Neutron activation analysis

Neutron activation analysis uses a flux of neutrons to excite the nuclei of chemical elements in samples, thus causing the excited nuclei to emit characteristic gamma radiation. The technique provides a sensitive method for measuring the amount of chemical elements contained in geological samples, particularly when there is only a small amount of the element in the sample.

Gamma Radiation

In neutron activation analysis, a sample of interest to the scientist is placed in a beam of neutrons produced by a radioactive source, such as an accelerator, a nuclear reactor, or an alpha-emitter bombarding beryllium. The neutrons interact with nuclei contained in the sample and alter their structures, frequently leaving the nuclei of the sample with excess energy. After a predetermined time, the sample is taken out of the beam of neutrons. The altered nuclei in the sample lose their excess energy by emitting nuclear radiation that is characteristic of each individual type of nucleus. The radiation is detected and allows identification of the nucleus that emitted it. The intensity of a particular characteristic radiation is directly related to the number of nuclei of that species in the sample. Usually, the radiation studied in neutron activation analysis is gamma radiation—that is, high-energy electromagnetic radiation emitted by nuclei without altering their chemical nature. The gamma rays emitted by the sample are directly related to the abundance of a particular chemical element in the sample.

Neutron activation analysis grew out of the systematic study of the interaction of neutrons and nuclei conducted by nuclear physicists to understand the structure of the nucleus, beginning with the work of Enrico Fermi in 1934. Interpretation of the patterns of gamma rays emitted by a sample following irradiation by neutrons requires several types of background information. First, not all types of nuclei react in the same way with a neutron beam. The cross-section for the nuclear reaction, which measures the chance that a particular nuclear species will be produced, depends not only on the structure of the nucleus involved but also on the energy of the neutrons used for bombardment of the sample. The reaction cross-section must be measured in a separate experiment as the energy of the neutrons is varied. Many reaction cross sections have been measured and are tabulated in the scientific literature.

Second, the gamma radiation from a particular nuclear species has characteristic energies that can be precisely measured using germanium-based detectors. The pattern of energies of gamma rays emitted by a particular type of nucleus identifies that nucleus just as the pattern of visible light emitted by an atom—its spectrum—identifies that atom. Tables of the gamma spectra of nuclei are an important input to neutron activation analysis. Such tables, along with the cross sections for nuclear reactions, are stored in computers and automatically recalled during analysis of the gamma spectra from neutron activation analysis.

Half-Lives of Radioactive Materials

Emission of nuclear radiation occurs gradually in time at a rate characterized by the half-life of the given decay—that is, the time for half the nuclei in a sample to emit their gamma radiation. Half-lives for nuclear species vary from picoseconds to millions of years. Nuclear species of interest for neutron activation analysis generally have half-lives ranging from seconds to days, as the species must live long enough for the sample to be transported to the detector and must decay quickly enough so that they can be detected in a reasonable amount of time. As the neutrons interact with the sample nuclei, producing new energetic nuclei, the energetic nuclei decay with their characteristic half-lives. Thus, the number of excited nuclei in a sample depends on the half-life of the nucleus as well as on the time it has spent in the neutron beam. Half-lives of nuclear radiation are measured in separate experiments and are tabulated. Analysts keep records of the exposure of the sample to neutrons.

The variations in the half-lives of excited nuclei can be used to identify nuclei present in the sample. Decays with short half-lives happen very rapidly so that the first gamma rays obtained from the sample are mostly those with half-lives less than about five minutes. If the sample then sits for half an hour, the short-lived nuclear species will have decayed, and the gamma rays from the sample will be those from nuclei with longer half-lives. Thus, in neutron activation analysis, gamma rays from the sample are measured at a series of carefully planned time intervals. Computers are used to calculate the effect of half-lives on the gamma spectra that have been recorded.

Isotopes

The number of excited nuclei produced during neutron irradiation (and thus the intensity of a particular gamma emission) depends on the concentration of a particular nuclear species in the sample. Because the chemical nature of an atom is not affected by the number of neutrons in its nucleus, most chemical elements are characterized by more than one type of nucleus or isotope—that is, nuclei with the same number of protons in them and thus belonging to the same element but with different numbers of neutrons. Different isotopes have different reaction cross sections and different gamma spectra. Thus, the analyst must know the relative amounts of each isotope of a given element to relate the intensity of a gamma emission to the abundance of a particular element in the sample. These data are well known and readily available.

