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    Nuclear Radiation - Nature and Dangers
    Experiments & Background Information
    For Senior High School Students
    For Science Labs, Lesson Plans, Class Activities & Science Fair Projects







    This experiment is courtesy of 

    An investigation into the nature and danger of nuclear radiation

    Developers:

    Alan Darion
    Kutztown Area School District
    Kutztown, PA

    Al Breaux
    Analytical Research
    Rohm and Haas Company

     

    Grade Level:

    Senior High School

     

    Discipline:

    Physics

     

    Goals:

    Upon completion of this unit, the student will

    1. Understand the random nature of nuclear decay and radiation
    2. Appreciate that risk assessment is a matter of probabilities
    3. Recognize the various ways to control exposure to radiation
    4. Deal with the problem of nuclear waste in a rational rather than emotional way

     

    Objectives:

    Upon completion of this unit, the student will be able to:

    1. Distinguish between ionizing and non-ionizing radiation
    2. Apply probability calculations to predict the outcome of random events
    3. Identify the three types of nuclear radiation and distinguish among their various penetrating powers and biological hazard
    4. Apply probability calculation to evaluate the risk benefit of a situation
    5. Determine the relationship between risk of exposure to radiation and time through an understanding of a substance's half-life
    6. Apply probability calculation to determine the nature of radioactive half-life
    7. Apply the principles of geometry to explain the inverse square law.

     

    Background and Overview:

    Students will be given the task of evaluating the claims of an imaginary nuclear power company that wants to dispose of its nuclear waste within their community. They will first investigate the nature of nuclear radiation, and then apply this to the problem of nuclear waste.

    Students should work in small lab groups. If enough radiation detectors are available, then the class can proceed with their activities in the order given. If the number of detectors is limited, then alternate paths through these activities need to be considered.

     

    Equipment:

    • Radiation detectors
      Geiger-Muller tube of sufficient sensitivity to detect alpha particles, and a scaler or computer and interface for counting.
      Pasco's Introductory Computer G-M system is adequate if you have a PC or Macintosh available (about $300). An older version for the Apple II's that inputs into the game port will also do.


    • Radioactive Sources: Alpha, beta, and gamma sources which do not require an NRC license.
      Available from Pasco (about $115)
      ONE Iso-generator Kit for Ba-137m
      Also available from PASCO (about $175)


    • Dice 800 to 1000 will do, but electronic dice are faster
      A computer program in LOGO plus for the Macintosh is included as an appendix and is on the disk


    • Jelly Beans (about 50 per student)

    • Electroscopes

    • Sources of Non-ionizing radiation

     

    Context

    The local power-company has proposed putting a high-level radioactive waste disposal site in your area. Hoping to eliminate opposition from the community, the company has offered it a very low rate for electric power, and has scheduled a public meeting in the high school auditorium to try to convince the public that they are in no danger from the site.

    However, your science teachers have gotten a copy of some of the claims they will make at that meeting. They have decided that you will check them out in your own laboratories first, and be prepared to present them at the meeting.

    Their assertion that nuclear waste can be disposed safely rests on the following claims:

    Nuclear radiation is not necessarily harmful. The danger depends on

    • Type of radiation
    • Dose — the amount of radiation received

    Dose may be controlled through

    • Shielding
    • Distance from the source
    • Time

    Your job is to now investigate whether or not these claims are true, and to decide how they would apply to the disposal of nuclear waste.

     

    ACTIVITY 1: SOME LIKE IT HOT

    Background

    Radiation is anything that radiates, that is, emerges from, and travels away from a source, in a continuous way. Bullets may be said to radiate from a machine gun although we usually do not describe it that way. Radiation usually refers to tiny particles or bundles of energy that emerge continuously from a source and travel through space away from it. Light radiates away from a light bulb. In fact, light is a form of electromagnetic radiation, about the only kind of radiation we can actually see. Radio signals radiate from the antenna of the radio station — these are another kind of electromagnetic radiation. Light — electromagnetic radiation we can see, ultraviolet and other electromagnetic radiation we cannot see, and various particles radiate from the Sun.

    All radiation is not dangerous. A few years ago, there was a hypothesis that the kind of electromagnetic radiation from power lines and even household appliances might be dangerous, but this has not stood up to any scientific research. The type of radiation that that is known to be dangerous is described as ionizing radiation.

    Ionizing radiation is made of those types of particles (which from now on will include "bundles" of electromagnetic energy which behave very much as particles) that have enough energy to knock the electrons off the atoms they pass through — turning them into electrically charged ions. Exposure to this kind of radiation can harm the cells of our bodies, either killing them outright, or altering them so that they can become cancerous.

    Activity

    Charge an electroscope.

