Gama Geçirgenlik Tekniği İle Yoğunluk Tayini

Abstract

Radyasyondan yararlanılarak oluşturulmuş teknikler den endüstrinin birçok dalında, üretimin her aşamasında önemli ölçüde faydalanılmaktadır. Radyasyona duyarlı Ölçme cihazları (radiogauging) ; radyoaktif ışınlar yar dımıyla ve bu ışınlarla ölçümler yapıp, bunları yorumla yarak çalışan sistemlerdir. Bu teknikler ve tekniklerle oluşturulmuş sistemler kullanılarak kalınlık, yoğunluk, seviye, nem, v.b. ölçümler kolaylıkla yapılabilmektedir. Bu çalışmanın konusu da gama radyasyonu ile gama geçirgenlik (transmisyon) tekniği kullanarak farklı mal zemelerin yoğunluk tayinlerini yapmaktır. Bu amaçla ön ce laboratuvarlarda kaynak, yoğunluğu bilinen çeşitli maddeler, dedektör, çok kanallı analizör ve diğer ilgili aletlerden oluşan deney düzeneği kurulmuş, sonra I.T.Ü Nükleer Enerji Enstitüsü TRIGA MARK-II reaktöründe ışın lama suretiyle istenen özelliklerde gama kaynağı olan Na-24 radyoizotopu üretilmiş, deney düzeneğine eklenmiş tir. Farklı yogunluklardaki katı ve sıvı maddeler kul lanılarak gama geçirgenlik prensibine göre sayımlar a- 1 inmiştir. Yinelenen bütün sayımların her deneyde iki şer kez ölçülmesi sonucu standart sapmaları hesaplanmış tır. Bu standart sapmalar da belirtilerek yapılan öl çümlerle farklı malzemelerin daha önceden konvansiyonel şekilde ölçümlenip hesaplanan yoğunluğu arasındaki deği şimi veren grafik çizilmiş ve yorumlanmıştır. Elde edilen bu kalibrasyon grafiği; yoğunluğu bi linmeyen maddeler için de gama geçirgenlik tekniğine gö re ölçümlerin yapılmasıyla elde edilen değerlerin uygu lanmasıyla duyarlı ve güvenilir sonuçların alınmasını sağlamıştır.

Radiogauging is a branch of non-destructive testing which is applied on a system (components or assemblies) to measure the particular properties of a system. Ba- sicly a radiogauging system consists of a radiation source, radiation detector and the related electronic equipment. Many radiation techniques are available for compo nent inspection. There are two types of measurement techniques which are widely used, namely; 1- Transmission 2- Scattering Each of these techniques require the use of radio active sources, namely; radioisotopes or machine sources latter has some disadvantages. The most important advantages of radioisotopic sources over machine sources of radiation are: 1- They are self contained sources of energy and do not require any power supplies, 2- They are portable, 3- Their radiations can be easily detected, - xi i - 4- Their performance is unaffected by heat.pressure and vibration, 5- Their properties are perfectly reproducible; i.e. the same radioisotope always has the same properties and gives the constant output, 6- They are less expensive, 7- They can be made quite small (some systems need very small radiation sources), 8- They can be used by people that are not highly trained in electronics Since radioisotopes have such big advantages, therefore they are preferred to machine sources. It is very important to determine the following three characteristics that belongs to the radiogauging system when it is projected. These characteristics are; 1- Functions of the system (i.e. what kind of a gauge is going to used to measure) 2- Principle of radiogauging (i.e. it is necessary to decide the type of the principle which will be used in the system) 3- Type of source (i.e. it is necessary to choose the suitable ra dioisotope for the gauging system) Considering on the major properties of radiation, different applications of radioisotopes can be classi fied in three groups as following: I. Since radiation can be detected with extremely high sensitivity, radioisotopes are useful as tracers. II. Ionization and excitation effects of their radi ation make possible the use of radioisotopes are possible: dissipa-ting electrostatic charge, improving operation of elec-tronic gas tubes, and exciting phosphors to produce light. III. Radiation emitted by some radioisotopes penetra tes great distances in solid materials, for example, - xii i - opening an immense field of gaging, radiography, moistu re, density, thickness, level, etc. measurements and the location of hidden objects, GARDNER, (1967). Industrial applications of radioisotopes utilize the radiation of radioactive materials (radiation sour ces). Depending on the mode of industrial application; information is the most cases obtained through the ef fects of materials on radiation, about, e.g., the quali tative material properties. The fundamental requirement to obtain and process is the detection and numerical evaluation of radiation. There are also some important factors that effect the choice of a radioisotope which are: 1- Availability and easy usage 2- Half-life suitable to the gauging system 3- Appropriateness of the energies in the lineer spectrums to the system 4- Activity 5- Cost 6- Radiation safety There are some other important factors effects the condition of the selection of the radioisotope systems are : 1- Purity of radioisotope 2- Changing of radioisotope 3- Effect of radiation 4- Effect of isotope In this work, the gamma-source radioisotope is used to measure the density of the different substances. Therefore, properties of gamma rays are briefly summari zed as fol low; Gamma rays are major class of indirectly ionizing particles. Gamma rays are electromagnetic radiations. Since gamma rays are indirectly ionizing particle, they - xiv - penetrate to deeper layer with in a medium comparing with the direct ionizing particles such as, beta par ticles. Gamma rays interact with the matter in three different types. These are : i- Photoelectric event 2- Compton scattering 3- Pair production Gamma rays lose energy through encounters which result in the ejection of electrons from atoms. The gamma ray may lose all of its energy in an encounter or only part. If only part of its energy is removed, the remainder part continues to travel through space with the speed of light, as a lower energy photon. On the average the higher the energies of the gamma photons, the higher the energies of the liberated electrons. Every gamma ray has a finite probabilty of passing all the way through a medium through which it is moving The probabilty that a gamma ray will penetrate through a medium depends on many factors. These factors are: 1- Energy of the gamma ray 2- Composition of the medium 3- Thickness of the medium If the medium is dense and thick, the amount of penet ration of gamma rays decreases, GARDNER, (1967). The aim of this work is to obtain a calibration curve by using gamma transmission technique. During the work, first the densities of different liquids and solid substances were calculated according to the conventional measurement techniques. Second step were the prepara tion of radioisotopes which were used in the measurement system. During the experiments 0,5 mCi Na-24 radioiso topes were used as a gamma ray source that have been produced by irradiation of Na-23 in I.T.Ü. TRIGA MARK- II Reactor. Coll imation and shielding design of the radioisotope were completed according to the safety - xv - criteria. The experiments were carried out in the Radioisotope and Radiotracer Laboratory in Nuclear Application Division of Î.T.Ü., Institute of Nuclear Energy. The obtained calibration curve was applied to the maesured number of gamma given by the substances with unknown densities, according to the gamma transmission techniques. The results taken from the calibration curve were sensitive and reliable.