University of Virginia
Physics Department

Protection from Radiation

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

  • define alpha particles, beta particles, gamma waves;
  • compare penetrating ability of alpha particles, beta particles, gamma waves;
  • identify various types of protective shielding for radiation;
  • identify hazards presented by radiation

Motivation for Learning

Driving Questions

What are nuclear reaction products? Are all forms of radiation equal? What types of protection exist for exposure to radiation? Ask the students some of these questions and try to obtain a response. What do they know about radiation? Most of them will think it is bad. Some of them may know that radiation is used to treat some forms of cancer, and from that standpoint, is considered good.

 

Background Information

Radiation is energy being emitted in the form of particles or waves. Radiation is emitted from atoms and nuclei that are changing their energy states. Most radiation is naturally occurring; some radiation is manmade. In general, the following kinds of radiation are monitored for purposes of radiation protection: alpha particles, beta particles, gamma rays, x-rays and neutrons.

Illustration used by permission of the Uranium Information Centre, Melbourne, Australia

An alpha particle (a) consists of two protons and two neutrons (a helium nucleus). It has a relatively large mass and a positive charge. Alpha particles are easily shielded by a piece of paper or human skin. Therefore, health effects of alpha exposure occur only when the particles are inhaled, ingested, or enter the body through a cut in the skin. More serious would be a material that is radioactive (a emitter) that is ingested into the body. The a particles emitted inside the body, for example in bone marrow, can be exceedingly dangerous.

Beta particles (b) are fast electrons produced following nuclear decay of certain radioactive materials. The amount of energy (speed) that a beta particle contains determines its penetrating capacity. Six millimeters of aluminum are needed to stop most b particles.

Gamma rays (g), an electromagnetic wave, are similar in form to visible light and radio waves. However, gamma waves are very energetic and have a far shorter wavelength. Gamma rays are produced from radioactive decay, in nuclear reactions, and in fission. Gamma rays are dangerous because they have great penetrating ability. Several millimeters of lead are needed to stop g rays.

Various types of shielding for different nuclear particles
Illustration used by permission of the Uranium Information Centre, Melbourne, Australia

Radiation is measured in two units - rads and rems. A rad stands for "radiation absorbed dose" and measures the amount of energy that is actually absorbed per unit mass. A rem stands for "roentgen equivalent man" and is a unit that measures the absorbed dose in human tissue and relates it to the effective damage done to your tissue. It is significant because not all radiation has the same biological effect. The radioactivity of a source is usually measured in how many rads or rems you would receive per hour; a geiger counter normally measures radiation in millirems per hour (mr/hr).

X-rays have the same characteristics as gamma ways, but they are produced differently. In 1895, Wilhelm Roentgen observed that when high-speed electrons hit metals, the electrons stopped and released an electromagnetic wave. He named this energy wave an x ray.

Neutrons are released during the nuclear fission process and during certain nuclear reactions. Neutrons trigger the nuclear chain reaction. Neutrons do not carry an electrical charge. However, when the neutrons hit the nucleus of hydrogen (a constituent of water molecules in cells), ionizations that can lead to damage can occur.

There are three basic forms of protection from radiation: time, distance and shielding. The amount of time spent near a source of radiation affects the amount of exposure received. The farther the distance from the radiation source, the less the amount of exposure will be. A shield (specific to the type of radiation) can limit the exposure to radiation.

When dealing with radiation, it is important for scientists to know where it exists, and how much is present. A Geiger counter is an instrument that does just this - it detects when radiation is present, and tells us how much is present by electronically counting the number of radioactive particles that interact with the counter. It is useful because it can measure very low levels of radioactivity.

To know how it works, you must first be familiar with the construction of a common Geiger counter. The counter uses a metal tube usually containing a wire at the center with high voltage and a gas that is ionized by particles passing through it. The ionized particles are collected as a current, and the electronics inside the box amplifies the pulse to where it can be recorded.

