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Neutron Activation Analysis (NAA)

Methods

• USGS INAA Procedure. Sample aliquants weighing ~0.5 g are irradiated for six hours in the TRIGA reactor at the USGS, Denver, Colorado, at a flux of 2.5×10¹² n/cm²/s. Standards consisting of spiked silicate powders are irradiated together with the samples. Following a decay period of 6-8 days, the samples and standards are counted for one hour on a pair of high purity germanium (HPGe) detectors, one coaxial and one planar, coupled to a multichannel analyzer system. After an additional 26-32 day decay period, the samples and standards are counted for two hours on a coaxial HPGe detector. The coaxial HPGe detectors have resolutions of about 1.9 KeV measured at 1.33 MeV, and efficiencies between 18 and 25%. The planar HPGe detector has a resolution of 0.80 KeV measured at 122 KeV. Gamma-ray peaks are integrated using the SAMPO program. Corrections for dead time, decay during counting, and spectral interferences are made using an in-house data reduction program to calculate elemental concentrations. Samples of SAR-L, SAR-M (USGS analytical standards) and/or NIST2710 are included in each irradiation as quality control standards. Details of the procedure may be found in Budahn and Wandless (2002).

• Becquerel INAA Procedure. 10-15 g sample aliquants are added to tared, plastic, watertight vials. Each vial has a flux monitor affixed to its base. The vials are stacked into one-foot long bundles for irradiation. The bundles contain randomly selected duplicate samples at the base of the bundle and standards inserted at random positions. Bundles are irradiated for 20 minutes in the core of the McMaster Nuclear Reactor at a flux of 8×10¹² n/cm²/s. In the RIFLS (Reactor Irradiation Facility for Large Samples) sites, the bundles are rotated during irradiation so that there is no horizontal flux variation. Vertical flux variation is monitored with the individual flux monitors. After a decay period of six days, the irradiated samples are loaded onto the counting system. High resolution, coaxial germanium detectors are used. The counting time is twenty to thirty minutes per sample. After an additional decay period of at least ten days, the samples are recounted for one hour. Commercially available library standards are used to quantify the analyses.

  Approximate detection limits (ppm) for elements analyzed by INAA at Becquerel Labs.

  Element Approx. Det. limit Na 200 Sc 0.2 Cr 20 Fe 2000 Co 5 Ni 10 Zn 100 As 0.5 Br 0.5 Rb 5 Zr 100 Mo 1 Sb 0.1 Cs 0.5 Ba 50

  Element Approx. Det. limit La 2 Ce 2 Nd 10 Sm 0.1 Eu 0.5 Tb 0.5 Yb 2 Lu 0.1 Hf 1 Ta 0.5 W 1 Au 0.002 Th 0.2 U 0.2

• Delayed Neutron Counting. ~10 g of powdered sample is placed in a small "rabbit" capsule, and irradiated in a Cd-lined (fast neutron) position in the USGS TRIGA reactor for 1 minute. The sample is counted in a neutron counter for 5 s after 5 s decay time, and then for 60 s after 20 s decay time. The sample is then irradiated a second time in an unshielded (thermal neutron) position, and recounted using the same scheme. Data are reduced as described in detail by McKown and Millard (1987) and Knight and McKown (2002), using doped rock standards for calibration. The same references provide a complete discussion of accuracy and precision.

  Data analysis

  Distribution maps Element QA/QC Conter- minous Alaska Hawai`i Na (wt%) In progress no data Sc (ppm) In progress no data Cr (ppm) In progress no data Fe (wt%) In progress no data Co (ppm) In progress no data Ni (ppm) In progress no data Zn (ppm) In progress no data As (ppm) In progress no data Br (ppm) In progress no data Rb (ppm) In progress no data Sr (ppm) In progress no data no data Zr (ppm) In progress no data Mo (ppm) In progress no data Sb (ppm) In progress no data Cs (ppm) In progress no data

  Distribution maps Element QA/QC Conter- minous Alaska Hawai`i Ba (ppm) In progress no data La (ppm) In progress no data Ce (ppm) In progress no data Nd (ppm) In progress no data Sm (ppm) In progress no data Eu (ppm) In progress no data Tb (ppm) In progress no data Yb (ppm) In progress no data Lu (ppm) In progress no data Hf (ppm) In progress no data Ta (ppm) In progress no data W (ppm) In progress no data Au (ppb) In progress no data Th (ppm) In progress no data U (ppm) In progress no data

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  U.S. Department of the Interior, U.S. Geological Survey
  This page is part of U.S. Geological Survey Open-File Report 2004-1001
  URL: http://www-tin.er.usgs.gov/geochem/doc/inaa.htm
  Maintained by Jeff Grossman
  Last modified: 15:03:11 Tue 06 Jan 2004
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