Answer to Question #784 Submitted to "Ask the Experts"

Q

I have two questions regarding radiation "penetration." (1) When a gamma ray penetrates say one inch of lead, does that mean it just (randomly) happened to pass between the lead atoms, or does it actually pass through the atoms? (2) When a gamma ray strikes a lead atom, what happens to both items? I am just a curious adult, so please answer any educational requests ahead of mine. Thanks for such a great forum!

A

Question 1: What most people do not realize is that most lead (or any matter) is made of space. Let's take your typical lead atom for instance. A natural lead atom has 82 protons and somewhere around 126 neutrons held together in its nucleus (center of the atom). Whizzing around the nucleus are 82 electrons in several different energy orbits. To give you an idea of the relative size, let's assume that nucleus with a total of 208 protons and neutrons is the size of a two-inch-wide ball. The first set of two orbiting electrons (in our expanded world their size would be 1/50 of an inch wide) would be 19 feet away from the ball, the next set of eight electrons would be at a distance of 76 feet and the farthest orbiting electrons would be one mile away. If we had two balls close together—bonded as found in lead metal—the distance between them would be over two miles apart in our relative world defined above. The main reason I want to throw these numbers out is that I want to make a point that most of an atom and most of a piece of lead is space! Now, on the other hand, every thin layer of lead has billions of atoms that the gamma must pass by. In a piece of lead one-tenth of an inch square there are 541 billion billion atoms. So, sooner or later, the probabilities are that the gamma will interact with a lead atom. For any one gamma, the chance of interaction depends on the thickness of the lead and the energy of the gamma. The gamma has a much, much higher probability of interacting with the electrons (more of them in their orbits), but it is possible for the gamma to interact directly with the nucleus. You used the term "random," and that is exactly what the process is, random; so we have a probability of interaction of a gamma with a lead atom. We call that probability a cross section. So, to answer your question—the gamma will almost assuredly pass through electron shells of some atoms and will pass between some atoms, as it "penetrates" past the billions and billions of atoms in the lead. Question 2: When a gamma ray strikes a lead atom, what happens to both items? Strike is a not exactly correct, but for ease of discussion, let's just use the term interact. Again, the most likely event will be the gamma interacting with one of the electrons. When the gamma interacts with an electron, it may transfer its energy to the electron, causing the electron to gain energy and the gamma to not exist anymore (gammas are energy). The electron now has the energy of the gamma, and can do several things, depending on how much energy it absorbed. It may remain in the electron shells, with higher energy. But, if given enough energy, it may leave the atom as a speeding particle, which will give off its own energy (slow down) by interacting with other electrons. It may also leave the atom and generate a secondary photon (gamma), in a process called scattering. The atom, having had an electron removed, is now an ion with a plus charge. This really means that it is chemically active and will want to gain back an electron through sharing or stealing from other atoms or by picking up a free electron. Depending on where the electron is missed from as the electrons shift around to fill in the orbits the atom may give off some x rays to help settle all the energy out to be at the ground state. Nowhere in this does the atom become radioactive, just ionized. With the possibility of a gamma hitting a nucleus, the nucleus may also become excited and give off its excess energy by radiation emission. For instance, at high enough energy, when a gamma hits a deuterium atom (hydrogen atom with a neutron), the neutron may be ejected from the nucleus. With heavier elements like lead, that is less likely, considering all the attractive forces going on in the nucleus. Bruce Busby RSO, Genentech