In-situ PGAA for catalysis

Alkynes can be selectively hydrogenated into alkenes on solid palladium catalysts. This process requires a strong modification of the near-surface region of palladium, in which carbon (from fragmented feed molecules) occupies interstitial lattice sites. In situ X-ray photoelectron spectroscopic measurements under reaction conditions indicated that much less carbon was dissolved in palladium during unselective, total hydrogenation. Additional studies of hydrogen content using in situ prompt gamma activation analysis, which allowed us to follow the hydrogen content of palladium during catalysis, indicated that unselective hydrogenation proceeds on hydrogen-saturated β-hydride, whereas selective hydrogenation was only possible after decoupling bulk properties from the surface events. Thus, the population of subsurface sites of palladium, by either hydrogen or carbon, governs the hydrogenation events on the surface. See also:

  • Detre Teschner, János Borsodi, Attila Wootsch, Zsolt Révay, Michael Hävecker, Axel Knop-Gericke, S. David Jackson, Robert Schlögl: The Roles of Subsurface Carbon and Hydrogen in Palladium-Catalyzed Alkyne Hydrogenation, Science, 320, 86-89.
  • Detre Teschner, Zsolt Révay, János Borsodi, Michael Hävecker, Axel Knop-Gericke, Robert Schlögl, David Milroy, S. David Jackson, Daniel Torres, Philippe Sautet: Understanding Palladium Hydrogenation Catalysts: When the Nature of the Reactive Molecule Controls the Nature of the Catalyst Active Phase, Angewandte Chemie International Edition Volume 47, Issue 48, Date: November 17, 2008, Pages: 9274-9278
  • Zs. Révay, T. Belgya, L. Szentmiklósi, Z. Kis, A. Wootsch, D. Teschner, M. Swoboda, and R. Schlögl, J. Borsodi, R. Zepernick, Anal. Chem. 80 (15) (2008) 6066-6071

New method to determine the spatial distribution of elements in the sample

A new technique is being developed to determine the distribution of elements in inhomogenous samples, first of all in archaeological ones. The method is called Prompt Gamma Activation Imaging (PGAI). If both the neutron beam and the gamma-detection are collimated, the analytical information is originating from a small volume of the sample. If the sample is scanned with the beam, and multiple spectra are recorded, the spatial elemental compositions could be reconstructed. The method is combined with neutron tomography to have high-resolution visualization of the sample. Some simple test objects and genuine archaeological findings have been investigated so far with success.

Visit the local homepage of the project ANCIENT CHARM for more details.

New high-energy efficiency data for detector calibration

The 14N(n, γ)15N reaction is a primary source of high-energy gamma-rays for use in the calibration of detectors for other neutron-capture reactions. The gamma-ray intensities of 15N produced by thermal neutron capture and the gamma-ray detection efficiency function have been simultaneously determined from gamma-peak areas alone using the basic principle of intensity balance. A least-squares fit was made to a new type of intensity balance calculation, combined with traditional efficiency fitting of radioactive sources. This latter ensures the compatibility with low-energy efficiencies, while providing an unbiased efficiency function for higher (up to 10 MeV) gamma-ray energies. The calculation is based on the assumption that the 15N decay scheme is complete. From the internal consistency of the resulting intensities, it is believed that they are more accurate than previously published values.

See also: Improved accuracy of gamma-ray intensities from basic principles for the calibration reaction 14N(n, gamma)15N, by T. Belgya, PHYSICAL REVIEW C 74, 024603 (2006). View the complete article

Upgraded PGAA analytical library

The main obstacle to the propagation of the PGAA method has been the lack of a suitable analytical library. The major reason for this is the large number of peaks appearing in the prompt gamma spectra of most elements, making the peak (hence element) identification rather difficult. The standardisation of the method means the determination of the relevant nuclear constants. A long series of PGAA measurements of almost all the elements are performed at the Budapest Research Reactor, to establish a new analytical data library that can be used in any laboratory for the determination of elemental compositions at the precision level of other analytical methods. The new database (being published in this paper series) contains all gamma peaks that are essential from the point of view of chemical analysis. The energy values and the relative intensities can be used to identify the elements and resolve the spectral interference from other elements.

See also: Standardisation of the prompt gamma activation analysis method, by Zs. Revay and G.L. Molnar, Radiochim. Acta 91, 361-369 (2003)

Uncertainty budget of the PGAA analysis

A complete uncertainty calculation is derived for prompt gamma activation analysis (PGAA). The uncertainty budget includes the calculation of the efficiency, the partial gamma-ray production cross-section and the component mass. It was shown that using the relative method of PGAA, the uncertainty values can be significantly reduced, especially taking into consideration the correlation between the efficiencies at close-lying energies. A method is proposed for the calculation of uncertainties of concentrations from those of component masses. The procedure is shown in an example and recommendations are given for keeping the uncertainties and error propagation of results originating from different laboratories under control.

See also: Calculation of uncertainties in prompt gamma activation analysis, by Zs. Revay, Nuclear Instruments and Methods in Physics Research A 564 (2006) 688-697

Spectrum unfolding

The response function of the detector can significantly differ from the true gamma-ray spectrum emitted by the measured sample due to the numerous interactions between photons and the detector material. Besides the full energy peak, a background related to pair production and Compton scattering events also contribute to the measured spectra. The single and double escape peaks caused by pair production appear at 511 keV and 1022 keV lower energies than the full energy peak. A continuous background can also be observed due to single and multiple Compton scattering events. The total Compton background is the sum of backgrounds corresponding to the different peaks. The annihilation peak appears at 511 keV, and the backscattering peak occurs at around 200 keV energy. The response function of the HPGe detector is determined by applying Monte Carlo simulations of certain monoenergetic gamma spectra. The response functions at all of the different energies are then determined from the spectra obtained by the simulations using a spectrum interpolation method.