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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, or , 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:

      (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.

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

                   (2)

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

                    (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 a 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.

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.

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:

                     (4)

                      (5)

                    (6)

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 a linear plot.  The information that you obtain from the plots will allow you to determine the mathematical relationship between distance and intensity.

Preparation

Every week, two items are due: Prelab for the current week and Results for the previous week. The Prelab and Results are in the form of worksheets, which are linked to in the Prelab & Results section, which is after the Experiment section of the lab manual.

In order to prepare for the new lab each week, you should read the Background information section of the lab manual, any reading assignment found in the Preparation section, and the Experiment section. You should print the experiment section if you want a hard copy in lab. After the Experiment section, you should find your Prelab worksheet, which you should print, fill out, and turn in. You may also want to print the Results worksheet, so that you can work on it in lab if you have time.

Reading Assignment:

• Description of this experiment—read Background and Experimental sections
• Nuclear Chemistry—Zumdahl¹, Sec. 21.1, pp. 978-984.  [Review Sec. 2.6, pp. 26-28.]
• Tutorials—Do the tutorials listed below.  Nothing needs to be turned in for the tutorials. However, you will need to understand the material in order to do your Results calculations.
  • Excel: http://www.wellesley.edu/Chemistry/Flick/excel1.html
  • Graphing in Excel 2007: http://www.wellesley.edu/Chemistry/Flick/graphinginExcel2007.pdf
  • Regression and Curve Fitting: http://www.wellesley.edu/Chemistry/stats/lesson6.html
  • Mean and Standard Deviation:  http://www.wellesley.edu/Chemistry/stats/lesson3.html
• Read the paragraphs below on significant figures

Significant figures

Significant figures is the number of digits reported, excluding zeroes to the left of the first nonzero number. When you report a measurement with correct significant figures, there will be some uncertainty in the last digit but the other digits are certain. There are two rules when determining significant figures in a calculated answer:

1. Multiplying or dividing: The number of significant figures in your answer is the same as that of the least precise number being multiplied or divided. For example, 23.4 x 1.0 = 23
2. Adding or subtracting: The answer should be rounded to the same decimal place as the least precise number being added or subtracted. For example, 2.467 - 1.0 = 1.5

Scientific notation should be used for reporting answers whose significant figures are unclear (e.g. numbers rounded to the tens or hundreds place).

When uncertainty is known, do not use the above rules for determining significant figures. Instead use the following rules:

1. Round your uncertainty (e.g. your standard deviation) to one significant figure.
2. Round your value to the same digit (i.e. 75.43 ± 0.02 are both rounded to the hundredths place).

Questions:

Fill out the prelab worksheet that can be found at the end of the Experiment section.

Print:

Print and bring to lab:

• Significant Figures Practice
• Workshop 02 Statistical Uncertainty

Look over, and print if you think it will be helpful:

• Summary of Significant Figures and Uncertainty (just read 1st paragraph, "Significant Figures")
• Excel tips for chem lab

Please print double-sided.

Experiment

Safety

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. You will measure 2 samples.  For each sample, you will record one background reading (no sample), and three readings with different materials between the sample and the detector (air, paper, and aluminum).

At Station I, a radiation counter fixed on top of a sample box will be used to measure radioactivity (see Figure 1) of an alpha, beta, or gamma radiation source.

Figure 1: Station 1 Apparatus

The Geiger counter output will be read on a handset (see Figure 2). The counter has two displays.  The upper display changes constantly, displaying the total number of radioactive events (“counts”) measured since the device was last turned on.  At the end of a minute, the total number of counts measured during the that minute is displayed in the lower window. The upper display will continue to increase as more counts are measured, and the lower display will remain the same until another minute has passed. 

After you make a change to your setup (e.g. add paper between your source and detector), wait for the lower number to change. Disregard this number because it includes measurements made before your final setup. Wait one more minute and record the new number that appears in the lower display. Do not try to get around this problem by turning the counter off between measurements, because the first one-minute reading after turning the counter on is typically erroneous.  

Figure 2: Handset for Geiger-Muller counter

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 sample will have its own counter and sample box. To ensure that the class data is reliable, please do not move samples to different counters/boxes.

1. Confirm that the radiation detector is in the hole on top of the sample box. 
2. On your handset, set Power = On, Sound = Off, and Time = Min.
3. Take a 1-minute background reading (remembering to discard the first reading).  Be sure there are no radioactive samples within one meter of the detector.
4. Place the α, β, or γ source sticker-down into the round indentation on the removable shelf, and place it in the top level of the sample box. It is important that the shelf be on the top level so that all measurement from the class will be comparable. Take a 1-minute reading. 
5. Without moving the source, place a piece of paper between the sample and the detector.  Take a 1-minute reading.
6. Remove the paper and replace it with the aluminum barrier provided. Be sure not to knock the sample out of place as you add the barrier. Ask your instructor for help if you are having trouble with this. Take a 1-minute reading.
7. Repeat steps d-f for another emitter if instructed to do so.
8. Before you leave lab, enter your data in the class spreadsheet. If there is no spreadsheet available, click here to download the Excel data table and save it on the desktop with your lab's day of the week in the title.

