Neutron Activation Analysis

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Chue Vue1, Gregory Rech2, Eric B. Norman3, Jeffrey Lee 4

1California State University of Fresno, Fresno, CA
2University of California at Berkeley, Berkeley,CA
3Lawrence Berkeley National Laboratory, Berkeley, CA
4University of California, Irvine, Irvine, CA


Basic nuclear science is an essential part of the high school science curriculum. Certain topics in nuclear science, like the concepts of neutron activation, gamma decay and beta decay, are something that every physical science student should know. However, it is very difficult to perform nuclear science experiments in a high school laboratory setting, and therefore hard to back up the theory with actual experimental results. With this web site, all that a classroom needs is Internet access in order to look at real nuclear science data. Then, by following the procedures described here, the students can used the real data to identify unknown elements. The idea is that by doing this, nuclear science will seem less abstract and easier to comprehend.

Purpose of Neutron Activation Analysis

Neutron activation analysis (NAA) is a useful technique for identifying unknown materials. 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 or lower). 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 all 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 requirements 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 sensitivity and precision, NAA is widely performed in many different fields of sciences.

What are neutrons?

Neutrons are one of two types of particles found in nuclei, the other being protons. The existence of neutrons were proposed by Rutherford in 1920 and finally discovered by Chadvick in 1932. Neutrons have about the same mass as protons but carry no electric charge, whereas protons carry a +1 unit of charge. Another major difference between neutrons and protons is that free (unbound) protons are thought to be stable. Neutrons, on the other hand, are unstable, and will decay with a half-life of 10.4 minutes.

How do we get them?

If we are going to perform NAA, obviously we are going to need neutrons. There are a number of ways to obtain neutrons, but the following is the method that we used. First we find a long-lived radioactive isotope that decays by emitting an alpha particle, like 241Am. An alpha particle is just a 4He nucleus (2 protons and 2 neutrons). We then mix this source with a sample of a light isotope, like 9Be and the following reaction occurs.

9Be + 4He -> 12C + 1n

Where 1n is the neutron that is produced by the reaction.

What nuclear reaction do they cause?

Since neutrons have no charge they only interact with the nucleus of an atom not the electrons. The most common type of neutron induced reaction is the neutron capture reaction (see figure 1 below). When a neutron fuses with the nucleus, a compound nucleus forms in an excited state. The excited compound nucleus will very quickly decay to a more stable state through emission of one or more gamma rays (also known as prompt gamma rays). The new state of the compound nucleus yields a radioactive nucleus, which will beta decay into an excited state of another radioactive nucleus, which will then decay by emission of one or more gamma rays (also known as characteristic delayed gamma rays). The emission rate depends on the half-life of each radioactive nucleus. The half-life of radioactive nuclei can range from nanoseconds to billions of years.


set up

Figure 1 Neutron capture

Beta Decay

Beta particles are electrons or positrons (electrons with positive electric charge, or antielectrons). Beta decay occurs when, in a nucleus with too many protons or too many neutrons, one of the protons or neutrons is transformed into the other. In beta minus decay, as shown in Figure 3, a neutron decays into a proton, an electron, and an antineutrino: n => p + e- + v- . In beta plus decay, as shown in Figure 4, a proton decays into a neutron, a positron, and a neutrino: p => n + e+ + v. These particular reactions take place because conservation laws are obeyed. Electric charge conservation requires that if an electrically neutral neutron becomes a positively charged proton, an electrically negative particle (in this case an electron) must also be produced. Similarly, conservation of lepton number requires that if a neutron (lepton number = 0) decays into a proton (lepton number = 0) and an electron (lepton number = 1), a particle with a lepton number of -1 (in this case an antineutrino) must also be produced. The leptons emitted in beta decay did not exist in the nucleus before the decay - they are created during the process of decay.

set up

Figure 2 - A representation of 14C beta minus decaying into 14N.

set up

Figure 3 - A representation of 18F beta plus decaying into 18O.

