02. Nuclear Chemistry: Properties of Alpha, Beta, and Gamma Radiation

Goals

- Compare the properties of alpha (α) particles, beta (β) particles, and gamma (γ) rays.
- Examine the relationship between distance and intensity as it applies to γ radiation.

Background

Most of the reactions studied in Chemistry 105 are chemical rather than nuclear in nature. This means that they involve motion of electrons surrounding stable nuclei. Because the nuclei do not change, and because the identity of an element is determined by the atomic number, Z, we can infer that in a typical chemical reaction the identities of the elements do not change.

Any process in which the nucleus itself undergoes change and the identity of the element is altered is called a nuclear reaction. Nuclear reactions have important applications in chemistry and can be sources of enormous amounts of energy. The energy of the sun is generated by a nuclear reaction, the fusion of hydrogen nuclei into helium nuclei. Although nuclear reactions can be used in destructive ways, they have positive applications in the generation of electric power, in medical diagnosis, and in a variety of industrial applications. Despite the beneficial applications, the use of nuclear reactions continues to be controversial because all nuclear reactions produce by-products, including radiation and nuclear waste (unstable nuclei). In this lab, some of the fundamental properties of nuclear radiation will be studied to help you acquire a better understanding of the safety issues surrounding nuclear reactivity.

All elements with atomic numbers greater than 83 are unstable. In addition, other nuclei can have an unstable mass:charge ratio. All unstable nuclei are said to be radioactive, meaning that they spontaneously disintegrate, or decay, into other nuclei. In this process, they typically emit electrons (beta particles, β), positrons (positive “electrons”, β⁺), or helium nuclei (alpha particles, α). Nuclear decay is sometimes accompanied by the emission of a photon of energy (electromagnetic radiation known as a gamma rays, γ). γ-ray emission does not affect the nuclear composition (number of protons and neutrons), but, rather, affects the energy of the nucleus. Alpha and beta particles and gamma rays are all ionizing radiation. As they pass through matter, they expend their energy by interacting with electrons on the molecules they encounter. In the process, electrons are ejected and reactive free radicals are formed. These free radicals can initiate chain reactions that disrupt previously stable systems.

The properties of the three types of nuclear radiation that will be investigated are described below:

1. Alpha particles (α) are helium nuclei, 4

2He or 4\2α, emitted by nuclei of high atomic number. For a given isotope they are monoenergetic (they all have the same energy). An example is the radioactive decay of radium-226:

226\88Ra → 222\86Rn + 4\2He (1)

The product nucleus has an atomic number that is two less, and a mass number that is four less, than that of the original nucleus. Alpha particles are ejected with high energy. They efficiently ionize atoms in their path. They do not travel far but produce intense ionization within a short path. They travel at 5% to 7% of the speed of light.

1. Beta particles (β) are high speed electrons. β-emission is equivalent to the conversion of a neutron to a proton:

1\0n → 1\1p + 0 \−1e (2)

An example of β-emission is the radioactive decay of carbon-14:

14\6C → 14\7N + 0 −1e (3)

The product nucleus has an atomic number that is one more than that of the original nucleus. The mass number remains the same. With their high speed, up to 90% of the speed of light, single negative charge, and extremely small size and mass, the high energy β particles pass through matter much more easily than do α particles. The β particles are not monoenergetic and a spectrum of energies is obtained, with a characteristic maximum energy that corresponds to the actual energy transition in the nucleus.

3. Gamma rays (γ) are a form of high energy electromagnetic radiation and travel at the speed of light. Because of their high energies and their ability to penetrate deeply into matter, they can do considerable biological damage. They have neither mass nor charge. γ-emission often accompanies α- or β-emissions, since these forms of radioactive decay frequently leave the product nucleus in an excited state. This unstable state can go to a lower energy state with the emission of electromagnetic radiation, which, for the nucleus, is in the γ-ray region of the spectrum.

