lesson 4 ionizing radiation | the harnessed atom (2016)

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The Harnessed AtomLesson FourIonizing Radiation


What you need to know about radiation:Types of radiationNon-ionizingIonizingForms of ionizing radiationAlpha particlesBeta particlesGamma raysRadiation Decay chainHalf-lifeDoseRadiation measurementsSources of radiationAverage annual exposure


You are surrounded.

You are always surrounded by radiation. Radiation is everywhere. The bricks in your school have natural minerals that are radioactive. Cosmic radiation from the stars showers us constantly. Radioactive minerals were in the banana you ate this morning.

Think of radiation as a natural energy that surrounds us all the time.


Ask students if they know what this image is. This photo of the Sun was taken by NASA's Solar Dynamics Observatory. Make the point that radiation is part of our natural world and has been since the birth of our planet. The Sun is one source of natural background radiation. Other sources include minerals in the soil, bricks on houses and buildings, even the potassium in bananas and other foods. Bananas contain potassium-40, also a naturally occurring radioactive material.


Radiation is moving around you.

Radiation is energy moving through space asWaves Particles.

Waves move in patterns like these. Electricity, for example, moves in wave patterns.

Particles travel more slowly than waves but still travel very fast. Examples of particles areAlpha particlesBeta particles.


This figure shows three electromagnetic modes (blue, green and red) with a distance scale in microns along the x-axis. Tell students that the wavy lines here show the wave forms of electric and magnetic (EM) fields. Light itself is an EM wave. EM energies are determined by the frequencies (the number of waves or cycles per second) not their speed. All types of electromagnetic radiation travel at the speed of light (186,000 miles/sec or 3 x 10^8 m/s). Alpha and beta particles are not part of the electromagnetic spectrum. They travel at very fast rates but are slower than the speed of light.4

Two types of radiation

Non-ionizing radiation is low energy. We use it to carry signals to our radios, TVs, and cell phones.

Ionizing radiation is high energy. We use it for medical x-rays. Its high energy can cause disease or it can treat disease.


Microwave ovens also use non-ionizing radiation. Other examples include electric light bulbs, microwaves, and radio waves.


The energy spectrum of radiation looks like this.6Low frequency radiation is non-ionizing radiation. High frequency radiation is ionizing radiation.

Spectrum may be a new word for your students. Tell them a spectrum is an orderly range of qualities or ideas. For example, a rainbow is a spectrum of color. This spectrum shows the orderly increase from non-ionizing to ionizing radiation. On the left of the illustration, low frequency radiation is non-ionizing radiation. On the right, high frequency radiation is ionizing radiation. Note the changing pattern of the waves. Ask students: Does this mean the ionizing waves are faster? (No. The wave patterns show the frequency.) Ask students to think of the rays of the Sun. At first they warm us. Then make us sweat or burn. Too much of that Sun over time can cause skin cancer. Radiation at the high-energy end of the UV spectrum can be as dangerous as x-rays.6

What to remember

Non-ionizing radiationIonizing radiation

Low energy High energy

Low frequencyHigh frequency


We divide non-ionizing and ionizing radiation when the radiation carries enough energy to remove an electron from a molecule or an atom.

Tell students: The lower the energy the lower the frequency of the non-ionizing radiation. The higher the energy the higher the frequency for ionizing radiation. The difference between ionizing and non-ionizing radiation is whether the radiation carries enough energy to remove an electron from a molecule.


What do unstable isotopes do?Some unstable isotopes make themselves more stable by shooting out energy rays or particles. We call this energy radiation. Substances that give off radiation in such a way are called radioactive.