    Touch it with your hand

    WHAT HAPPENS TO THE LEAVES?

    line

    line

    WHY?

    line

    line

    Charge the electroscope again

    AS LONG AS THE AIR SURROUNDING THE ELECTROSCOPE REMAINS AN INSULATOR, WHAT WILL HAPPEN TO THE LEAVES OF THAT ELECTROSCOPE?

    line

    line

    Open the chamber with the leaves and bring a flame near them

    WHAT HAPPENS TO THE LEAVES?

    line

    line

    WHAT DOES THAT INDICATE HAPPENED IN THE AIR AROUND THE LEAVES AS A RESULT OF THE FLAME BEING NEAR?

    line

    line

    Charge the electroscope again

    Put the following sources of radiation, one at a time, into the chamber with the leaves for about five minutes (the flame itself consists of ions — it works much faster).

    1. The "red" (alpha) nuclear source
    2. A light bulb
    3. An electric motor

    WHICH SOURCE EMITS IONIZING RADIATION?

    line

    EXPLAIN WHY YOU THINK SO

    line

    line

     

    ACTIVITY 2: WANT TO BET?

    Background

    The nucleus of an atom is made of positively charged protons, and neutral neutrons. Since like charges repel each other, and the strength of the force increases as the square of the distance decreases, there is an extremely large repelling force among the protons squeezed to within .0000000000001 cm of each other. They are held within the nucleus by the nuclear force. In large nuclei, the balance between these two forces is unstable, and it is possible for a small part of the nucleus, called an alpha particle consisting of two protons and two neutrons, to fly away, leaving the decayed nucleus behind.

    Neutrons are inherently unstable; they can only exist for long periods of time if joined with an appropriate number of protons in a nucleus. If there are too many neutrons, then one of them may decay into a positive proton, and a negative particle. This negatively charged beta particle (which has the same characteristics as an electron) escapes from the nucleus, leaving behind a nucleus that now has one less neutron, and one more proton. This is another form of nuclear decay.

    In either of these processes, or if a nucleus has too much internal energy to be stable, a "bundle" of energy may be released called a gamma ray. (These are not really particles but rather extremely short wavelength electromagnetic radiation, and so they are referred to as rays. However, this distinction is very complicated and, for the purposes of this discussion, not very important.) While the release of this uncharged particle does not change the type of nucleus it came from, it does allow its internal energy to reach a lower, more stable, state.

    Materials made of these unstable nuclei are said to be radioactive. In any of these radioactive types of nuclear decay, it is impossible to predict when a particular nucleus will actually decay. However we can determine the probability that it will decay in some period of time. Let us see what it is like when an event is controlled through its probability of happening.

    Activity

    Consider a single die (one dice).

    WHAT ARE THE ODDS THAT ON A SINGLE ROLL YOU WILL GET A 4? line 

    Roll it. WAS IT A 4? line 

    Now roll it again.

    WHAT ARE THE ODDS THAT ON THIS ROLL YOU WILL GET A 4? line 

    Past events do not effect future probability. As a result, we can never predict when a particular roll will give a particular result. However, we can determine what is the most likely outcome of many rolls.

    WHAT IS THE MOST LIKELY NUMBER OF TIMES A 4 WILL COME UP IF THE DIE IS ROLLED 60 TIMES? line 

    WHAT IS THE MOST LIKELY NUMBER OF 4's YOU WILL GET IF YOU ROLL 60 DICE? line 

    Teacher Note:
    Make sure you emphasize that the number expected is the number of dice rolled times the probability of getting the desired outcome.

    Try it — either with real dice, or electronic ones.

    HOW MANY 4's DID YOU GET? line 

    Try it again.

    HOW MANY 4's DID YOU GET? line 

    Roll the 60 dice 8 more times, and record the number of 4's below.

    line line line line line line line line 

    Determine the absolute value of the difference between the most likely outcome and the actual outcome on each of these ten rolls.

    line line line line line line line line and the first two line line 

    WHAT IS THE AVERAGE VALUE OF THESE DIFFERENCES?

    line

    WHAT PERCENT OF THE NUMBER OF DICE ROLLED EACH TIME (60) IS THIS?

    line

    Rolling 60 dice 10 times is the same as rolling 600 dice once.

    WHAT IS THE MOST LIKELY TOTAL NUMBER OF 4's TO COME UP?

    line

    HOW MANY ACTUALLY CAME UP?

    line

    THE DIFFERENCE BETWEEN PREDICTED AND ACTUAL WAS

    line

    The difference between these values should not be equal to the sum of the differences calculated above, because those were the absolute values of the differences.