Neutron Flux

The number of gamma rays at a particular energy level also depends on the number of neutrons that are aimed at the sample. If the neutrons are produced by a radioactive source or an accelerator, they form a beam and are described as a particular number of particles per unit of area and time. If the sample is placed inside a nuclear reactor, the neutrons bombard it from all directions, and the irradiation is described in terms of a neutron flux, or the number of neutrons crossing a square centimeter of the sample each second. The energy distribution of the neutrons that strike the sample must also be recorded because reaction cross sections depend on energy. In an accelerator or a radioactive source, the neutrons produced usually have a well-defined single energy. In a reactor, they will have a distribution of energy levels that must be measured for an individual reactor and for a particular location in the core of that reactor.

Costs and Benefits of the Analytical Method

Neutron activation analysis is expensive because it requires a neutron source and specialized detectors and counting systems, all of which are run by a computer in modern laboratories. The technique is not sensitive to the chemical state of an atom but merely determines the number of atoms of a particular element to be found in the sample; therefore, it is not suitable for determining the chemical state in which elements are present. At the same time, neutron activation analysis is very fast compared to standard quantitative analysis, thus compensating for the expense when fast results are needed. For example, neutron activation analysis using a radioactive source to produce the neutrons has been conducted in the field to determine the copper and manganese content of ores. The technique is advantageous in that samples do not have to be transported to a laboratory for analysis, and results of the analysis can be used to guide drilling operations.

Neutron activation analysis cannot detect every chemical element in the sample, as not all chemical elements produce gamma rays with suitable half-lives or have large cross sections for nuclear reactions. In some cases, strong gamma radiation from abundant elements may mask the weaker signals from less abundant species. The technique for producing neutrons may strongly influence the elements that are detected. For example, the high-energy or fast neutrons produced in accelerators using tritium targets interact strongly with oxygen and silicon, while lower-energy neutrons characteristic of nuclear reactors interact very little with these elements. Thus, fast neutrons are characteristically used for rapid determinations of the silicon and oxygen content of minerals. Frequently, samples are subjected to analysis using more than one sort of neutron source to detect different elements. Rock samples will first be subjected to fast neutron analysis to determine their content of silicon and oxygen and then to analysis using a reactor to detect about twenty-five other elements. Finally, neutron activation analysis may be supplemented by chemical separation of elements for the detection of very rare elements.

Neutron activation analysis is ideally suited for the study of trace elements—that is, relatively rare elements present in samples in small quantities. Because neutrons easily penetrate geological materials, samples for neutron activation analysis require little preparation, and the technique does not destroy the sample, which can thus be saved for display or subjected to further analysis. Therefore, this technique is often applied to samples of archaeological importance, where samples are too precious to destroy in analysis. It offers the researcher the further advantage that many elements in the same data can be identified, and thus elements may be found whose presence in the sample was not initially expected. This scanning for many elements at once is an advantage in problems such as the search for pollutants in river water. For example, extensive studies of environmental mercury in Sweden have been conducted using neutron activation analysis of minerals, coal, and plant and animal tissues.

The advent of computer-based systems has automated much of the tedious calculation needed to analyze data from neutron activation analysis. The technique is thus accessible to a much wider variety of researchers than was previously the case and promises to find increasing use as a probe of the elemental composition of samples of interest to earth scientists.

Types of Earth Science Applications

Neutron activation analysis provides a powerful technique for simultaneously determining the amounts of many chemical elements in a geological sample without destroying the sample. Although it requires a source of neutrons and fairly complex instrumentation, it is much faster than conventional chemical analysis and can analyze a sample for several elements at the same time with little sample preparation. This technique is also uniquely sensitive to very small amounts of particular elements and can often detect minute amounts of such elements present in samples that would escape all but extremely detailed and time-consuming chemical analyses using atomic spectroscopic techniques designed to search for that element.

