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The Geiger Counter

Objective

Note for Teachers: Give out this lab handout at least one day before doing this lab and assign the problems for homework.

Also, there is another, slightly different version of this 'lab' which leaves out the data collection tables.

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

Geiger.Counter (37K)
Attribution: Wikimedia Commons (click here for information)

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 strongly ionizing it is, 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. Alpha and beta radiation involve the ejection of a particle with mass from the nucleus. Alpha particles are helium-4 (42He) nuclei moving at extremely fast speeds. Alpha particles are not atoms because they do not have electrons. Beta particles are electrons (0–1β) moving at immensely high speeds. Gamma particles are photons (00γ) emitted by a nucleus in an excited energy state. Gamma particles have no mass and because they are a form of electro-magnetic radiation (light) they are also called gamma waves.

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 passes through the detector tube window) causes a spike in the current in the Geiger counter. This happens because when the ionizing radiation strikes an atom of the low-pressure gas inside the tube it causes that atoms to lose an electron. The high voltage inside the tube accelerates that electron toward the positively charged anode. This creates the electrical current detected by the circuitry in the counter. Geiger counters typically display the number of counts per minute. Each count represents the decay of one radioactive atom. 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.

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. This is because half-life is a measure of the rate of decay. If an isotope has a short half-life it means that the sample decays to half of its original intensity more quickly than an isotope with a longer half-life. The number of atoms present also 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. If the sample is small then there will be correspondingly fewer decays per minute. If the sample is large, even for atoms with a very long half-life, the radiation may still be very intense. All isotopes of uranium have very long half-lives, some of them several billion years. Even so, a large enough piece of pure uranium still produces very intense radiation because there are so many atoms.

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. Using mathematical symbols: I ∝ 1/d2 where I is intensity in counts per minute and d is distance.

The ionizing power of radiation depends on the type of radiation. Alpha particles are really the nuclei of helium-4 atoms. As such, they have a positive two charge. They are the most massive kind of radioactive decay particle with a mass of 4 amu. Given their large mass and high electrical charge they are by far the most ionizing kind of radiation. Beta particles are really electrons (or, in the case of beta-plus particles, they are positrons). Electrons and positrons have a mass almost 2,000 times smaller than the mass of a hydrogen atom. However, they have a full unit of electrical charge and this leads to beta radiation being the second-most strongly ionizing form of radiation. Gamma rays are photons of very-high-energy light. They have zero mass and zero charge. As a result, they are the least ionizing form of radiation. Other forms of electro-magnetic radiation (in other words, light) can also cause atoms to become ionized. Ultraviolet light and x-rays can also cause ionization. This is why exposure to sunlight, which includes UV light, causes sun burns. It is also why the number of x-rays a person receives each year should be limited. Other forms of light cannot cause ionization and are therefore extremely unlikely to cause harm. For example, the microwave radiation used to allow cell-phone communication has far too little energy per photon to cause the kind of genetic damage that could lead to cancer.

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. This means that their strong ionizing power is usually not important, at least for a sample outside the body. In fact, alpha particles can barely penetrate skin. But if an alpha-emitter should find its way inside the body it is by far the most damaging. 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 (average atomic mass = 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. Using your notes or other materials define the following types of ionizing radiation:
    1. Alpha:

    2. Beta:

    3. Gamma:
  2. List several types of non-ionizing radiation. (Hint: think about the spectrum you studied in your lesson on light).
  3. What is ionizing radiation? What does it do at the atomic/molecular level?
  4. What are the three features that characterize the radiation given off by a radioactive substance?
  5. What effects does ionizing radiation have on living things?
  6. Which type of radiation is the least ionizing? Why?
  7. Which type of radiation is the least ionizing? Why?
  8. Humans do not have senses that can perceive ionizing radiation directly. What happens inside a Geiger-counter tube that allows it to detect ionizing radiation?
  9. Which do you expect would have a higher radiation intensity: a sample of atoms with a half-life of 1 hour or another sample with the same number of radioactive atoms with a half-life of 1 year. Why?
  10. Which do you expect would have a higher radiation intensity: a large sample of atoms with a half-life of 1 year or another sample with half the number of radioactive atoms with a half-life of 1 year. Why?
  11. Describe the best way to shield against the three major types of radiation.
  12. 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?
  13. Write nuclear symbols (AZX) for each of the following. For example, a beta-plus (or positron) has the symbol 0+1β
    1. an α-particle ________
    2. a β-minus particle ________
    3. a proton ________
    4. an electron ________
    1. a neutron ________
    2. an isotope of C with A=14 ________
    3. an isotope of Fe with 32 n0 ________
    4. a γ-particle ________
  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.
  16. 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 12 decays to become 20983Bi. Each decay in order is (1) alpha, (2) alpha, (3) beta-minus, (4) alpha, (5) alpha, (6) beta, (7) alpha, (8) alpha, (9) alpha, (10) beta, (11) alpha, (12) beta. Write a balanced nuclear equation for each of these decays. Two have been done for you.

    24195Am --> 42He + 23793Np
    23793Np --> 42He + 23391Pa



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Materials

  1. Geiger counter
  2. household smoke detector (disassembled)
  3. standard source for &alpha radiation
  4. standard source for β-minus radiation
  5. standard source for γ radiation

  1. sheets of aluminum
  2. sheets of lead
  3. paper

Safety

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.

For the standard sources and the smoke detector place the end of the Geiger tube in contact with the source. Set the Geiger counter to continuous count. Start a one-minute timer at the same time as the count is reset. If the count rolls over to the next 10,000 counts-per-minute then keep track of how many times this happens. When the timer goes off switch the counter to rate mode and the display will freeze. Use the data to calculate counts per minute.

Source Counts per Minute
background  
smoke detector  
21084Po  
9038Sr  
6027Co  
   



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Part II: 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. Try allowing air space between the Geiger tube and the sample. Air blocks a fair amount of both alpha and beta radiation. Try using sheets of papers, aluminum, and lead. Collect counts-per-minute data in order to do a proper comparison.


    Source      Particles Emitted Shielding
Material and Amount
Result
       
       
       
       

Questions

Answer the following questions using complete sentences.

  1. What are the likely sources of the radiation detected as part of the background radiation? Your teacher discussed this with you in class.
  2. According to the demonstration in class, which type of radiation is the most penetrating? Support your answer by referring to data and observations made in class.
  1. What shielding used in the lab blocked all of the alpha rays?
  2. 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?
  3. What shielding used in the lab did it take to block the radiation from the β-emitter?
  4. What shielding used in the lab did it take to block the radiation from the γ-emitter?
  5. Would a very bright lamp be able to make the Geiger counter click? Why or why not?
  6. Which is a better strategy for avoiding exposure to radiation: putting more distance between yourself and the source or using shielding? Why?
Last updated: Mar 09, 2016 Home
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