Journal of Radioanalytical and Nuclear Chemistry, Vol. 271, No.2 (2007) 345–351 0236–5731/USD 20.00 Akadémiai Kiadó, Budapest © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht INAA with Compton suppression: How much can the analysis of plant materials be improved? M. A. Bacchi, 1 * L. G. C. Santos, 1 E. A. De Nadai Fernandes, 1 P. Bode, 2 F. S. Tagliaferro, 1 E. J. França 1 1 Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Caixa Postal 96, 13400-970 Piracicaba, Brazil 2 Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands (Received April 13, 2006) The effectiveness of a Compton suppression system (CSS) for instrumental neutron activation analysis of plant materials was evaluated. Suppression factors were measured with 137 Cs sources. Five certified reference materials were analyzed and the detection limits calculated from both suppressed and unsuppressed spectra were compared. The CSS demonstrated to be useful for lowering the detection limits of ten out of sixteen elements tested, showing a maximum improvement factor of 3.9. The system performance was strongly influenced by the sample composition and also by the measurement conditions, indicating the importance of testing each individual sample type and analytical protocol. Introduction Instrumental neutron activation analysis (INAA) is known for being nearly independent on effects arising from the sample matrix. However, the sample composition still has some influence on the analytical results, since the degree of accuracy can be affected by interfering reactions or spectral interferences. Furthermore, the sample composition can also lead to acquisition of γ-ray spectra with peaks on a high background, increasing the uncertainty of peak areas and causing negative impact on the precision of results and on the detection limits. The Compton scattering of γ-rays in the detector is often the most important source of the background under low energy γ-ray peaks if measuring in the presence of high energy γ-rays. For such a case, the use of anti-Compton techniques can significantly reduce the background, improve the detection limits and also lower the measurement uncertainty. The performance of a Compton suppression system (CSS) is usually characterized by means of suppression factors determined from spectra of either 137 Cs or 60 Co sources. The suppression factor is defined as the ratio of the peak-to-Compton ratios for suppressed and unsuppressed spectra. The effectiveness of a CSS in reducing the γ-ray spectrum background depends first of all on the equipment configuration itself and on the radionuclide being measured. Nevertheless, the source– detector arrangement and the source composition, i.e., the presence of other radionuclides, are also factors affecting the effectiveness of the equipment for a specific measurement. Consequently, it is difficult to estimate the improvement obtained for the determination of specific elements in a real sample, if only the suppression factor for single radionuclide sources is measured. * E-mail: mabacchi@cena.usp.br Since the first developments of anti-Compton techniques in the late 1960's, the use of CSS in INAA has being considered. Although COOPER and BROWNELL 1 explored the applicability of CSS for activation analysis early in 1967, most publications are from two decades later, when dedicated systems were described 2,3 and some applications were investigated. 4,5 In 1994, LANDSBERGER 6 published an overview about the use of anti-Compton techniques in INAA for environmental samples. With exceptions, 7 publications about Compton suppression INAA applied to biological materials have been focused on the determination of one or a few elements. Moreover, the anti-Compton techniques have been often used 8–10 in combination with epithermal neutron flux for improving the determination of elements like As, Cd, Cu, I and Sb in the presence of high amounts of Br, Cl and Na. Considering such a scenario, there is still a need for studies evaluating the analysis of different matrices by Compton suppression INAA with thermal neutrons, in order to further explore its multi-element characteristics. Here, the usefulness of a CSS for the determination of sixteen elements in plant materials was assessed. Five certified reference materials were analyzed by INAA using different measurement conditions and the detection limits calculated from both suppressed and unsuppressed spectra were compared. Experimental Description of the Compton suppression system The CSS was integrated by Ortec and consists of a main HPGe detector surrounded by two secondary detectors as to provide a near 4π configuration (Fig. 1).