Nuclear Decay Experiment 5 Lab Report

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Virtual General Chemistry Laboratory

 

Activity

Age

Sample

(counts/sec)

(yr)

Present-day materials

  
 

wood

 

0

paper

 

0

bone

 

0

Archaeological objects

  
 

bone, Bering land bridge

  

wood, tomb of pharoah Zoser

  

bone, La Brea tar pits

  

paper, Dead Sea scrolls

  

skull, Laguna Beach, CA

  

wooden timber, Stonehenge

  

bone, Pedra Furada, Brazil

  

wooden beam, India

  

word image 802Nuclear Chemistry

Experiment 5 – Radiocarbon Dating

Name ___________________

Nuclear Decay

The action of the particles in the nucleus of atoms are not very well understood, but we do have a picture of what these particles can do.

We do know that a nuclear particles can convert to other particles. Unstable nuclei (radioisotopes), which means that the nucleus will change to become more stable, decay by emitting radiation to get rid of excess energy. The radiation can be emitted as positively charge alpha particles (α), negatively charged beta particles (β), or gamma rays (γ) Neutrons can also be released and x-rays are sometimes a byproduct of a decay.. These are all high energy ionizing radiation, meaning they have enough energy to remove an electron from an atom. We know this as electron binding energy or ionization energy.

Individually, we know an α particle is the same as a helium nucleus with two protons and two neutrons. A β particle is an electron, and γ rays are not particles at all but are electromagnetic radiation. A neutron is the nuclear particle we are familiar with.

Other particles involved in nuclear decay are positrons (an antielectron) and neutrinos (and antineutrinos). We give these names to particles and events that we do not completely understand, but research is being done to better know what is going on in terms of quantum physics.

We use the decay of radioisotopes to perform radiocarbon dating. By using the β-emitter, C-14, we can estimate the age of an artifact. In nature the C-14 reacts with oxygen in the atmosphere to form carbon dioxide, which eventually makes its way into the biosphere where it is taken up by plants during photosynthesis. Animals eat the plants—and we eat both animals and plants—ultimately exhaling the C-14

again as CO2. In this way a steady concentration of C-14 is maintained in living tissue and in the atmosphere. (Although C-14 emits ionizing radiation, and can therefore damage cells, the natural abundance of C-14 is very small: there exists only one atom of C-14 in every 1012 atoms of carbon.)

Once an organism dies the C-14 is no longer replenished, and the amount of C-14 steadily decreases as it undergoes β-decay. Thus, when a sample of that scroll, the mummy, or the bone is measured for its C-14 content, there is less of it than in a living organism. If we assume that the person who was mummified had the same amount of C-14 in her tissues when she was alive as we do today, and we know the rate at which C-14 decays, we can determine her age.

The half-life of a radioisotope is the time it takes for one-half of a sample to decay. Carbon-14 has a halflife of 5730 years, which means that after that period of time, only half of the C-14 that was originally present in an organism would remain. After another 5730 years, only half of that quantity would be present, or one-quarter would remain, and so on. If we were to start with 12.0 g of C-14, the amount remaining after several half-lives is shown below.

Time Elapsed

0 yr

5,730 yr

11.460 yr

17,190 yr

Number of half-lives

0

1

2

3

Amount of C-14

12.0 g

6.0 g

3.0 g

1.5 g

Fraction remaining

1

½

¼

 

 

Similarly, the radioactivity of a sample (as measured by a Geiger counter) will decrease by the same fraction. Note that if a sample is either too young or too old, this method will not work. If a piece of bone is only a couple years old, the process cannot be used because there will have been too little decay of C-14 for us to be able to measure a decrease. Likewise, if it is more than about 40,000 years old, there will be too little C-14 left to measure accurately.

What we do know is about the energy and the effects of these decay processes and in this investigation we will look at the individual decay particles. This investigation is adapted from an activity written by David R. Anderson at the University of Colorado at Colorado Springs.

