|Central Region Geology|
|Irradiation only*||Lesser of $30 per sample or $250 per operating hour|
|Irradiation and analysis||Varies from $80 to $130 per sample, depending on analytical complexity.|
These prices are subject to change without notification. Call 303-236-4726 or send an email to firstname.lastname@example.org for details.
Tours of the GSTR are available to members of the general public as well as to high school, college, and university groups. Group size is limited to 25 persons. Tours normally last 60 - 90 minutes. Call 303-236-4726 or email email@example.com to schedule a tour.
This technique measures the spatial distribution of trace amounts of uranium in a variety of solid materials, at a microscopic scale. Fission of 235U is induced during slow neutron bombardment. Fission fragments emitted from the polished surface of a small sample are recorded as a visible image of fission tracks in a "detector" of mica that is placed over the sample. The density of recorded fission tracks is directly proportional to the uranium concentration in the sample. This low cost spatial-chemical measurement of U can only be duplicated by the most advanced microbeam analytical instruments. Recent applications include screening of suitable samples for dating by U-series or U-Pb methods, measuring U distribution in small particles of coal fly ash, and determining mineral hosts of U in polished thin sections of natural or contaminated rocks and soils. In some cases the distribution of U can be used as a proxy to suggest the residence of other trace elements of environmental concern such as radium, or arsenic.
This is the science that deals with measuring past time and assigns
events to their proper dates based on geologic data.
40Ar / 39 Ar Geochronology
The 40Ar / 39Ar isotopic dating technique is a variant of the conventional K-Ar method and is based on the formulation of 39Ar during irradiation of potassium-bearing samples. It is used to date terrestrial rocks and minerals as well as meteorites and lunar samples ranging in age from approximately 30,000 years to the age of the Solar System (4.65 billion years). The method is derived from the natural occurrence of the radioactive isotope of potassium, 40K, which has a dual decay to 40Ca and 40Ar and a half-life of 1250 million years. Radiogenic 40Ar ideally accumulates in a mineral over geologic time. So by irradiating a sample of unknown age with a standard of known age and then measuring the abundances of argon isotopes, we can determine an 40Ar / 39Ar date, that is, the sample's geologic age. As a result of irradiation, 39Ar serves as a proxy for potassium since it is produced from 39K by fast neutron bombardment. After irradiation, argon isotopes are extracted from samples and standards and separately measured on a gas-source mass spectrometer. Apparent ages for the samples then are calculated by comparing the sample's 40Ar / 39Ar ratio with that of the standard. Isotopic measurements on modern mass spectrometers are highly sensitive and precise. Thus, very small amounts of material, ranging in size from a single mineral grain to a few millimeters, are analyzed commonly with small associated analytical errors (less than 0.25% absolute).
40Ar / 39Ar geochronology has evolved rapidly over the past 25 years into one of the most commonly used isotopic dating techniques because of its applicability to a broad range of geologic problems. At the U.S Geological Survey, 40Ar / 39Ar geochronology is used primarily for mission-related scientific research funded through congressionally mandated programs. Currently, the method is applied to geologic studies on the origin and thermal histories of mineral deposits; emplacement, cooling, and uplift history of plutonic rocks; formation of metamorphic belts; development of volcanic terranes, formation and amalgamation of the Earth's crust; age and development of the landscape; and the timing of catastrophic events in earth history, such as the K/T boundary event. Argon laboratories have hosted more than 200 guest investigators from other agencies, academia, and other countries.
Neutron activation analysis (NAA) is an analytical technique that relies on the measurement of gamma-rays emitted from a sample that was irradiated by neutrons. The rate at which gamma-rays are emitted from an element in a sample is directly proportional to the concentration of that element. The major advantages of NAA are that:
The process for analyzing samples by NAA involves irradiating them with a neutron source. Neutrons are captured by elements in the sample to produce unstable radioactive isotopes (radio-nuclides). Beta particles, and in some cases gamma-rays, are emitted from the radio-nuclides as they decay. The energies of these gamma-rays are, in general, distinct for a specific nuclide and the rate at which these photons are emitted with a particular energy can be measured using high-resolution semiconductor detectors. Because the production and decay rate of gamma-ray radiation are dependent on the half-life of the nuclide, elemental measurements can be optimized by varying the irradiation and the decay times (i.e., how long the sample is near a neutron source and when the sample is counted).
