Part 1: How Radiation Was Discovered
A story is told of a young physicist, Fritz Houtermans. One summer evening he was feeling exhilarated as he strolled down a quiet street in the old university town of Gottingen, Germany. His girlfriend was at his side. She sensed his mood, looked up at the night sky and said softly, "Look up at the stars. Aren't they beautiful."
"Yes," replied the young physicist, "and do you know you are holding hands with the only man alive who knows why they shine?"
He was telling the literal truth. That very day Houtermans had discovered just what the nuclear reactions were that gave the stars (and our own sun) their shining radiant energy.
It is good to start a lesson on radiation with this story because many people think of radiation today mostly in terms of its dangers. Like everything else in science and society, there are at least two sides to the story, and we have a lot to learn about both. Though radiation is as old as the universe itself, penetrating the mysteries of radiation has only just begun.
The first kind of radiation to be studied was light visible light. From ancient times people have been fascinated by light. And with good reason. Some people worshipped the sun, the source of this radiant gift. Farmers knew that light was needed to grow crops. Sailors knew that light brought changes in the weather. And everyone knew that light brought warmth, sight and beauty.
But no one knew what light was. How it was made. How it moved. What happened to it when it disappeared.
At the dawn of science in ancient Greece, scientists like Anaximander, Aristotle and Democritus wondered about light and made up theories to explain it. Democritus, for example, said light was made of tiny little bullets streaming away from sources like the sun. These little bullets would bounce off whatever they hit, warming it in the process. Euclid even formulated some laws to describe how light reflected from mirrored surfaces and bent as it went through water and glass.
One of the first scientists to solve some of the mysteries of color-almost two thousand years after Democritus and Euclid-was Isaac Newton. Working alone at his country home in Woolsthorpe, England, where he retired to escape the plague in 1665, Newton used a prism to break up sunlight into colors.
The colored rays, he found, could not be further separated. They could be put back together, however, to make white light again. Thus, said Newton, contrary to what was commonly believed at the time, it is white light that is the mixture. Each of the colors is its own pure ray.
Newton, like Democritus, said that each colored ray must be a stream of tiny particles since light always cast sharp shadows and did not bend to go around corners the way a wave would.
It took over a hundred years to prove Newton wrong on this last point. It was on November 24, 1803 that another English scientist, Thomas Young, performed a simple, elegant experiment in front of the Royal Society in London.
Light was not a particle, but a wave, said Young. And here is the proof. He shone a thin beam of light through two small holes. He pointed triumphantly to the telltale bands of light and dark streaks that appeared on the screen behind the holes. Just look, said Young if you split a beam of light you do get bending! Just like a wave, light produces patterns of light and dark bands as the light waves cancel and add to one another. If light was a simple stream of tiny particles, these bands of dark and light could not happen.
Young later figured out in his experiments the actual wavelengths of light, which turned out to be very small indeed-smaller than a millionth of a meter.
This did not end the controversy over light being a wave or a particle, however. In fact, it has continued right down to our own day. And the scientific view today is that light is both a wave and a particle!
Let's look at how Newton's and Young's work opened the door to the widening spectrum of radiation. They studied the visible spectrum from red to yellow to blue and violet. Soon other scientists were finding there was invisible light. That is, radiation that fanned out on either side of the visible spectrum in a much broader rainbow of color.
There was not only violet light, there was an ultraviolet light. Ultraviolet light could not be seen with the eye, but when it shone on certain minerals, it made these minerals glow with what came to be called a black light.
And off the other end of the spectrum, after red light, there was found to be an infrared light that one could not see, but one could feel as warmth. More progress was made in the middle of the nineteenth century, expanding the spectrum of radiation.
Pioneers like the Englishman Michael Faraday, the Dane Hans Christian Oersted, the Italian Alessandro Volta and the American Joseph Henry made slow progress in discovering new facts and laws about electricity and magnetism.
Electricity moving through a wire, it was found, would affect a magnetic needle. And vice versa! A moving magnet would make electricity flow through a wire. The basic principles were there for later construction of electric motors and generators. And more important for our subject, new questions arose.
What was the field surrounding magnets and electrical conductors? Something seemed to radiate out from these strange magnets and electrical devices? Did it have anything to do with the radiation we already knew something about? In the middle of the nineteenth century, the Scottish physicist and mathematician James Clerk Maxwell found some answers to these questions.
Surrounding any current carrying wire, said Maxwell, is a moving-out field of electromagnetic waves. And-amazing fact-these electromagnetic waves act very much like visible light waves.
