Neutron activation analysis

From Wikipedia, the free encyclopedia

1 How Neutron Activation Analysis Works
2 General information
3 NAA Overview
4 Possible Applications for NAA
5 External links
6 References

[edit] How Neutron Activation Analysis Works

Neutron Activation Analysis (NAA) is a nuclear process used for determining certain concentrations of elements in a vast amount of materials. NAA allows discrete Sampling (statistics) of elements since it disregards the chemical form of a sample, and only focuses on its nucleus. The reactor is at the bottom of a deep tank of water. Vials are placed and transported in and out of the core through an air-driven pneumatic transport system. The nuclear reactor consists of 60 lead insulated rods of uranium (20% ²⁸⁵U) whose decay creates power, and therefore become a constant source of neutrons proportional to their concentration in the core of the reactor. The usage of H₂O during NAA is a key component of “bombarding” or constantly hitting a sample with neutrons. By placing the core underwater, it creates a natural biological shield to repel possible radiation from neutron bombardment. Furthermore, hydrogen molecules share energy when they ricochet against uranium molecules, therefore creating a slower fission and a continuous chain reaction of ricocheting molecules. Since neutrons have no charge they will interact with the nucleus of the atom, rather than the electron shell. Consequently, if a neutron particle comes in contact with the nucleus of an atom, it might become absorbed. When this happens, the element will become a different isotope of the same element. This alteration in the nucleus of the atom causes the isotope to become unstable, and ultimately radioactive.

[edit] General information

In chemistry, neutron activation analysis is a technique used to very accurately determine the concentrations of elements in a sample. The particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artifacts.

The sample is introduced into the intense radiation field of a nuclear reactor. The sample is thus bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known. Using this information it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it.

[edit] NAA Overview

Neutron Activation Analysis is a sensitive multi-element analytical technique used for both qualitative and quantitative analysis of major, minor, trace and rare elements. NAA was discovered in 1936 by Hevesy and Levi, who found that samples containing certain rare earth elements became highly radioactive after exposure to a source of neutrons [1]. This observation led to the use of induced radioactivity for the identification of elements. NAA is significantly different from other spectroscopic analytical techniques in that it is based not on electronic transitions but on nuclear transitions. To carry out an NAA analysis the specimen is placed into a suitable irradiation facility and bombarded with neutrons, this creates artificial radioisotopes of the elements present. Following irradiation the artificial radioisotopes decay via the emission of particles or more importantly gamma-rays, which are characteristic of the element from which they were emitted.

For the NAA procedure to be successful the specimen or sample must be selected carefully. In many cases small objects can be irradiated and analysed intact without the need of sampling. But more commonly a small sample is taken, usually by drilling in an inconspicuous place. About 50mg (one-twentieth of a gram) is a sufficient sample, so damage to the object is minimised [2]. It is often good practice to remove two samples using two different drill bits made of different materials. This will reveal any contamination of the sample from the drill bit material itself. The sample is then encapsulated in a vial made of either high purity linear polyethylene or quartz [3]. These sample vials come in many shapes and sizes to accommodate many specimen types. The sample and a standard are then packaged and irradiated in a suitable reactor at a constant, known neutron flux. A typical reactor used for activation uses uranium fission, providing a high neutron flux and the highest available sensitivities for most elements [4]. The neutron flux from such a reactor is in the order of 10¹² neutrons cm^-2 s^-1 ^[1]. The type of neutrons generated are of relatively low kinetic energy (KE), typically less than 0.5eV [5]. These neutrons are termed thermal neutrons. Upon irradiation a thermal neutron interacts with the target nucleus via a non-elastic collision, causing neutron capture. This collision forms a compound nucleus which is in an excited state. The excitation energy within the compound nucleus is formed from the binding energy of the thermal neutron with the target nucleus. This excited state is unfavourable and the compound nucleus will almost instantaneously de-excite (transmutate) into a more stable configuration through the emission of a prompt particle and one or more characteristic prompt gamma photons [6]. In most cases this more stable configuration yields a radioactive nucleus. The newly formed radioactive nucleus now decays by the emission of both particles and one or more characteristic delayed gamma photons. This decay process is at a much slower rate than the initial de-excitation and is dependent on unique half-life of the radioactive nucleus. These unique half-lives are dependent upon the particular radioactive species and can range from fractions of a second to several years. Once irradiated the sample is left for a specific decay period then placed into a detector, which will measure the nuclear decay according to either the emitted particles, or more commonly the emitted gamma-rays ^[1].

