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PGAA is a very widely applicable technique for determining the presence and amount of many elements simultaneously in samples ranging in size from micrograms to many grams. It is a non- destructive method, and the chemical form and shape of the sample are relatively unimportant. Typical measurements take from a few minutes to several hours per sample.

The technique can be described as follows. The sample is continuously irradiated with a beam of neutrons. The constituent elements of the sample absorb some of these neutrons and emit prompt gamma rays which are measured with a high-resolution gamma-ray spectrometer. The energies of these gamma rays identify the neutron-capturing elements, while the intensities of the peaks at these energies reveal their concentrations. The amount of analyte element is given by the ratio of count rate of the characteristic peak in the sample to the rate in a known mass of the appropriate elemental standard irradiated under the same conditions. Typically, the sample will not acquire significant long-lived radioactivity, and the sample may be removed from the facility and used for other purposes.

Design of the facility has focused on three related factors. First, the high quality of the neutron beam and the low background in the guide hall will allow closer sample-detector spacing, resulting in higher counting efficiency and better sensitivity, especially in the energy region below 1 MeV. Second, the high count rates possible with this high efficiency (greater than 50k counts per sec) can be measured without loss of quality with recent advances in instrumentation. Finally, the improved efficiency will make attractive the use of gamma-gammaand gamma-conversion-electron coincidence counting in analytical measurements, with considerably improved specificity.

Structural and shielding materials for this and neighboring instruments have been chosen to avoid generating a background of capture and decay gamma rays. Hydrogenous absorbers are avoided. The section of the beam tube adjacent to the sample position is made of boron-free glass. Li-6 is used wherever possible for collimators and absorbers, and antimony-free lead is used for gamma shielding. As a result, the sensitivity for most elements is at least tenfold better than with any thermal beam in existence. The detection limit for hydrogen is less than 2 micrograms. Detection limits to be attainable with the new cold source for selected elements are given in the following table. The list is not exhaustive; most elements can be detected and quantified by PGAA.

Limits of detection for 1 g sample counted for 24 hours, after installation of liquid H2 cold source.

    Range (micrograms)---Elements
  • 0.01 - 0.1----------B, Cd, Sm, Gd
  • 0.1 - 1-------------Eu, Hg
  • 1 - 10--------------H, Cl, In, Nd
  • 10 - 100------------Na, S, K, Sc, Ti, V, Cr, Mn, Co, Ni,
    ------------------------Cu, Ge, As, Se, Br, Mo, Ag, Te, I, Au
  • 100 - 1000----------Mg, Al, Si, P, Ca, Fe, Zn, Ga, Rb, Sr,
    ------------------------Y, Zr, Nb, Sb, Ba, La
  • 1000 - 10000--------C, N, F. Sn, Pb
Other instrument characteristics:
  • Location: Neutron guide NG-7
  • Max beam area and sample size: 50 mm x 50 mm
  • Collimation: Li-6 glass
    Full-time access to beam.
    Beam filtered through Bi and Be at 77 K.
  • Approximate reaction rate per atom at sample position: 8 x 108 sigma, where sigma is the thermal neutron cross section in sq. cm.
  • Gamma detector: High-resolution Ge, designed for high-rate acquisition; BGO Compton suppressor
The apparatus is controlled and data are acquired through an Ethernet-based ADC-MCA and workstation. An automatic sample changer will be added in the future. The counting system is designed to handle multiple gamma detectors with Compton suppression. Future modifications will include multiparameter coincidence counting.

Contact: R. Lindstrom (301) 975-6281; R. Paul (301) 975-6287
Hydrogen Peak in K3C60
Determination of hydrogen content in a sample of K3C60, potassium fulleride, used in a neutron scattering experiment. Small amounts of hydrogenous solvents, which are used in the purification of C60 and are very difficult to remove completely, were a major concern because of the large scattering cross section of hydrogen. The intensity of the strong peak at 2223 keV in the lower scan implies the peak implies the presence of 250 micrograms of hydrogen. After a drying treatment in which the sample was heated in vacuum, the hydrogen content has been reduced to 65 micrograms. The "after-drying" spectrum has been displaced upward for clarity.

Last modified 15-October-2001 by website owner: NCNR (attn: )