaginary space ship, the Geiger counter clicks faster and faster. More and more of the primary protons are being stopped by atoms of air, and more and more electrons, protons, neutrons, and mesons are appearing around us. This secondary radiation, in turn, creates more cosmic rays. The secondary protons, you see, produce the whole list of cosmic particles, just as the primary ones do. And the neutrons and mesons can also ionize and break up atoms.

You eat radioactive meals because of cosmic rays. Radioactive carbon, produced from the air's nitrogen by cosmic rays, is in all animal and vegetable carbon.

But Dr. Heitler calls our attention, as we descend, to some new kinds of pictures in the cloud chamber. One type shows the death of a meson. Mesons, like radium and similar substances, are radioactive. They decay into an electron and a neutrino (a neutral electron) after a lifetime of only about two-millionths of a second.

But a little radioactivity doesn't seem to hurt us. The normal human body even contains some radium. There's more than half an ounce in the world's total population.

These decay electrons travel very fast, and they cause the second new type of picture in the cloud chamber. This picture looks like a heredity chart - the streak of a single electron branches again, and again until there is a host of new particles. This is called cascade radiation.

Mass into Energy - and Back

In a cascade, a high-energy electron hits an atomic nucleus and emits a photon, which is a tiny package of pure light energy. Photons consist of energy alone and do not have weight like other particles. When they hit atomic nuclei, they change back into electrons. Then those electrons emit more photons, and so on and on.

These processes fill the sky with particles of all kinds. This shower from the heavens is thickest, however, about 12 miles above the earth. As our space ship moves down lower than that, Dr. Heitler points out that the Geiger counter registers fewer and fewer particles because the atmosphere is becoming dense and the cosmic energy is thinning out. By the time we reach the earth, the tremendous energy originally present in the primary protons has been divided and redivided so many times that relatively few individual particles are powerful enough to continue through the air, and the Geiger counter clicks only a few times a second. Most of these clicks are caused by mesons.

If we unloaded the Geiger counter and cloud chamber and carried them down into a coal mine, Dr. Heitler adds, we would find that the cosmic rays lessened as we descended, because the earth too absorbs them. About a mile below ground, the rays would disappear almost completely.

Learning these things has taken a long time, much expert craftsmanship, and some very fancy head-scratching. About 50 years ago, when Becquerel and the Curies found that certain elements were radioactive - that is, that they emitted strange particles - scientists began to look for radioactivity in whatever was handy, even the air. They found it, too. So they supposed that there was a trace of some radioactive substance in nearly everything - the earth, air, water, buildings, and so on.

Then a couple of very inquisitive fellows, Y.F. Hess of Austria and W.H.V. Kolhörster of Germany, sent their instruments up in a balloon. They reasoned that the radiation detected should diminish as the balloon went farther away from the earth. But it didn't. The higher the balloon rose, the more intense the radiation became. So there was only one plausible explanation: the radiation came from somewhere out in space.

A Japanese physicist, Hideki Yukawa, was responsible for the next big advance, though not working on cosmic rays at the time. He was trying to figure out why the nucleus of an atom stays in one piece. His computation of the force necessary to hold it together revealed that it was a type of force that could be represented by a particle. That particle, he said, should weigh about 200 times as much as an electron, have a single negative charge, and last only a few millionths of a second.

So the cosmic-ray experts began looking for such a particle, and Dr. Carl D. Anderson of the California Institute of Technology soon found it. Dr. Yukawa had forecast the existence of mesons.

The most recent discoveries resulted indirectly from World War II. Two of the deadliest weapons ever devised, the B−29 plane and the V−2 rocket, enabled the physicists to measure cosmic rays at altitudes higher than were previously attainable. '

Flights by Dr. Anderson in a B−29 made a pretty sure thing out of what had been a strong suspicion: the basic radiation from out in space is almost exclusively protons. And V−2 experiments directed by Dr. James A. Van Allen of Johns Hopkins University settled another years-old question: where does the cosmic-ray intensity level off? A 100-mile flight last July finally pegged this point at 34 miles up.

More Puzzles to Solve

There are still a lot of cosmic-ray puzzles for smart young men to answer. The most intriguing is the problem of where these rays come from. Most scientists believe they come from far outside our solar system - maybe some place in the Milky Way. There are other theories, too, such as the "solar cyclotron" proposal of Winfield W. Salisbury, research director of the Collins Radio Co., and Dr. Donald H. Menzel, Harvard astronomer. They suggest that the movement of atoms in the sun generates very long radio waves. These radio waves then accelerate particles that exist in space near the earth - in much the same way particles are accelerated in a laboratory cyclotron.

But more evidence is needed. How long it will take the scientists to obtain it no one knows. But the planes and rockets that may help them tremendously are being developed now. Scientific progress, like cosmic radiation, is overtaking us day and night, winter and summer.

Posted January 16, 2024