    WHAT PERCENT OF 600 IS THE DIFFERENCE BETWEEN THESE VALUES?

    line

    Summary

    It is impossible to predict exactly what will occur when events are controlled by probability. For example, we cannot predict which individual dice will come up 4. However we can always calculate what is the most likely total outcome of a series of events if we know their probabilities. The more events, the smaller the percentage difference between the most likely outcome and the actual outcome.

    If we have a collection of unstable (radioactive) nuclei, it is impossible to predict which nuclei will decay in a given amount of time. However, if we know the probability that a nucleus will decay in a some amount of time, we can predict the total number of that collection that are most likely to decay in that time. Since a macroscopic sample of some material would have at least somewhere around 1025 nuclei, we can predict, with a very low percentage error, the total number that will decay in that time. Of particular interest is the amount of time it would take for the odds to be 50:50 that the nucleus of a material will decay. This will be important in a later activity.

     

    Activity 3: COUNT ON IT!

    Background

    The ionizing property of nuclear radiation may be used to detect, and count, the nuclear particles and rays in the surrounding environment. (For the sake of simplicity in our language, discrete bundles of electromagnetic energy referred to as rays may also be included when reference is made to particles). The device used to detect the particles is called a Geiger-Muller tube, (G-M tube) and when it is combined with a method of counting those particles it is called a Geiger Counter.

    The Geiger-Muller tube operates in the following manner


    Geiger-Muller tube diagram

    When the particle passes through the "window" into the tube, it ionizes the gas inside. (Some G-M tubes have no window so as not to block out the least penetrating radiation, and others have thick windows that only allow the most penetrating to be detected). This completes the circuit, and a pulse of current passes through it. The counter, which is called a scaler, or a computer programmed to act as a scaler, then counts the number of such pulses.

    Because of the random nature of radioactive emission, you can never be "sure" that the number of counts you have is the amount you should expect, i.e., the most likely amount. However, the uncertainty can easily be calculated; it is the square root of the total number of counts that have been made. So

    if the count is 4 the uncertainty is plus or minus 2 or 50%
    if the count is 25 the uncertainty is plus or minus 5 or 20%
    if the count is 100 the uncertainty is plus or minus 10 or 10%
    if the count is 1,000 the uncertainty is plus or minus 32 or 3.2%
    if the count is 10,000 the uncertainty is plus or minus 100 or 1%

    10,000 or more counts would be ideal, however you should try to get at least 1000 to be fairly certain of your results.

    Activity

    Set up your G-M tube so it is held vertically above the source as shown below. Set the value of d to be just big enough to allow five thicknesses of lead to be inserted above the source.

    Geiger-Muller tube diagram

    Remove all sources and count the number of "background" particles that pass through in 5 one-minute time intervals.


    line line line line line
    minute 1 minute 2 minute 3 minute 4 minute 5

    Calculate the average background radiation per minute line 

    (You could simply count for five minutes and divide this total by five, but your counter may not be set up to do this.)

    Place the "red" source directly below the G-M tube with the "hole" facing up. Count the number of particles to get into the tube in one minute with nothing between the source and the tube. Record in data.

    Put successive thicknesses of paper between the source and the tube. Record the number of particles to get through in one minute for each thickness.

    Remove the paper, and repeat for successive thicknesses of plastic, and then of lead.

    Any time your count has dropped to the background level, STOP, and begin this procedure again with the "green", and then the "orange" source.

      Activity 3 Worksheet  (PDF, 5K)  Get Acrobat Reader

    WHAT CAN YOU CONCLUDE ABOUT THE RADIATION COMING FROM THE THREE SOURCES? USE THE ABOVE DATA TO EXPLAIN YOUR ANSWER

    line

    line

    WHAT DO YOU SUPPOSE HAPPENED WHEN THE FIRST PIECE OF PAPER WAS PUT OVER THE RED SOURCE?

    line

    line

    WHAT MAY HAVE CAUSED THE GRADUAL DROP AS THE THICKNESSES OF PAPER OVER THE GREEN SOURCE WERE INCREASED?

    line

    line

    WHAT DID THE PIECE OF PLASTIC DO TO THE RADIATION COMING FROM THE GREEN SOURCE?

    line

    line

    WHAT EFFECT DO PAPER AND PLASTIC HAVE ON THE RADIATION COMING FROM THE ORANGE SOURCE?

    line

    line

    WHAT MIGHT HAVE CAUSED THE GRADUAL DROP AS GREATER THICKNESSES OF LEAD WERE PUT OVER THE ORANGE SOURCE?

    line

    line

    WHAT MIGHT YOU NEED TO STOP ALL THE RADIATION FROM THE ORANGE SOURCE FROM GETTING TO THE DETECTOR

    line

    line

    THE LEAST PENETRATING RADIATION COMES FROM THE line SOURCE

    THE SECOND LEAST PENETRATING RADIATION COMES FROM THE line SOURCE

    THE MOST PENETRATING RADIATION COMES FROM THE line SOURCE

    Summary

    The three types of particles are:

    Alpha Beta Gamma

    The alpha (Alpha) is doubly charged and extremely dense.
    The beta (Beta) is singly charged and has several thousand times less mass than the alpha.
    The gamma (Gamma) has no charge and is a "bundle" of pure energy with zero mass.