Radiation (α and β particles and γ-rays) is detected using a radiation monitor. The most familiar type of radiation monitor is a Geiger-Müller counter. High-energy radiation, such as α, β, or γ-radiation, enters through the window of the monitor and ionizes the argon gas enclosed within. The electrons and ions are attracted to positive and negative electrodes, respectively. This generates a small current flow between the electrodes. The current is amplified and used to activate a counter, a flashing light, and/or a clicking sound. The output of the counter can be directed to a computer for automated recording.

Note: The Geiger counter measures "counts." These counts include both radiation from our sample of interest and some unavoidable background signal from other sources. We only care about the signal that comes from our sample, which we will call "activity":

activity = counts – background

Activity is a measure of radiation intensity, so activity and intensity can be used interchangeably.

A Quick Note About Standard Deviation:

This lab will also introduce you to how we use the uncertainty of any measurement to express the precision of our answers correctly (i.e.-use the correct number of significant figures in our answer). For example, to obtain the height of a Wellesley College student we could have every student in lab take measurements with a meter stick and then average all the measurements together to obtain the most likely height for this student. It is highly unlikely that all the measurements would be the same. So, to report our answer with the correct significant figures, we would need to look at the "spread" of the measurements. The larger the spread in the measurements, the less precise we will be able to report our final height for the student. This "spread" creates a standard deviation for our average value. The standard deviation depends on the accuracy of our instrument (i.e.-number of tick marks on the meter stick) and how well we use it to measure the student's height. Note: the one constant in this experiment would be the student's height, yet this is not always the case when taking measurements of other phenomena in lab. For example, in our lab today, not only will there be a "spread" due to our instrument (the Geiger-Muller Counter), but our radioactive sources will be varying the number of radioactive particles they produce from one minute to another. Their decay is not constant like a student's height and therefore, will create a contribution to the standard deviation in addition to the contribution from our instrument's ability to measure radioactive particles. Figure 1 below shows the random nature of the radioactive decay for a 137Cs sample. (A measurement of the number of radioactive particles produced by the Cs-137 sample was measured every minute over a 10-hour period, i.e.-one measurement was 44,620 decay particles/min). The plot shows how many times a distinct value of "radioactive particles produced per minute" (i.e.-9 times in 10 hours) versus the various measured values of 'radioactive particles measured per one minute' (i.e.-44,620 particles/minute). From this experiment, we get a “Bell-shaped” curve with an average in the middle, about 45,000. This type of distribution is very common and is referred to as a Gaussian or Normal Distribution.

In conclusion, the random nature of radioactive decay and the uncertainty in our instrument will create the "spread" (standard deviation) for any average obtained on the number of particles produced by our radioactive sources.

Just for fun!

Do you have nuclear phobia?

Take this brief quiz to see if you perceptions about nuclear energy are valid.

Frontline: Nuclear Reaction

(This website also contains excellent information about nuclear energy, such as finding the closest nuclear reactor to your home).

Figure 1: Normal distribution of CPM for Cs-137 sample (See cool mp4 of data collection!)

Synopsis of the Experiment

This 2-part experiment will explore various properties of α, β, or γ-radiation.

In Part I you will determine how well different types of radiation are blocked by physical barriers. You will measure the % transmission through paper and aluminum for each type of radiation, and compare their relative penetrating abilities using class data.

In Part II, you will determine whether or not physical separation (distance from a radiation source) is an effective way of protecting yourself from radiation. You will determine the relationship between intensity and distance for a γ-ray source. You will place a Geiger-Müller counter at different distances from a γ-emitter and measure counts at each distance. It is postulated that the intensity (I) of many physical properties, such as light or radioactivity, decreases as the distance (d) of the source (of light or radioactivity) from the detector increases. There are many possible relationships between (I) and (d), but we will narrow it down to one of the following three mathematical relationships:

All three equations have the general form of an equation of a line (y = mx + b), with x = 1/d², d, or 1/d. Your task will be to plot your experimental data in three ways: I vs. 1/d², I vs. d, and I vs. 1/d. The correct relationship should give an R² value (regression value) close to 1.0. The information that you obtain from the plots will allow you to determine the mathematical relationship between distance and intensity.