To the best of our knowledge, an isolated proton does not decay. However, within a nucleus, the beta decay process can change a proton into a neutron. An isolated neutron is unstable and will decay with a half-life of 10.5 minutes. A neutron in a nucleus will decay if a more stable nucleus results; the half-life of the decay depends on the isotope. If it leads to a more stable nucleus, a proton in a nucleus may capture an electron from the atom (electron capture), and change into a neutron and a neutrino. Beta plus decay, beta minus decay, and electron capture are three ways in which protons can be changed into neutrons or vice-versa; in each decay there is a change in the atomic number (number of protons), so that the parent and daughter atoms are different elements. In all three processes, the number of nucleons, A, remains the same, while both proton number, Z, and neutron number, N, increase or decrease by 1.

In beta decay, the change in binding energy appears as the mass energy and kinetic energy of the beta particle, the energy of the neutrino, and the kinetic energy of the recoiling daughter nucleus. The energy of an emitted beta particle from a particular decay can take on a range of values because the energy can be shared in many ways among the three particles while still obeying energy and momentum conservation.

What are gamma rays and gamma decay?

Gamma rays are electromagnetic radiation (photons) emitted from an unstable nuclei of a radioactive isotope. In gamma decay, as shown in Figure 2, a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter isotopes are the same. In the gamma decay of a nucleus, the emitted photon and recoiling nucleus each have a well-defined energy after the decay. The characteristic energy is divided between only two particles.

LOVE

Figure 2 - A representation of the gamma decay of excited 152Dy.

Gamma ray spectra

The energy levels of a nucleus are different for every isotope. Therefore, gamma rays emitted from one isotope will not have the same properties (such as energy and intensity) as gamma rays emitted from any other isotope. In other words, gamma rays are characteristic for each isotope. Fortunately, many isotopes that we make through neutron activation emit gamma rays. All we need then, is a device for measuring gamma rays and another device to sort these measured gamma rays by energy. Then we would know what isotope is being produced since these gamma rays are characteristic.

The Setup

In theory, every element can be neutron activated, but in practice certain conditions must be met in order to get good data. First, the element must have an isotope that is able to appreciably react with the incoming neutrons; we say that the isotope must have a high "neutron cross-section". Second, the isotope itself must be relatively easy to get in sizable quantities. Third, the half-life has to be long enough so the amount of activity is measurable. Lastly, gamma rays must be produced that are measurable by our detectors, so they must be reasonably intense and in a limited energy range .
To measure the gamma rays emitted from each isotope, we used a coaxial germanium detector 5 cm thick and 5 cm in diameter sitting directly in front of the sample. We acquired the gamma ray spectra in 4096 channels using an ORTEC, PC based data acquisition system. We used lead bricks to shield the detector from outside sources. The shield did not seal all out sources but does give us enough shielding to produce reasonable data.

Materials and Methods

Each sample was bagged separately in ziplock bags and placed together in a small plastic container. A string of about 60 inches was tied to the top of the container for lifting the samples from the neutron source. The samples were placed on top of the neutron source in a shielded container for about 48 hours. The samples were lifted from the source using the string and taken to the counting room for counting. All the samples were counted separately.

The experimental procedure for students

Solutions are available for teachers, send e-mail to :Eric Norman or :Chue
Lesson plan for teachers also available

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: ie.lbl.gov/NAA/page.htm
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: 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 ± 2 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 isotopes will appear on the screen. The second column on the list indicates the intensity of the gamma ray.
10) Click on the isotope 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 : isotopes.lbl.gov/toips/greatch.pdf and see if the isotope can be made by neutron activation. * Once you're 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.

Applications.

i) Biochemistry
ii) Archaeology
iii) Environmental restoration
iv) Nutritional Epidemiology --Thyroid Cancer Study
v) Soil Science

Acknowledgements

If you would like to learn more about x-ray fluorescence or radioactive decay, please visit our other websites: http://ie.lbl.gov/xray and http://ie.lbl.gov/gamma

This work was supported by the US Department of Energy under contract number DE-AC03-76SF00098.