Like visible light waves, electromagnetic waves reflect, refract and are absorbed. And-here was the clincher-they move at the same speed and follow the same mathematical laws as light.
Suddenly the spectrum of radiation was multiplied manyfold. Not only do we have red, yellow and blue light; not only do we have ultraviolet and infrared light; now we have a whole new world of radiating electromagnetic waves. All with longer wavelengths and lower frequencies than visible light. Later these longer & radar wavelengths, lower-frequency waves of radiation came to be known as the familiar radio, television, radar and microwave frequencies.
How about the other end of the spectrum? Could there be electromagnetic waves with shorter wavelengths than ultraviolet? Yes, indeed. Though it took another fifty years to discover any such waves.
The first ones to be discovered were the electromagnetic waves just above ultraviolet on our spectrum. X rays.
The discovery of X rays by the German physicist Wilhelm Roentgen in 1895 was an international sensation. Working with vacuum tubes, he bombarded a metal plate with high-speed electrons. He found that invisible rays were produced. These mysterious new rays could go through skin and flesh and give a picture of a person's bones.
His X rays became world famous-as well as world misunderstood. A legislator in New Jersey introduced a bill banning their use in opera glasses in order to preserve the modesty of ladies.
X rays soon found many valuable uses in medicine and in industry. It was, unfortunately, quite a few years later that a handful side of X rays was also discovered. They could be harmful to living tissue, or even cause cancer if the exposures were too great or too prolonged.
So, too, with the next discovery in our radiation story- radioactivity. Here the first breakthroughs were also made around the turn of the century. Shortly after Roentgen discovered X rays, a Frenchman, Henri Becquerel, stumbled upon another kind of radiation. He found that the element uranium spontaneously gave off rays that would darken photographic plates, as did X rays.
Marie Curie and her husband, Pierre, worked long hours in a drafty garage laboratory in Paris to isolate a new element, radium, that gave off very powerful new rays.
What were these new rays? New forms of radiation? Experiments were done all over the world. Scientists found that some of the rays coming out of radium and uranium acted like electromagnetic waves. They were weightless. They traveled at the speed of light. And they obeyed all of the mathematical equations Maxwell had discovered for the electromagnetic spectrum.
These were later called gamma rays. And indeed they were a part of the electromagnetic spectrum, with still shorter wavelengths and higher frequencies than X rays. Besides the gamma rays, there were at least three other kinds of penetrating particles that came out of the radioactive atoms of radium and uranium. Each of the three kinds of rays acted quite differently from the others. And none of these three new kinds of radiation was a part of the electromagnetic spectrum.
What were they then?
Well, one kind of ray turned out to be made of heavy particles that were easily stopped by a piece of paper or by the skin. A stream of them could be bent by a magnetic or electrical field.
They were charged positively. They were named alpha particles. These alpha particles were later found to be identical to the nuclei of the element helium.
Another kind of radiation coming out of radioactive elements was also made of particles. But these particles were much lighter, moved much faster, were more penetrating and were bent in the opposite direction than alpha particles by magnetic or electrical fields. These rays were named beta rays. Beta particles were found later to be identical to electrons.
And finally, in certain rare cases, another kind of radiation came out of elements like uranium . These were particles, too, but they were not affected at all by magnetic or electrical fields. They were named neutrons. Neutrons were emitted when a uranium nucleus fissioned. That is, when it split in two or more parts, and in the splitting emitted very large amounts of energy as well.
In the twentieth century, physicists discovered where electromagnetic waves come from. All of them, except the gamma rays, come from leaps of electrons inside of atoms, or from the vibrations of atoms and molecules.
Engineers soon learned to control these electron leaps, these atomic vibrations. The result was radio, television, microwave ovens, computer circuits and the whole new world of electronic marvels.
It was a somewhat different story with the other basic kind of radiation. The kind that comes out of radioactive elements like radium and uranium. The kind that is associated nowadays with nuclear energy.
Here the radiation came not from the leaping electrons of the outer atom, but rather from the cracking apart of the nucleus of certain atoms. In the case of naturally radioactive elements like radium and uranium, their nuclei are continually and randomly breaking apart. From this breaking apart we get alpha, beta and gamma rays, exploding out at high speeds and dangerous momentums.
When special kinds of uranium atoms' nuclei actually split in two, still another kind of radiation is emitted, high speed neutrons. This happens in a controlled way in a nuclear power plant, and in an uncontrolled way in a nuclear bomb. This atom splitting process is called nuclear fission.
