Alpha, Beta, and Gamma Radiation: an Exercise in Shielding

Quantifying and Shielding Radiation 01/15

Integrated Science 3 Name Per.

Ø Introduction

The term radioactivity refers to the activity of unstable atoms. Radioactive substances send out very energetic particles or rays in an effort to become more stable. The different types of radiation are alpha, beta and gamma radiation. All of these have the ability to change the chemical makeup of matter including living tissue. When matter is changed, the molecules that make up the matter are changed. The molecules are broken apart as radiation knocks electrons out of the atoms, leaving them with a positive charge. Charged atoms are called ions. Therefore, these types of radiation are known as ionizing radiation. These types of radiation are further described in the table below.

Radiation Type / What they are / Range in Air / Stopped By / Comments
Alpha (a) / Positively charged nuclei of Helium (42He) / A few centimeters / A sheet of paper / Because they are so massive and carry a double positive charge, they easily affect atoms. They make lots of ions and don't travel far. Alpha emitters are quite safe unless they get into the body
Beta (b) / Negatively charged fast-moving electrons / A few tens of centimeters / A few millimeters of aluminum / Electrons are small and so can travel further than alpha particles, as they don’t collide as often. Dangerous, but easily stopped
Gamma (g) / Uncharged, very short wavelength of electromagnetic waves; high energy photon / They go on indefinitely / A meter or two of concrete / These are genuine “rays”. They don't ionize atoms very easily and so travel a long way. They travel at the speed of light. Dangerous because they are hard to shield against

§ Detecting and Measuring Radioactivity

Natural radioactivity is identified using an electric field. When a radioactive source is placed within the electric field, the radiation given off is affected as illustrated in the Figure 1 below. (Remember, opposites attract.)

Figure 1 Figure 2

The instrument most commonly used to detect radiation is called a Geiger counter. It contains a gas filled tube called a Geiger tube. Within the tube, there is also a high voltage wire. When radiation enters the tube, it ionizes the gas inside (the gas particles become charged). The wire attracts these ions which sends an electric pulse down the wire. The Geiger tube is connected to a counter that counts these electrical pulses as counts or clicks. Geiger tubes (see Figure 2 above) can also be connected to a rate meter that records the rate of decay.

A certain hesitation about working with radioactive material is natural. After all, radiation can be dangerous to living things. Therefore, the ability to detect and measure the amount of radiation is very important. Radiation is sometimes measured in a unit called a rem (radiation equivalent for man). This is a measure of the power of radiation to cause damage (ionization) to human tissue. The average person receives about 0.2 rem of radiation per year. This includes radiation from various sources. Background radiation is around us all of the time, in the air, in rocks and soil, in our water, and from the sun. This type of radiation accounts for approximately 68% of the average annual exposure to radiation. For your information, typical levels of gamma radiation from the sun for active people living in Marin County is about 0.05 rems per year. Other sources of radiation include medical x-rays, television tubes, consumer products, the nuclear industry, etc. The radioisotope samples used in this lab emit between 1.0 X 10-5 (0.00001) rems and 5.0 X 10-4 (0.0005) rems per hour. This is approximately 10,000 times less than a dental x-ray. The radioisotope samples you will be using are very small and cannot be detected with the naked eye. The samples are encased in polyethylene, insuring that you cannot come in physical contact with them.

Ø Purpose

This lab is designed to familiarize students with radioactive substances that undergo radioactive decay until they reach a stable ending point. Students will use a Geiger Counter to quantify the amount of radiation emitted from radioisotopes as they break apart. Students will also learn how distance from a Geiger Counter affects the amount of radiation detected by the device.

Ø Additional Background Research

1. Read pp. 229-237 in your Integrated Science textbook.

2. Answer the following questions:

a. Describe how alpha and beta particles usually change the atoms they smash into.

b. Describe how mutations affect organisms.

c. Explain why alpha and beta radiation is more dangerous inside your body than outside of you.

Ø Experimental Organizer

1. You will be conducting an experiment that tests the relative ability of different shield types to prevent the penetration of alpha, beta, and gamma radiation.