Variations of NAA

NAA can vary according to a number of experimental parameters. The kinetic energy of the neutrons used for irradiation will be a major experimental parameter. The above description is of activation by slow neutrons, slow neutrons are fully moderated within the reactor and have KE <0.5eV. Medium KE neutrons may also be used for activation, these neutrons have been only partially moderated and have KE of 0.5eV to 0.5MeV, and are termed epithermal neutrons. Activation with epithermal neutrons is known as Epithermal NAA (ENAA). High KE neutrons are sometimes used for activation, these neutrons are unmoderated and consist of primary fission neutrons. High KE or fast neutrons have a KE >0.5MeV. Activation with fast neutrons is termed Fast NAA (FNAA). Another major experimental parameter is whether nuclear decay products (gamma-rays or particles) are measured during neutron irradiation (Prompt Gamma), or at some time after irradiation (Delayed Gamma). PGNAA is generally performed by using a neutron stream tapped off the nuclear reactor via a beam port. Neutron fluxes from beam ports are the order of 10⁶ times weaker than inside a reactor [7]. This is somewhat compensated for by placing the detector very close to the sample reducing the loss in sensitivity due to low flux. PGNAA is generally applied to elements with extremely high neutron capture cross-sections; elements which decay too rapidly to be measured by DGNAA; elements that produce only stable isotopes; or elements with weak decay gamma-ray intensities [8]. PGNAA is characterised by short irradiation times and short decay times, often in the order of seconds and minutes. DGNAA is applicable to the vast majority of elements that form artificial radioisotopes. DG analyses are often performed over days, weeks or even months. This improves sensitivity for long-lived radionuclides as it allows short-lived radionuclide to decay, effectively eliminating interference [9]. DGNAA is characterised by long irradiation times and long decay times, often in the order of hours, weeks or longer.

If NAA is conducted directly on irradiated samples it is termed Instrumental Neutron Activation Analysis (INAA). In some cases irradiated samples are subjected to chemical separation to remove interfering species or to concentrate the radioisotope of interest, this technique is known as Radiochemical Neutron Activation Analysis (RNAA).

NAA Detectors

There are a number of detector types and configurations used in NAA. Most are designed to detect the emitted gamma radiation. The most common types of gamma detectors encountered in NAA are the gas ionisation type, scintillation type and the semiconductor type. Of these the scintillation and semiconductor type are the most widely employed. There are two detector configurations utilised, they are the planar detector, used for PGNAA and the well detector, used for DGNAA. The planar detector has a flat, large collection surface area and can be placed close to the sample. The well detector ‘surrounds’ the sample with a large collection surface area.

Germanium Type Gamma Radiation Detectors

Scintillation type detectors use a radiation sensitive crystal, most commonly Sodium Iodide NaI (TI), which emits light when struck by gamma photons. These detectors have excellent sensitivity and stability, and a reasonable resolution.

Semiconductor detectors utilise the semiconducting element germanium. The germanium is processed to form a p-i-n (positive-intrinsic-negative) diode, and when cooled to ~77K by liquid nitrogen to reduce dark current and detector noise, produces a signal which is proportional to the photon energy of the incoming radiation. There are two types of germanium detector, the lithium drifted germanium or Ge(Li) (pronounced ‘jelly’), and the High Purity Germanium or HPGe. The semiconducting element silicon may also be used but germanium is preferred, as its higher atomic number makes it more efficient at stopping and detecting high energy gamma-rays. Both Ge(Li) and HPGe detectors have excellent sensitivity and resolution, but Ge(Li) detectors are unstable at room temperature, with the lithium drifting into the intrinsic region ruining the detector. The development of undrifted high purity germanium has overcome this problem.

