Title: Neutron Activation Analysis

Author: Chue Vue, Gregory Rech, Eric B. Norman

At the completion of the lesson students should be able to:

· Understand the difference between alpha, beta and gamma decay.

· Understand how Neutron Activation Analysis (NAA) allows elemental

identification to be done.

· Understand how radioactive nuclides can be created in the laboratory

and how they can be studied.

· Learn how to access information on the Internet.

From: Science Content Standards for California Public Schools, Kindergarten Through Grade Twelve, Adopted by California State Board of Education October, 1998

Content Standard 3a: Structure of Matter

Content Standard 7b: Periodic Table

· Students know each element has a specific number of protons in the

nucleus ( the atomic number) and each isotope of the element has a

Content Standard 11a, 11c, 11d: Nuclear Processes

· Students know the three most common forms of radioactive decay

(alpha, beta, and gamma) and know how the nucleus changes in each

type of decay.

· Access to the Internet and a notebook.

Neutron activation analysis (NAA) is a useful technique for identifying the elemental composition of materials in a non-destructive way. This can be done by irradiating a sample with neutrons and then studying the decays of the radioactive nuclei that are produced. It can be performed quantitatively or qualitatively depending on the purpose of the experiment. Sensitivities of the method are sufficient enough to measure certain elements down to extremely low concentrations (parts per trillion). NAA can be performed to determine the concentration of several different elements within a single sample of a material. Since neutrons have no charge they only interact with the nucleus of an atom, not the electrons. In addition, this technique sees many of the elements in a sample, regardless of their chemical form or oxidation state. NAA is one of the most accurate techniques for identifying elemental abundances known. The basic requirement to carry out analysis of samples by NAA are: the detailed knowledge of the reactions that occur when neutrons interact with the target nuclei, a source of neutrons, and an instrument that can detect gamma rays accurately. Because of its accuracy and sensitivity, NAA is widely performed in many different fields of sciences.

In this lab you will be given a set of unknown gamma ray spectra acquired from the Lawrence Berkeley National Laboratory (solutions are available for teachers). You will identify each spectrum by using the information provided on the internet website and following the steps of the procedure (the steps are also available on the website). Some of the spectra may contain more than one element. All the background information is provided on the website.

If you are first time user of this activity we strongly encourage you to go through the Lab before assigning it to the students. Solutions are available upon request. Some spectra are more difficult to identify and will take more time than stated in this lesson plan. We strongly recommend that you do one example in class before having students do this activity.

Vocabulary: Students need to know these vocabulary before starting this project.

Alpha decay
Beta decay
Gamma decay
Neutron
Proton
Electron
Nuclide
Intensity
Isotope
Half-life

Answer the following questions in your lab notebook:

1. What is a half-life? Explain in your own words.
2. What are alpha, beta and gamma decay? Explain.
3. What is an isotope?
4. What is a gamma ray spectrum?

The following is a step by step procedure on how to identify an unknown sample using NAA.

1. Go to the following Website to obtain spectra: http://ie.lbl.gov/NAA
2. Pick any spectrum from the set of spectra.
3. Find all the energies of the peaks of the spectrum and record these energies in your note book (some spectra will have more then one peak).
4. Go to the following Website: http://ie.lbl.gov/toi/
5. Click on Radiation search, located in the middle of the page under WWW Table of Radioactive Isotopes
6. Enter the energy of the largest peak into the box provided (kev), use ± 1 as your uncertainty.
7. Enter the half life of the isotope (the half life of these samples are between 5 minutes to 300 days).
8. Click on search.
9. A list of all the possible istopes will appear on the screen. The second column on the list indicates the intensity of the gamma ray.
10. Click on the element that has the highest intensity and look for other strong g-rays that came from this isotope and compare to your list.
11. Now go to the following Website : http://isotopes.lbl.gov/toips/greatch.pdf and see if the element can be made by neutron activation. * Once you’r on the chart of nuclides home page you have to zoom in to see the nuclide. The zoom in button is located to the left of the text select button on the tool bar.
12. If the conditions of making the isotope apply, try to match the spectrum with the element (look for other intense peaks). If the energy peak or peaks match, then you have found the element.
13. If the conditions of making the isotope do not apply, or the spectrum does not match the element, then go back to the list on # 8, choose the next most intense peak and repeat # 10 and 11. Repeat this process until you find an element that matches the spectrum.
14. For each sample, several spectra are shown. You can use the time at which the spectrum was collected and the run length to estimate the half-life of the isotope.

* Only the stable isotopes of an element can be neutron activated. In the chart of the nuclides the stable isotopes are colored black, so the only possible isotopes you can see in the spectra are to the right of these stable isotopes. In the chart of nuclides, the number of protons increase as you moved from the left to the right. The number of neutron increase as you moved from the bottom to the top.

After completion the activity students are required to do a lab-write up addressing the followings.

1. What are the elements in the spectra? Justify your answer.
2. What isotopes do the neutron activated elements decay into?
3. What is the usefulness of NAA?