The Geiger Counter

Objective

The lab work has two objectives: first, to gain some experience with how radioactivity really works. Students will collect data about levels of activity, the effect of distance on radioactivity counts, and shielding. Second, students will learn about how a Geiger counter works (also called a Geiger-Müller counter).

Overview

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In this activity you will collect data as part of a demonstration of a Geiger counter. Geiger counters are designed to measure the number of nuclear decays per minute of radioactive substances. Usually the displayed numbers are in fact only proportional to the true number of radioactive decays per minute. Geiger counters are sensitive to ionizing radiation

Radiation from a given sample of a substance is characterized by three features: (1) how intense it is, (2) how its intensity drops with distance, and (3) how easily it can be blocked. In this lab you will collect data about each of these three features.

Background

Radioactivity that can be detected by Geiger counters comes in three types, which you have already learned about: Alpha radiation, beta radiation, and gamma radiation. All three types of radiation (α-rays, β-rays, and γ-rays) cause ionization. Ionization is an atomic-scale process in which atoms lose one or more electrons and therefore come to have a positive charge. When a radioactive particle strikes matter it cause electrons to become ionized. These electrons can then cause damage to neighboring atoms. Also, the positively charged atoms and molecules need to become neutral again and so steal electrons from neighboring atoms and molecules. This causes a cascade of electron-stealing as more molecules are affected by the lack of an electron. In living things these ionized molecules no longer work properly and can make cellular processes fail to function. Also, ionization can lead to damage to DNA, causing mutations and (with long exposure) cancer. Geiger counters take advantage of this ionization because the motion of electrons can be detected as an electrical current. Each ionization event (i.e., whenever a radioactive particle strikes the detector tube) causes a spike in the current in the Geiger counter. Geiger counters typically display the number of counts per minute. Some counters display a number which the user must multiply by 10, 100, or 1,000 to get the true number of counts per minute. Each count represents the decay of one radioactive atom.

The intensity of radiation from a sample depends on the half-life of the substance and the number of atoms that are present. In general, if the substance has a short half-life the intensity will be higher. If the half-life is long, the intensity will be lower. Perhaps more importantly, the number of atoms present plays a role in how intense the radiation will be. If there are a large number of atoms then there are more atoms that can decay during a given period of time, say every minute. If the sample is small then there will be correspondingly fewer decays per minute.

The intensity of radiation drops quickly with increasing distance. As the distance from the source increases the intensity decreases with the square of the distance. So if you double the distance the intensity decreases to 1/4 of the original amount. If you triple the distance the intensity decreases to 1/9; quadruple it and it decreases to 1/16…and so on. The best way to describe this relationship is to say that the intensity is inversely proportional to the square of the distance.

Shielding can reduce the intensity of radiation as well. Alpha-rays are the easiest to shield against and they can be blocked with a piece of paper. In fact, they can barely penetrate skin. Beta-rays are more penetrating and will penetrate up to 1 cm into a human body. They can be blocked with a relatively thin piece of metal such as a few layers of aluminum foil. Additional concerns with beta-rays involve the x-rays produced by the interaction of the particles with the shielding but we won’t study that in this lab. Gamma-rays are by far the most penetrating type of radiation. They are similar to x-rays but are more powerful. Gamma-rays can pass all the way through a human body and are difficult to shield against. The best shielding is done with atoms that have heavy nuclei, such as lead (A = 207 amu). Lead is best for shielding against gamma-rays but anything will do, as long as it is thick enough.

Pre-lab Questions & Problems

The following questions and problems will help you to understand the concepts be able to do the math required for the analysis of your lab results.

1. What are the three features that characterize the radiation given off by a radioactive substance?
2. Name and define the three types of radiation that can be detected by a Geiger counter.
3. What is ionizing radiation? What does it do at the atomic/molecular level?
4. How does ionizing radiation damage living things?
5. In your own words describe how a Geiger counter works.
6. What effect does the half-life of an isotope have on the intensity of radiation?
7. What effect does the number of radioactive atoms in a sample have on the intensity of radiation?
8. What effect does increasing the distance from a radioactive substance have on the measured intensity of the radiation?
9. A substance has an intensity of 1,000 counts per minute at 1 cm. What is the intensity at 2 cm? At 10 cm?
10. Describe the best way to shield against the three major types of radiation.
11. See the procedure (which comes after this section) to read about safety precautions for this lab. What steps should be taken to prevent and/or minimize the received dose of radiation during this lab?
12. Write nuclear symbols (^A_ZX) for each of the following :
    1. an α-particle ________
    2. a β-minus particle ________
    3. a proton ________
    4. an electron ________
    5. a neutron ________
    6. an isotope of C with mass number 14 ________
    7. the isotope with A=42 and Z=20 ________
    8. a γ-particle ________
13. The radioactive isotope in the common household smoke detector is ²⁴¹₉₅Am. It is produced artificially specifically for this purpose. It decays by a series of decays (about 13) to become ²⁰⁹₈₃Bi. The first five decays are (1) alpha, (2) alpha, (3) beta-minus, (4) alpha, and (5) alpha. Write a balanced nuclear equation for each of these decays.
14. The radioactive source used for β-minus radiation in this lab is made of ⁹⁰₃₈Sr. Write a balanced nuclear equation for its decay.
15. The radioactive source used for γ radiation in this lab is made of ⁶⁰₂₇Co. It decays by β-minus decay and emits two gamma rays in the process. Write a balanced nuclear equation for its decay.

Materials

Part I: Measuring Counts per Minute

Except in specially shielded spaces in labs where radiation is studied there is no place on Earth that is completely free of all radiation. This radiation is usually at very low levels (say, around 40 counts per minute). Turning on the Geiger counter and setting it to report counts per minute when it is not oriented toward a radioactive source will allow you to measure the background rate. This rate should be subtracted from all other rates to get the true count.

Part II: The Effect of Distance

In this part of the lab data will be collected to compare radioactive intensity at different distances.

Part III: Shielding

In this part of the lab different shielding materials will be used to find out how much is required to reduce the count to background.

Questions

Answer the following questions using complete sentences. Make graphs on separate graph paper or using Excel or a similar program.

1. What was the background rate measured using the Geiger counter? Why does it need to be subtracted from all other measurements?
2. Which sample had the most intense radiation? Did anyone bring anything that turned out to be really radioactive? If so, what was it and how intense was the radiation?
3. Make a graph of the data in Part II. Put Counts per Minute on the y-axis and distance on the x-axis. Set up your axes so that the data cover as much of the graph as possible. (Use a whole sheet). Sketch a line of best fit for your data. This graph should make a curve that is called an Inverse Squared curve.
4. Calculate the inverse square of all of the distance values. Do this by squaring each distance then dividing one by the result: 1/d². Add them to the table in Part II.
5. Make a new graph. Put the inverse square of the distance on the x-axis (be careful setting up that scale!). Counts per minute goes on the y-axis, as before. Again, fill as much of the page as possible. If the data really do follow an inverse square law then your graph will now have the form of a straight line when you draw the line of best fit. Is your graph a straight line? If so, you have confirmed that the decrease in the intensity of radiation is proportional to the inverse square of the distance.

6. What shielding used in the lab blocked all of the alpha rays from the smoke detector?
7. What kind of shielding was required to block all of the radiation from the smoke detector? Why was there a difference between blocking the alpha rays and blocking all of the radiation from the smoke detector?
8. What shielding used in the lab did it take to block the radiation from the β-emitter?
9. What shielding used in the lab did it take to block the radiation from the γ-emitter?