Radiation

Introduction

• BIRTHDAY
  
  
  
• ICE CREAM
  
  
  
• RADIATION

What is "radiation"

• Sound - sound is a pressure wave (a compression and depression of air molecules) that travels from the source to the receiver. It is radiation, since the wave contains energy.
• Light - light is an "electromagnetic" wave, a vibrating electric and magnetic phenomenon that requires no medium to travel (unlike sound or water waves). It transports energy.
• Heat - heat is just light that we cannot see because its waves are too long for our eyes to detector. But we can feel it. The atoms in our bodies respond to heat and light by vibrating and oscillating, and this we sense as "heat".
• X-rays - x-rays are just light that has a wavelength so short that we cannot see it; unlike heat, x-rays can do physical damage to atoms. This has implication for chemistry, and thus biology.

• Ionizing: such radiation has the ability to remove one or more electrons from an atom. This is called "ionization," and that radiation is said to be "ionizing." When you remove electrons from an atom, you alter its chemical properties; it may become highly reactive, and those reactions can cause changes to nearby molecules. Such processes are capably of altering a biological system, with potentially negative consequences that depend on the AMOUNT of ionizing radiation to which you are exposed. More about that later.
• Non-ionizing: such radiation CANNOT remove electrons from an atom, and therefore cannot alter their chemical properties. However, this radiation can still cause atoms inside of molecules to vibrate, can cause the molecules to spin (rotate), and can excite an electron up into a higher orbit before the electron then "de-excites" and drops down to its original energy state. Such radiation has no known negative health effects beyond heating. More about this later.

Different Examples of Potentially Dangerous Radiation

• Particle radiation - so-called "alpha" and "beta" radiation, discussed below. There are other kinds of particles than can also ionize, but these are very common and result from the radioactive decay of naturally occurring elements on Earth.
• Electromagnetic radiation - above a certain frequency, electromagnetic radiation can ionize. This range is right around the ultraviolet end of the electromagnetic spectrum. Any wave with a wavelength shorter than UV can ionize an atom. "X-rays" and "Gamma rays" are two classes of ionizing electromagnetic radiation.

• Alpha Particles: these are just the nuclei of helium atoms (2 protons and 2 neutrons) that have been stripped of their two orbiting electrons. They have a large electric charge (+2e) and can ionizing atoms in material by removing electrons from atoms in the material. However, they are heavy and can be easily stopped in material. Even a thin sheet of paper can be enough to stop most alpha radiation! We demonstrate this in class.
• Beta Particles: these are just very fast electrons. They are often ejected from unstable radioactive nuclei. They have less electric charge than the alpha particles (-1e) but they are much less massive and can penetrate further through material before being stopped. Paper, for instance, is not enough to stop beta particles; but your hand is enough material to stop them.
• Gamma Radiation: these are electromagnetic waves with wavelengths much shorter than UV. They carry a lot of energy and EASILY ionize atoms. They are also very penetrating; because they are just light, which itself has no mass, they can easily penetrate through a lot of material. Lead is required to stop them; your hand, or a sheet of paper, are easily traversed by gamma rays. These, too, result from the decay of unstable atomic nuclei.

How much is too much?

The following chart is used by the US Nuclear Regulatory Commission (NRC) in their web page illustrating the typical ionizing radiation dose in the US, per person, per year. The chart uses units of exposure called "rems" - 100 milli-rem is equivalent to 1 milli-Sv, so 620 milli-rem is 6.2 milli-Sv.

SOURCE: "Fact sheet on Biological Effects of Radiation" ( http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/bio-effects-radiation.html) and NCRP Report 160 (2009).

We see that radiation is all around us, and as such our bodies are use to handling some typical amounts of ionizing radiation, even though ionizing radiation can in principle be hazardous. This is because biological systems on Earth evolved under conditions of constant exposure to such radation - in fact, a biological consequence of ionizing radiation, called "mutation," is a necessary component in order to explain the diversity of life on Earth. So ionizing radiation is, in part, responsible for the vast diversity of life on Earth.

A Demonstration of Ionizing and Non-ionizing Radiation

We demonstrate the quantum nature of atoms in class. Please see this video below for the demonstration.

VIDEO

We learn some very valuable things from this:

1. Mobile phone radiation is INCAPABLE of ionizing.
2. Green or red laser light are INCAPABLE of ionizing.
3. In fact, all the visible light and heat from a set of bright incandescent bulbs (each about 60 Watts!) is INCAPABLE of ionizing.
4. But, the UV-C light from the sanitizing wand, with a wavelength of about 253 billionths of a meter (nanometers) and a bulb wattage of just 6 Watts, is easily capable of removing the charge from the aluminum can (ionization!).

The Physics of EM Waves - Energy, Wavelength, and Frequency

The wavelength and frequency of EM waves are related to the speed of light, denoted c = 2.998 x 10⁸ m/s:

where λ is the wavelength (the distance between like parts of neighboring waves) and f is the frequency of the wave (the number of times per second that the same part of neighboring waves passes a common point in space). Wavelength is measured in meters (m) and frequency in a unit called the "Hertz" (Hz) which is just given by "per second" or 1/s.

Quantum physics also relates the ENERGY carried by the wave to its frequency:

where h is Planck's Constant, h = 4.136 x 10^-15 eV*s. An "eV" is an "electron-Volt," a convenient measure of energy on the scale of an atom. 1 eV is the energy gained by a single electron when it is accelerated between the two terminals of a 1 Volt battery (for example).

Here is a table illustrating the energy carried by EM waves of different wavelengths and frequencies:

Let's see if we can get Cas Milner's notes Let's play word association: I say RADIATION, you say ... Different kinds alpha - Apollo mission Pu for thermal generator (movie?) beta neutron gamma, X-ray (chiropractors, mammograms, CT scans) lethal overdose in the news etc. Ionizing versus non-ionizing Demonstration with Geiger-Mueller counter absorption in materials 1/r^2 law half-life (dice) cell phones *YOU* radiate in the IR Naturally occurring radiation cosmic rays - demonstration, has anybody ever flown in a plane? evolution granite bananas - B.E.D. salt substitute KCl brazil nuts How to dispose of the waste Yucca Mountain - 1 million years? C'mon! NIMBY