k0-PGNAA of pollutants in aqueous samples using MCNP code

ISSN 1818-331X ЯДЕРНА ФІЗИКА ТА ЕНЕРГЕТИКА 2014 Т. 15 № 1 ТЕХНІКА ТА МЕТОДИ ЕКСПЕРИМЕНТУ УДК 539.125.5 Ashraf Hamid1, Hesham Shahbunder2 1 Department of Radioactive Environmental Pollution, Hot Laboratories Center, Atomic Energy Authority, Cairo, Egypt 2 Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt k0-PGNAA OF POLLUTANTS IN AQUEOUS SAMPLES USING MCNP CODE Prompt γ-neutron activation analysis (PGNAA) using the k0 method by employing the 1951.1 keV γ-line of the Cl(n, γ)36Cl thermal neutron reaction as monostandard comparator was described. The method has been applied and evaluated using the anti-Compton prompt γ-ray neutron activation analysis facility using 252Cf neutron source with a neutron flux of 6.16 · 106 n · cm-2 · s-1. A well-type HPGe detector as the main detector surrounded by NaI(Tl) guard detector has been arranged to investigate the performance of the Compton suppression spectrometer using the simplified slow circuit. The properties of neutron flux were determined by MCNP code calculations. In order to determine the efficiency curve of an HPGe detector, the prompt γ-rays from chlorine were used and an exponential curve was fitted. AC-PGNAA method has been used for the determination of high neutron absorbing elements like Cd, Sm and Gd as well as 20 light and heavy elements (Na, Mg, Al, Si, P, K, Ca, Ti, V, Mn, Sc, Fe, Co, Zn, La, Rb, Cs, As and Th) in standard reference materials (IAEA, Soil-7) and ten sediment samples collected from El-Manzala lake in northern part of Egypt. The reference material IAEA, Soil-7 was analyzed for data validation and good agreement between the experimental values and the certified values have been obtained. Keyword: k0-PGNAA, AC-PGNAA, 252Cf neutron source, water pollution, MCNP code, self shielding. 35 Introduction Neutron induced prompt γ-ray activation analysis (PGAA) is based on the observation of instantaneous γ-radiation following the neutron reaction, which is mostly radiative capture, usually denoted as (n, γ) reaction. The energies of the emitted γ-rays are characteristics of the element (more precisely, it's radiating isotope), whereas the corresponding spectral intensities are proportional to the elemental content. PGAA is a rapidly developing technique, pursued with intense reactor beams as well as portable neutron sources. It is applicable to all elements including the light elements, which are usually difficult or even impossible to measure in neutron activation analysis (NAA) utilizing radioactive decay γ-rays. Moreover, for certain nuclides PGAA provides substantially higher sensitivities. Further advantages are its nondestructive nature due to the substantially lower neutron flux, the lack of sample preparation needs, and the ability to obtain results promptly [1]. A major obstacle to the use of the PGAA method for quantitative multi-element analysis has so far been the absence of a high-quality database of characteristic prompt γ-ray energies and intensities. In a prompt spectrum, the very large number of peaks (order of 1000) tremendously increases the chance for spectral interference [2]. Because the escaping energy is a photon, it is possible to collect that energy with another detector. This is typically done with a larger detector made of a less expensive material such NaI(Tl) surrounding the main detector, and is known as a surrounding shield “guard”. By correlating events in the main detector and the surrounding shield detector with timing electronics, events counted in the shield detector can be used to reject simultaneous events in the main detector (veto signal). Therefore, a veto action is made to discard the signal from being counted as a true data. This results in the suppression of the Compton continuum which results in removing the Compton-associated background, therefore, enhancing the signal-to-noise ratio and improving the detection capability of the system [3]. In the present work, the k0-PGNAA has been applied to the analysis of major and trace elements of sediment samples collected from the Manzala Lake in northern parts of Egypt, as well as a careful and complete characterization of the neutron flux parameters in the selected irradiation positions. Theory Internal Single Comparator (monostandard) Method (k0-PGAA Method) [4] The usual analysis for PGAA was carried out using a comparative method. Indeed, element concentrations (or masses) were determined by comparing the specific γ-ray count rate (Asp), usually given in (counts · s-1 · g-1) units of an element in an unknown sample to the corresponding element. The relevant k-factor is easily obtained as a ratio of specific count rates for sample x and comparator or rather its simplified form [4]: © Ashraf Hamid, Hesham Shahbunder, 2014 82 k0-PGNAA OF POLLUTANTS IN AQUEOUS SAMPLES Asp , x (θσI/M) x ε x ⋅ , (θσI/M) c ε c (1) Nx / t (counts · s-1 · g-1), mx (2) Kc ( x) = = Asp ,c where Asp , x = where θ – the abundance of the capturing isotope of the element of interest; I – the number of γ-rays emitted per capture; M – the atomic weight, σ (Eγ) the capture cross-section for neutron energy Eγ while ε (Eγ) is the counting efficiency (full energy or double-escape peak) of the detector at energy Eγ. It has been assumed that, for nearly all elements at neutron energies less than 5 MeV, the neutron cross section (σ) is inversely proportional to velocity v, hence σ = σ0·v0/v, where v0 is the standard velocity of thermal neutron (2200 m/s) [4]. Following the convention used in neutron activation analysis as defined by [13]: Cx = Asp , x 1 ε . c Asp ,c k0,c (x) ε x . ppm , (3) where, Cx – the concentration of the matrix element under consideration; Asp – the count rate per unit weight of the elemental sample x and comparator c. The indexes “x” and “C” stand for the analyte and the co-irradiated chlorine comparator, k0,c is the k0 factor and ε is the relative γ-ray efficiency of the elemental sample εx and comparator εc. The k0-prompt γ-factors for the majority of the elements that can be determined via PGAA were experimentally measured with a high accuracy and were tabulated [6 - 8]. The only difference with respect to NAA is that no saturation, decay and counting correction factors appear in the prompt case. However, in Eq. (3), the weights of element (x) and sample are expressed in µg and gram, respectively, so that Cx is in μg/g. Experimental Samples Preparation Ten short sediment and aqueous core samples were recovered manually in the El-Manzala lagoon, i.e. the core samples were collected at 3 - 5 cm interval and placed into well-numbered plastic bags. Sediment description was also made while sampling. After removing extraneous materials, the collected samples were air-dried in the laboratory at room temperature and then oven-dried at 105 °C until they reached constant mass. The samples were ground, sieved to 200 mesh and prepared for prompt γ-neutron activation analysis, (10 g) of sedime (...truncated)