University of Virginia
Physics Department

Finding Radiation

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

  • define alpha particles, beta particles, gamma waves;
  • become familiar with the use of a geiger counter;
  • identify various objects that are radioactive from those that are not;
  • identify types of radiation emitted from a source;
  • become familiar with the concept of half-life.

Motivation for Learning

Driving Questions

What is radiation? Are all forms of radiation equal? Is all radiation dangerous? How do we measure radiation around us? Is it more dangerous to watch a soccer match in Charlottesville or in New York? What are nuclear reaction products? Why does the dentist's assistant put a heavy apron below your chin when you have an x-ray taken of your teeth? What is in the apron (lead)?

Asking lots of questions like this will hopefully cause the student to become interested in the finding out more about radiation.

 

Background Information

Radiation is a process of energy being emitted in the form of particles or waves. Radiation is emitted from atoms that are changing. 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 injested 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; your geiger counter will measure radiation in millirems per hour.

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.

Every type of radioactive material decays into another (or the same) element as it loses particles due to radiation. The rate at which the source decays is called its "half-life." The half-life is simply the average amount of time it takes for half of the source to decay into something else. Half-lives can range from very short time periods to as long as billions of years. For instance, if you have 2 grams of a substance at 12:00 pm and only 1 gram left undecayed at 2:00 pm, then your substance has a half-life of 2 hours.

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. Earth's atmosphere shields us from radiation, making it more dangerous to watch a soccer match in Denver than in New York, because Denver is at a higher altitude and has less atmosphere above it to shield 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, because alpha particles are easily shielded. For this reason, a Geiger counter tube capable of detecting alpha particles has to be made with a special window, or else the window itself will block the alpha particles and they won't be detected. In general, 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.

Another instrument you may hear of 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 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. 

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

How can you obtain a Geiger Counter? There were probably hundreds of thousands of Geiger Counters distributed in the 1940s, 1950s, and 1960s during the cold war with the former Soviet Union. A good, common Geiger Counter is the CDV 700 model. You can usually find one or more for sale on the eBay auction website. Your local emergency manager (city or county, see above) will have them.. Perhaps ask to borrow one. The American Nuclear Society has an outstanding outreach program with workshops; if you obtain their workshop, they give you a free Geiger Counter!

Another possibility is to purchase a Student Radiation Monitor from either PASCO or Vernier. Both companies make probe devices that connect to computers or CBLs for graphing calculators. Possibly the Vernier Student Radiation Monitor might be more user friendly, because it has connectors that fit directly to the CBL. These are excellent devices and have Geiger tubes in them. The Student Radiation Monitor detects betas and gammas, and the more expensive model also detects alphas.

Student Activity - Analyzing Radiation

To print out the Student Copy only, click here.

Materials

 Procedure

  1. Set up the Geiger counter according to the unit's instructions, and calibrate it if possible.
  2. Test each source item separately by placing the item 5 cm (or the same precise distance) 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. The audio units are not very useful for precise measurements. Some counters have a meter with a needle moving on it like in a car's speedometer. Others may have a digital meter that registers a count with every particle.
  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. Which items were not radioactive at all?

 

4. Using what you know about the penetrating abilities of the different particles, what types of particles does each source emit? (Hint: Compare the readings you received for paper, aluminum, lead, and with no shielding.)

 

 

 

 

 

 


 

Student Activity - Half-Life

Materials

Procedure

  1. Flip each penny once. For each flip, record whether it was heads or tails. Set aside the pennies that indicated tails.
  2. Once you have flipped all the pennies, repeat step one for each penny that landed on heads. For example, if you had 29 heads, flip those 29 pennies a second time. Record again how many heads and tails you obtained for the second round of flips. Again set aside the pennies that came up tails.
  3. Keep flipping and recording the pennies that land on heads after each round. Set aside the ones with tails each time.
  4. Stop when you have only one penny left.

 

Data Sheet

Round
Heads
Tails
Total

1.

     
2.      
3.      
4.      
5.      
6.      
7.      
8.      
9.      
10.      
Total Number of Rounds to get One Penny :

 

The teacher should then compile the results of each group and find the average number of flips it took to "decay" all the pennies until only one was left. Since mathematically only half of the pennies should be left after each round, you can show that the "half-life" of the pennies in our experiment was equal to one flip.

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 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?

 

 

  1. If you flipped 1000 pennies in the manner described here in the half-life experiment, how many heads would you expect to obtain during the fourth cycle? That is, you remove the pennies coming up tails each time. You do this three times and then what do you find on the fourth time?