Experiment

To print instructions, select the portion that you wish to print, right click and choose "print" to prevent printing the entire web page.

Safety

Sealed sources will be used in this experiment and present virtually no hazard on contact.

However, as a precaution, they should be handled as little as possible and kept away from the body, especially the eyes, which are particularly sensitive to radiation. If you are pregnant or think you may be pregnant, please tell your instructor.

Instructions

Students will work in groups of 2 unless otherwise specified by your instructor.

There will be 2 stations set up, with different tasks to accomplish at each. All students must go through both stations. If there is no station available, start working on Workshop 2.

For each station, you will be using sealed sources. These sources must be used with the label facing away from the detector (Why?). At each station, write down in your notebook which isotope your source contains (found on the label).

Station I. Penetrating ability of α, β, and γ-radiation

The penetrating ability of α, β, and γ-radiation will be examined by quantitatively determining how much radiation is blocked by paper and aluminum for each type of source. For each source, you will record one background reading (no sample) and readings with different materials between the sample and the detector (air, paper, and aluminum).

Record all of your data in your lab notebook as well as in the class spreadsheet, which will be posted on the lab conference. Results calculations will be performed on compiled class data.

Each source will have its own set up. To ensure that the class data is reliable, please do not move sources to different set ups.

1. Connect the Vernier Geiger counter to the Vernier LabPro device using the DIG/Sonic 1 port.

2. Connect the Vernier LabPro device to the computer via the USB port.

3. Place the Geiger counter on the wooden block and confirm that it is turned on.

4. Open the Logger Pro program icon on the computer.

5. Ensure that the program recognizes that the Geiger Counter is attached.
If not, Choose Experiment from the menu bar at top of the screen.
Click Set Up Sensors and choose Show All Interfaces
Click on the white square under DIG/Sonic 1 and then Choose Sensor
Choose Radiation
Close out of this window by clicking on the x in the upper right-hand corner.

6. Choose Experiment from the menu bar at top of the screen.

7. Choose Data Collection

Mode: Time-based

Duration: 1 minute

Sampling Rate: 1 minute per sample

8. Click Done

9. Be sure all radioactive sources are at least 3 feet away and click on the green arrow to begin the one-minute background reading.

10. After a minute, the counts per minute will appear in the box in the lower left hand corner of the screen.

11. Record the counts. This is your background counts.

12. Place the radioactive sample in the white rubber feet directly in front of the Geiger counter with the label facing away from the counter, as shown in Figure 1, exactly 1 cm away from the detector.

Figure 1: Set up for measuring radiation using Vernier Geiger Counter

13. Click on green arrow (shown below) to begin the one-minute background reading with air as the barrier.

14. Choose “Erase and continue”.

15. Once the new reading appears in the box, record the counts. This is your counts with air.

16. Now, place a piece of paper in the same set of feet with the source, such that it is facing the Geiger Counter as shown in Figure 2. Again, make sure that the label is facing away from the Geiger counter. Also, try to make sure that the feet are the same distance away from the Geiger counter as before.

Figure 2: Radioactive source with paper in-between source and detector

17. Click on green arrow to begin the one-minute background reading with paper as the barrier.

18. Again, choose “Erase and continue”.

19. Record the counts. This is your counts with paper.

20. Repeat steps 16-19, but replace the piece of paper with the piece of aluminum. These are your counts with aluminum.

21. Repeat this experiment so that you have measured counts for air, paper, and aluminum for alpha, beta and gamma sources.

Questions for thought:

1. Which of your samples had the highest counts in air? Why? (Hint: There are two possible reasons.)
2. Based on your Station 1 data, what is one way to protect yourself from radiation?

Figure 3: Set up for Station II Data Collection

Station II. Intensity vs. distance relationship

The activity of a γ-emitting sample will be measured at different distances from the radiation detector. The data will be interpreted to determine the relationship between γ-radiation intensity and distance.