Planar Semiconductor Detector and Liquid Nitrogen Dewar

Particle detectors can also be used to detect the emission of alpha (α) and beta (β) particles which often accompany the emission of a gamma photon but are less favourable, as these particles are only emitted from the surface of the sample and are often absorbed or attenuated by atmospheric gases requiring expensive vacuum conditions to be effectively detected. Whereas gamma-rays are not absorbed or attenuated by atmospheric gases, and can also escape from deep within the sample with minimal absorption.

NAA Analytical Capabilities

NAA can detect up to 74 elements depending upon the experimental procedure. With minimum detection limits ranging from 0.1 to 1x10⁶ng g^-1 depending on element under investigation. Heavier elements have larger nuclei, therefore they have a larger neutron capture cross-section and are more likely to be activated. Some nuclei can capture a number of neutrons and remain relatively stable, not undergoing transmutation or decay for many months or even years. Other nuclei decay instantaneously or form only stable isotopes and can only be identified by PGNAA.

Sensitivity (picograms)	Elements
1	Dy, Eu
1 - 10	In, Lu, Mn
10 - 100	Au, Ho, Ir, Re, Sm, W
100 - 1E3	Ag, Ar, As, Br, Cl, Co, Cs, Cu, Er, Ga, Hf, I, La, Sb, Sc, Se, Ta, Tb, Th, Tm, U, V, Yb
1E3 - 1E4	Al, Ba, Cd, Ce, Cr, Hg, Kr, Gd, Ge, Mo, Na, Nd, Ni, Os, Pd, Rb, Rh, Ru, Sr, Te, Zn, Zr
1E4 - 1E5	Bi, Ca, K, Mg, P, Pt, Si, Sn, Ti, Tl, Xe, Y
1E5 - 1E6	F, Fe, Nb, Ne
1E7	Pb, S

Table source [10]

Gamma Spectra from a Sample of Pottery Irradiated for 5 Seconds, Decayed for 25 Minutes, and Counted for 12 Minutes with an HPGe Detector.

Gamma Spectra from a Sample of Pottery Irradiated for 24 Hours, Decayed for 9 Days, and Counted for 30 Minutes on a HPGe Detector.

Summary of NAA

NAA can perform non-destructive analyses on solids, liquids, suspensions, slurries, and gases with no or minimal preparation. Due to the penetrating nature of incident neutrons and resultant gamma-rays the technique provides a true bulk analysis. As different radioisotopes have different half-lives, counting can be delayed to allow interfering species to decay eliminating interference. Until the introduction of ICP-AES and PIXE, NAA was the standard analytical method for performing multi-element analyses with minimum detection limits in the sub-ppm range ^[1]. Accuracy of NAA is in the region of 5%, and relative precision is often better than 0.1% ^[1]. There are two noteworthy drawbacks to the use of NAA; even though the technique is essentially non-destructive the irradiated sample will remain radioactive for many years after the initial analysis, requiring handling and disposal protocols for low-level to medium-level radioactive material; also the number of suitable activation nuclear reactors is declining, with a lack of irradiation facilities the technique has declined in popularity and become more expensive.

[edit] Possible Applications for NAA

1. Archaeology

The use of neutron activation analysis to characterize archaeological specimens (e.g., pottery, obsidian, chert, basalt and limestone) and to relate the artifacts to sources through their chemical signatures is a well-established application of the method. Over the past decade, large databases of chemical fingerprints for clays, obsidian, chert and basalt have been accumulated through analysis of approximately thirty elements in each of more than 42,000 specimens. The combination of these databases with powerful multivariate statistical methods (i.e., principal components analysis, factor analysis, discriminant analysis, and Mahalanobis distance probabilities) allows many archaeological materials to be sourced with a high degree of confidence. The sourcing information can help archaeologists reconstruct the habits of prehistoric peoples. For example, the "fingerprinting" of obsidian artifacts by NAA is a nearly 100 percent successful method for determining prehistoric trade routes since sources of obsidian are easily differentiated from one another through their chemical compositions. For more information concerning this application of NAA, see the Archaeometry Laboratory at MURR. A recommended charge schedule for instrumental neutron activation analysis of archaeological and geological specimens is available (See Charge Schedule).