And finally, when certain kinds of very light hydrogen atoms combine, that is, when their nuclei combine we also get radiation. In this case, we net the kind of radiation that Fritz Houtermans had figured out on that summer evening in Gottingen. The kind of radiation that our sun and our stars emit. The kind of atom combining process that is called nuclear fusion.
How all of these kinds of radiation affect living creatures like us is the subject of the second apart of this program, Radiation and You.
Part 2: Radiation and You
The world is made of atoms. The world is powered by radiation. And the world includes you and me.
In other words, we too are made of atoms and powered by radiation.
Like some kinds of atoms, some kinds of radiation can be harmful to living things. Like most atoms, most radiation is not only helpful, it is absolutely essential to life on this earth.
Like light, for instance. Visible light.
Light rays have been streaming in to earth from our brother sun for over five billion years. Those light rays from our sun have been the power behind all life on earth throughout these billions of years, and are the power behind all life on earth today.
Like other kinds of radiation, light is itself weightless and invisible. Like other kinds of radiation light travels at 186,000 miles per second. Though the sun is over ninety-three million miles away from earth, it takes only about eight minutes for light rays to get from the sun to us.
Like other kinds of radiation, light, even though invisible in itself, can be detected by its effects on other things. Green leaves, blue sky, photographic paper, our eyes, to mention just a few.
And finally, like other kinds of radiation, light can hurt as well as help. Anyone who has ever been sunburned can attest to one kind of hurt, and anyone who has ever eaten food can attest to a major kind of help.
Let's get more scientific now. Just what is this "light" we all know intuitively, but few know scientifically?
Visible light, scientists tell us, is a form of energy. Energy in the form of electromagnetic waves. What is an electromagnetic wave?
It is not easy to describe. Imagine a wave in a pond or in the ocean. Notice how it changes, moves up and down, forward and back, in and out, in regular cycles. Now keep the movement, and take away the water! An electromagnetic wave is like that. A mysterious "field" of electromagnetic energy that moves in regular cycles.
Electromagnetic waves are hard to describe in words, or picture in images, but they can be accurately described in numbers. Two important measures are wavelength and frequency. How long the waves are and how frequently they move back and forth, up and down, out and in.
In the case of light, these wavelengths are so small they are measured in micrometers-that is, millionths of a meter, about the diameter of a bacterium. Another way to visualize this small size-it would take between this-three thousand and sixty six thousand visible light waves to make up an inch.
The frequency of visible light waves is as fast as the wavelengths are short. On the order of 10 to the tenth power cycles per second.
Visible light makes up the center of the electromagnetic spectrum charted below. Moving to the left, we come to waves with longer wavelengths. These are called infrared waves. They are invisible to the eye, but you can feel them as heat when they strike your skin.
We get infrared radiation from the sun to warm us here on earth. We also get infrared radiation from fires, from radiators in our homes, from light bulbs and from just about any other hot object. As a matter of fact, we also give out infrared radiation ourselves, generated by our own bodily heat.
The next longer wavelengths are in the microwave region. These are useful to us today in many ways. Microwaves are used to cook our food, to open garage doors, to communicate via cell phones, to guide airplanes and missiles and to trap speeders with radar.
Move to still longer wavelengths (and lower frequencies) and we enter the band of broadcasting. Television, radio, short-wave radio, FM and AM radio, and finally the electromagnetic waves from electric power lines.
These last and longest waves have lengths measured in thousands of miles and frequencies as low as sixty cycles per second.
It can give you an eerie feeling to think of all the thousands of electromagnetic waves that hit and pass through your body every second of every day. This radiation comes from all directions and you are powerless to escape it. It comes from your local and far away radio and TV stations. It comes from your local power lines, telephone lines, international satellites orbiting the earth, and the personal computer next door. It comes from the ninety-three million mile away sun, the ten million light year away star, and the candle in your romantically lighted dining room.
It may give you an eerie feeling to have so many electromagnetic waves invading your body but, so far as we know today, you don't have much to worry about healthwise.
Looking directly at the sun or any other intensely bright light can harm the retina of your eyes. Visible light, infrared light and microwaves can be intense enough to overheat and burn living tissue. But these effects are fairly easily avoided by common sense, fail-safe doors on microwave ovens, etc. (Some very recent studies do show possible harm from long exposure to high voltage power lines.)
As we move in the opposite direction on our electromagnetic spectrum the dangers definitely do increase.
