Applications of Nuclear Analytical Techniques in Geoscience

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1 Applications of Nuclear Analytical Techniques in Geoscience J. Aspiazu a,1, J. López a, J. Ramírez a, M. E. Montero b, P. Villaseñor a a Intituto Nacional de Investigaciones Nucleares (ININ), Carretera México-Toluca Km 36.5, Ocoyoacac c.p , Estado de México, México. b Centro de Investigación en Materiales Avanzados S.C., Miguel de Cervantes 120, Complejo Industrial Chihuahua, c.p , Chihuahua, Chih., México. 1 Corresponding Author: Head of Accelerator Department ININ; Telephone ; jaaf@nuclerar.inin.mx Abstract This paper describes some advantages of analytical nuclear techniques associated to ion accelerators for analyzing geological samples. As example, it is carried out an application of PIXE to decide Sr/Ca ratio in different stages during stalactites' growth. This ratio depends on temperature and its value provide information about atmospheric temperature variations during thousands years. 1. Introduction Analyzing elements of geological sample whose concentrations are parts per million (traces), it can be performed studies on geological processes, minerals' genesis and their relationship with geological structures. Elemental analysis is also useful for prospecting for minerals and geo-chemical studies on clays, silts or superficial waters. In such studies have been identified around 30 trace elements and major constituents as O, Na, Mg, Si, Al, K and Ca [1]. The interaction between energetic ions (MeV) and sample's atoms, it can produce X or gamma ray emission and nuclear particles. This radiation is characterized by its emission's energies whose relative intensities are unique for the excited atom. Then based on this, by spectroscopy it is possible to conduct elemental analysis with precision smaller than 5% and accuracy approximately of 100%. This kind studies, are superficial (~150 microns), with space resolution of millimeters, non destructive and multi-elemental. Now there are focusing systems for ion beams with spatial resolution of microns (microbeams) or even nanometers. [2]. The techniques associated to ion accelerators as PIXE (Proton Induced X-ray Emission), PIGE (Proton Induced -ray Emission),

2 ERDA (Elastic Recoil Detection Analysis), NRA (Nuclear Reaction Analysis) and RBS (Rutherford Backscattering), being applied in a complementary way can cover elemental analysis from hydrogen until uranium. For sample analysis, commonly it is required a minimum of preparation, at least that analyzed surface is plane. Then due to physical properties of the stalactites, analytical advantages of mentioned techniques above, turn out to be the most suitable for their study [3]. 2. Experimental PIXE technique is based on ionization of inner atomic shells by atomic charged particles with speed magnitude similar to the corresponding ones of orbital electrons. Commonly protons or deuterons are use as "projectiles" for PIXE applications, but for applications of other techniques, ionized atoms of C, Li, B, O, Au, etc., are used mainly to carry out diverse studies on materials properties. Fig. 1. Atomic florescence induced by charged particle. The atomic internal ionization produces characteristic X-ray emission with energy and relative intensities well defined (Fig. 1). This emission allows identification of elements in sample with atomic number at least equal to aluminum. Those X-rays corresponding to slighter elements than aluminum are easily absorbed, not being possible for that reason its analysis for PIXE. Using also protons, elements with Z 13 can be analyzed by NRA, PIGE or ERDA.

3 Accelerating System Protons Irradiation Chamber Sample Computing Analysis Characteristic X-rays Detection and Electronic Systems Fig. 2. Diagram for experimental set up of PIXE technique. Fig. 3. Image of studied stalactite. The studies performed by PIXE are located at specific sample's zones of few millimeters of diameter. Then in this way obtained information as elementary concentrations, must be considered as averages for irradiated region. Fig. 2 shows typical experimental arrangement used for PIXE technique. A stalactite was sampled at "Majaguas-Cantera" caverns in San Carlos' mountain in Cuba [4]. According to its size, the stalactite dates at least 50,000 years. This, taking into account that a growth with thickness of 1 cm, is completed in approximately 6,000 years [5]. It was practiced a longitudinal cut to stalactite so that was visible its internal structure of "rings" (Fig. 3). This structure obeys to changes of atmospheric temperature in relatively short periods [6]. PIXE was applied to conduct elemental analysis for stalactite. Several points along time axis defined by growth direction of stalactite, were irradiated using protons of 5 MeV. These points had a minimum spacing of 3 mm, which was the size of proton beam. The protons were obtained from Tandem Van de Graaff accelerator (EN type) at nuclear center of ININ (Fig 4). Fig. 5 shows vacuum chamber where the stalactite was irradiated.

4 Fig. 4. Lateral view of Tandem Van de Graaff accelerator and ion sorces (ININ). Accelerating tank is around 20 m long. Fig. 5. Vacuum irradiation chamber for simultaneous application of PIXE, PIGE, NRA and RBS techniques (60 cm of diameter). 3. Results Fig. 6 shows typical X_ray spectrum of stalactite when is irradiated by protons. The X_ray spectra were analyzed using GUPIX code [7]. The identified elements were Ca, Fe, As and Sr. The traces of these was possible to observe, integrating a minimum charge of 5?C over especímen. It was also necessary to use a X_ray absorber made of aluminum of 25.4 microns thick. The aluminum absorber was placed between target and X_ray detector ( Si(Li) type with 230 ev of resolution). Using aluminum, X_rays of low energy are significantly absorbed favoring this way for X_ray spectrum, the definition of picks corresponding to heavy elements as Sr, which are not intense due to its low concentrations. Using picks' areas obtained by GUPIX, elemental concentrations were calculated using PIXCO code [8]. It was taken CaCO3 as sample matrix. This way, C and O, which are not visible in X_ray spectrum, were determined when it was reached stoichiometric condition corresponding to CaCO3. Potassium concentration in stalactite was estimated in 1.2% [4]. In this case, the difficulty for determination of K, it is its proximity to Ca whose pick is high intense in comparison with the corresponding one to K, being hindered this way precise determination of its area.

5 Fig. 6. Typical X-ray spectrum of the stalactite. Fig. 7. X-ray spectrum of reference material SL1 used for determination of experimental parameters for detection efficiency of X- rays.

6 Experimental parameters required by total efficiency function for X-rays detection used by PIXCO code were decided by a genetic algorithm GIPXIN [9]. In combination with PIXCO in its simulation mode, GIPXIN selects approximated values for experimental parameters like geometric or intrinsic ones corresponding to X-ray detector. Such determination, it is achieved minimizing error of calculation for concentration values of a known reference material when its X-ray spectrum is analyzed. In this case, it was used Sediment Lake SL1, which is certificated material by IAEA. This way, average error per element for certified concentrations of SL1 was smaller than 2% (Fig. 7). Time in thousands of years K Ca Fe As Sr C O Sr/Ca