Bentonitik killerin nötron aktivisyonu ile analizi

Abstract

Ana kil minerali montmorillonit yada montmorillonit- ten izomorfik iyon değişimi ile türeyen semektit grubunun bir başka elemanı olan bentonitik killerin fiziksel, kimya sal ve dolayısıyla teknolojik pek çok özellikleri birim hücreler aralığında ve kenarlarında bulundurdukları değişe bilir katyonların cins ve miktarına bağlıdır. Bentonitin içeriğinde temel kristal yapıdaki (TOT, tetrahedral- oktahedral-tetrahedral ) Si4+, Al3+, Fe3+, Mg3+, O2-, OH" iyonlarının yanında birim hücreler aralığındaki alkali ve toprak alkali iyonlar ve eser elementler vardır. Bentonitin içerik tespiti için kullanılan pek çok analitik metod vardır; Atomik Absorpsiyon Spektrofotometresi, DTA (Diferansiyel Termal Analiz), İR (Infrared) gibi. An cak bu metodlerm uygulamaları çok zaman alır ve hassasiyetleri konusunda bazı kuşkular vardır. Çalışmada kullanılan Nötron Aktivasyon Analizi ise diğer yöntemlere göre daha kısa sürede yapılması, ışınlama öncesi herhangi bir kimyasal işlem gerektirmemesi, tahrip edici olmaması ve bir defada birçok elementi belirleyebilmesi gibi özelliklere sahiptir. Bu çalışmada iki farklı yöreye (Ünye ve Kurşunlu) ait bentonitik kil mineralleri ÎTÜ Nükleer Enerji Enstitüsü reaktöründe (n,y) reaksiyonuna maruz bırakılmış ve numunelerin içerikleri S100, çok kanallı analizörü ve Sampo 90 y analiz programı kullanılarak kalitatif ve kantitatif olarak tespit edilmiştir.