2. Study the Redistribution of Uranium and Thorium due to Ore Processing

In the 10 years between 1948 and 1958 uranium ore mining in the United States expanded from a cumulative total of 38,000 tons to 5.2 million tons involving more than 400 mines. Ores from these mines were chemically processed at an estimated 50 to 100 sites. These activities have resulted in the contamination of hundreds of square miles of surface and subsurface soils, and their corresponding ground waters, with uranium-238, thorium-232 and their radioactive daughters. In most cases these sites have various radioactive waste materials such as ore tailings, processing residues and leachates mixed with soil and generally covered with an uncontaminated soil layer several feet in thickness. In other cases the tailings or wastes have remained exposed and hence have been further distributed by wind and erosion. EBNAA has been developed as a methodology suitable for automation by which contaminated ore-processing sites can be characterized and their restoration monitored.

3. The Use of Radiotracers to Study the Fate of Hazardous Elements in Waste Material/Coal-Char Admixtures under Gasification - an Emerging Waste Management Technology

With an increasing emphasis being placed on the cleanup of hazardous waste sites from past technological operations, science has had to come up with ideas on how to simultaneously decompose and stabilize a variety of mixed wastes which include radioactive materials. The most difficult of all wastes to cleanup is mixed waste. These may contain organic materials (dichlorobenzene, naphthalene, etc) along with hazardous elements (cadmium, mercury, plutonium, thorium, uranium and other transuranics). Current waste destruction technology relies on oxidation of the waste (incineration); an alternative is the storage of this waste. Both have drawbacks. Incineration requires high temperatures and the resulting high fuel costs and volatilization of some hazardous components. Storage costs are proportional to the amount of space taken up by the waste and the regulations surrounding the waste, therefore, long term storage of some existing radioactive waste is unacceptable without volume reduction and stabilization. A process developed at MU, ChemChar gasification, helps minimize these problems. ChemChar gasification using a coal char (a triple reverse burn coal product which is very porous) acts as both a surface for chemical reactions and a sequestering agent of hazardous elements. Initially the process uses the char as an absorber of the waste stream which is mixed on a 2:1 weight ratio (char: waste). A flame front is established which moves opposite the direction of oxygen flow and in the flame front organics are decomposed and other species tend to be chemically reduced. The volatile organics are carried off and trapped later while the immobile metals, metalloids and some non-metals are sequestered on the char for final disposal or other disposition. The gasification process also results in volume reduction, partitioning of potentially usable organic solvents and the production of a gas that could be used as an energy source. The objective of the MURR component of this project is to develop a procedure by which radiotracers can be used to determine the fates of hazardous elements during gasification in support of the use of gasification as a means of decomposing, stabilizing and reducing the volume of hazardous mixed wastes including radioactive wastes resulting from the nuclear weapons program. To date the fates of arsenic, strontium, cadmium, cesium, mercury, uranium, thorium, neptunium and protactinium, chlorine (as an organochloride) and phosphorus (as organophosphorus surrogates for military wastes) have been studied in this reducing atmosphere using MURR produced radiotracers.

4. Selenium Distribution in Aquatic Species in Selenium-contaminated Fresh-water Impoundments

This work is being done in collaboration with both federal and state agencies. Fresh-water ecosystems in California have been grossly contaminated with selenium as a result of irrigation run-off from heavily used agricultural areas. The effect has been observed throughout most of the food chain. The objective of these studies is to evaluate the extent of the contamination and evaluate methodologies that might be efficacious in the reduction of selenium in these ecosystems.

5. In-situ Radiotracers for Dosage-Form Testing

Over the last several years there has been a growing interest in the use of in-situ radiotracers to test new pharmaceuticals and dosage forms being developed for commercial distribution. This work has been done in collaboration with several pharmaceutical companies and university research centers. The in-situ radiotracers are produced through carefully designed irradiations done at the MURR and used in laboratory, animal, and in a few cases, human experiments. These methodologies offer significant advantages in the evaluation of encapsulations, time release, clearance and the distribution of the pharmaceutical in animal and human models.

