Neutron Radiography

Neutron radiography provides a very efficient tool for investigations in the field of non - destructive testing as well as for many applications in fundamental research. A neutron beam penetrating a specimen is attenuated by the sample material and detected by a two dimensional imaging device. The image contains information about material and structure inside the sample because neutrons are attenuated according to the basic law of radiation attenuation. Contrary to X rays, neutrons are attenuated by some light materials, as i.e. hydrogen, boron and lithium but penetrate many heavy materials. Neutrons are able to distinguish between different isotopes and neutron radiography is an important tool for studies of radioactive materials.

At the Atominstitute der österreichischen Universitäten (ATI) neutron radiographic investigations are performed for more then 35 years. The detectors mainly used are converter / film assemblies. However, these detectors are limited regarding their sensitivity, dynamic range and linearity. Due to rapid development of detector and computer technology as well as deployments in the field of digital image processing, new technologies are nowadays available which have the potential to improve the performance of neutron radiographic investigations enormously. Therefore, the aim of this work is to identify and develop two and three dimensional digital image processing methods suitable for neutron radiographic and tomographic applications, and to implement and optimize them within data processing strategies.

In neutron radiography (NR), a neutron beam is recorded on a two dimensional integrating position sensitive detector after penetrating a sample. The output of the detector is an image representing the macroscopic structure of the sample interior, as the neutron beam is attenuated depending on the sample material and geometry according to the law of radiation attenuation:

F (x,y,E) = F0(x,y,E) · e-S (x,y,E) · d

F0(x,y,E): flux density leaving the collimator in [cm-2s-1]
F (x,y,E) : flux density transmitted through the object in [cm-2s-1]
d : the thickness of the sample in [cm]
S : the total macroscopic neutron cross-section in [cm-1] of the sample material
Because the interaction takes place with the nucleus of the atom, the cross-section depends not only on the chemical element, but also on the specific isotope. Cross-sections of isotopes of the same element may vary over several orders of magnitude. For a chemical element consisting of a mixture of several isotopes, the overall macroscopic cross-section is the sum of the cross-sections of the isotopes weighted with their mass fractions, and for a material consisting of several chemical elements, the overall cross-section is the sum of the cross-sections SEl of the component elements designated by an index El, weighted with their mass-fractions pEl:

As mentioned above, the S of any isotope or element also depends on the neutron energy, i.e. the material of a sample interacts differently with neutrons of different energies from a typical polychromatic NR beam.

Basic Experimental Set-Up of Neutron Radiography

The basic experimental layout of NR consists of a neutron source, of a collimator functioning as a beam formatting assembly, of a detector and of the object of study, which is placed between the exit of the collimator and the detector.

Neutron Beam Line

A neutron beam line consists of all components from the neutron source to the exit of the collimator. The following performance criteria are most relevant:

These characteristics depend on the nature of the source and of the beam formatting assembly, and are also partly correlated. From these performance criteria the strength of the source (S) and a parameter (Q), representing the quality of the beam line can be defined:

  and 

At stationary NR facilities, neutron sources are usually fission reactors or accelerator / target assembles, including neutron spallation sources . In all of these sources, neutrons are generated with high initial energies, between 2 MeV ( mean value in nuclear reactors) and several 100 MeV (in spallation sources). Due to the low cross-section of fast neutrons, some applications, which require extremely high transmission of neutrons (i.e. thick steel samples, several cm of water,...) can only be radiographed by fast neutrons. For other materials (i.e. thin layers), the use of cold neutrons provides the most suitable technique. But nevertheless, thermal NR is the most frequent technique because thermal neutrons provide good contrasts with a large number of elements, and they are more easily available at most large neutron sources (in a thermal reactor neutrons have to be moderated to maintain the chain reaction).
 

Neutron Radiography detectors

NR detectors are generally plane integrating position-sensitive imaging devices containing material with a high neutron cross-section functioning as neutron converter and a recorder, which has the task of collecting the signal emitted by the converter during exposure. After the measurement the detector signal is read out and may be digitized. In the context of the neutron radiographic measurement the output of the detector will be called detector signal or radiographic image, whereas the terms signal and image are considered equivalent.

Typical NR-detectors are:

Camera / Scintillator Systems

This type of NR detector consists of a neutron sensitive scintillator screen which has the task of converting the neutrons in photons and a camera which is the recording device for the light emitted by the scintillator. Typical scintillator materials are ZnS(Ag)-6LiF or ZnS(Cu)-6LiF. The first step of the detection mechanism for these scintillator materials is an (n,)-reaction:

Then these emitted a -particles cause a secondary converter emission in the form of visible light which is recorded by the camera. The camera can either be a video- or a CCD-camera, with or without intensifier. The performance of a CCD-camera can be enhanced by cooling, which can be done either by peltier elements using air or water circulation (approximately 30° C - 40° C), or by liquid nitrogen (approximately 120° C - 130° C). The cooling reduces the dark current and therefore enhances the signal to noise ratio. Especially for applications requiring longer exposure times (several minutes), the optimal temperature of the CCD-chip is in the range of around -120° C, which can be achieved only by liquid nitrogen cooling. To avoid radiation damage of the semiconductor material of the CCD-chip the camera has to be placed outside of the direct neutron beam. Therefore the light emitted by the scintillator is reflected to the camera by one or several mirrors.

Advantages of these detectors: An excellent linearity, high sensitivity, easy handling (readout of information and digitization do not cause additional working steps), good reproducibility (for the use of a cooled CCD-camera) and the possibility of real-time measurements (for the use of an adequate camera with high repetition rate and sensitivity). Disadvantages are the high costs of the detector system and the lower spatial resolution compared to film and IPs, which depends on the chosen chip size to image size ratio.

