4.  Nuclei and Radioactivity

 

 

 

 

1. This book is radioactive. 

 

2. You are radioactive too, unless you have been dead for a long time.

 

3. The United States Bureau of Alcohol, Tobacco, and Firearms tests wine, gin, whisky, and vodka for radioactivity. If the product does not have sufficient radioactivity, it may not be legally sold in the United States.

 

4. Of those killed by the Hiroshima atomic bomb, the best estimate is that fewer than 2% died of radiation-induced cancer. 

 

Those anecdotes are all true, and yet they surprise most people. That reflects the confusion and misinformation that pervades the public discussion of radioactivity.  I hope that when you finish this chapter you can come back and read those four anecdotes and say, ÒOf course.Ó 

 

 

Radioactivity

 

Radioactivity is the explosion of the nucleus of the atom. What makes this explosion so important and fascinating is the enormous energy released, typically a million times greater than in chemical explosions for the same number of atoms.

      Atoms are small but not completely invisible. A device called a Scanning Tunneling Microscope (called an STM by experts) can pass over individual atoms, feel their shape, and then present that on a computer screen in the form of an image.  A similar device can pick up individual atoms, carry them, and place them at new locations. In the photo below we show 35 xenon atoms arranged to form the letters ÒIBMÓ on the surface of a nickel crystal.  (Guess what company did this work.) 

 

ÒVisibleÓ atoms.  The letters IBM were written by arranging individual xenon atoms on the surface of a nickel crystal. The atoms were manipulated and photographed using a scanning-tunneling microscope. This work was done by a team led by Donald Eigler. Guess which company they worked for. (Copyright IBM)

 

 

It is this ability to manipulate individual atoms that has led to the excitement about the new field called Ònanotechnology.Ó  The name comes from the fact that an atom is about 1/10 of a nanometer (a billionth of a meter) in diameter.

 

      To put the size of an atom in perspective, consider the following examples: a human hair has a thickness of about 200,000 atoms and a human red-blood cell has a diameter of about 10,000 atoms. These numbers are big, but not huge. I didnÕt have to use scientific notation. So atoms are small, but they are not infinitesimally small.    

      Each atom consists of a cloud of electrons with a tiny nucleus in the center.  The nucleus has a radius of about 10-13 cm, which means it is 100,000 times smaller than the atom itself. To visualize this ratio, imagine that an atom were enlarged until it was the size of a baseball or football stadium (300 m). Then the nucleus, similarly expanded, would only be the size of a mosquito (3 mm). Since its linear size is 10-5 times the size of the atom, then its volume is 10-15 times the volume of the atom (since you calculate volume by taking the cube of the linear size). ThatÕs like the volume of the stadium compared to the volume of the mosquito. This enormous disparity often gives rise to the statement that the atom is mostly Òempty space.Ó Some could argue, however, that the space isnÕt really empty; it is filled with the electron wave. WeÕll talk more about that in Chapter 10 ÒQuantum Physics.Ó Yet, even though it has only 10-15 of the volume of the atom, the nucleus contains more than 99.9% of the mass of the atom. The nucleus is very small, but very massive. That was not predicted; try to imagine the surprise and disbelief of scientists in 1911 when Ernest Rutherford discovered this incredible fact. It seems completely implausible. But it is true.

      Within 20 years of RutherfordÕs discovery, we learned that the nucleus itself was made up of even smaller pieces. The most important of these are protons and neutrons:

Protons weigh almost 2000 times as much as electrons and have the same magnitude of electric charge, but they are opposite in sign. (WeÕll discuss the sign of the charge in Chapter 6. Electrons, by convention, have negative charge, and protons have positive charge.) 

 

Neutrons are similar in mass than protons (they are actually about 0.3% heavier), but have no electric charge, i.e. they are ÒneutralÓ--hence their name.

 

So here is the basic picture of the atom: it has a very small nucleus made of protons and neutrons. Surrounding this is a relatively large volume occupied by electrons. But most of the mass is in the tiny nucleus. The nucleus of an atom weighs almost exactly the same as the entire atom itself since the electrons are so light.

      Scientists love to deconstruct. So it is natural for them to wonder whether protons and neutrons are made of smaller objects. The answer was uncovered in the last few decades of the 20th century: protons and neutrons are made of particles called quarks[1] and a variable number of lightweight gluons that hold the quarks together (they are named after glue). WeÕll discuss these further in an optional section at the end of the chapter. What are quarks made of? According to the unproven string theory, they (as well as electrons) are made of something called strings. I summarize this in the list on the next page.