    WHICH DO YOU SUPPOSE IS THE MOST PENETRATING? line 

    WHY?

    line

    WHICH DO YOU SUPPOSE IS THE LEAST PENETRATING? line 

    WHY?

    line

    WHICH DO YOU SUPPOSE CAN DO THE MOST DAMAGE TO YOU? line 

    WHY?

    line

    line

    UNDER WHAT CIRCUMSTANCES WOULD EXPOSURE TO THIS RADIATION BE MOST HARMFUL?

    line

    line

    The fact that a particle is "stopped" by some material means that it has interacted with something in that material. For this reason, the very heavy, highly charged alpha particles are the ones that are the least penetrating. However, they are also the ones that could do the most damage. If an alpha particle were to enter, or contact your body, it will almost certainly interact with the cells of your body. This could damage them immediately, causing immediate radiation damage, or alter the genetic material of the cell so that it is more likely to become cancerous in the future.

    However, since alpha particles are stopped by even a few inches of air, they are only likely to interact with the cells of your body if they get on or inside of you. This is only going to happen if you touch, inhale or eat a source of alpha particles. Beta particles are slightly more penetrating but they too pose a serious risk only if there is ingestion, inhalation or direct physical contact with a source of these particles.

    As a result, if you are exposed to nuclear radiation from outside of your body, it will most likely be to gamma rays. Because they have no charge or mass, gamma radiation is the most difficult to stop. That also means a gamma ray may pass completely through your body without interacting with any of your body's cells. If that occurs, it has done you no harm. Once again, we are dealing with probability, or the odds, that you will be harmed by exposure to gamma radiation.

    Current theory holds that there is no absolutely safe amount of radiation that we can tolerate. Any exposure carries with it some risk of harm. The amount of risk depends on the amount of radiation, that is, the total number of particles you are exposed to.

    Before investigating the factors other than shielding that affect the number of particles you might be exposed to, let us examine this issue of risk more carefully.

     

    ACTIVITY 4: IS IT WORTH THE RISK?

    Background

    The current approach to the biological hazard of radiation is that any exposure carries with it a risk of damage. In other words, it is possible for a single particle to interact with the DNA in one of your cells and trigger the cascade of events which results in cancer. The more particles one is exposed to, the greater the risk that such an adverse event will actually happen.

    However, eliminating all risk is impossible. There is the constant source of background radiation. There is risk in crossing the street, eating bacon, getting too much exercise, getting too little exercise. In fact, some research has found that you have the greatest risk of having a heart attack when you get out of bed in the morning — usually on Mondays. It is impossible to live without risk. The question is not eliminating risk, but keeping it low enough so that the risks involved in some activity are less than the benefits that may be received.

    This is not a scientific question. The value of the benefit, and the hazard of the risk, are really personal values. However, the following activity might give you a sense of the balance that you might strike, and a sense of how probability governs your decisions.

    Activity

    You have two assortments of jellybeans — the "sweet" and the "spice".

    Wash your hands.

    Pick out fifty of your favorite flavors and put them in the jar.

    Now pick out a flavor you absolutely hate.

    Replace one of the jellybeans in the jar with one of these and shake the jar to mix them up.

    WHAT ARE THE ODDS YOU WILL GET ONE YOU LIKE? line 

    WHAT ARE THE ODDS YOU WILL GET ONE YOU HATE? line 

    Close your eyes, and pick one bean out of the jar.

    ARE YOU WILLING TO EAT IT? line 

    If not STOP here
    If yes, then you MUST eat it whatever it is.

    WHAT KIND WAS IT? line 

    If it was a "bad" one you may replace it and repeat the trial before moving to the next one.

    Replace another one of the "good" beans with one of the "bad".

    WHAT ARE THE ODDS YOU WILL GET ONE YOU LIKE? line 

    WHAT ARE THE ODDS YOU WILL GET ONE YOU HATE? line 

    Close your eyes, and pick one bean out of the jar.

    ARE YOU WILLING TO EAT IT? line 

    If not STOP here
    If yes, then you MUST eat it whatever it is.

    WHAT KIND WAS IT? line 

    If it was a "bad" one you may replace it and repeat the trial before moving to the next one.

    Now replace three more (a total of five)

    WHAT ARE THE ODDS YOU WILL GET ONE YOU LIKE? line 

    WHAT ARE THE ODDS YOU WILL GET ONE YOU HATE? line 

    Close your eyes, and pick one bean out of the jar.