Components of the detector of the ATI-Austria at the Station-2:

·Nitrogen cooled slow scan CCD-camera
Chip: SITe SI502A/T
         512x512 pixels
         24x24 µm2 pixel size
QE : up to 90 %
·16 bit digitalization « 65535 gray-level
Lenses:    Nikon NOKT 58 mm F 1,2
               Nikon NOKT 180 mm F 2,8
·Scintillator
Levy Hill: ZnS(Ag)-6LiF
·Mirror: 2 mm thick glass plate coated with Al and TiO2
·Al-Box, lighttight
Area of the object projected on one pixel:
58 mm F1,2 : 410x410 µm2
180 mm F 2,8 : 80x80 µm2
image size:
58 mm F 1,2 : 21x21 cm2
180 mm F 2,8 : 4x4 cm2
 Illustration of image digitization: a physical image and the corresponding digital image










Comparison of X-Ray and Neutron Radiography

X-ray radiography has a long tradition as an important tool for medical applications as well as in the field of non destructive testing. While penetrating matter, X-rays interact with the electron clouds of the atoms in the object. Therefore the attenuation of X-rays depends on the charge density of the electron cloud and increases with the atomic number of the sample material, respectively. Unlike X-rays, neutrons interact with the nuclei. Thus, the total neutron cross-section subject to the neutron energy depends on the properties of the nuclei and varies from element to element, even from isotope to isotope. Fig. b shows very impressively the quite random attenuation behaviour of thermal neutrons for different elements, whereas the attenuation of X-rays with an energy of 125 kV clearly depends on the atomic number. Fig. a. and b. demonstrate impressively that NR can yield different information then obtainable with X-rays, because these two methods are able to visualize other materials. Fig. b shows the image of a camera, made with X-rays and Fig. a shows an image of the same camera made with NR. These two images express the complimentary result obtainable with these two methods very clearly. As neutrons penetrate the metallic enclosure of the camera easily, plastic components (containing hydrogen) inside the camera get able to be seen, whereas in the image obtained with X-rays mainly the metal parts of the camera are visualized.
 


Fig. a: Neutron radiography of a camera

Fig. b: Radiographic image of a camera made X-rays

 

Neutron Radiography Facilities at the Atominstitute

At the Atominstitute of the Austrian Universities neutron radiographic examinations have been carried out for more than 35 years at various different facilities at a 250 kW TRIGA Mark II reactor. Presently, Two beam lines at the 250 kW TRIGA Mark II reactor at the Atominstitute are dedicated to NR investigations.

Neutron Radiography Station-1 at ATI

The NR station-1 at the Atominstitute is located at a radial thermal beam channel of the TRIGA reactor. The collimator assembly penetrates into the reactor pool, leaving only a thin layer of water between the graphite moderator of the reactor and the collimator entrance. The g - background emitted by the reactor is reduced by a bismuth filter which is mounted at the collimator entrance. The size of the entrance aperture (D) is 5 cm. The collimator continues as a divergent conical space with walls lined with boron carbide. Part of the collimator can be filled with water to close the beam channel. At the beginning of operation, this water is pumped out of the channel. The length of the collimator (L) is 250 cm. Therefore the L/D-ratio of this facility is 50. During measurement, the samples are placed in a closed room shielded with concrete walls around the collimator exit, which is not accessible during reactor operation. Therefore the samples have to be brought to the irradiation position with a vertical elevator system connecting the inside of the shielded room to a platform atop of it. The elevator system consists of two independent devices that can be lowered into the shielding and brought back to the platform. One of these devices is a table to place the objects, the other one is a metal frame intended for mounting a cassette containing X-ray film and a gadolinium converter. A shutter located between collimator exit and sample position is opened and closed by a pneumatic mechanism before and after each individual measurement. A control unit atop of the facility enables control of the elevator and of the shutter. The beam diameter at the collimator exit is 40 cm.

Neutron Radiography Station-2 at ATI

The NR station-2 at the Atominstitute is located at the thermal column of the TRIGA reactor. This is a cube of nuclear grade graphite with dimensions of about 1,2 m at each side. A collimator is mounted inside this cube with an aluminum shell of the dimensions (10 x 10) cm2. The conical collimator inside this shell is made of cadmium sheet metal. The space between the collimator and its shell is filled with a polycrystalline material consisting of paraffin and boron carbide. The diameter of the aperture blend is 2 cm. It is a sandwich construction made of 5 mm thick lead and boron carbide layers. A 4 cm thick bismuth crystal is used to reduce the g -background emitted by the reactor. The second part of the collimator is mounted inside the concrete door in front of the thermal column. This part of the collimator has a cylindrical shape and a steel shell. This collimator design (2 parts) was chosen to enable access through the concrete door to an irradiation position at the end of the thermal column. The total length of the collimator is 257 cm. Therefore the L/D-ratio of this facility is 128. The beam diameter at the collimator exit is 8 cm. Both parts of the collimator are lined with boron carbide to absorb neutrons to extract a useful neutron beam.
Reasonable exposure times at the thermal column of the TRIGA reactor at the Atominstitut with the thermal flux 1,3x105neutrons/(cm2s) can get as low as 20s per image. This means, that it is possible to perform a set of measurements necessary for neutron tomography (about 200 images) within several hours. Therefore, in a second working step, an entirely automated neutron tomography facility has been built at the Atominstitut with this detector. The whole fission chamber and SEQ 5 dosimeter images are reconstructed from  2 or 3 images because of the small beam size at the ATI. 3D reconstruction of a diode by neutron tomography was done at the ATI NR station-2 with CCD-camera for 20s explosure time.
 
 
 


NR of a fission chamber

NR of a SEQ 5 dosimeter

NT of a diode