Radyografide Farklı Ortamların Poz Süresine Etkisi

Abstract

Bu çalışmada standart radyografi tekniğinde olageldiği gibi ışın kaynağı ve parça arasındaki ortamın hava veya boşluk dışında > daha farklı bir malzemeden olması durumunda» ( yoğunluğunun artık ihmal edilememesi hali için ) pozlandır- ma sürelerini degi^t irmeksizin film yoğunluğunun bu ara ortam kalınlığının etkisiyle değişimine bakarak bu sürelerin daha doğru olarak saptanabilmesine çalışılmıştır. Bu tür durumlar günümüzde radyografinin gittikçe genişleyen kullanım alanları içinde artan bir sıklıkla ortaya çıkmaktadır. Böyle bir durumun şartlarını belirleyebilmek için bölmeli bir kap -kullanılmış ve içerisine belirli kalınlıklarda su)tahta,cara gibi farklı yoğunlukta malzemeler konarak ortaya çıkan değişimler incelenmiştir. Görüntü elde edebilmek için iki ayrı metalden basamaklı türde penetremet- ler yaptırılıp bunların film üzerindeki yoğunluk defterlerin deki değişimler incelenmiştir. Sonuçta beklenen ve hesaplanan değerlerden farklı olarak bazı malzemelerde çok 5 bazılarında ise az zayıflama saptanmış. Bunun nedenleri belirlenmeye çalışılıp, uygulamadaki etkileri ve bunların giderilmesi için gereken poz süresi değişimleri saptanmaya çalışılmıştır.

In this study» the X- and Gamma-ray radiographic techniques have been used by detecting the changes with dif ferent media. Normally > a radiograph is taking by using the air as a media between the source and the object which has nearly no weightness. But in some cases, one needs to make a measure through another kind of a media. Then it must be known what kind of differences can occur. On the other hand the effects of these rays on materials have been summarised in this study. Three effects of the rays on materials which are photoelectric event, Compton scattering and pair produc tion and absorption process of both rays also been explained For both rays, different sources have been used. There are limitations for some equipments as far as the different media concern rather than air. So for all types of radiation there will be a kind of shielding and will reduce the amount, In other words more exposure time is needed to have the same x results.lt must be detectable on the film, if the same exposure time is used as -in the air» which lowers the film density 5when having a different media between source and film. When it was a media with a great shielding factor »one can calculate that, the time must be multiplied on the inverse factor value. But it is very important to know the other effects on this phenomena. The most important effect is the "Build-up" factor, which is normally given according to material density, that can not be known. If there is an another value then the calculated » it must be the result of this Build-up factor. In a radiograph with a thin thickness of the media between the source and the film, the effect causes because of this media is not detectable. But for the thicker media, there is a need of a multiplication factor for the exposure time correction. When the thickness is getting bigger and bigger the total factor will be calculated from the inverse of the shielding factor and of the Build-up factor. By using the equipments on this work one can find a way to have a factor according to the film densities. Finally by using this factor one can multiply the normally exposure time to have the corrected value for a good radiograph. xi Normally in the use of X and gamma-rays, the radiograph is taken in a special place, in a shielded room or in a labratory.So the radiographer needs only to use the tables for exposure time calculations. But sometimes the specimen can not be moved. It can be a big piece or a pipeline or it can be placed in concrete or in the soil. Then there would be a shielding effect and this must be corrected. That means that the exposure time must be increased and the film must have enough radiation dosage on itself. The exposure time will be much greater then the normal calculated time and to reduce the difference, the radiation source must have high energy levels and high radioactivity. The half life of the radioisotopes or the energies of the X- rays should be carefully considered. As it is well known the half lifes are determined by the nature and the energy of the X-ray tubes can not be made more than a technical value. In the applications, it is very important to know how the changes in time can occur. Since the time is long enough and if it is not the right choice, it must be repeated and recalculated. The repeating process means more time and higher cost. The best thing is to have calculated time for a good radiograph with the accepted film densities. XII The radiographic techniques are not difficult and can be found with the help of the normally used exposure tables. The multiplying factor can not to be calculated easily» for this there must be tables for each different media and diffe rent thicknesses giving these factors. In many cases the radiographer should make more exposures than a single to have a good result. Usually the first good result will give the right factor to calculate the other exposure times. This factor is the effect of the shielding in this media. When one takes a radiograph in the dense media>the distance between the source and the film must become as little as possible, considering the geometrical unsharpness. At the last step »one can have the good radiograph as a result of correction of all the factors mentioned above. Therefore > using of this method gives the right result easier and faster, but it is only for use in special applications. In the future applications one can use it to correct the bad radiographs or to take X- and gamma tomographies. The technique needs portable radiation sources and an experienced radiographer who can use them. The radiographer must be in special cases a good diver for underwater applica tions or anybody at another branch depending on the scenario. xiii In the next time, one can be sure, that this method will be widely used for many applications. However it is clear that the radiographers, who works on this kind of NDT 5 are much more specialist than a simple radiographer, a diver, a civil engineer or an architect. In other words > it can be commonly usefull for many sciences and for new dimensions. The geometric factors, like unsharpness and shadow are the same importance like without the media between radia tion source and film >but all the procedure about placement will stay as the same.