    ARE YOU WILLING TO EAT IT? line 

    If not STOP here
    If yes, then you MUST eat it whatever it is.

    WHAT KIND WAS IT? line 

    If it was a "bad" one you may replace it and repeat the trial before moving to the next one.

    Continue to replace five at a time until the BENEFIT of getting one you like is not worth the RISK of getting one you hate, and you decide to stop.

    WHEN YOU QUIT:

    WHAT WERE THE ODDS YOU WOULD GET ONE YOU LIKE? line 

    WHAT WERE THE ODDS YOU WOULD GET ONE YOU HATE? line 

    Questions for Discussion

    1. Why did you decide to stop?

    2. Why do you suppose other people chose to stop at other times?

    3. Did you decide to stop after getting a bad one?

    4. Suppose the "bad" one did not just taste bad, but made you ill. Would you have stopped at the same place?

    5. Suppose the "bad" one did not just taste bad, but was a deadly poison, and was mixed in with a million "good" ones. How many would you eat from those million?

    6. The following have been estimated to increase your risk of dying by one in a million chances (Adapted from DOE Radiation Worker Training, based on work by B.L Cohen)
      • Smoking 1.4 cigarettes (lung cancer)
      • Eating 40 tablespoons of peanut butter
      • Spending 2 days in New York City (air pollution)
      • Driving 40 miles in a car (accident)
      • Flying 2500 miles in a jet (accident)
      • Canoeing for 6 minutes
      • Receiving 10 mrem of radiation (cancer)
      Which of these risks have you been taking? Which ones will you continue.

    7. Do you think the willingness to take these risks of those who have had personal experience with cancer (either themselves or someone close to them) might be different from that of those who have not?

     

    ACTIVITY 5: THAT'S ONLY THE HALF OF IT

    Background

    Teacher Note:
    Check the maximum counting rate your detector can handle and still be accurate. With the Pasco SE — 7981 for the Apple II's do not have an initial counting rate of over 80 per sec. (4800 per Min). You only want to "milk" the iso-generator of 1 ml.

    Radioactive materials do not emit radiation forever. Remember that the release of a particle changes the nucleus it came from. Let us consider the simplest situation; the one in which the new nucleus is not radioactive. Once every nucleus in a macroscopic amount of this material has decayed by emitting a particle, the material would stop being radioactive.

    You will be given a sample of an isotope of Barium, Ba 137m. Remember, while you cannot predict which nuclei will decay in any particular amount of time (let's use one minute), you can predict that the number of nuclei that will decay during that time. It will be the total number of radioactive nuclei you have times the probability that any one of them will decay in that time.

    Activity A

    Place the radioactive material given to you by your teacher under the G-M tube as quickly as you can (but no so fast you spill any).

    Record the number of particles emitted every 30 seconds for ten minutes, and then calculate the rate (counts per minute) during each of these intervals.

      Activity 5A Worksheet  (PDF, 4K)  Get Acrobat Reader

    Graph the rate vs. time

    WHAT HAPPENS TO THE RATE AT WHICH PARTICLES ARE EMITTED AS TIME GOES ON?

    line

    THE RATE OF EMISSION DEPENDS ON THE NUMBER OF RADIOACTIVE NUCLEI AND THE PROBABILITY ONE WILL EMIT A PARTICLE IN ONE MINUTE. WHICH OF THESE FACTORS WILL CHANGE AS TIME GOES ON?

    line

    line

    CONSIDER THE RATE AT WHICH PARTICLES WERE EMITTED DURING THE FIRST HALF-MINUTE INTERVAL. USE THE GRAPH TO DETERMINE HOW LONG IT TOOK FOR THIS RATE TO DROP TO HALF THIS NUMBER. LET US CALL THIS TIME INTERVAL Tau.

    Tau = line 

    OF THE ORIGINAL NUMBER OF NUCLEI, WHAT DOES THIS TELL US IS THE FRACTION THAT WERE STILL RADIOACTIVE AFTER THE INTERVAL Tau? line 

    AND SO, WHAT FRACTION WAS NO LONGER RADIOACTIVE? line 

    PREDICT WHAT THE RATE WILL BE AFTER AN ADDITIONAL TIME INTERVAL Tau. line 

    LOOK AT THE GRAPH. WHAT WAS THE ACTUAL RATE? line 
    ARE THESE VALUES SIGNIICANTLY DIFFERENT? line 

    CONSIDER THE RATE AT ANY TIME. WHAT HAPPENS TO THAT RATE AFTER AN INTERVAL Tau FROM THAT TIME?

    line

    line

    Summary Activity A    Background Activity B

    The rate drops in half because the number of radioactive nuclei drops in half during the time interval Tau. For this reason Tau is referred to as the half-life of the substance. It is the time it takes for half of all the nuclei of a radioactive substance that you have at the moment to emit their particle and decay into different nuclei.