6. Nutritional Epidemiology - Nutritional and Biochemical/Genetic Markers of Cancer

This project is being done in collaboration with the Harvard Medical School stemming from the rather striking findings reported in JAMA (December, 1996) showing a significant protective effect due to a 200 microgram/day supplement of selenium against cancers of the colon, lung and prostate. One specific aim of the project is to evaluate the relationship between selenium status and cancer of the colon and prostate in the Physicians= Health Study. As proposed, this is a five-year study using prospectively collected blood samples as the biologic monitor of selenium status. If fully funded, this would be the largest prospective case-control study to date of selenium status and incidence of prostate cancer. Over the five years 2470 prostate cancer cases and 850 colon cancer cases are anticipated.

7. Nutritional Epidemiology - A Cohort Study of the Relationship Between Diet, Molecular Markers, and Cancer Risk: the Canadian Study of Diet, Lifestyle and Health

A pilot study to characterize the nail as a biologic monitor for the dietary intake for selenium in Canada has been undertaken to extend our work to the major Canadian provinces. Suspected selenium determinants such as smoking, use of dietary supplements, age and gender will be statistically evaluated in a population of over 700 subjects. Results will be compared with our earlier study of a female population drawn from 11 states in the U.S. The objective of this work is to expand the collaborative MURR nutritional epidemiology program, that has enjoyed good success in the U.S., to Canada. This pilot will serve as a basis for two grant applications that will be submitted to the National Cancer Institute of Canada and the Canadian Research Council. The work is being done in collaboration with the Faculty of Medicine, University of Toronto.

8. Nutritional Epidemiology - Thyroid Cancer Study

This study is being done in collaboration with the Northern California Cancer Center, Stanford University and the University of California, San Francisco. Asian women who have immigrated to the U.S. west coast experience a significantly increased incidence of thyroid cancer which is hypothesized to be a consequence of their substantial change in diet. Specifically, iodine intake is substantially increased and may stress a hypersensitive thyroid --a condition observed in many of these subjects presumably due to chronically low iodine intakes prior to immigration. MURR is responsible for two specific aims: characterizing the nail as a biologic monitor for iodine; and to measure iodine in nails obtained in a nested case-control study of thyroid cancer. In these subjects, the use of iodine-containing contrast agents can confound analytical interpretations of any biologic monitor. From our work we have found that biologic monitors from subjects exposed to iodine-containing contrast agents will be influenced by the exposure for over a year. Consequently, these subjects should be screened out of nested case-control studies having an iodine hypothesis. The case-control comparison is blinded to our laboratory and will be evaluated beginning in 1999.

9. Nutritional Epidemiology - Non-Melanoma Skin Cancer Study

This study is being done in collaboration with Dartmouth Medical School. There is some evidence that higher than normal intakes of arsenic may increase the risk of non-melanoma skin cancer. This hypothesis is being studied in a New Hampshire population routinely ingesting comparatively high levels of arsenic in their drinking water supplies which are typically wells serving a single residence or just a few residences. We have shown that the arsenic concentration in nails is directly correlated with drinking-water arsenic levels. An NAA procedure to measure arsenic in nails has been developed and applied in this nested pseudo-prospective case-control study. Over 1000 samples have been analyzed over the last 2 years which is approximately the midpoint of the project. The case-control status will remain blind to our laboratory until the project is concluded.

10. Nutritional Epidemiology - Molecular Epidemiology of Prostate Cancer

In this study, being conducted in collaboration with Johns Hopkins University, School of Hygiene and Public Health, we are investigating the relationship of dietary and occupational exposures to selenium, cadmium and zinc and the incidence of prostate cancer. This is the most extensive study to date in which we have the opportunity to compare two biologic monitors, nails and blood sera or plasma, in a nested case-control study having a specific disease outcome.

11. Knock-Out Gene Mouse Model for Cystic Fibrosis

Mineral characterization studies with the knockout gene cystic fibrosis (CF) mouse model have been done in collaboration with the MU Department of Veterinary Biomedical Science. Bone and tooth mineralization differences between CF and normal mice were studied using NAA to measure Ca, Mg, P, Na, K, F, Cl, Br and Mn. NAA was also used to measure mineralization differences in the whole-body of the CF and normal mouse.