Prelab Questions:

  1. Write the balanced equation for the beta decay of !”C.

 

 

  1. Write the balanced equation for the alpha decay of #$%U.

 

 

  1. Another type of decay is electron capture. Write the balanced equation for electron capture in #&’Bi.

 

 

  1. Another type of decay is neutron capture. Write the balanced equation for neutron capture in !”N.

 

 

 

Procedure (experiment 1):

  1. Go to: https://uccs.edu/vgcl/nuclear-chemistry/experiment-1-radiation-and-matter. You will need to have Flash enabled.
  2. Click on the apparatus to start the lab. (If you are using Internet Explorer, you will have to click once to activate the control, then click again to start the lab.) The experimental setup includes three radioactive sources (one each of an α, β, and γ emitter), different types of shielding, and the apparatus to measure the radioactivity. On the right is a Geiger counter that will measure radiation in counts per second. On the left is a holder for the radioactive source, and in the middle is a holder for the shielding material.
  3. Click on the drop-down list of radioactive sources and choose radon-222, an α emitter. Drag the source into the sample holder. (There is information on each nuclide in the box that pops up. After reading it, you can close the box, or it will close on its own when you click on something else.)
  4. Click on the Geiger counter switch to turn it on. Read the activity from the gauge. (The needle on the gauge may move around a bit. You should try to get an average reading. Note that the scale on the gauge is not linear.) Record the activity of the sample in counts/sec in the data table below.
  5. Click the switch again to turn it off.
  6. Click on the drop-down list of shielding materials and choose one. Drag the shielding into its holder in the apparatus.
  7. Again click on the Geiger counter switch, and again record the activity in the appropriate column in the data table.
  8. Repeat steps 6 and 7 using the other types of shielding material. Record the activity values in the appropriate cells of the data table.
  9. Repeat steps 3–8 for the other radioactive sources: iron-59 (β) and strontium -85 (γ).

Data Table:

 

Source

Type of Radiation

  

Activity (counts/sec)

 

No shielding

Paper

1 mm Cardboard

1 mm

Aluminum

1 mm Lead

Radon-222

α

 

 

 

 

 

Iron-59

β

 

 

 

 

 

Strontium-85

γ

 

 

 

 

 

 

Analysis:

  1. What was the effect of the type of shielding on alpha particles? What kinds of materials would provide sufficient to protect a person working with alpha emitting sources?

 

 

  1. What was the effect of the type of shielding on beta particles? What kinds of materials would provide sufficient to protect a person working with beta emitting sources?

 

 

  1. What was the effect of the type of shielding on gamma particles? What kinds of materials would provide sufficient to protect a person working with gamma radiation?

 

 

  1. From your data, which particle has the greatest penetrating power? Justify your answer considering the physical decay particles.

 

 

 

  1. Iodine-131, used for the imaging of the thyroid and for the treatment of goiter, hyperthyroidism, and thyroid cancer, emits both beta particles and gamma rays. What type of shielding should be used when working with I-131? Why?

 

 

 

 

 

Procedure (experiment 2):

    1. Go to: https://uccs.edu/vgcl/nuclear-chemistry/experiment-2-types-of-radiation. You will need to have Flash enabled.
    2. Click on the apparatus to start the lab. (If you are using Internet Explorer, you will have to click once to activate the control, then click again to start the lab.) The experimental setup includes a number of radioactive sources, different types of shielding, and the apparatus to measure the radioactivity. On the right is a Geiger counter that will measure radiation in counts per second. On the left is a holder for the radioactive source, and in the middle is a holder for the shielding material.
    3. Click on the drop-down list of radioactive sources and choose a nuclide. Drag the source into the sample holder. Record the name (e.g., iron-59) or the symbol (e.g., Fe) of the source in the data table. (There is information on each nuclide in the box that pops up. After reading it, you can close the box, or it will close on its own when you click on something else.)
    4. Click on the Geiger counter switch to turn it on. Record the activity of the sample in counts/sec. (The needle on the gauge may move around a bit. You should try to get an average reading. Note that the scale on the gauge is not linear.)
    5. Click the switch again to turn it off.
    6. Click on the drop-down list of shielding materials and choose one. Drag the shielding into its holder in the apparatus.
    7. Again click on the Geiger counter switch, and again record the activity in the appropriate column in the data table.
    8. Repeat steps 6 and 7 for the other types of shielding material.
    9. Repeat steps 3-8 until you have identified the type of radiation given off by several nuclides. Try to find at least one for each type of radiation: α, β, and γ.

