Nötron Aktivasyon Analizi Yöntemi İle Cam İçinde 59 27 Co Belirlenmesi

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Tarih
1998-01-15
Yazarlar
Çelik, İsmail
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Enerji Enstitüsü
Energy Institute
Özet
Bu çalışmada, dünyada son otuz yıl içinde büyük bir gelişme gösteren Nötron Aktivasyon Analizi'ni incelemek, temel bilgileri vermek ve dünyada yapılan öğretim çalışmalarını aksettirerek bu konuda ulaşılan sınırlar konusunda fikir vermek amaçlanmıştır. Nötron Aktivasyon Analizi 'nin teorik incelendiği bölümde temel bilgiler verilmiştir. Bunun yanısıra, bir Nötron Aktivasyonu uygulama çalışması yapılırken kullanılan metotlar ve uygulama adımlarından sözedilmektedir. Yine bu anlatılandan pekiştirmek ve metodun ne kadar önemli olduğunu belirtmek amacıyla, dünyada çeşitli üniversitelerde yapılan gerek eğitim - öğretim çalışmalarına gerekse deneysel çalışmalara yer verilmiştir. t.T.Ü. Nükleer Enerji Enstitüsü TRIGA Mark - II reaktörü ile ışınlama yapılarak, NAA yöntemi kullanılarak bir örnek madde içinde iz element belirlemesinin bir uygulaması yapılmıştır. Söz konusu örnek madde, ülkemizdeki yüksek tirajlı gazeteler tarafından satışı arttırmak amacıyla yaygın bir biçimde dağıtılmış olan koyu mavi renkli cam gıda kaplarından bir bardaktır. Örnek maddede bulunup bulunmadığı ve varsa miktarı belirlenecek olan iz element, belli ölçülerden fazlasının bulunması halinde insan sağlığı için zararlı olduğu bilinen Co-59 maddesidir. Ancak, burada gözetilen amaç, iz elementin ilgili örnekteki miktarının sağlığa zararlı olup olmadığının belirlenmesi ile kesin hükme varmak değil, bir yüksek lisans tezi çerçevesinde böyle bir hükme temel olabilecek verilerin saptanması lmuştur. Cam örnek içindeki Co-59 miktarı Nötron Aktivasyon Analizi Yöntemi ile belirlenip^ elde edilen sonuç diğer çalışmalarla karşılaştırıldı. Bulunan (443 ± 14,3) ppm değerinin karşılaştırma yapılan x-ışım fluoıesan yöntemi ile yapılan bazı çalışmalara göre duyarlık bakımından daha doyurucu olduğu görülmüştür.
The main objective of this study is to confirm the role of Neutron Activation Analysis (NAA) by using its fundamental aspects and to progress the applicant's ability. An adaptation of determination of a trace element in a defined sample has been carried out by using of the method Neutron Activation Analysis in our institute's TRIGA Mark-II type reactor. The defined sample is selected to be a glass, which is a part of dark blue colored translucent kitchen ware set, that was commonly promoted a few years ago, throughout the country by the highest circulated newspapers of Turkey, for a period of almost two years. The trace element whose presence - and if present, its amount - is to be determined in the sample, is cobalt, "Co which is known to be harmful for human health, if exceeds some certain amount. However; the aim of the research here is not to give a final decision concerning human health, but to supply data for likely decisions; by determining the amount of trace element in the sample. This research also includes the investigation of some of the research and education programs from different universities. The analytical technique of Neutron Activation Analysis has played an important role in research and education fields for the last thirty years; and still continues to be one of the primary methods used at technical laboratories all over the world. The NAA technique has natural capabilities such as high sensitivity, great selectivity, multielement capability, freedom from most blank problems and the ability to analyze many samples nondestructively. NAA technique also has a redundant quality assurance capability that can lead to high accuracy even at very low elemental concentrations. In order to get the best accuracy and precision in trace element analysis, the Neutron Activation Analysis technique can be separated into five distinctive functional areas. These are the criteria that are to be taken into consideration throughout the research studies. The five functional areas are the sampling and sample handling; irradiation parameters; radiochemical separations (if used); detection and counting and, data reduction and calculation. The first stage of irradiation is the sampling and sample handling. Thus; before the irradiation, a sample should be prepared properly in an adequate container. At this point, the analysts should be extremely careful not to contaminate the container to be used. The instruments used for the irradiation are made of polyethylene, silica and aluminum foil. It should be evident that the samples are to be handled in dry and clean areas. The analyst should also use clean materials, clean plastic gloves and cutting materials such as knives, files, tweezers and etc. One of the five distinctive functional areas that concerns this study is the irradiation parameters. A NAA analyst has to consider five primary irradiation parameters throughout the irradiation facilities. These are the neutron energies; neutron flux density variations over the irradiation volume; neutron fluency rates as a function of time over each irradiation and between irradiations; sample temperatures generated within the irradiated samples; and, sample pressure generated within a sealed irradiation container. For the experiment that is carried out in this research, two of the irradiation parameters that, affect the induced radioactivity in the samples, needed to be discussed. These are the neutron energies and the neutron flux density variations. It is evident that the fourth and the fifth items, sample temperatures generated within the irradiated samples and sample pressure generated within a sealed irradiation container, are safety factors. One of the irradiation parameters that, affects the induced radioactivity in the samples, is the neutron energies. The neutron energy distribution can often change as a result of variations in the reactor's core loading, control rod or shim arm locations, fuel element burnup, and so forth. This type of distribution fluctuations may cause changes in the epithermal-to-thermal neutron ratios. Such changes should cause no problem if the analyses are adequately monitored for the appropriate neutron energy. The second one of the irradiation parameters that, affects the induced radioactivity in the samples, is the neutron flux density variations. The variation of neutron