 

 

 

 

Matter is made of molecules (e.g. water is made of H2O)

Molecules are made of atoms (e.g. H2O = hydrogen and oxygen)

Atoms are made of electrons orbiting a nucleus

Nuclei are made of protons, neutrons, and other light particles (e.g. gluons)

Protons and neutrons are made of quarks and gluons

Quarks and electrons may be made of strings

 

Elements and Isotopes

 

The number of protons in the nucleus is called the Òatomic number.Ó This number also indicates the number of electrons orbiting the nucleus. An atom of hydrogen has 1 proton in the nucleus (and 1 electron in orbit), and we say it has atomic number 1. An atom of helium has 2 protons in the nucleus and 2 electrons in orbit.  It has atomic number 2. An atom of uranium has 92 protons in the nucleus and 92 electrons in orbit. We say it has atomic number 92. Each element has a different atomic number. Here is a list of some of the atomic numbers of elements we will be discussing in this chapter:

 

Element

Atomic Number (Np)

hydrogen

1

helium

2

lithium

3

carbon

6

nitrogen

7

uranium

92

plutonium

94

 

      As mentioned earlier, the nucleus consists primarily of protons and neutrons.  Neutrons donÕt have electric charge, so they donÕt change the behavior of the atom (at least, not much). But they do make the nucleus heavier. Atoms of an element with different numbers of neutrons are called different ÒisotopesÓ of that element.

      For example, the nucleus of ordinary hydrogen (the abundant kind) always contains one proton and no neutrons. But about 1 in every 6,000 hydrogen atoms has a nucleus that contains an extra neutron. That kind of hydrogen is called deuterium, or Òheavy hydrogen.Ó Water made from heavy hydrogen weighs more; it is called heavy water. Heavy water was very important during World War II in the development of the nuclear reactor. In fact, Hitler had a special plant to purify deuterium (useful to make a nuclear reactor), and the Allies sent a team to blow that plant up.

      About 10-18, or a billionth of a billionth, of ordinary hydrogen has 2 neutrons in the nucleus. This kind of extra-heavy hydrogen is called tritium. Tritium is the only kind of hydrogen that is radioactive. It is used in medicine and in hydrogen bombs.

      WeÕll be talking a lot about deuterium and tritium, especially when we get to nuclear reactors and bombs.  So learn those terms.  Here are useful memory tricks:

 

            In deuterium the proton and neutron in the nucleus form a duo.

            In tritium the proton and neutrons in the nucleus form a trio.

 

      Over 99% of uranium found in the Earth has a nucleus with 92 protons and 146 neutrons, making up a total of 92+146 = 238 particles in the nucleus. This is called U-238. But about 0.7% of the uranium has only 143 neutrons in the nucleus instead of 146. This is a different ÒisotopeÓ of uranium called U-235. It is very important, because U-235 plays a key role in the atomic bomb and nuclear reactors. 

      Both U-238 and U-235 have 92 protons. That means they both have 92 electrons. Since it is the electrons that play the major role in ordinary chemistry, both isotopes react very similarly with other elements, such as oxygen and water.  ThatÕs why they are both called uranium. But when we are interested in the properties of the nucleus, particularly in nuclear explosions, then the difference in neutrons becomes extremely important. 

 

 

Radiation and Rays

 

 

Now letÕs return to the radioactivity, the explosion of the nucleus. A common chemical explosion (e.g. TNT) takes place when a large molecule suddenly breaks up into smaller molecules. In a similar way, a radioactive explosion takes place when a nucleus breaks up into smaller parts.

      WeÕll begin with the most common type of radioactivity, in which a relatively small particle is thrown out from a big nucleus. It flies out like a bullet, at very high speed, sometimes approaching the speed of light. When this process was first discovered, nobody knew what was coming out. The projectiles couldnÕt be seen directly, but they passed through matter and could expose photographic film. The projectiles were called Òrays,Ó probably because they travel in nearly straight lines.  They had properties similar to X-rays, which Wilhelm Conrad Roentgen had discovered a few years earlier in 1895. Different kinds of rays were found, with somewhat different properties, and they were named after the letters of the Greek alphabet.[2] Some rays (e.g. from uranium) could be stopped by a piece of paper; these were called alpha rays. Rays with more penetration were called beta rays.  The most penetrating of all were called gamma rays. (There were delta rays too--but those turned out to be the same as low energy beta rays so the term is little used.) 

