Gama Geçirgenlik Tekniğiyle Yoğunluk Ölçüm Sistemi Tasarlanması Ve Kurulması

Abstract

Bu çalışmanın amacı, y-geçirgenlik tekniğiyle yoğunluk ölçümü yapan bir sistemin tasarımım gerçekleştirerek elektronik olarak kurmaktır. Bunun için önce sistem bileşenlerinin tümü ayrı ayrı oluşturulup, çalışmaları kontrol edilmiş sonra bunlar biraraya getirilerek sayım düzeneği oluşturulmuştur. Kurulan düzeneğin güvenilirliğini kontrol etmek amacıyla seçilen 5 malzemenin her biri için beşer sayım alınarak doğrusal kalibrasyon eğrisi çıkarılmıştır. Radyoaktif kaynak olarak İTÜ Nükleer Enerji Enstitüsü TRIGA MARK-II reaktöründe, sodyum karbonatın (Na2CO3) ışınlanmasıyla elde edilen Na-24 radyoizotopu kullanılmıştır. Ölçüm sonuçlarının doğrusal kalibrasyon eğişinden sapmalarının küçük olması ve sayımlardaki yüzde hatanın ortalama %2,5 olması sistemin güvenilir olduğunu açıkça göstermektedir.

Radiogauging techniques are more sensitive than conventional techniques. Radiogauges work with interaction of radiation by matter. Thus they could be used for thickness, density (density thickness), homogenity, humidity or level measurements. In some conditions there is not any other alternative except radiogauging. Radiogauges are economic, portable, sensitive and stable systems therefore they were preferred in many branches of industry. It could be said that there are two types of radiogauging system considering the source. X-ray machine sourced and radioisotope sourced radiogauges. Radioisotope sourced radiogauges have more advantages than machine sourced gauges. These advantages are here: 1) Less expensive 2) Portable 3) Could be made quite small 4) Not require any power supply Vlll 5) Could be used by people who are not highly experienced in electronic. Ofcourse they have some disadvantages too, which are: 1) Cannot turn off 2) Constant energy 3) Constant activity for short time intervals Then radiogauging systems are classified about their measurement techniques. There are two important techniques here: 1) Transmission technique 2) Scattering technique In transmission technique, sample (whose one of its parameter can be measured) is between the detector and the source. Therefore radiation passes through the sample and reaches the detector. In scattering technique detector and source are on the same side of the sample thus radiation scatters from the sample then reaches the detector. Shield must be used between the detector and source in this technique. In this thesis a radioisotope sourced (y-source) radiogauging system which works by transmission technique was planned and produced. When a radiogauging system is planning the following properties must be considered: 1) The function or goal of system 2) Principle of radiogauging (transmission or backscattering) 3) Type of source (type of radiation, activity etc.) ix 4) Type of the detector. Function of the system and physical parameter which will be measure are known because of industrial necessities. Thus principle, source and detector types must be determined. The criteria of selection were examined in this thesis. The principal goal of this thesis is planning and producing a radiogauging system which could be used for industrial purposes. This system that was produced is only a prototype. Every radiation detection pulse counting system (detector plus preamplifier) exhibits a characteristic delay time in the nature of a recovery time. By definition no counts can be recorded during this time. This recovery time is a function of the characteristics of the detector, preamplifier, amplifier and recording system (eg scalers or ratemeters) but is ultimately defined by the detector-preamplifier combination. Two limiting types of recovery time are exhibited by detector systems: The paralyzable and the nonparalyzable. Although no detection systems exhibit perfect paralyzable or nonparalyzable behaviour, it is useful to discuss these as limiting cases Gardner (1967). The paralyzable detector system has a recovery time that exists for a finite time after a radiation discharge occurs in the detector. Most GM-counter systems exhibit recovery times that closely approximate to this condition. If radiation-discharge interactions occur at rates so high that the time between them is always less than the recovery time, then no counts are recorded and the system is paralyzed. The respond of such a system would go through a maximum observed counting rate as the radiation flux is increased Gardner (1967) The nonparalyzable detector system has a recovery time that exists for a finite time after a radiation discharge occurs in the detector that results on a recorded count. Most scintillation counters exhibit recovery times that closely approximate this condition. For the nonparalyzable system the observed counting rate would approach a limiting value as the radiation flux is increased Gardner (1967). The adequacy of an electronic circuit depends on its semiconductor devices. High frequency characteristics of semiconductors determine the fitness of system. There is a limiting counting rate for a counting system about its detector and circuits high frequency characteristics. Cost of a semiconductor device is increasing by its maximum working frequency. In these thesis IC741 and IC555 integrated circuits are used. Thus this system frequency limit stayed in a mean level. This system can be used for normal density measurement process in spite of it cannot works by high counting rates. In this work, GM-tube was preferred because it is a low priced, stable and rugged detector. Even though this preferation makes this system a paralyzable one. High voltage supply, preamplifier, timer and counter circuits were produced in separate parts. All circuits have their own voltage supply except counter. Counter is electrically dependent to the timer by an opto- coupler. Opto-coupler used for protection against the high voltage pulses. Therefore counter should not be used alone. There are two needs for making the system in separate parts. One of them is to produce a multi tasking system which parts can be use for other needs because they are separate and complete sub-circuits. Other one is pedagogical. Some density measurement experiments were done for checking the reliability of this system. Na-24 radioisotope was chosen for being a radioactive source. Therefore lg Na2C03 (sodium carbonate) which consists 0,4g Na-23 has been irradiated in İTÜ TRIGA MARK-H Reactor's rabbit system for 1 minute in 250 kW. After 20 minutes of cooling, irradiation experiments were done. At first step density of liquid and powdered solid samples were measured and calculated by conventional methods then they xi measured by radiogauge. By the responses of radiogauging system a linear calibration curve was determined. It was showed that this system reliable and fit for normal (not too sensitive) density measurement processes.