Many clays which contain motmorillonit type minerals like calcite, dolamit, pirit feldspat, gibsit have been termed bentonite. The structure of the clay minerals consist of three layers; Silica-oxygen atoms in tetrahedral sheet and Alumina-oxygen-hydroxil atoms in octahedral sheet that between two tetrahedral sheets. Figure 1. shows the structure of a bentonitic clay mineral. In the tetrahedral sheet, tetravalent Si is sometimes partly replaced by trivalent Al. In the octahedral sheet, there may be replacement of trivalent Al by divalent Mg without complete filling of the third vacant octahedral position. Al atoms may also be replaced by Fe, Cr, Zn, Li and other atoms. In many minerals an atom of lower positive valance replaces one of higher valance, resulting in a deficit of positive charge, or, in other words an excess of negative charge. The axcess of negative lattice charge is compansated by the adsorption on the layer surfaces of cations which are too large to be accommondated in the interior of the lattice. © © 0 0 Kxchangeable Cations > © © Positive Double Layer Ions Unsaturated 0"2 Bonds © © © © Al"3 Mg'2 Tons Expansiable Space © © Negative Double Layer Ions Fig.l. The structure of a bentonite clay mineral VI In the presence of water, the compensating cations on the layer surfaces may be easily exchanged by cations when available in solution; hence, they are called exchangeable cations. Exchangeable cations capacity very importanat to determine their characteristic purposes and using fields of bentonite. In the presence of water, these compensating cations have a tendency to diffuse away from the layer surface since their concentration will be smaller in the bulk solution. On the other hand, they are attracted electrotatically to the charged lattice. The result of these opposing trends is the creation of an atmosferle distribution of the compensating cations in a diffuse electrical double layer on the exterior layer surfaces of a clay particle. The diffuse character of counterions called Gouy Layer, or Diffuse Layer. The Zeta potential is an electrical potential in the double layer at the interface between a particle which moves in an electric field and surrounding liquid and is the measure of stability of a clay solution which has agreat importance in many of industrial processes with respect to f loccullation or def locculation phenomena. From the different physical properties of bentonite; kolloidal,süspansion, plastisity, viscosity properties, bentonites have various type of physical properties such as strong colloidal properties, swelling and bleaching. Hence, they have extremly large industrial usage (i.e. ceramic, medicine, food, detergent, paint, paper industries ). In this work, neutron activation analysis was used for bentonite clays to make quantitative and qualitative analysis. Neutron activation analysis achives a qualitative and quantitative analysis of an unknown sample by irradiating the sample and thus producing radioactive nuclides from stabil or unstabil isotops present in the sample. The activation analysis method consists of the following major steps; 1. Selection of the optimum nuclear reaction 2. Preparetion of the sample for irradiation 3. Irradiation of the sample 4. Counting of the irradiated sample 5. Analysis of the counting results Most commonly used neutron reaction is the (n,y) reaction which takes places with almost all isotopes and has no threshold. In general, the (n,y) cross section is higher for thermal than for fast neutrons. vii One of the greatest advantages of activation analysis is its ability to dedect most of the isotopes with on extremely high sensitivity and the others are that its is nondestructive, needs a sample with a very small mass, can dedect than one element at a time, identifies different isotopes of the same element, is not affected by the chemical form of the element of interest. Therefore a lot of advantages of activation analysis, neutron activation analysis has wide use in many different fields like science, engineering, industry, minerals expo- lation, medicine, environmental monitoring....i.e. Two different bentonitic clays (Ünye and Kurşunlu bentonitic clays) were used. First of all samples were irradiated with 1.66 1012 neutron /cm2sec neutron flux in Triga Mark II reactor at Nuclear Energy Enstitute, ITU. After 5 minutes irradia tion, spectra were taken using a germanium dedector, multi channel analyzer Canberra System 100 and a fitting program called Sampo 90. Germanium detectors are semiconductor diodes having a P-I-N structure in which the intrinsic region is sensitive to ionizing radiation, particularly X-rays and gamma rays. Detector used is the Canberra XtRa is a coaxial germanium detector having a unique thin-window contact on the front surface which extends the useful energy range down to 3 keV. Convational coaxial detectors have a lithium-diffused contact typically between 0.5 and 1.5 mm thick. This dead layer stops most photons below 40 keV or so rendering the detector virtually worthless at low energies. The XtRa detector, with its exclusive thin entrance window and with a Beryllium cryostat window, offers all the advantages of conventional standard coaxial detectors such as high efficiency, good resulation, high energy, rage capability and excellent timing resolution. The multichannel analyzer is the heart of most experimental measurements. It performs the essential functions of collecting the data, providing a visiual monitor, and producing output, either in the form of final results or data for later analysis. The System 100 MCA features a single slot printed circuit board with a 16k channel x 32 bit data memory. It has many esinesses for user like using the screen with groups, energy calibration, live or true time settings, peak information options and region of interest processes. Sampo 90 is intended for the research spectroscopics who requires the maximum flexibility in analysis of complex viii pulse -hight spectra. It provides two modes of operation: completely automatic and interactive peak search and fitting. Sampo 90 provides three distinctive spectral calibrations, the first is energy calibration to establish peak energy versus cannel relationships, the second is peak shape calibration to determine the proper mathematical representations for spectral peaks. The last is detector efficiency calibrations to establish relationships between measured peak areas and photon intensities. Before the spectra were analyzed in Sampo 90, detec tor efficiency was done to find out the performance of the detector at new count geometry (35 cm) with using 1.5 mm Al shield between the source and the detector. To calculate absolute detector efficiency values, Ba133, Cs137, Am241 and Co60 standart calibration sources were used. In addition to these standart sources Eu152 source was used to determine behavior of the effiency curve near 100 keV. Our samples have peaks at extremely high energies, the peaks at 1368 keV and 2754 keV of Na24 from our samples were also used as calibration sources. For the reason that we do not know activities of Eu152 and Na24, calculated relative efficiency points of these sources were shifted in order to have better efficiency by checking of the curve RMS devia tion of the curve. Detector efficiency curve fitting was possible with a suitable fitting function. The fitting function is, e=exp (a1~(a2 +a3 e ~a<-E ( e ~a*B) ) InE and parameters found after fitting are; a1=1.1952 a2=0.7952 a3 = -1.7302 a4=39.9227 a5=-0.1454 By using, new detector efficiency, shape and energy calibrations, our spectrums were fitted in Sampo 90. Then fitting results were used in calculation of relative guantitative of bentonitic clay minerals. Counting of Kurşunlu bentonitic clay mineral was repeated several times. Firstly sample was counted 15 min after. 10 min waiting time to be able to see short lived - radioisotopes, then counted 2 hours later to see long-lived ix radioisotopes fig.2.b. Those spectra are shown fig. 2. a and In order to take qualitative results from these spectra, we classified elements in the sample according to energies, intensities and half lives of radioisotopes.