Neutron activation analysis (NAA) is the
nuclear
Nuclear may refer to:
Physics
Relating to the nucleus of the atom:
* Nuclear engineering
*Nuclear physics
*Nuclear power
*Nuclear reactor
*Nuclear weapon
*Nuclear medicine
*Radiation therapy
*Nuclear warfare
Mathematics
*Nuclear space
*Nuclear ...
process used for determining the concentrations of
elements in many materials. NAA allows discrete
sampling of elements as it disregards the chemical form of a sample, and focuses solely on atomic nuclei. The method is based on
neutron activation
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emit ...
and thus requires a
source
Source may refer to:
Research
* Historical document
* Historical source
* Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence
* Source (journalism), a person, publication, publishing institute o ...
of
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s. The sample is bombarded with neutrons, causing its constituent elements to form radioactive isotopes. The
radioactive emissions and radioactive decay paths for each element have long been studied and determined. Using this information, it is possible to study
spectra of the emissions of the radioactive sample, and determine the concentrations of the various elements within it. A particular advantage of this technique is that it does not destroy the sample, and thus has been used for the analysis of works of art and historical artifacts. NAA can also be used to determine the
activity of a radioactive sample.
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 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
Inductively coupled plasma atomic emission spectroscopy (ICP-AES), also referred to as inductively coupled plasma optical emission spectroscopy (ICP-OES), is an analytical technique used for the detection of chemical elements. It is a type of emiss ...
and
PIXE
Particle-induced X-ray emission or proton-induced X-ray emission (PIXE) is a technique used for determining the elemental composition of a material or a sample. When a material is exposed to an ion beam, atomic interactions occur that give off E ...
, NAA was the standard analytical method for performing multi-element analyses with minimum detection limits in the sub-
ppm range.
Accuracy
Accuracy and precision are two measures of ''observational error''.
''Accuracy'' is how close a given set of measurements ( observations or readings) are to their ''true value'', while ''precision'' is how close the measurements are to each oth ...
of NAA is in the region of 5%, and relative
precision
Precision, precise or precisely may refer to:
Science, and technology, and mathematics Mathematics and computing (general)
* Accuracy and precision, measurement deviation from true value and its scatter
* Significant figures, the number of digit ...
is often better than 0.1%.
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.
Overview
Neutron activation analysis is a sensitive multi-
element analytical technique used for both
qualitative and
quantitative
Quantitative may refer to:
* Quantitative research, scientific investigation of quantitative properties
* Quantitative analysis (disambiguation)
* Quantitative verse, a metrical system in poetry
* Statistics, also known as quantitative analysis ...
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
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
became highly
radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
after exposure to a source of
neutrons.
[Overview of NAA](_blank)
/ref> 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
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
decay with emission of particles or, more importantly gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s, 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 50 mg (one-twentieth of a gram
The gram (originally gramme; SI unit symbol g) is a Physical unit, unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Originally defined as of 1795 as "the absolute weight of a volume of pure wate ...
) is a sufficient sample, so damage to the object is minimised. 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
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including bo ...
or quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
. 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
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
. A typical reactor used for activation uses uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
fission, providing a high neutron flux and the highest available sensitivities for most elements. The neutron flux from such a reactor is in the order of 1012 neutrons cm−2 s−1.[Pollard, A. M., Heron, C., 1996, ''Archaeological Chemistry''. Cambridge, Royal Society of Chemistry.] The type of neutrons generated are of relatively low kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
(KE), typically less than 0.5 eV. 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
In physics and chemistry, binding energy is the smallest amount of energy required to remove a particle from a system of particles or to disassemble a system of particles into individual parts. In the former meaning the term is predominantly use ...
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. 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 the 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.
Variations
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 <0.5 eV. Medium KE neutrons may also be used for activation, these neutrons have been only partially moderated and have KE of 0.5 eV to 0.5 MeV, 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 >0.5 MeV. 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, DGNAA). 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. 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 isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
s; or elements with weak decay gamma ray intensities. 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. DGNAA is characterised by long irradiation times and long decay times, often in the order of hours, weeks or longer.
Neutron sources
A range of different sources can be used:
* A nuclear reactor
* An actinoid
The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The inform ...
such as californium
Californium is a radioactive chemical element with the symbol Cf and atomic number 98. The element was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding ...
which emits neutrons through spontaneous fission
* An alpha source such as radium
Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rather t ...
or americium
Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was na ...
, mixed with beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...