      The old terminology has changed; instead of saying rays, we now say radiation. It is worthwhile to learn this formal terminology:

     

radioactivity refers to the explosions of atomic nuclei

radiation consists of the pieces that get thrown out in the explosion

 

      Each ray (or particle) is like a tiny bullet, so tiny that you donÕt feel it if it hits your body. Alpha rays and beta rays bounce off many atoms before they stop; with each bounce, they can knock apart a molecule or mutate a gene. The slowing bullet leaves a trail of damaged molecules along its wake. The damage is small but, if you are hit by a large number of particles, the total effect can make you ill or even kill you. Gamma rays tend to be absorbed by a single atom; however, they frequently break up the atom or even its nucleus, so secondary radiation is emitted.  It is that secondary radiation that often does the most damage.

 

 

ÒSeeingÓ radiation

              – the Cloud Chamber

 

When alpha or beta rays pass through a gas, they knock electrons off atoms, creating a trail of charged particles called ions. If the gas has a lot of water vapor or alcohol vapor mixed in, and the gas is cool, then the water or alcohol tends to form little droplets on these ions. In essence, they form clouds along the path of the radiation. White tracks suddenly appear when an alpha or beta ray passes through a chamber set up in this way. In the image below, we show some images taken in the original cloud chamber, invented by Charles Wilson (for which he got the Nobel Prize in 1927). The streaks are the cloud particles along the path of the radiation.

 

 

Cloud Chamber with antimatter

The thin curved line is a trail of cloud particles left behind by a positron.  This image was the first antimatter ever observed, and it won Carl Anderson a Nobel Prize. The horizontal broad white region was a divider that the positron passed through.  (Department of Energy photo)

 

      The tracks are very similar in nature to the vapor trails left behind when a jet airplane engine passes overhead. The track doesnÕt actually show the airplane itself, but it shows where it has passed.

      A cloud chamber is a marvelous thing to watch. Radiation from a radioactive source will show short white lines of cloud particles suddenly forming along a path. The droplets are heavy, so they drift to the bottom of the chamber.  Meanwhile, new paths are suddenly appearing above them. Every once in a while, radiation all the way from space will cause a long path to appear in the cloud chamber. This radiation is known as cosmic radiation.

 

 

Radiation and Death: the rem

 

The biological damage done to cells hit by radiation is measured in a unit called the rem. I can give you a rough idea of how big a rem is from the following example. Suppose each square centimeter of your body is penetrated by 2 billion gamma rays. If that happens, then the radiation dose to that part of your body is approximately 1 rem.[3] A rem usually refers to the amount of damage to each gram of your body. If your whole body is exposed, then we say you got a whole body dose of 1 rem. It means that each gram of your whole body suffered the same damage.

      Two billion gamma rays sounds like a lot of radiation, so it might make you think that a rem is a huge amount of damage. But remember, the nucleus is very small. When it emits energy, the energy is big only in a relative sense. For example, the gamma rays entering your body will deposit their energy, and that will cause your body to warm up. But the amount of warming can be calculated; it is less than 1 billionth of a degree C.[4] The radiation does damage individual molecules, and that is the source of all the real danger. Most of the time, the damage can be repaired by the cells in your body. But your body is not always successful. Many people estimate that the exposure to this much radiation (1 rem to every cell in your body, i.e. 1 rem whole body) will increase your chance of getting cancer by about 0.0004, or 0.04%. WeÕll talk more about that shortly.[5]  

      The term rem was originally an acronym.[6] Physicists will never miss a chance to honor one of their own, so it was inevitable that a new unit would be introduced, the Sievert.[7] The conversion is simple: there are 100 rem in 1 Sievert.  If you look at modern textbooks, youÕll see Sieverts used more and more often.  But most public reports still persist in using the rem, so we will too. (Memory trick: the Sievert is capitalized and it is the big unit. It consists of 100 of the smaller, uncapitalized rem.)

 

 

Radiation poisoning

 

If every cubic centimeter in your body is exposed to 1 rem, then we say you have received a Òwhole-body doseÓ of 1 rem. If the whole-body dose is more than 100 rem, the damage done to the molecules of the cells is enough to disrupt the metabolism of the body and the victim becomes sick. This is called radiation poisoning. If you know someone who has undergone radiation therapy (to kill a cancer), then you know what the symptoms of mild radiation sickness are: nausea, listlessness (popularly referred to as Òloss of energyÓ), and loss of hair. The severity of the illness depends very sensitively on the dose, as the table on the next page shows. 

