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Prompt Gamma-Ray Activation Analysis (PGAA)

Cold Neutron PGAA
The use of lower energy "cold" neutrons rather than thermal neutrons ( 0.025 eV) improves PGAA detection limits and sensitivities. The values for neutron absorption cross-section (sa) for most elements increase with decreasing neutron energy so that the use of lower energy cold neutron increased the reaction rate for most elements. In addition, the cold neutron beams at the NIST Center for Neutron Research are guided meters away from the neutron source to a region of lower gamma-ray background resulting in improved detection in the low energy region of the spectrum.
The CNPGAA instrument was designed to minimize background and optimize detection of H and B. Structural and shielding materials for this and neighboring instruments were chosen to avoid generating a background of capture and decay gamma rays. Hydrogenous absorbers were avoided, where possible. 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 the NIST thermal beam facility and higher than anywhere else in the world. Limits of detection of CNPGAA for several elements are listed in Table 1.
Table 1. Limits of detection for 1 g sample counted for 24 hours, using CNPGAA with the liquid hydrogen cold source.
Table 1
Analysis of Hydrogenous Materials:
The CNPGAA is optimized for analysis of trace amounts of hydrogen. The detection limit for hydrogen is less than 10 micrograms for most materials. However, in most materials that contain percent levels of hydrogen, neutron scattering by hydrogen decreases sensitivities for CNPGAA but increases those for TNPGAA.
Richard Lindstrom, 301-975-6281,
R. Paul, 301-975-6287,
NCNR Cold Neutron Instruments

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