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 mounted on an alignment rail so that their separation distance can be readily adjusted. Refer to figures 3 and 4.

Figure 3: Station II View 1

Figure 4: Station II View 2

1. Verify that the Geiger counter is mounted on the laser alignment rail and turned on. 
2. Take note of the isotope being used as a γ source at your lab bench.
3. Mount the γ source in the sample holder on the laser alignment rail.  Make sure the source label is facing away from the Geiger counter.  Make sure the Geiger counter active area is centered in front of the γ source.  Slide the post holding the source until source itself is 4 cm (0.04m) from the active area of the detector.  Use a ruler to confirm this. (Do not measure from base to base.)
4. If you are the first person at the computer, check to make sure that the file Distance vs. Activity Temp.xmbl is on the desktop. If the template is not on the desktop, right-click (ctrl-click on Mac) on this link: Distance vs Activity Template.xmbl, select Save Link As..., and save it to your desktop.  To run the program, start Logger Pro. From within Logger Pro, open the file Distance vs. Activity Temp.xmbl from the Desktop. If this file is already open, clear the last set of data by selecting Clear Last Run from the Experimental pull down menu.  Do not select File/New; this will get rid of all the settings, and you will need to re-open the template file from within Logger Pro.
5. Don't worry about the background. The average background radiation has been entered into the computer already.  (7 counts/30 sec.)  This value will be used to automatically convert the counts into the activity (intensity).  (activity = counts – background)
6. You are now ready to begin collecting your Activity vs. Distance data.  The computer/detector will measure the counts at 30-second intervals.  Click Collect to begin your first 30-second interval. Notice that the Keep button (upper right) has gray lettering. When the computer is finished reading data for 30 seconds, the letters on the Keep button will turn from gray to black.  When this happens, click on the Keep button.  Enter your first distance of 4 cm (0.04m) and click OK. Do not click on Continue yet!
7. Move your source 4 cm further away from the detector. Rather than using your ruler, use the measurement markings on the alignment rail to move the base 4 cm. Click Continue to start the next 30 second interval. When the computer finishes its reading, click on the Keep button. Enter the distance between sample and source and click OK. Do not click on Continue yet!
8. Repeat step g until you have entered your final distance of 0.24m or until the readings that you obtain are as low as the background reading (7 counts/30 sec), whichever comes first.  Then, click on Stop Collection.
9. Copy your data to your lab notebook either by manually copying the data or including a computer printout/spreadsheet.
10. If you want an electronic copy of your data, select (highlight) all the data in the data table and copy it to the clipboard. 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.

Workshop 2: Significant Figures, Statistical Uncertainty, and Plotting in Excel

Links to reference materials:

• Summary of Significant Figures and Uncertainty
• Excel tips for chem lab

The workshop has three parts:

1. Significant figures: Read through the first table ("Significant Figures") of Summary of Significant Figures and Uncertainty. Then do the worksheet Significant Figures Practice, which you should have printed and brought with you. Check your answers here. 
2. Statistical uncertainty: Work through Workshop 02 Statistical Uncertainty, which you should have printed and brought with you.
3. Plotting in Excel: Click here to download the Workshop 02 Excel template that you will use to 1) practice using Excel for plotting, calculating, and displaying significant figures; and 2) learn how to determine significant figures both for calculated values and for replicated measurements (you will need both methods for your Results calculations).  There are three worksheets. Tabs in the lower left of the screen can be used to select a worksheet. You do not need to turn anything in. However, be sure to check your answers so that you can use these skills correctly in your Results calculations.

Prelab & Results

Click here for Prelab worksheet: pdf or Word format

Click here for Results worksheet: pdf or Word format

A question to think about for your quiz:

1. Based on the experiments you performed, think of at least 2 ways to protect yourself from each of the three types of radiation.

Before Lab 3: Complete Safety Training

• Please complete the online safety tutorial before lab 3. You cannot pass CHEM 105 if you do not complete the tutorial. The tutorial must be completed from an on-campus computer.
• Choose Science Center as your department.
• Your instructor can check your status online, so you do not need to print out your certificate. If you have already completed the tutorial for another course, tell your instructor and she can check your status online. You do not have to do it again.

References

¹ Zumdahl, S.S.; Chemical Principles; Houghton Mifflin: Boston, New York, 5^th ed., 2005.  Use this reference for any future reading assignments that refer to Zumdahl

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.”

Created By: Adilia James '07 and Sarah Coutlee '07
Maintained By: Nick Doe
Date Created: July 3, 2006
Last Modified: September 15, 2009
Expiration Date: July 31, 2009