    There is another way to calculate the half-life of a material. It is the time it takes for the probability of decay to become 1 in 2 (50:50). This time does not depend on the number of nuclei decaying. It is a constant characteristic of the material. Let us use our dice (real or electronic) to see this. Since the probability of an even number coming up are 1 in 2, let us say that any die that comes up even represents a nucleus that has emitted a particle and decayed. Each roll of the dice would then represent the time it takes for the odds of decay to become 50:50.

    Activity B

    Start with 800 dice and begin rolling them. (If these are real dice splitting them up among the entire class might be a good idea).

    Count the number of even dice — these represent the number that decayed and so would also give you the number of particles emitted during this roll. Calculate the number of odd dice — these represent the number of nuclei that did not decay and are therefore still radioactive.

    Remove them — they represent nuclei that are no longer radioactive, and roll the remaining ones.

    Continue for 5 rolls and record your results on the worksheet.

      Activity 5B Worksheet  (PDF, 4K)  Get Acrobat Reader

    WHAT WOULD YOU HAVE PRDICTED THESE VALUES TO BE?

    line

    WERE THE ACTUAL VALUES SIGNIFICANTLY DIFFERENT THAN THE PREDICTED VALUES? REMEMBER, THE PERCENTAGE UNCERTAINTY FOR THESE NUMBERS IS FAIRLY HIGH.

    line

    The number of "radioactive" nuclei, and therefore the number of radioactive particles emitted during each roll, should have dropped by close to one half on each roll. How does the time represented by each roll (which is the time in which the probability for decay becomes 1 in 2) compare to the half-life Tau?

    line

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    Questions for Discussion

    1. If you have a collection of radioactive nuclei, will they be most radioactive at first, or after a few half-lives of time?

    2. You have the same number of nuclei of two different substances - one with a half-life of two weeks, Substance A. The other, Substance B, has a half-life of two years. The number of nuclei in both substances is such that after 10 half-lives, any radiation emitted is indistinguishable from background

      1. Which will be the most radioactive during the first month?

      2. Which will be the most radioactive after a year?

      3. If you are constantly exposed to both sources, which one will give you the greatest dose of radiation in one year?

      4. If you are constantly exposed to both sources, which one will give you the greatest dose of radiation in twenty years?

      5. If both substances emitted the same type of radiation at the same energy, which would you say is the most dangerous. Why?

    3. If you have a substance with an extremely long half-life, let us say several hundreds of years

      1. Would it remain radioactive for a long time or a short time?

      2. Under what conditions would a sample of such a substance present a high risk to those exposed to it?

    Summary

    The situation in this investigation is the simplest we can have. When these radioactive nuclei decay, the resulting nuclei are not radioactive. Often, this is not the case. These daughter nuclei may well be radioactive, emitting different particles and having a different half-life. There may be a cascade of decays before a stable arrangement is finally reached. As a result it is possible for a single radioactive nucleus to be the ultimate source of several particles — maybe up to ten or so.

     

    ACTIVITY 6: DON'T BE SQUARE

    Background

    If you happen to be walking along one day and an evil executive from the power company rolls a ball of nuclear waste at your feet (a possibility you may lose sleep worrying about), what do you suppose you should do?

    A. Pick it up to bring to the authorities.
    B. Run away
    C. Call your best friend over to have a look.

    That's right! B is correct.

    You already know that alpha particles do not go very far. A little bit of air will stop them, and almost any kind of barrier will stop beta particles. But what about gamma rays.

    Activity

    Set up your equipment as it was in Activity 3, shown below.

    Diagram of equipment as set up in Activity 3

    Teacher Note:
    You may have to use some trial and error until you find the correct calibration point for the distance your detector.

    The source should be the orange, gamma source. The distance d should be measured to the window in the G-M tube, not to the bottom of the protective sleeve.

    Find the counts per minute at each of the distances d shown in the Activity 6 worksheet. You may have to count for several minutes to get statistically useful data at the larger distances. If so, make sure all data points are normalized to the same amount of time.

      Activity 6 Worksheet  (PDF, 4K)  Get Acrobat Reader

    Calculate the square of each of the distances and record in the table above.

    FROM YOUR DATA, TRY TO DETEREMINE THE FRACTION OF THE COUNTS AT 2 CM YOU WERE GETTING AT 4 CM. REMEMBER THE UNCERTAINTY OF THESE VALUES.