12. Calcium Metabolism Study

In support of the MU Department of Child Health we have studied calcium absorption in pig and lamb models using Ca-47 produced and processed at the MURR from an enriched Ca-46 target. In support of human studies with low birth-weight infants, juvenile rheumatoid arthritics and cystic fibrotics, a dual-enriched-isotope methodology, based on stable isotopes measured via NAA, has been developed to measure true absorption of calcium from experimental diets.

13. Geological science

Analysis of rock specimens by neutron activation analysis assists geochemists in research on the processes involved in the formation of different rocks through the analysis of the rare earth elements (REEs) and other trace elements. About thirty elements can be measured routinely in almost any geological sample. An additional 15-20 elements can be measured by applying specialized procedures. In addition to modeling geochemical processes, other applications include location of ore deposits and tracking elements of environmental importance. For example, the discovery of anomalously high iridium concentrations in 65-million-year old limestone deposits from Italy and Denmark could only have been accomplished by NAA. The NAA findings support the theory that extinction of the dinosaurs occurred soon after the impact of a large meteorite with the earth. For more information about this application for NAA, see the Archaeometry Laboratory at MURR. A recommended charge schedule for instrumental neutron activation analysis of archaeological and geological specimens is available (See Charge Schedule).

14. Semiconductor materials and other high-purity materials

Neutron Activation Analysis (NAA) is used to measure trace- and ultra trace-element concentrations of impurities and/or dopants in semiconductors and other high-purity materials. The behavior of semiconductor devices is strongly influenced by the presence of impurity elements either added intentionally (doping with B, P, As, Au, etc.) or contaminants remaining due to incomplete purification of the semiconductor material during device manufacture. Small quantities of impurities present at concentrations below 1 ppb can have a significant effect on the quality of semiconductor devices. The objective of the client is to demonstrate that a chemical or material meets or exceeds purity requirements required by the end user. In some instances the MURR Nuclear Analysis Group conducts a multi-element qualitative and quantitative analysis which then is used as the certificate of analysis for that substance. In other cases the NA staff work with clients to establish purification factors at various stages in the production process of high-purity materials such as silicon or, the efficacy of cleaning and leaching procedures. In still other cases we are ask to demonstrate that a specific impurity or set of impurities is below the level of technical or regulatory concern. NAA is the elemental analysis method of choice for these projects because of the limited sample handling required and the high sensitivity for many elements of interest. For more information about this application of NAA, see NAA of Semiconductor Materials.

15. Soil Science

Many agricultural processes and their consequences, such as fertilization and herbicidal and pesticidal control, are influenced by surface and sub-surface movement, percolation and infiltration of water. Stable activatable tracers, such as bromide, analyzed by NAA, have allowed the soil scientist to quantify the distribution of agricultural chemicals under a wide variety of environmental and land use influences. In soil science (ca. 1970), the use of bromide ion (Br-) in various forms (e.g. KBr, NaBr, SrBr2) was introduced as a non-reactive stable tracer in solute transport studies normally moving freely with the flux of water without substantial chemical or physical interactions with the soil. Typically, Br- is extracted from soil and quantified using either a bromide selective electrode (sensitivity is ~10 mg/mL) or by high-performance liquid chromatography (sensitivity is ~0.010 mg/mL). Where the sensitivity is adequate, the selective conductivity method, which is simple, affordable and fast, is preferred. More recently (ca. 1990), workers have reported that 20% of Br- tracers, at low groundwater pH, may be absorbed by iron oxides and kaolinite when present in the alluvial aquifer. We investigated the use of epithermal neutron activation analysis (ENAA) as a means of measuring Br- directly in soil samples without an extraction. ENAA was chosen because of its high theoretical advantage factor over aluminum (i.e. ~20), the principal interfering soil constituent, calculated for the 79Br(n,g)80Br reaction compared to 27Al(n,g)28Al. Br- was measured (sensitivity is ~0.050 mg/g) in one gram soil samples from a five-second irradiation (fth = 2.5 x 1012 n/cm2/sec) using a BN capsule.