Geiger counters are more useful for detecting beta particles and gammas. Most counters cannot detect alpha particles. Remember that alpha particles are easily shielded. For this reason, the Geiger counter tube has to be made with a special window, or else the window itself will block the alpha particles and they won't be detected. Secondly, the counter must be held steady for several seconds at the same distance in order for us to obtain a good reading. Moving the counter around will change the number of particles that enter the tube; so make sure that you hold the tube the same distance from each object that you are trying to measure, or else your results will not be accurate. A third disadvantage of a Geiger counter is that it cannot measure very high amounts of radiation; in fact, the machine can be damaged if you expose it to an extremely high radiation, but that is unlikely in our case.

An excellent Geiger counter to use in student experiments is the CD V-700, because thousands of them were made in the 50s and 60s for civil defense. The American Nuclear Society gives each teacher a Geiger counter who takes their workshop. For directions to add a sound speaker to a CD V-700 Geiger counter, please go to the following site on the American Nuclear Society website: sound speaker

Another similar instrument is called a Survey Meter. A survey meter is similar to a Geiger counter, except that it is used to measure very high levels of radiation. However, a survey meter cannot measure very low levels of radiation, so it is not very useful in classroom experiments.

The Environmental Protection Agency has a nice site called "Students and Teachers' Radiation Protection Pages" that is full of useful information. It has a quiz similar to the one in this activity. Click here to see it.

The United States Nuclear Regulatory Commission has a teacher lesson site that can be downloaded. It contains the figure that we have in this site. Click here to see it.

You can find the name of your local emergency manager in Virginia (which includes radiation hazards) by clicking here.

 

Student Activity

To print out the Student Copy only, click here.

Materials

Procedure

  1. Set up the Geiger counter according to the unit's instructions; be sure it is calibrated.
  2. Test each source item by placing the item 5 cm away from the Geiger counter probe. Keep the probe steady for 15 seconds, and find the average reading on the meter. The needle will shift around some, so choose a value in the middle of its oscillations. Record your findings in the data table.
  3. Select the three source items with the highest readings. One at a time, place the source far enough away from the Geiger counter probe so that you will be able to fit the thickest piece of shielding in between the probe and the source. Test each of the shielding materials by placing them between the source and the counter. Follow the same procedure as in step 2 when taking the reading. Remember to keep the probe at the same distance for each measurement. Record your findings in the data table along with the thickness of each type of shielding that you use..
  4. Answer the questions on the worksheet after you have recorded your findings.

 

Data Sheet

To print out the Data Sheet only, click here.

 

Data Table

Source

Without Shielding

Paper

Aluminum

Brick

Jar of Water

Wood
Lead

Gas lantern mantel

 

 

 

 

 

 

 

Fiesta ware pottery

 

 

 

 

 

 

 

Luminescent clock face

 

 

 

 

 

 

 

Smoke detector

 

 

 

 

 

 

 

 

Questions 

1. Which item had the highest reading?

 

2. Which item had the lowest reading?

 

3. Do you think the density of the shield was important? Why?

 

4. Do you think the thickness of the shield was important? Why?

 

 

 

 

 

 

Resources

 
The American Nuclear Society has an excellent site devoted to radiation at www.ans.org .
 
http://www.uic.com.au/ral.htm

The Enivronmental Protection Agency has a nice site called "Students and Teachers' Radiation Protection Pages" that is full of useful information. It has a quiz similar to the one in this activity. Click here to see it.
 
The United States Nuclear Regulatory Commission has a teacher lesson site that can be downloaded. It contains the figure that we have in this site. Click here to see it.
 

 Extensions

  1. Do experiments for different thickness of the same absorbing material.
  2. Do the experiment for various distances from the source. Make a graph and describe the result.

 

Students with Special Needs

Each student should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

 

Assessment

  1. Each student should complete a data table and the questions on the worksheet.
  2. If you were designing a building to protect the occupants from alpha particles, what type of shielding would you need to consider?

     

  3. If you were designing a building (or a room) to protect the workers from beta particles, what type of shielding would you include in your project?

     

  4. Why do x-ray technicians stand behind a lead barrier when they take an x-ray of someone?