    CONTINUE WITH THE OTHER DISANCES INDICATED

    AT 4 CM THERE WERE line THE COUNTS AS AT 2 CM

    AT 8 CM THERE WERE line THE COUNTS AS AT 2 CM

    AT 16 CM THERE WERE line THE COUNTS AS AT 2 CM
    DO YOU SEE THE RELATIONSHIP BETWEEN DISTANCE AND RATE OF EXPOSURE?

    TRY GRAPHING THE COUNTS DETECTED VS. THE SQUARE OF THE DISTANCE, AND THEN AGAINST THE INVERSE OF THE SQUARE OF THE DISTANCE.

    THE INTESITY OF RADIATION (NUMBER OF COUNTS) AT A PARTICULAR LOCATION DECREASES AS THE line OF THE DISTANCE FROM THE SOURCE INCREASES.

    Summary

    We call this the inverse square law. Anything that radiates from a source uniformly in all directions obeys this law. Doubling the distance (multiplying it by 2) drops the intensity by one-fourth (1 divided by 2 squared). Increasing the distance 10 times (multiplying it by 10) makes the intensity 100 times less (dividing it by 10 squared). It is really a matter of geometry, not science.

    diagram of the inverse square law

    All eight particles travel through the cone, and all eight pass through the ends of the cone at distance d and at 2d from the source at the apex of the cone. However the radius of the circle at the end of the cone at 2d is twice as big as that at d (similar triangles), and so its area is four times bigger since area goes as r squared. As a result, the number of particles passing through an area at 2d that is the same as the area of the circle at the end of the cone at d, is four times less — in this case only two. If the source is not limited to a point, the effect of distance may not follow this pattern exactly for small distances.

     

    ACTIVITY 7: SPREAD IT AROUND

    Background

    In nuclear explosions, within nuclear reactors and particle accelerators, radioactive particles are brought to very high energy levels. These high energy particles can interact with stable nuclei and change them into the unstable nuclei of radioactive isotopes. There is an underlying fear that all radiation, even the lower energy particles that are not being accelerated, can do the same thing. In other words the quality of being radioactive can be caught, like a contagious illness. Let us see if this can happen.

    Activity

    Take three coins and check them with a rate meter (or your computer set to METER)

    IS THE RATE GREATER THAN BACKGROUND? line 

    Put one on each of the sources and leave them there over night.
    Check them the next day with a rate meter.

    IS THE RATE GREATER THAN BACKGROUND NOW? line 

    FOR WHICH ONE(S) line 

    While exposure to radiation may not make something radioactive, contact is another matter.

    Use a rate meter to see if there is any radiation coming from a paper towel.

    IS THE RATE GREATER THAN BACKGROUND? line 

    Make sure you have gloves on or have your teacher do this!

    Let a drop from the iso-generator (from the half-life activity) fall on the paper towel. Quickly, check the rate coming from the wet spot on the towel.

    IS THE RATE GREATER THAN BACKGROUND? line 

    Summary

    While radioactivity cannot be spread like a virus or bacteria through contagion, it can be spread through contamination. That is, the radioactive material can mix with or adhere to other substances. It can then spread throughout the environment through the natural migration of those substances.

    Questions for Discussion

    1. What are some examples of ways this can happen?

    2. Which of these do you consider the most likely?

          Why?


    3. Which of these do you consider the most dangerous?

          Why?


     

    ACTIVITY 8: CONCLUSION

    Background

    You should now be in a position to evaluate the claims of the power-company. These were:

    Nuclear radiation is not necessarily harmful. The danger depends on

    • Type of radiation
    • Dose — the amount of radiation received

    Dose may be controlled through

    • Shielding
    • Distance from the source
    • Time

    You need to consider the validity of these claims as they apply to nuclear waster. In order to evaluate the risk of having a high-level waste disposal sight in your community, you will need to deal with a unit of measure called the rem. This is the unit used to express the amount of damage radiation can do to your body. It is a found by multiplying the amount of radiation you are exposed to by the biological effect that that particular radiation might have. For example, alpha particles are between 10 and 20 times more damaging than an equal number of gamma rays.

    Activity

    Do an Internet search on the risk of exposure to nuclear radiation, or read over the three articles included in the Appendix. One of these articles is obviously anti nuclear, one seems to be from the nuclear power industry, and another from the science department of a university — although they may all claim academic affiliation. You should be aware of these biases when you evaluate and use their content.

    Prepare a written statement of the position you would take at the town meeting to discuss the power-company's proposal. Make sure this statement includes your own evaluation of the risks involved. This evaluation must be supported with the evidence of your investigation. It may include information from your Internet search, but the validity of this information must be evaluated in terms of the observations and conclusions you made in your activities.

