Gama geçirgenlik prensibi ile malzemelerin ağırlıklarının ölçülmesi

Abstract

Endüstrinin birçok dalında üretim ve kontrol aşamalarında radyasyon ile ölçüm teknikleri (radiogauging) kullanılmaktadır. Radyoaktif ışınların farklı metodlarla kullanılması ve alınan ölçümlerin yorumlanmasıyla çalışan bu sistemler yardımıyla, kalınlık, yoğunluk, ağırlık, seviye, nem, gibi ölçümler ucuz, duyarlı, güvenilir bir şekilde yapılabilmektedir. Bu çalışmanın konusu, gama geçirgenlik prensibi kullanarak farklı malzemelerin ağırlıklarının tayinini yapabilmektir. Bu amaçla Radyoizotop labaratuarında, yürüyen bant sistemlerinin deneysel bir modeli kurulmuştur. Deney düzeneği içinde Na-24 kaynağı, ağırlığı ölçülecek farklı malzemeler, dedektör, çok kanallı analizör, elektronik hassas terazi, analog terazi, malzemelerin konulduğu kap ve diğer ilgili ekipmanlar yer almıştır. İ.T.Ü. Nükleer Enerji Enstitüsü TRIGA MARK-II reaktöründe, istenen aktiviteye göre belirlenmiş ağırlıktaki Na2C03 örnekleri ışınlanmış ve Na-24 radyoizotopu(kaynak) üretilmiştir. Bu kaynak deney düzeneği içine konulmuş ve farklı malzemelerin, gama geçirgenlik prensibine göre, önceden analog terazi ile ölçülen farklı ağırlıkları için sayımlar alınmıştır. Aynı ağırlık için, yürüyen bant modeli olarak kullanılan kap hareket ettirilmek suretiyle altı noktadan ölçüm alınmıştır. Bütün malzemeler için ortalama değerler ve standart sapmaları hesaplanarak, sayımların ağırlık ile değişimlerini veren kalibrasyon eğrileri çizilmiştir. Daha sonra, deney düzeneğinde aynı malzemelerin bilinmeyen ağırlıklarının sayımlan alınmış ve bu sayımlar elde edilen kalibrasyon eğrilerine uygulanarak malzemelerin ağırlıkları hesaplanmıştır. Sonuçların duyarlı ve güvenilir olduğu görülmüştür.

