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, “Of course.”

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 “IBM” on the surface of a nickel crystal. (Guess what company did this work.)

“Visible” 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 “nanotechnology.” 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 didn’t 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). That’s 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 “empty space.” Some could argue, however, that the space isn’t really empty; it is filled with the electron wave. We’ll talk more about that in Chapter 10 “Quantum Physics.” 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 Rutherford’s discovery, we learned that the nucleus itself was made up of even smaller pieces. The most important of these are protons and neutrons:

Protons