Example:Most smoke detectors contain americium-241. This radioactive element emits energy. When the energy is blocked by smoke, the detector activates the alarm. Heres what it looks like inside the detector.8

Important safety note: Tell students not to dismantle a smoke detector. Photo released in public domain by original copyright holder: 2010-11-05 22:10 36482736 (2974515 bytes) {{Information |Description = Photo of the Americium container in a smoke detector. The copyright holder grants any entity the right to use this work for any purpose, without any conditions, unless such conditions are required by law. However, as a courtesy, a link back to Wikipedia (http://www.wikipedia.org/) would be appreciated


What else do unstable isotopes do?Other unstable isotopes may give off particles from their nuclei and change into different elements. The process of isotopes emitting particles or rays to become more stable is called radioactive decay.


Tell students that uranium is a radioactive element. This decay chain shows some of the steps in the uranium decay chain. The half-life of uranium-238 is 4.5 billion years.9

One strange thing about radioactive isotopesNobody knows exactly when a radioactive isotope will decay and release radiation. Some isotopes may change in the next second, some in the next hour, some tomorrow, and some next year. Other isotopes will not decay for thousands of years.In a group of atoms, a pattern can be seen. We describe this pattern by using the term half-life.Half-lives range from fractions of a second to several billion years.


Naturally occurring uranium is composed of three major isotopes, uranium-238, uranium-235, and uranium-234. All three isotopes are radioactive and create radioisotopes as they decay. The most abundant and stable is uranium-238 with a half-life of 4.51109 years (just about as old as the Earth itself). Uranium-235 has a half-life of 7.13108 years, and uranium-234 has a half-life of 2.48105 years.


What is a half-life?The amount of time it takes for a given isotope to lose half of its radioactivity is known as its half-life.

An unstable isotope will eventually decay into a stable element. However, this process is often drawn out into something called a decay chain.

We measure the decay in half-lives. 11

Make sure students understand that when an atom decays, it changes into another element. It does not disappear.11

What is happening here?12

A jet at high altitude is leaving behind a condensation trail. The trail is caused by water vapor in the aircraft's exhaust condensing in the cold atmosphere.


What is happening here?We can indirectly observe ionizing radiation in this cloud chamber.

Radiation cannot be seen. But the cloud chamber allows you to see the tracks that ionizing radiation leaves in cold alcohol vapor.


Hover over the image above. When the media bar appears, click on the play button. Ask students if they can detect different trails in the video. Refer to Lesson 4 in the Teachers Guide of The Harnessed Atom. You can segue into the Cloud Chamber lab from this slide or at the end of the presentation. The photo above links to a video recording of a cloud chamber. They should understand that they are not seeing radiation. Its an indirect observation. They are seeing the tracks of atom pieces the atoms are throwing out as they break down. Ask them to try to identify these types of footprints:Alpha bright, short, straight tracks about 1 cm longBeta thin, twisting tracks about 3 cm to 10 cm long. Betas small mass and negative charge cause it to bounce away from air molecules it strikes.Gamma long, thin, scattering tracks. Gamma rays can knock off air molecules, causing a scattering track.13

Ions have an electric charge.An ion is an atom or a group of atoms that have gained or lost an electron.

Ions have an electric charge. Ions are formed when an electron or proton joins or leaves an atom, giving it a positive or negative charge.


Ask students: Remember when we talked about the structure of the atom? We said the nucleus is in the middle and its made up of the positive proton and neutral neutron. Whirling around it are the electrons. They are negatively charged. The electrons want to stay in orbit around the electrons. Thats what the positive and negative forces do. They make them stay together.Ionizing radiation, from say the isotope uranium-235, has enough energy that when it interacts with an atom, it can remove those tightly bound electrons from their orbit. When that happens the atom is no longer balanced with equal numbers of protons and electrons: the difference creates an electric charge. Remember from an earlier lesson: unstable isotopes emit energy as radiation. Unstable isotopes are constantly emitting ionizing radiation. Radiation is the term given to the particles and/or energy emitted by radioactive material as it disintegrates.as it becomes more stable. We call that process decay.


How do the types of ionizing radiation deposit energy?Types of ionizing radiation differ widely in their abilities to penetrate tissue and deposit energy through ionization. Because


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