The sample and detector will be set up next to a ruler so that their separation distance can be readily measured.

Setting up the Geiger counter and source

1. Verify that the Geiger counter is turned on and properly connected to the Vernier LabPro device as described for Station 1.
2. Confirm that the LabPro radiation device is connected to the computer as described for Station 1.
3. Take note of the isotope being used as a γ source at your lab bench.
4. Place the Geiger counter on the block of wood.
5. Place the γ source in the white rubber feet.
6. Set the ruler down on the benchtop such that the "0 cm" mark is lined up with the face of the active counting area on the Geiger counter.
7. Place the feet and source such that the face of the source is 4 cm (0.04m) from the active area of the counter and the source label is facing away from the Geiger counter.
8. Make sure the γ source is centered in front of the active area of the counter. See Figure 3.

**Please follow these instructions IF the "Distance vs. Activity Temp.xmbl" file in LoggerPro is NOT OPEN on your computer.**

- Open the Logger Pro program from the icon on the desktop of the lab computer.
- Download the file "Intensity vs Distance.cmbl" which is attached to the bottom of this page to the Desktop of the computer.
- From File in LoggerPro, select Open and open the "Intensity vs Distance.cmbl" file from the computer Desktop.
- The following "Sensor Confirmation" window should appear (see Figure 4 below).
- Click the Connect button (as seen in Fig. 4). The green button should be lit up and you are all ready to go!

Figure 4: Sensor Confirmation Window

9. You do not need to measure a background reading. The average background radiation has been entered into the computer already. (7 counts/60 sec.) The computer will use this value to automatically convert the counts into the activity (intensity) for you. (activity = counts – background).

10. You are now ready to begin collecting your Activity vs. Distance data. The computer/detector will measure the counts at 1-minute intervals.

11. To begin your first 1-minute interval, click the Collect (green arrow) shown below button, at the top of the screen. If the program is working properly, the button will turn into a Stop (red icon below) button.

12. After 1 minute, the 'wheel' symbol to the right of the Play/Stop button will turn blue (as seen above).
When this happens, click on the blue button and enter your first distance of 4 cm (0.04m). Then, click OK.
Do not click on Continue yet!

13. Move your source 1 cm further away from the detector using your ruler (i.e.-5 cm).
NOTE: The key to great data is not making sure you are at exactly "4 cm" or "5 cm", but that all your measurements are exactly 1 cm apart from one another.

14. Now, click Continue to start the next 1-minute interval.

15. When the computer finishes its reading, click on the blue wheel button. Enter the distance between sample and source and click OK. Again, wait to move the source away another 1 cm before clicking Continue.

16. Repeat the previous step until you have entered your final distance of 0.12 m or until the readings that you obtain are as low as the background reading (7 counts/60 sec), whichever comes first.

17. Select (highlight) all the data in the data table and copy it. Then open Excel and Paste the copied data table into the Excel spreadsheet. Save the Excel spreadsheet on the Desktop and send copies to yourself and your lab partner to analyze at your leisure.

18. Please clear your data from the program, but do not close it. Clear your data by selecting "Clear Latest Run" under "Experiment" in the menu bar.

Question for thought:

1. Based on your Station 2 data, what is one way to protect yourself from radiation?

References

¹ Brown, T.L., LeMay, E.H., Bursten, B.E., Murphy, C.J., and Woodward, P.M.; Chemistry; Prentice Hall: Boston et al., 12^th ed., 2012. Use this reference for any future reading assignments that refer to Brown et al.

Chase, G.D., Rituper, S., Sulcoski, J. W., Experiments in Nuclear Science, 2^nd Ed., 1971.

Morss, L.R., Boikess, R.S., Chemical Principles in the Laboratory, 2^nd Ed., 1981.

Bauer, T., Wellesley College, Department of Physics, handout on “Radioactive Decay.”

Intensity vs Distance.cmbl

Please try to download the above LoggerPro file (Intensity vs Distance.cmbl) to the Desktop of the computer you are using in lab. This is the correct template for Station 2. :)

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