SHIELDS AVAILABLE: Polyvinyl 4 mil; Polyvinyl 8 mil; Plastic 0.030”; Plastic 0.040”;

Aluminum 0.025”; Aluminum 0.090”; Lead 0.064”; Lead 0.250”

2. Considering both the physical properties of alpha, beta and gamma radiation and the data collected from the Counting Lab, complete the Experimental Organizer for the experiment your lab team will conduct. Be thorough, making sure to reference the particular properties you are basing your assumptions and expectations on.

Title:

Hypothesis: (make predictions about which type of shield will effectively block each type of radiation)

Independent Variable:

Category:

IV Levels
# Trials

Dependent Variable (include units):

Category:

Constants:

Ø Procedures

1. Plug in the Geiger counter and turn it on using the switch on the back of the unit.

2. Press and release the H.V. button - then press the UP button until you reach the value 800. [this is the voltage being applied in the Geiger tube itself]

3. Press and release the TIME button - then press the UP button until you reach the value 20. [this is the time in seconds that each reading will last]

4. Make sure that the plastic tray in the Geiger counter is empty (is not holding a radioactive sample).

5. Press and release the COUNT button - the timer will begin to count from 1-20.

6. Once the counter has stopped at 20, press the TIME button again and the display will indicate the number of counts recorded for that reading. Pressing the TIME button allows you to switch the display between TIME and COUNT.

7. Take 3 readings with each of the following radioisotopes (Sr-90 emitting beta particles and Co-60 emitting gamma rays), matched with each shield your group has been assigned. Data for alpha radiation has been provided for you.

8. To take readings place the appropriate shield into the top slot of the Geiger counter. Then, place one radioisotope, label side down, into the loading tray and slide the tray into the slot directly below the shield (2nd from the top). Take and record a twenty-second reading.

9. Complete the data tables and calculate mean counts detected.

Ø Data Tables

Directions: Record the data collected for each radioisotope in the space below. For alpha radiation you only need to calculate the mean values.

Data Table 1: Radiation Detected for Radioisotope emitting Alpha (a) Radiation (Po-210)

Type of Shield / Geiger Counter Reading (counts)
Trials / Mean Reading (counts)
Trial 1 / Trial 2 / Trial 3
No Shield (control) / 0 / 0 / 0
Polyvinyl 5 mil / 0 / 0 / 0
Polyvinyl 10 mil / 1 / 0 / 0
Plastic .030” / 2 / 0 / 0
Plastic .040” / 0 / 0 / 2
Aluminum .025” / 1 / 0 / 0
Aluminum .090” / 0 / 0 / 0
Lead .064” / 0 / 0 / 0
Lead .250” / 0 / 0 / 0

Data Table 2: Radiation Detected for Radioisotope emitting Beta (b) Radiation (Sr-90)

Type of Shield / Geiger Counter Reading (counts)
Trials / Mean Reading (counts)
Trial 1 / Trial 2 / Trial 3
No Shield (control)
Polyvinyl 5 mil
Polyvinyl 10 mil
Plastic .030”
Plastic .040”
Aluminum .025”
Aluminum .090”
Lead .064”
Lead .250”

Data Table 3: Radiation Detected for Radioisotope emitting Gamma (g) Radiation (Co-60)

Ø Graph

Construct the appropriate graph(s) to present the data collected for each radioisotope. The graph(s) will present the average count for each radioisotope with each shield tested.

Ø Discussion

Write a Discussion that summarizes the results of this experiment. Use the “three paragraph” approach.

· For paragraph one, describe WHAT happened during the experiment. Use ACTUAL DATA VALUES in your answer. Discuss trends and anomalies in your data.

· For paragraph two, discuss WHY each type of radiation either penetrates or is blocked by each shield tested. Consider how the different materials used to shield the radiation affected the radiation detected by the Geiger counter and how the mass and/or charge of each type of radiation affected the results.

· For paragraph three, discuss HOW you would improve this experiment. Include a discussion of errors you encountered and suggestions for further study.

· Based on what you’ve learned about the biological effects of radiation, include in your discussion the importance of using effective shielding materials when constructing a Nuclear Power Plant.