flux densities over the irradiation volume, is one of the major remaining sources of error for activation analysts. Experimenters have to carefully evaluate the neutron distribution and other fluctuations that may occur as a result of core configuration changes, time spent intp the fuel cycle, control rod or shim arm movements. In addition to these, the analysts should be aware of a secondary affect of neutron density by using adequate variations, which results from different activation levels within different parts of the sample. As for the detection and counting of the irradiation sample; which is among the five distinctive functional areas, it is one of the main supporters of the Neutron Activation Analysis technique. After radiation is completed, the sample is to be counted by using an adequate system. The counting system depends on the detection of gamma and x - rays and very seldom on other particles. A basic counting system for activation analysis consists of a detector Ge(Li) or Si(Li), and XI multichannel analyzer (MCA). These are mini computers that analyze the recorded data. It should also be emphasized that using the standards is an important factor for obtaining high accuracy in Neutron Activation Analysis technique. The standards that are very significant for NAA are also specifically mentioned throughout this research. The standard to be used is defined as a material or substance, whose value, that should be within specific limits, can be accepted without question, when it is used in order to establish the value of the same or different material with a related property. This definition is adapted for Neutron Activation Analysis so as to determine "a well-characterized pure element or compound, that will be known as to give out a high degree of accuracy". The International Union of Pure and Applied Chemistry states that the degree of purity for a primary standard should be (100+0,02) %. In this research, the standard to be used is set to be cobaltchloride (C0CI2). Standard and glass samples are irradiated at the central thimble of the reactor by the usage of 250 kW pulse type TRIG A Mark-Il Nuclear Research Reactor of Institute for Nuclear Energy of İTÜ. Standard and glass samples are also irradiated and counted exactly under the same conditions. As the general explanation of Neutron Activation Analysis is presented, the experimental part of the study is carried out. Thus, in the fifth chapter, the experiment to be done, and the weight of materials and standards used in the tubes are explained. Institute for Nuclear Energy of ITÜ has a 250 kW pulse type TRIGA Mark-II Nuclear Research Reactor and other necessary equipment such as; multichannel analyzer, calibration sources, standards, solid state detector and electronic modules Xll and all of these facilities have been available throughout this study. Central thimble is used for the irradiation of samples. In our study, the sample is a piece of glass and our aim is to determine the amount of Co-59 in the sample. For the determination of the trace element Co-59, there are three possible methods named as the absolute method, the comparison method and the coincidence metfiod. Out of the these three counting methods, the experimental jstudy is set to base on the two techniques that are the comparison method and the coincidence method. The counting of y - rays; that is the derivation y - spectrum, by using the multichannel analyzer, gives out some values. By the help of the comparison method, these values are used to figure out the activity and thus the presence of the amount of element searched. As for the coincidence method, by the derivation of the P and y and or the y and y - rays, using the 0 - y or y - y coincidence respectively, the absolute activity and thus the amount of the element searched should be figured out. Carrying out the application of the NAA technique, there are certain equipment that can be used throughout the experiments. These are the scintillation detector and the germanium semiconductor detector of multichannel analyzer (MCA). As for the determination of trace element Co-59, in this research, it has been decided to realize five different experiments. Unfortunately because of the inappropriate condition of the P and y detectors of our laboratory, it was impossible to use the coincidence method for the counting experiment. Therefore, the experiment had to be executed only by using the multichannel analyzer method. By this method, the.derivation of Jwo radioactive nuclei 602TCoand T Co with 10.47 min and 5.24 y half lives respectively, is aimed. Before it can be detected out among the other undesired radioactive nuclei around, the 10.47 min "short" half life 60TCo nucleus has decayed; thus died. On the Xlll other hand, by using the multichannel analyzer, 5.24 y "long" half life ^Co nucleus is detected and by this derivation the amount of the element searched could easily be determined. However, since the other methods of derivation could not be executed, it was not possible to provide a control of assurance cross-check. In spite of this unfortunate situation the values that were obtained could be compared with the other experiments done by using the x - ray fluorescent method, in Istanbul Technical University. In a similar study with a similar sample, but with a different method of atomic absorption, the value iş calculated as (500 ± 100) ppm value. Since the results obtained within this research are (443 ± 14,3) ppm; it shall be clearly seen that the precision of our estimated value is evident. In another research done by the using of the very same glass sample, also with the technique of atomic absorption, the estimated data was (800 ± ?) ppm value. This value is again, greater than the calculated value of this research. As it is cited in the literature; according to the frame of our research and the compared ones, it has been roughly experienced that the Method of Neutron Activation Analysis is an evidently more sensitive technique than the method of the x - ray fluorescent; concerning the determination of the trace element.
Açıklama
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Enerji Enstitüsü, 1998
Thesis (M.Sc.) -- İstanbul Technical University, Energy Institute, 1998
Anahtar kelimeler
Eser elementler, Nötron aktivasyon analizi, Reaktörler, Trace elements, Neutron activation analysis, Reactors
Alıntı