The electromagnetic waves with wavelengths just shorter and frequencies higher than visible light are called ultraviolet light. This is the kind of radiation that makes what are called black light displays. While your eyes cannot respond directly to ultraviolet waves, certain minerals can. They will glow, that is, change the ultraviolet radiation into visible light radiation that your eyes can see.
The sun sends out a strong dose of ultraviolet radiation to us here on earth every day. Fortunately there is a zone of ozone gas that surrounds the earth and filters out much of this potentially harmful ultraviolet light. But not all of it.
The part that gets through the ozone is responsible for sunburn and for much of the skin cancer that plagues humans. Especially people who spend a lot of time in the bright sun, either from choice or from necessity. Some scientists are worried today that this ozone shield is being destroyed by some modern industrial chemicals. If the ozone should decrease significantly, it would mean an increase-maybe a catastrophic increase-in human skin cancers. It could also result in other harmful effects on the living world.
As we keep moving in this direction toward still shorter wavelengths and higher frequencies, we run into still more powerful waves of radiation that scientists call ionizing radiation.
An ion is an atom that has lost or gained electrons. An ion, thus, has an electrical charge and is much more chemically reactive than a neutral atom. Ionizing radiation can penetrate deep into cells and knock electrons out of atoms in the cell. If cell atoms are ionized, unpredictable things can happen in the cell.
If too many atoms are ionized, the cell's metabolism may get so out of whack that the cell dies. Or, worse, it may be that the ionized atoms are a part of the controlling DNA molecule in the cell's nucleus. Since it is the DNA that controls the cell's activities, especially in reproducing, this may result in uncontrolled cell reproduction. In other words, cancer.
The first band of ionizing electromagnetic waves beyond ultraviolet is the zone of X rays. X rays can penetrate skin and flesh to give a picture of bones, or show up cavities in the teeth.
Over ninety percent of the total human made ionizing radiation that most people will ever receive in their lifetime will come from these medical and dental x rays. In most cases the benefits far outweigh the risks. However, there are some risks. Your own doctor or dentist is your best source for help in avoiding these risks.
Beyond X rays, physicists have discovered another range of ionizing radiation with even shorter wavelengths and higher frequencies. These are called gamma rays.
Gamma rays are emitted by radioactive elements on earth. We can control gamma rays in special industrial applications in order to see where no one had ever seen before-through thick metal plates for instance. This gives you a new way to check on the safety of bridges, of tankers and other heavy metal machines and structure.
At the opposite end of the materials spectrum, gamma rays are useful in medicine. They can be focused, for instance, to deliberately kill fast-multiplying cancer cells deep within the body-without harming normal cells.
Besides this spectrum of electromagnetic waves there is still another kind of energy radiation that is important today. This is the radiation that comes from radioactive elements-natural and human-made. As well as from nuclear fission and fusion, natural and human-made.
Most atoms on earth are stable, non-radioactive ones. However, a very small number of atoms are what we call radioactive. That is, the nuclei of these atoms are unstable. They spontaneously break apart. In that breaking apart they release radiation of four different kinds.
These four kinds of nuclear-generated radiation are alpha particles, beta particles, gamma rays and high-speed neutrons. Each of these four types of nuclear generated radiation has its own dangers and its own potential for good. Alpha particles are comparatively large in size and travel at high speeds.
An alpha particle is the same thing as a helium nucleus, two protons and two neutrons. It has a positive charge of two. Ordinarily alpha particles cannot penetrate the skin, but if radioactive atoms that produce alpha particles are taken in through the mouth or lungs, they can then disintegrate in the lungs, stomach, blood stream or other organs, and cause serious damage to body cells.
Particularly dangerous are radioactive isotopes like iodine 131 which can accumulate in the thyroid gland. After many years of such exposure, cancer may result. Cesium 137 tends to accumulate in the liver, spleen and muscles. Barium 140 in the bones. Other radioactive atoms also have the potential to invade and harm the body from inside.
Beta particles are much smaller. Beta particles are high speed electrons that shoot out of a radioactively disintegrating nucleus. Being smaller, they are also more penetrating.
Gamma rays also come from the radioactive decay of certain elements. Gamma rays, remember, are electromagnetic waves of very short wavelength and very high frequencies. And finally in the special case of nuclear fission or fusion-whether in bombs or power plants-high speed neutrons are emitted that can also cause ionizing destruction to atoms in cells. Like other kinds of radiation, they can also be useful in medical and industrial applications.
That completes our list of kinds of radiation known today. With one exception-cosmic rays.