 

 

Whole-Body Dose

Resulting Radiation Illness

below 100 rem

(below 1 Sievert)

no short-term illness

100 to 200 rem

(1 to 2 Sievert)

slight or no short-term illness; nausea, loss of hair; rarely fatal

300 rem

(3 Sieverts)

50% chance of death, if untreated within 60 days

more than 1000 rem (more than 10 Sieverts)

incapacitation within 1 or 2 hours, survival unlikely

 

 

In medical terminology, 300 rem is said to be the ÒLD50Ó--which stands for the lethal dose that will kill 50% of those exposed. Memorize the following:

 

LD50 for radiation is 300 rem = 3 Sieverts

 

      You will frequently hear the term millirem being used when people talk about radiation leakage. One millirem is 1 thousandth of a rem. LD50 for radiation is 300,000 millirem. The reason that I want you to memorize these numbers is that you will very likely encounter radiation in your life, maybe at a doctorÕs office, maybe elsewhere. YouÕll probably hear the term millirem, rather than rem. A dental x-ray gives, typically, a few millirem to your jaw (not to your whole body).  It is useful to understand how small a millirem really is. 

 

 

Radiation and Cancer

 

Get ready for a paradox: the average dose that it takes to induce cancer is a whole body dose of approximately 2500 rem.[8] But a dose of 1000 rem will kill you within a few hours from radiation illness. So how can anyone get cancer from radiation? You might (incorrectly) think that the victim would die first. The solution to this paradox is found in the linear hypothesis.

 

 

The ÒLinear HypothesisÓ

 

The solution to the paradox lies in the Òlinear hypothesis.Ó We canÕt give 2500 rem to a single person without immediate death. But if we spread it out among 2500 people, each of whom will be exposed to 1 rem, then the same total damage will be done. Not one person will get radiation illness, but the same number of mutations will be induced--just spread out over many people. The linear hypothesis is a fancy name for the belief that the 2500 rem, even though distributed over a large number of people, will still cause 1 cancer--not 1 cancer each, but 1 cancer total among the 2500 people. 

      The basis for belief in the linear hypothesis is the fact that most mutations are harmless; they may cause the cell to die, but we have lots of cells, and many of the cells can be replaced. But after 2500 rem of damage, the chances are good that a particularly bad mutation will be created in one of the victims,[9] a mutation that causes cancerous multiplication of cells.

      Not all experts believe the linear hypothesis. They argue that the cell can repair minor damage, so if spread out enough, every body will be able to recover.   We know that the linear hypothesis doesnÕt work for radiation sickness. Although 1,000 rem will cause fatal radiation illness, 1 rem per person, spread over 1000 people, will cause no radiation illness whatsoever. Moreover, the linear hypothesis doesnÕt work for most other kinds of poisons, such as arsenic. 

      Others counter by saying that radiation sickness, like other symptoms of poisoning, is different from cancer. Radiation illness occurs when the radiation damage overwhelms the bodyÕs ability to recover. Cancer appears to be a much more probabilistic illness. You develop cancer when you get, by chance, exactly the worst possible kinds of mutations, creating cells that grow and divide and will not be turned off by normal bodily controls.

      In fact, we donÕt really know if the linear hypothesis is valid at low levels (e.g. 1 rem) of radiation exposure. ThatÕs because cancer is a common disease. Even without radiation, 20% of people die from cancer. So with 2500 people, you expect 500 cancers anyway, even with no exposure to radiation. 

      Now letÕs look at what will happen if each of the 2500 people is exposed to 1 rem. According to linear hypothesis, we expect one additional cancer in the group, i.e. 501 cancers. Statistical fluctuations make such a small effect virtually impossible to verify. Even with large numbers of people exposed (e.g. in the Chernobyl nuclear reactor accident; see the next section) the effect tends to be obscured by statistical fluctuations and systematic uncertainties.

      But a premature death from cancer is a tragedy for anyone, even if it doesnÕt appear in the statistics. One additional cancer is significant (especially to the person affected), even if it is not Òstatistically significant.Ó ThatÕs why many people think we should assume the linear hypothesis, even if it is experimentally unverified, and use it as a basis for public policy. Since the linear hypothesis forms the basis for much public discussion, it is important for future presidents to know what it implies. It is equally important to know that it may not be accurate.

 

 

The Chernobyl Disaster

 

In 1986, the Chernobyl nuclear power plant in Ukraine had a violent accident.  There was an explosion in the vessel that contained the radioactive fuel, and a huge amount of radioactivity was released into the atmosphere. WeÕll discuss the innards of nuclear reactors in coming chapters; for now, all you need to know is that a vast amount of radioactive material was released. Several of the firefighters who put out the Chernobyl blaze died from radiation sickness.   Radioactivity from the plant was carried by wind over populated areas. This was one of the biggest news items of the 1980s; everybody who was an adult at that time remembers it. There was fear around the world as the radioactive plume drifted.  Some radiation from this event was detected in the United States.

      The Chernobyl event was one of the most famous and important events of the entire decade. It was in the newspapers for months. It is cited today by many people, those who oppose the further use of radioactive processes (such as nuclear power), and by those who are in favor of nuclear power.