Nötron aktivasyon analizi yöntemi ile besin maddelerinin analizi

Abstract

Bu çalışmada, son yıllarda çok geniş uygulama alanı bulan nötron aktivasyon analizinin esasları ve besin maddeleri üzerinde yapılan uygulamalar incelenmiştir. Nötron aktivasyon analizinin teorik olarak incelendiği bölümde, analizin basit teorisi, uygulanma şartlan, kullanılan deteksiyon metodları anlatılmıştır. Bu bölümün son kısmında nötron aktivasyon analizinde karşılaşılabilecek hatalara, yöntemin avantaj ve dezavantajlarına değinilmiştir. Deneysel çalışma bölümüne geçmeden evvel bir kaç değişik ülkede besin maddeleri üzerinde bu yöntem ile yapılan çalışmalar anlatılmış ve sonuçlan verilmiştir. Deneysel çalışma bölümünde ise Türkiye 'de üretilen ve tüketilen Ankara armudu, sarımsak ve semizotunda instrumental nötron aktivasyon analizi ile Mangan (Mn), Klor (Cl), Sodyum (Na), Potasyum (K) ve Alüminyum (Al) elementleri kantitativ olarak belirlenmiştir. Yapılan deneyler etraflıca anlatılmış elde edilen spektrumlar çalışmaya eklenmiştir. Alınan sonuçların ötesinde, araştırmacı öğrenci, yöntemin uygulanması sırasındaki değişik adımlarda ; örneğin, örnek hazırlama, ışınlama, sayma ve değerlendirme aşamalarında deneyim kazanmış, değerli bilgiler edinmiştir. Ayrıca bu tür analizlerin daha iyi bir şekilde yapılabilmesi için enstitünün gereksinimleri ortaya çıkmıştır.