The most common procedure for NAA involves encapsulating the samples and suitable standards in heat-sealed polyethylene or quartz vials and simultaneously irradiating them. Ideally, the samples are irradiated in a "lazy susan" facility that revolves around the core thereby insuring that the samples and standards experience the same neutron fluence. Following sequential decay periods, each standard sample is counted utilizing high resolution germanium detectors coupled to a multi-channel analyzer system. Such counting of samples over sequential decay periods optimizes the determination of 35 to 50 elements in various types of samples. The analyzer system converts the signals that result from gamma-ray photons impinging the detector into digital electronic pulses. Gamma-ray counts accumulated in an energy region above the background counts produce photo peaks. After counting is complete, these data are processed using sophisticated computer programs that smooth the spectral data and determine the net areas of gamma-ray photo peaks and translates the area into count rates (counts per minute or cpm). These programs are capable of resolving overlapping and complex photopeak energy regions. Additional data for decay time differences, electronic dead time losses and unresolved interferences and compares the sample data (cpm/weight) to the standard data (cpm/ug) to calculate elemental abundance in the sample.
The principal error in the analysis of materials by NAA is the counting statistic error, which is based on the signal to background ratio at the gamma-ray energy region of interest. A one sigma error for a photo peak area determination is approximately equal to the square root of the total counts (background plus net counts) divided by the net counts. For example, a peak area determination with 2000 total counts and a net area of 1000 counts produces a counting error of about 4.5%.
An additional source of error for some elemental determinations is due to unresolved interferences. The most serious interferences are those that result when identical radio nuclides are produced from different nuclear reactions. For example, high concentrations of U fission products can interfere with the accurate determination of La, Ce, Nd, Mo, and Zr abundances. In addition, the determinations of some elements can suffer because the photo peaks produced by gamma-rays emitted by two or more radio nuclides is not readily resolved. In these cases, empirical corrections are integrated into the data reduction procedure.
Self-shielding is a phenomenon that occurs when the neutron flux experienced by a sample is attenuated, thereby reducing the neutron activation of the nuclides in self-shielded samples. This effect occurs when a sample contains extremely high concentrations of elements with very high probabilities that a neutron will be captured, which is measured in terms of a cross section.
Elemental detection limits for NAA are variable because the production of radioactive nuclides depends on the cross section of a specific element. Also important is the number of gamma-rays that are emitted by a radio nuclide. In some cases, only a small fraction of the total emissions from a specific nuclide is in the form of gamma-rays. In general, elements that have large cross sections will have low detection limits. Detection limits are also matrix dependent. Samples with high concentrations of elements that are readily activated and emit a considerable number of gamma-rays, such as Na and Sc, can generate high background count rates and raise the detection limits.
Neutron activation analysis remains at the forefront of techniques for the quantitative multi-element analyses of major, minor and trace elements in hundreds of different types of materials. The following list includes some of the materials that can be readily analyzed by NAA:
Instrumental Neutron activation analysis (INAA) is a non-destructive, highly precise and accurate analytical technique capable of determining up to 48 elements in almost all types of sample matrices. The INAA procedure involves irradiating samples and appropriate standard reference materials with neutrons in the GSTR to produce unstable radioactive nuclides. Many of these radio nuclides emit gamma-rays with characteristic energies that can be measured utilizing high-resolution semiconductor detectors. The rate that the gamma-rays are emitted from an element in the sample is directly proportional to its concentration. Samples as small as 1 mg can be quantitatively measured by INAA. Detection limits are in the parts per million to parts per billion range depending on the element and sample matrix.
|Elements and Detection Limits (ppm)|
|Al, 25||Au, .001||Ag, 1.0||As, .1||Ba, 4.3||Br, 0.3||Ca, 3000||Cd, 2|
|Co, .02||Cr, .5||Cs, .01||Cu, 500||Fe, 20||Hf, .02||Hg, 1.0||Ir, .002|
|K, 200||Mg, 5000||Mo, 3||Na, 3.1||Ni, 1.0||Rb, .65||S, 2500||Sb, .035|
|Sc, .0015||Se, 0.2||Sn, 100||Sr, 8.1||Ta, .002||Th, .05||Ti, 500||U, .05|
|V, 10||W, .25||Zn, .5||La, .02||Ce, .2||Nd, .45||Sm, .002||Eu, .008|
|Gd, .15||Tb, .06||Dy, 1||Ho, .1||Tm, .01||Yb, .02||Lu, .002|
Form: Any (Fuel and oil samples are evaporated to dryness)
Precision: Varies, typically 1-5%
Instruments: Canberra GENIE-ESP, ND6600, ND76 multichannel analyzers; 6 Ge(Li) high-energy semiconductor and 6 low-energy photon (LEP) detectors.
Costs: Varies from $80 to $130 per sample, depending on analytical complexity.
Sample Size: About 100 mg is preferred, but samples as small as 1 mg are acceptable.
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