; this generates neutrons by a (α,12C+n) reaction
* A D-T fusion reaction in a gas discharge tube
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric d ...
Reactors
Some reactors are used for the neutron irradiation of samples for radioisotope
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
production for a range of purposes. The sample can be placed in an irradiation container which is then placed in the reactor; if epithermal neutrons are required for the irradiation then cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
can be used to filter out the thermal neutrons.
Fusors
A relatively simple Farnsworth–Hirsch fusor can be used to generate neutrons for NAA experiments. The advantages of this kind of apparatus is that it is compact, often benchtop-sized, and that it can simply be turned off and on. A disadvantage is that this type of source will not produce the neutron flux that can be obtained using a reactor.
Isotope sources
For many workers in the field a reactor is an item which is too expensive, instead it is common to use a neutron source which uses a combination of an alpha emitter and beryllium. These sources tend to be much weaker than reactors.
Gas discharge tubes
These can be used to create pulses of neutrons, they have been used for some activation work where the decay of the target isotope is very rapid. For instance in oil wells.
Detectors
There are a number of detector types and configurations used in NAA. Most are designed to detect the emitted gamma radiation
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...
. The most common types of gamma detectors encountered in NAA are the gas ionisation type, scintillation type and the semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
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.
Scintillation-type detectors use a radiation-sensitive crystal, most commonly thallium-doped sodium iodide (NaI(Tl)), 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
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...
. The germanium is processed to form a p-i-n (positive-intrinsic-negative) diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
, and when cooled to ~77 K by liquid nitrogen
Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
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
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
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
In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass ...
region ruining the detector. The development of undrifted high purity germanium has overcome this problem.
Particle detectors can also be used to detect the emission of alpha
Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whic ...
(α) and beta
Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...
(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
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
conditions to be effectively detected. Gamma rays, however, are not absorbed or attenuated by atmospheric gases, and can also escape from deep within the sample with minimal absorption.
Analytical capabilities
NAA can detect up to 74 elements depending upon the experimental procedure, with minimum detection limits ranging from 0.1 to 1x106 ng 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.
Applications
Neutron Activation Analysis has a wide variety of applications including within the fields of archaeology
Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscap ...
, soil science
Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to th ...
, geology
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
, forensics
Forensic science, also known as criminalistics, is the application of science to criminal and civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standards of admissible evidence and crimina ...
, and the semiconductor industry
The semiconductor industry is the aggregate of companies engaged in the design and fabrication of semiconductors and semiconductor devices, such as transistors and integrated circuits. It formed around 1960, once the fabrication of semiconduct ...
. Forensically, hairs subjected to a detailed forensic neutron analysis to determine whether they had sourced from the same individuals was first used in the trial of John Norman Collins
The Michigan Murders was a series of highly publicized killings of young women committed between 1967 and 1969 in the Ann Arbor/ Ypsilanti area of Southeastern Michigan by an individual known as the Ypsilanti Ripper, the Michigan Murderer, and t ...
.
Archaeologists use NAA in order to determine the elements that comprise certain artifacts. This technique is used because it is nondestructive and it can relate an artifact to its source by its chemical signature. This method has proven to be very successful at determining trade routes, particularly for obsidian, with the ability of NAA to distinguish between chemical compositions. In agricultural processes, the movement of fertilizers and pesticides is influenced by surface and subsurface movement as it infiltrates the water supplies. In order to track the distribution of the fertilizers and pesticides, bromide ions in various forms are used as tracers that move freely with the flow of water while having minimal interaction with the soil. Neutron activation analysis is used to measure bromide so that extraction is not necessary for analysis. NAA is used in geology to aid in researching the processes that formed the rocks through the analysis of the rare-earth elements and trace elements. It also assists in locating ore deposits and tracking certain elements. Neutron activation analysis is also used to create standards in the semiconductor industry. Semiconductors require a high level of purity, with contamination significantly reducing the quality of the semiconductor. NAA is used to detect trace impurities and establish contamination standards, because it involves limited sample handling and high sensitivity.
Applications of NAA
/ref>
See also
* High Flux Isotope Reactor
The High Flux Isotope Reactor (HFIR) is a nuclear research reactor at Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, United States. Operating at 85 MW, HFIR is one of the highest flux reactor-based sources of neutrons for conden ...
(HFIR) at Oak Ridge National Labs has NAA capabilities.
* Neutron flux
The neutron flux, φ, is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total length travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travellin ...
* Neutron howitzer
References
{{DEFAULTSORT:Neutron Activation Analysis
Analytical chemistry
Scientific techniques