Gama rays are major class of indirectly ionizing particles called electromagnetic radiation. Since gama rays are indirectly ionizing particle, they penetrate to deeper layer with in a medium comparing with direct ionizing particles such as beta particles. Gama rays interact with the matter in three different types. These are : -Photoelectric efect -Compton scattering -Pair production Gama rays lose energy through encounters which result the ejection of electrons from atoms. The gama ray may lose all of its energy in an encounter or only a part. If only a part of its energy is removed, the remaining part continues to travel trough space with the speed of light as a lower energy photon. On the average, the higher energies of gama photons, the higher energies of the liberated electrons. Every gama ray has a finite probability of passing all the way through a medium through which it is moving. The probability that a gama ray will penetrate trough a medium depends on many factors. These are: -Energy of the gama ray -Composition of the medium -Thickness of the medium If the medium is dense and thick, the amount of penetration of gama rays decreases, GARDNER, R.P., ELY, R.L., (1967). 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. Basicly a radiogauging system consists of a radiation source, radiation dedector and the related electronic equipment. It is very important to determine the following three characteristics that belong to the radiogauging system when it is projected. These characteristics are: XI -Function of the system (It is neccessary to decide what kind of gauge is going to be used to measure) -Principle of radiogauging (It is neccessary to decide the type of the principle which will be used in the system) -Type of source (It is neccessary to choose the suitable radioisotope) Many radiation techniques are available for component inspections. There are two measurement techniques which are widely used. These are : -Transmission technique -Scattering technique Considering on the major properties of radiation, different applications of radioisotopes can be classified in three groups as following: - Since radiation can be detected with extremely high sensitivity, radioisotopes are useful as tracers. -Ionization and excitation effects of their radiation make the use of radioisotopes possible; dissipating electrostatic charge, improving operation of electronic gas tubes, and exciting phosphors to produce light. -Radiation emitted by some radioisotopes penetrates great distances in solid meterials, for example,opening an immense field of gauging, radiography, moisture, density, thickness, level, weight, etc. measurements and the location of hidden objects Industrial applications of radioisotopes utilize the radiation of radioactive materials (radiation sources). Depending on the mode of industrial application, information, is obtained in the most cases through the effects of materials on radiation e.g., the qualitative material properties. The fundemental requirement to obtain and process is detection and numerical evaluation of radiation. There are some important factors that effect the choice of a radiosotope which are: -Availability and easy usage -Suitable half-life for the gauging system -Appropriateness of the energies in the linear spectrums to the system -Activitiy -Cost -Radiation safety There are some other important factors that effect the condition of the selection of radioisotope systems are: -Purity of radioisotope Xll -Effect of radiation -Effect of radioisotope. The techniques require the use of radioisotopes has some advantages. The most important advantages of radioisotopic sources over machine sources of radiation are written below: -They are self contained sources of energy and do not require any power supplies -They are portable -Their radiations can be easily detected -Their performance is unaffected by heat, pressure and vibration -Their properties are perfectly reproducible; the same radioisotope always has the same properties and gives the constant output -They are less expensive -They can be designed quite small that some systems can need very small radiation sources -They can be used by people that are not highly trained in electronics The industrial use of radioisotopes have wide application fields. One of the frequently occuring problem is that of measuring material flow in transportation systems. This problem can be solved by nuclear means for solid transportation by belt conveyors. The measurement is based upon the radioation absorption dependence on the density thickness of the material. The nuclear conveyor belt weigher used for quantity measurement at belt transporters consists of a linear radiation source with its length equal to the belt width and a dedector array of the the same length. In the fork-shaped measuring line arranged perpendicularly to the belt advance direction, the conveyor belt loaded with material moves between the source and the detector. The system does not contain any mechanical parts and there is no contact with the transported material. The Co-60 or Cs-137 y source of the isotopic belt weigher is placed underneath the belt and the dedector is positioned above it. The radiation emitted by the source (assumed to be constant in intensity from the measurement aspect) is partly absorbed by the substance transported on the belt. An unloaded belt absorbs approximately 5 to 10 % of the radiation and the variation in radiation attenuation by the transported material depends on the instantaneous thickness ( or, sometimes density) of the layer at the measurement point. Therefore, the detected radiation intensity is proportional to the transported material thickness at the measurement point or, indirectly, to the instantaneous quantity passing through the measurement lines, FOLDIAK, G., (1986). In this work, gama ray emitting radioisotope source was used to measure the weights of different substances by using gama transmission principle. The aim of the work was to mount an experimental model of nucleonic belt weighers. These weighers offer the advantages of simple installation, easy frequent standardisation Xlll without production delays, low mechanical maintenance and independence from position, inclination and movement of conveyors. An experimental model of nucleonic belt weighers was mounted in Radioisotope labaratory. This experimental set consists of : -Gama ray emitting radiation source : Na-24 -Nal (Tl) detector -Multi Channel Analyser -Lead collimator -Lead blocks for shielding -Different substances for weight measurements -A plastic case as a model of conveyor belt -A metal rail which the plastic case stand on it After mounting the experimental set, several experiments were carried out with different substances. These were: -Coal -Soil -Pebble -Sand -Aliminyum As it was written before, the most important factors that effect the choice of a radiosotope were; availibility and easy usage, suitable half-life for the gauging system, appropriateness of the energies in the linear spectrums for the system, activitiy, cost, radiation safety. Belonging to these criterias, it was decided to choose Na-24 radioisotope. It was calculated that one experiment would take approximately 5 hours and a radioisotope with a short half-life would be suitable. According to the tables, 14.96 hours half-life and 1.37 MeV and 2.75 MeV gama energies showed that Na-24 could be used. If also, the cost of the radioisotope and availibility specifications were thought, it was realized that Na-24 gama radiation source would be the suitable selection. Na-24 was produced by irradiation of Na2CC»3 with 1.65xl012 n/cm2.sn thermal neutron flux. Carbon element can not answer to the (n,y) reaction. The half- life of the oxygen radioisotope is very short. According to these reasons, a short time after the irradiation, only Na-24 radioisotope remains. After choosing the radioisotope, the irradiation time was calculated as 60 seconds for 0.2 mCi activity. By the help of this time, 1 gr. of Na2C03 samples irradiated at İ.T.Ü. TRIGA MARK-II Reactor with 250 kW power and 1.65xl012 n/cm2.sn thermal neutron flux before every experiment. Collimation and shielding design of the radioisotope were completed according to the safety instructions. XIV 6 different experiments were carried out and measurements were taken. It was arranged that the cooling period after the irradiation would be 1.5 hours, for every experiment. Before every experiment, background measurements were taken twice. The aritmetic mean of this measurements were subtracted from the weight measurements and net amount of measurements were calculated. The substances used in the experiments had different particle shapes and sizes. Coal was consists of the particles between 1 to 4 cm. diameter. Pebble was consists of the particles between 1 to 2 cm. diameter. Aliminyum had particles which were approximately 6-7 cm. length, like bar shape. Soil and sand had the known characteristics: Soil; moist and gluey Sand; yellow sea sand In every experiment, every substance was measured for 4 different weights (that were weighted by the help of analog weigher before the measurements) and from 6 different points of the plastic case for every weight to decrease the statistical error. All these measurements were taken for a period of 1 minute. It took approximately 55 minutes to measure 1 substance and whole experiment took approximately 5 hours. Because of the half-life, the activity of the radioisotope decreases by the time passes. To decrease the error coming from this situation, all the net amount of measurements were standardized. Before every measurement, the empty plastic case measurements were taken and these results were used to standardize the net amount of measurements. At the end, by the help of these measurements, calibration curves for every substance were found. The obtained curves were applied to the substances that were measured in experimental set with unknown weights. The results taken from these calibration curves were sensitive and reliable.