Applications of neutron activation analysis to earth science fall into two broad categories. The first consists of cases in which researchers take advantage of the speed of neutron activation analysis to obtain immediate results on the elemental compositions of their samples. Such work is often done in the field, using a radioactive source to produce the neutrons, and the results of the analysis guide field operations. Similarly, neutron activation analysis may be used to screen a very large number of samples rapidly on a production basis.

The second category of applications of neutron activation analysis utilizes the ability of the technique to determine rapidly very small concentrations of certain chemical elements. One example of this application has been the systematic study of trace elements in rocks of various ages. In the energy industry, neutron activation analysis has been applied to the study of trace elements in coals, thereby providing clues to the origin of particular coal beds. The quality of coal as a heat source varies widely from bed to bed. An understanding of why this variation occurs might lead to new methods of treating coals before burning them in order to reduce pollution. Finally, the ability of neutron activation analysis to scan large numbers of samples for minute quantities of chemical elements has been put to use in the study of sources of pollution, particularly by metals, in the environment. Large numbers of samples of river water or runoff near landfills can be checked for the presence of a wide variety of metals quickly and efficiently using neutron activation analysis.

A particular application of neutron activation analysis is autoradiography. In this case, the sample is irradiated and then placed in contact with a piece of film. The film is developed and records concentrations of radioactivity, showing how particular chemical elements—for example, uranium and thorium—are distributed within the sample.

Study of

Probably the most famous example of the use of neutron activation analysis in earth science is the study of the lunar rocks brought back to Earth by the Apollo astronauts. Scientists wished to know the chemical composition of these rocks to obtain clues as to their history and to learn whether the moon formed from the same original material as did the earth. At the same time, only relatively small samples were available, as the lunar rocks had to be carried back from the moon in circumstances where the amount of weight was critical; in addition, scientists wanted to save the lunar rocks for future analysis and for public display. Neutron activation analysis does not damage the sample it studies. Even when combined with chemical separation techniques to aid analysis for very scarce elements, the samples needed are very small—on the order of milligrams—as opposed to the gram-sized samples needed for standard chemical analysis. Thus, the lunar rock samples could be subjected to neutron activation analysis to determine their basic elemental composition and still be left intact for display or analysis by other methods. Some of the surprising results from neutron activation analysis of the lunar rock samples include the fact that lunar rocks and soils are very low in oxygen compared to their terrestrial counterparts.

One of the problems in studying the lunar samples was to determine their content of rare-earth elements such as europium, neodymium, or gadolinium. These elements are chemically very similar and thus difficult to separate by quantitative chemical analysis. In contrast, their nuclear structures are very different; therefore, neutron activation analysis is an ideal tool for distinguishing among them. Results of the analysis showed that overall abundances of the rare-earth elements in lunar rocks were fifty to one hundred times greater than is standard for chondritic meteorites, which are meteorites believed to represent the primordial composition of the material from which the solar system formed. At the same time, lunar rocks were depleted in the element europium compared to chondritic meteorites and terrestrial rocks. Explanation of these strange patterns of elemental abundances uncovered by neutron activation analysis supports the theory that the moon formed from a disk of material spun off from the very early Earth by a grazing collision with a very large planetesimal.

Principal Terms

cross section: the effective area that a nucleus presents to an oncoming nuclear particle, which determines the chance that the particle will strike the nucleus, causing a nuclear reaction

gamma decay: the emission of high-energy electromagnetic radiation as a nucleus loses excess energy

gamma spectrum: the unique pattern of discrete gamma energies emitted by each specific type of nucleus; it identifies that nucleus

half-life: the time during which half the atoms in a sample of radioactive material will decay

isotope: atoms of the same chemical element containing equal numbers of protons whose nuclei have different masses because they contain different numbers of neutrons

neutron: an uncharged particle that is one of the two major nuclear constituents having nearly equal masses and different electric charges

nuclear reaction: a change in the structure of an atomic nucleus brought about by a collision of the nucleus with another nuclear particle such as a neutron

nucleus: the tiny central portion of an atom that contains all the positive charge and nearly all the mass of the atom

proton: a particle that carries a single unit of positive charge equal in size to that of the electron; one of the two major nuclear constituents having nearly equal masses and different electric charges

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