PGAA method developement and applications
Standardization
PGAA method can be performed without the use of standards. However the spectroscopic data have to be standardized in advance. One of our major projects has been the standardization of all the elements. Each element, occurring in nature, was irradiated in elemental form (or as a simple compound, where the other elements are almost invisible to neutrons) and in the forms of stoichiometric compounds. (If no appropriate compound was available, then homogeneous mixtures were prepared.) The characteristic peaks were identified using the elemental spectra, their intensities were determined using internal standardization from the peak areas obtained from the spectra of compounds or mixtures. Using the accurate counting efficiency function and the compositions of the samples, the partial gamma-ray production cross-sections were determined relative to that of the comparator.
See also: Révay, Z. and G. L. Molnár: Standardisation of the prompt gamma activation analysis method. Radiochimica Acta 91 (2003) 361-369.
PGAA Analytical Library
The spectroscopic data library is in continuous improvement since 1997. The energies of the characteristic peaks are determined using the energy difference method, irradiating the element of interest in the presence of another element with well-known energies (typically chlorine, whose prompt gamma lines were measured at the crystal spectrometer at ILL). The partial gamma-ray production cross-sections of the major peaks for each element were determined using internal standardization. The library is available in the Handbook of PGAA with neutron beams.
Quantitative analysis
The determination of the composition is based on the assumption that all the elements in the sample appear in the prompt gamma spectrum. The mass ratios of the elements are calculated from the peak area ratios, the efficiency ratios and the ratios of the partial gamma-ray production cross-section, taken from the library, measured at the Budapest PGAA facility. The actual concentrations are determined as weighted averages from several lines.
Applications in analytical chemistry, safeguards and material science
- Identification and characterization of fissile materials in containers. (The project is supported by the Hungarian Atomic Energy Authority and the IAEA.) Slow neutrons deeply penetrate into most materials, thus several centimeters of the most common shielding material still transmit significant fraction of the neutron beam. Though the internal flux is not exactly known inside the shielding containers, the samples with high cross-section, hidden in the shielding, still give strong gamma signals. Fissile materials always show a characteristic shape of gamma spectrum. This is true for both, prompt and also for the decay spectra. This latter one can be measured in a chopped neutron beam, while the decay spectrum is collected during the beam-off phases. Chopped beam measurements make possible the investigation of nuclear materials in shielding with higher cross-section as well, since in the decay phase the decay gammas from the fissile material will give a much stronger signal than the container itself. Based on the characteristic shape fissile materials can easily be identified in many commonly used shielding material, lead, iron etc. The characteristic gamma lines from uranium isotopes and from fission products enable the determination of the enrichment of the uranium sample. The mass of the hidden material can also be estimated.
- Investigation of soil and meteoritic samples from paleo-Indian excavation sites of North America. (Collaboration with Richard B. Firestone, Lawrence Berkeley National Laboratory.) Elemental compositions of soil and iron-nickel meteoritic samples were determined with PGAA method. The results seem to confirm a new interesting theory: the impact of a gigantic meteorite in North-America approximately 13,000 years ago, which might have caused the extinction of several animal species.
- Investigation of fullerene samples. (Collaboration with Tibor Braun, ELTE University.) C60 and C70 fullerene samples were measured with PGAA to determine the minor constituents besides carbon. The identified elements (H, S, Cl) provided information on the separation and purification techniques of these materials.