Cosmic rays are mysterious particles that come from unknown sources in outer space and bombard our earth continually. Our atmosphere shields us from these rays to some extent, as does earth's magnetic field. We can't do much to escape these cosmic rays. And we get more radiation from them if we ride in an airplane, live at a high altitude or spend much time at the North or South Poles. In all of these cases we get less atmospheric or magnetic field shielding.
If you had to rate the best and worst of all these forms of radiation from our human point of view, it would be impossible. Like most things in life, forms of radiation have good sides and bad sides.
Scientists who specialize in the study of ionizing radiation have invented ways of measuring the bad side--the potential of radiation for harming living tissues. The most widely used measure for this is called a rem (short for Roentgen Equivalent in Man). Since a rem is a rather large unit measurements are usually expressed in millirems, that is, thousandths of a rem.
Rems and millirems are measures of the amount of ionizing radiation-no matter what the source-needed to produce a particular amount of damage in living tissue.
At the lowest level, everyone on earth receives between one hundred to four hundred millirems of radiation a year from natural sources like cosmic rays, naturally radioactive atoms in the air, soil, water, rocks, building materials and practically all food. Indeed we get about twenty millirems a year from our own bodies. And we radiate some of that to our family and friends.
By far the most significant ionizing radiation exposure from natural sources is from the radioactive element radon. Radon is a decay product from naturally occurring uranium that is present in most rocks and soils. Uranium and radon occur naturally almost everywhere on earth and have no relation to nuclear fall-out from bombs or power plant emissions.
Radon exposure, like any ionizing radiation, can lead to lung cancer. The average radiation dose from radon in the United States and Canada is about 200 millirems a year. Some areas of the country, and some homes may have significantly more than that however.
The EPA (Environmental Protection Agency) recommends that people test the amount of radon in their home using simple kits available from most hardware stores. If it is above what is considered a relatively safe level, there are various ways to ventilate rooms so that the level is decreased.
Besides this natural radiation, everyone on earth gets very small amounts of radiation due to fallout from past nuclear bomb tests and from nuclear power plant accidents. At present, this is much less than the natural background rate described above. People living closest to the nuclear accident at Three Mile Island in Pennsylvania, for instance, received no more additional radiation than they would get one chest xray.
The famous Chernobyl nuclear accident in the former Soviet Union (now a part of Ukraine) resulted in significantly higher background radiation for a few days or weeks in many parts of Europe. Outside the Soviet Union these levels were still far below those considered safe by most radiation scientists.
If you lived within 20 miles of Chernobyl you would have been exposed to about 1500 extra millirems the first year after the accident. This is roughly three times the safety standard for the general public, but less than a third the standard for radiation industry workers in the United States. (By way of comparison, smoking 1 ½ packs of cigarettes a day, exposes your lungs to 8,000 millirems a year.)
Besides the dangers from radon (or from cigarette smoking), the next most significant ionizing radiation exposure most people get in their lifetime is from medical and dental X rays. One chest X ray, for instance, gives about 25 millirems. A dental X ray gives 25 to 35 millirems. Neither of these is given to the whole body, however, which means a great deal less potential damage.
A total of five rems of whole body radiation a year is the government safety maximum for radiation industry workers. For the general public the standard is lowered to one half a rem (five hundred millirems) a year.
As exposures go higher than this, there is more significant risk, but at present it is very difficult to estimate accurately. We know that over one thousand rems at one time is almost always fatal. However, one thousand rems over a period of years would not be fatal. It might not even be harmful! We know that over one hundred rems a year to the whole today is dangerous, however, and does increase the long term risk of cancer.
But we should not close, as we did not open, with the negative. With all the emphasis on risks in radiation, we may be in danger of forgetting the other sides, the positive sides.
Not only are X rays, gamma rays, alpha rays, beta rays, neutrons and even cosmic rays potentially dangerous, you could also call all of them windows of opportunity.
Where would the living world be, for instance, if green plants had not opened up a few billion years ago to the opportunity visible light radiation presented to the world? The opportunity to invent photosynthesis and make food and energy available for themselves first, and for animals later.
And what about animals? Where would we be if eyes had not been invented to take advantage of the opportunity visible light radiation offered to see and to explore our world?
So, too, we have only begun to explore the opportunities that new kinds of ionizing radiation present to our world. Opportunities to understand our place in this radiant universe-and to make a better place of our spaceship earth home.
And if you are ever tempted to forget these elementary facts, take a walk today-on the sunny side of the street.