The objective of this study is to apply instrumental neutron activation analysis (N.A.A.) to the quantitative analysis of some food items using the facilities of our Institute. Such analysis were made few times in the past at the Institute. Our desire is to start and gain experience in food analysis with the purpose of scientific research and to answer the demands of the commercial institutions in Turkey. I.T.U. Institute for Nuclear Energy has 250 kw pulsed type TRIGA Mark II Nuclear Research Reactor. Necessary equipment; like multy- channel analyser, calibration sources, standarts, solid state dedector and electronic modules are available. The reactor rabbit system and central channel is used for the irradiation of the samples. The fundamental aspects of N. A. A. and its some of the applications are given in the study. Necessary conditions for the application of the method and detection system are given. The probable errors, advantage and disadvantage of the N. A. A. are also explained. Activation analysis achieves a qualitative and quantitative analysis of an unknovvn sample by irradiating the sample and thus producing radioactive v nuclides from stable ör unstable isotopes preseni in the sample. The radioactive nuclides can be identified from properties of the radiations they emitt: 1- Type of radiation 2- Energy of radiation 3- Intensity of radiation 4- Half-life The activation analysis method consists of the follovving majör steps, 1- Selection of the optimum nuclear reaction 2- Preparation of the sample for irradiation 3- irradiation of the sample 4- Counting of the irradiated sample 5- Analysis of the Results The optimum nuclear reaction is chosen with these considerations in mind: 1- Production of large activity should occur within a reasonable irradiation time. 2- The radioisotope produced should have a reasonable half life (T > min). 3- The type and energy of the radiation emitted by the radioisotope should not pfesent great counting difficulties. 4- A minimum number of interfering reactions should be involved. The most commonly used neutron reaction is the (n,y) reaction, which takes place with almost ali isotopes (although with different probability) and has no threshold. in general, the (n,y) cross section is higher for thermal than for fast neutrons. Other neutron reactions are (n,a), (n,p), and (n,2n) reactions. -Preparation Of The Sample For irradiation : A sample should be prepared properly and placed in a container before it is irradiated. The person who prepares the sample should be extremely careful not to contaminate it. To avoid contamination, samples should be handled in dry vi boxes ör in clean rooms. The person who prepares the sample should use clean instruments (knife, file, tweezers, ete.) and also wear clean plastic gloves. The materials that are used for packaging frequently are pplyethylene, silicia, and aluminium foil. -Irradiation Of The Sample: Depending on the selected reaction, irradiation of the sample may take place in a reactor, in an accelerator, ör with an isotopic source. After the selection of an irradiaton facility, the next step is a decision about the irradiation time. If the sample contains known isotopes at approximately known amounts, it is easy to estimate the proper irradiation time. If, on the other hand,the sample is completely unknown, öne irradiates the sample for an arbitrary time, checks some of the isotopes present (from the emitted radiations), and the irradiates the sample ağam for a time that will provide enough activity for proper isotope identification with the accuracy. The equation that gives the activity produced after irradiating the sample for time t0 is: A(t0)=a1.m^a,.^(l-e-At») Aı m = Mass of the element of interest in the sample a; = Weight fraction (abundance) of isotope with atomic mass A; (A; is an isotope of the element with mass m) K = Decay constant of the radioisotope produced O; = Cross section for the reaction that makes the isotope with atomic vveight Aİ radioactive 4> = Partide flux (particles/m2. s) NA= Avogadro's number vii If the half-life of the radioisotope is much shoter than t0 (t0 > 6.Tı^), saturation activity (A^.) is obtained as, N A,at = ai.m-^-cri.^ Samples are always activated together with a standard of known composition, and the radioactive species generated in each are compared. If identical activation conditions are used, and we designate subscripts l and 2 for sample and standard respectively JÜ = ^L m2 a2 where m = mass of element of interest in sample ör standard a = activity due to this element in sample ör standard -Counting of the sample : After irradiation is completed, the sample is counted using an appropriate system. The counting system depends on the dedection of gamma rays and X- rays and very seldom on detection of other particles. A basic counting system for activation analysis consists of a detector [Ge(Ii) ör Si(li)], electronics (i, e., preamplifier, amplifier) and a multichannel analyzer (MCA). Modern MCAs do much more than record the data. They are minicomputers that analyze the recorded spectra. Examples are Canberra's SCORPIO computer Analyzer and Trac ör Northern's Spectroscopy System TN-4000. Before the explanation of the experiments some experimental results which are obtained in different countries are given. viii In this study the N.A.A. results of Ankara pear, garlic and purslane which were grown in Turkey are given. In samples of these items Manganeze (Mn), Chlorine (CI), Sodium (Na) and Aliminium (Al) were found and their abundances have been calculated. It is expected that some more elements would also be found but longer irradiations could not be performed because of the reactor cooling problems which would be very useful to increase the number of element found in samples. The preparation of samples is as follows: 250gr of each item washed in distilled water and dried. Then, cut into small pieces and placed into freeze-dry for three days. Dried samples put into powder form and small tablets were produced by pressing them under 3 tons force. In each step of the production necessary precautions are taken against the contamination. The weight of the samples were also taken in each step of the preparation. The same precedure was applied to the standarts. Samples and standarts were kept in Desikator. After irradiation and counting of the samples the result were obtained. Rabbit system and central channel of the reactor were used for irradiation. The results were given below: Chlorine (CI) in Garlic : 0,96 mg/g Chlorine (CI) in Purslane : 20,656 mg/g Chlorine (CI) in Ankara Pear : 0,946 mg/g Sodium (Na) in Garlic : 04 mg/g Sodium (Na) in Purslane : 18 mg/g Sodium (Na) in Ankara Pear : 0,059 mg/g IX Potasium (K) in Garlic : 1,5 (%) Potasium (K) in Purslane : 6 (%) Potasium (K) in Ankara Pear : 0,8 (%) Manganaze (Mn) in Garlic : 5,75 ug/g Manganaze (Mn) in Purslane : 20,276 ug/g Manganaze (Mn) in Ankara Pear : 5,8 ug/g Aluminium (Al) in Garlic : 2,03 ug/g Aluminium (Al) in Purslane : 50,85 ug/g Aluminium (Al) in Ankara Pear : 35,478 u,g/g