Data Table:

Source

No shielding

Paper

1 mm

Cardboard

1 mm

Aluminum

1 mm Lead

Type of Radiation

Justification

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis:

Table 1 – Approximate Penetration Depths of Radiation in Various Materials

 

α

β

γ

Body Tissue

0.05 mm

5 mm

>50 cm

Aluminum

0 mm

2 mm

30 cm

Lead

0 mm

0.4 mm

30 mm

 

1) From your data and the information in Table 1, determine the type of radiation emitted by each of the radioactive sources and enter it in the table and give a justification.

 

Procedure (experiment 4):

    1. Go to: https://uccs.edu/vgcl/nuclear-chemistry/experiment-4-radiation-and-distance. You will need to have Flash enabled.
    2. Click on the apparatus to start the lab. (If you are using Internet Explorer, you will have to click once to activate the control, then click again to start the lab.) The experimental setup includes the radioactive source (gallium-67, a γ emitter), a ruler to measure the distance from the source to the detector, and a Geiger counter to measure the radiation in counts per second.
    3. Click on the drop-down list to retrieve the source. Drag the source into the holder.
    4. Click on the 1-cm mark on the ruler to position the source holder at 1 cm distance from the detector.
    5. Click on the Geiger counter switch to turn it on. Record the activity of the sample in the appropriate cell in the data table. (The needle on the gauge may move around a bit. You should try to get an average reading. Note that the scale on the gauge is not linear.) 6) Click the switch again to turn it off.
    6. Click at a point on the ruler to move the source a distance away from the detector.
    7. Again click on the Geiger counter switch, and record the activity in the appropriate cell in the data table.
    8. Repeat steps 6 and 7 for the other distances in the data table.

 

Data Table:

Distance (cm)

Activity (counts/sec)

1

 

2

 

3

 

4

 

5

 

6

 

7

 

8

 

9

 

10

 

 

 

 

 

 

 

 

 

 

 

 

 

1000

Activity (counts/sec)

 

 

0

0 2 4 6 8 10

Distance (cm) Analysis:

  1. Draw a graph of your data.
  2. What is the relationship between radiation intensity and distance? Derive a mathematical expression for the relationship.

 

 

 

 

 

  1. When a sample of phosphorus-32 (a β-emitter used to treat leukemia and pancreatic cancer) was placed 1 cm from the detector, the intensity of the radiation was measured to be 491 counts/sec. What would the activity be 2 cm from the detector? 5 cm? The effect of distance on all types of decay particles are similar.

 

 

 

 

 

Procedure (experiment 5):

    1. Go to: https://uccs.edu/vgcl/nuclear-chemistry/experiment-5-radiocarbon-dating. You will need to have Flash enabled.
    2. Click on the apparatus to start the lab. (If you are using Internet Explorer, you will have to click once to activate the control, then click again to start the lab.) The experimental setup includes a number of samples of C-14 isolated from objects discovered in archaeological excavations, and the apparatus to measure the radioactivity. On the right is a Geiger counter that will measure radiation in counts per second. On the left is a holder for the sample.
    3. Click on the drop-down list of samples and choose one. Drag the sample into the sample holder. (There is information on each sample in the box that pops up. After reading it, you can close the box, or it will close on its own when you click on something else.)
    4. Click on the Geiger counter switch to turn it on. Record the activity of the sample in counts/sec. (The needle on the gauge may move around a bit. You should try to get an average reading. Note that the scale on the gauge is not linear.) 5) Click the switch again to turn it off.
    5. Repeat steps 3-5 for the other samples.
    6. For one of the present-day materials, type its activity in the “Initial Activity” cell and hit Enter. This will generate a decay curve for C-14 based on that starting activity.
    7. For each of the archaeological objects made of that material, estimate its age using its activity and the C-14 decay curve.
    8. Repeat steps 7-8 for the other materials.
    9. Submit the completed excel file along with this lab report.

Data Table: Record your data in the Excel file nucl_expt_5_1.

 

Analysis:

  1. In what year, approximately, did the person die whose skull was discovered in Laguna Beach, CA?

In what year was the tree cut down that was used for the wooden timber in Stonehenge? (Include A.D. or B.C.)

 

 

 

  1. Given the inaccuracies in reading the Geiger counter scale and in estimating the age of an object from the graph, give a rough estimate of the error (+/- years) in the age you determine in question 1.

 

 

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