ISO 11537 pdf download.Non-destructive testing — Thermal neutron radiographic testing — General principles and basic rules
1 Scope
This International Standard specifies the basic practices and conditions that are to be observed for thermal neutron radiography of materials and components for flaw detection. It is concerned with techniques using photosensitive film as a recording medium. However, it recognizes that alternative methods of imaging may be used more widely in the future. The scope includes neutron production and collimation methods, converter screen selection, radiographic film, neutron radiographic inspection techniques and the type of material to be inspected. This practice is generally applicable to specific material combinations, processes and techniques.
2 Background material
A glossary of terms relating to neutron radiography is presented in annex A. Attenuation of neutrons in matter is presented in annex B.
3 Neutron radiography method
Neutron radiography and X-radiography share some similarities but produce different results when applied to the same object. Neutrons replace X-rays as the penetrating beam of radiation whose intensity is modulated by an object, resulting in a film image of the features of the object. Since the absorption characteristics of materials for X-rays and neutrons are very different, the two techniques generally tend to complement one another. Neutron and X-ray attenuation coefficients, presented in figure 1 as a function of atomic number, are a measure of this difference.
5 Neutron sources
Neutron sources suitable for thermal neutron radiography can be classified into three general categories: — radioactive isotopes; — sealed tubes and accelerators of particles; and — nuclear reactors. Each of these sources produces high-energy neutrons that require moderation (slowing down) to thermal energies. This can be accomplished by surrounding the neutron source with beryllium, graphite, water, oil, plastic or some other moderator material.5.1 Isotopic sources Isotopic sources have the advantage of being small and portable but because of their relatively low neutron yield require long exposure times to achieve a given radiographic quality. Many isotopic sources have been used for neutron radiography and the most common of these are shown in table 1. Californium ( 252 Cf) is one of the most popular isotopic sources used for thermal neutron radiography because of its low neutron energy and small physical size which permit efficient moderation and high total neutron yield.5.2 Accelerator sources Sealed tubes and low-voltage accelerators utilizing the 3 H(d,n) 4 He reaction, high-energy X-ray machines utilizing the (x, n) reaction, and Van de Graaff accelerators and cyclotrons using charged-particle neutron reactions have been used as neutron sources for thermal neutron radiography. The targets of these accelerators are surrounded by materials that will moderate the neutrons to thermal energies. The thermal neutron fluence rate of accelerator sources before collimation can be as high as 10 9 neutrons cm -2 sec -1 . 5.3 Nuclear reactors Nuclear reactors are a preferred neutron source for thermal neutron radiography because of their high neutron yield. The high neutron intensity makes it possible to provide a tightly collimated beam and high-resolution radiographs can be produced with a relatively short exposure time. Some of the disadvantages of using nuclear reactors for neutron radiography are the high cost of installation and operation, lack of portability, and vulnerability to strict and complex regulation.
6 Neutron collimators
Neutrons are emitted in all directions from a source and are further scattered randomly by the moderator. A means of collimating thermal neutrons into a beam is provided to produce a high quality neutron radiograph. A well collimated thermal neutron beam coupled with the ability to place the object being inspected close to the imaging system will provide the best radiographic resolution.