    Suggested Web Sites

    http://www.umich.edu/~radinfo/introduction/risk.htm
    http://whatisnuclear.com/articles/waste.html

    Suggested Search Words

    radiation + risk
    nuclear waste
    hormesis

     

    Appendix One: Electronic Dice

    TO POUT :S :E
    TELL 0 HT
    TELL :S
    SETTFONT "GENEVA
    SETTSTYLE [1 12]
    IF :S > :E [ODDEVEN STOP STOP]
    MAKE "A ( STRING `THE NUMBER OF ` :S `'s = ` THING WORD :S "S )
    TELL :S STAMPTEXT :A 200 29
    POUT :S + 1 :E
    END

    TO SETCOUNT :S :E
    MAKE WORD :S "S 0
    IF :S > :E [STOP]
    TELL :S PU HT SETX -100 SETY 100 - 25 * :S
    SETCOUNT :S + 1 :E
    END

    TO DICE :N
    HT
    CT
    SETWFONT "LOGO [GENEVA 14]
    SETCOUNT 1 6
    DICE.DO 1 :N
    POUT 1 6
    AGAIN
    DICE :NUMB
    END

    TO DIE
    MAKE "XX ( 1 + RANDOM 6 )
    ( PRINT1 :XX ` ` )
    END

    TO NUMBER
    PR `How many dice do you want to roll?`
    MAKE "NUMB READWORD
    IF NOT NUMBER? :NUMB [NUMBER STOP]
    END

    TO ODDEVEN
    MAKE "ODD :1S + :3S + :5S
    MAKE "EVEN :2S + :4S + :6S
    TELL [7 8]
    SETTFONT "GENEVA
    SETTSTYLE [1 12]
    TELL 7 PU
    SETXY -100 -75
    STAMPTEXT STRING `EVEN = ` :EVEN 150 30
    TELL 8 PU
    SETXY -100 -100
    STAMPTEXT STRING `ODD = ` :ODD 150 30
    END

    TO MANYDICE.DO :S :E
    IF :S > :E [STOP]
    MANYDIE
    CHECKIT 1 6
    MANYDICE.DO :S + 1 :E
    END

    TO MANYDICE :N
    HT
    CT
    SETWFONT "LOGO [GENEVA 14]
    SETCOUNT 1 6
    MANYDICE.DO 1 :N
    POUT 1 6
    AGAIN
    MANYDICE :NUMB
    END

    TO MANYDIE
    MAKE "XX ( 1 + RANDOM 6 )
    END

    TO SETUP
    CT DRAW HT
    SETWPOS "LOGO [5 100]
    SETWSIZE "LOGO [400 300]
    SETWPOS "TURTLE [400 100]
    SETWSIZE "TURTLE [300 300]
    END

    TO VISIBLE
    PR `Do you want to see the numbers on the dice? `
    PR `It goes faster if you do not.`
    PR []
    PR `Press Y or N and then return`
    MAKE "AAA READWORD
    IF :AAA = "Y [DDD DICE :NUMB STOP]
    IF :AAA = "N [DDD MANYDICE :NUMB STOP]
    PR []
    PR `Y or N only please!`
    WAIT 10
    VISIBLE
    END

    TO AGAIN
    PR []
    PR `Do you want to roll this number of dice again`
    PR `Press Y or N and then return`
    MAKE "BBB READWORD
    IF :BBB = "Y [STOP]
    IF :BBB = "N [DOIT STOP]
    PR `Y or N only please!`
    WAIT 10
    AGAIN
    END

    TO DICE.DO :S :E
    IF :S > :E [STOP]
    DIE
    CHECKIT 1 6
    IF ( REMAINDER :S 15 ) = 0 [PR []] [PRINT1 ` `]
    DICE.DO :S + 1 :E
    END

    TO DOIT
    SETUP
    NUMBER
    VISIBLE
    END

    TO CHECKIT :S :E
    IF :S > :E [PR "????? TOPLEVEL STOP]
    IF :XX = :S [MAKE WORD :S "S 1 + THING WORD :S "S STOP]
    CHECKIT :S + 1 :E
    END

    TO DDD
    CT
    TELL 0
    SETTFONT "GENEVA
    SETTSTYLE [5 12]
    PU HT
    SETXY -100 100
    STAMPTEXT ( STRING `ROLLING ` :NUMB ` DICE` ) 150 30
    END

    MAKE "A `THE NUMBER OF 6's = 58`
    MAKE "AAA "N
    MAKE "XX 6
    MAKE "BBB "N
    MAKE "1S 71
    MAKE "ODD 208
    MAKE "3S 65
    MAKE "2S 46
    MAKE "EVEN 168
    MAKE "5S 72
    MAKE "4S 64
    MAKE "7S 0
    MAKE "6S 58
    MAKE "NUMB 376

    This experiment is courtesy of 



    My Dog Kelly

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    Last updated: June 2013
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