Radiological and nuclear methods

Goal(s)

Main objective

Radiological methods are very popular for the non-destructive determination of defects, which are not visible to the naked eye in various types of structures. By means of industrial radiography, it is possible to detect and document fractures inside elements or ensure that there are no cracks in an object for quality control aspects. Radiological methods are also used for the assessment of reinforcement distribution in reinforced concrete structures. This group of methods is based on directing radiation from sources such as radioisotopes and X-ray generators against or through fresh or hardened concrete samples (in case of bridge maintenance purposes), taking advantage of the dependence of attenuation of radiation on material thickness and density. As a result, it is possible to obtain qualitative and quantitative Information about the object under study such as cracks dimensions, early signs of corrosion, microcracking progress. The durability of concrete can be quantified by certain characteristics such as porosity, sorptivity, and permeability. The quantification of neutron radiography images of concrete structures validates conventional measurements .

Nuclear methods are commonly used for the evaluation of the dampness and water absorption rate in different materials. Nuclear Magnetic Resonance and Neutron Radiography are really powerful in accurate detection on the smallest scale, but at the same time need a lot of technical practice and knowledge that requires specific training. For laboratory applications, NMR is used for the analysis of mixtures, analysis of molecular structures, dynamics of chemical reactions, determination of polymers conformations in fields such as food or pharmaceutical industry, or conservation of works of art. NMR techniques allow also to detection of free chloride molecules in the structure of concrete.

For industrial purposes both – radiological and nuclear methods are widely used in different areas for estimation of corrosion of aluminum components, water flow in plants, a study of nuclear fuels, combustion in engines. For civil engineering purposes, those are used for quantification of water movement in building materials.

Description

Functioning mode

• Computed X-ray/Gamma Tomography – reconstruction of a cross-sectional image from its projections; sample subjected to X-ray/gamma radiation at a given intensity. A detector registers the intensity of the ray received - sample revolves in front of the emitter and detector, emitting rays in all directions on the plane Another functioning mode consists of system with flat detector and conic beam of X-rays and in this case only the specimen has to revolve, relative displacement between detector and emitter is unnecessary; process is repeated for different sections of the sample and tri-dimensional information is being registered in the software .
• Neutron Radiography – objects bombarded with neutrons become radioactive and emit gamma radiation. In neutron radiography, imaging screen can retain their radioactivity and indirectly transfer the test object image to the radiographic film. Crucial parameters that can be controlled in this method are: neutron energy, exposure time, film type, L/D ratio source to target distance divided by beam diameter and type of conversion screen. Neutron radiographic imaging is a complex process, therefore in order to fully understand the methodology it is necessary to search for a detailed source of information such as - .
• Nuclear Magnetic Resonance Spectroscopy - sample subjected to a magnetic field and pulses of radiofrequency radiation to induce precession of the nuclear spin; The electromagnetic signal is measured by electronic device in the system inside, which is then converted to an NMR spectrum by applying a Fourier transform. NMR Spectroscopy is a complex technology as Neutron Radiography - details on the method functioning can be found for example n

Types

• Computed X-ray/Gamma Tomography - system composed of an emitter of X-ray radiation at a given intensity, a detector, computed radiography reader, cassettes with imaging plates and control station with monitors.
• Neutron Radiography – a system composed of the neutron beam, collimator, scintillator screen, mirror, and camera.
• Nuclear Magnetic Resonance Spectroscopy – solid-state, liquid-state NMR applied in continuous and pulsed wave spectrometers or Fourier –Transform spectrometers.

Process/event to be detected or monitored

• Computed X-ray/Gamma Tomography images to evaluate the structural integrity of concrete samples.
• Neutron Radiography – neutron flux density transmitting through the object, neutron flux density leaving the collimator, water absorption time.
• Nuclear Magnetic Resonance Spectroscopy – the electromagnetic response produced as the nuclei relax back to their equilibrium states (relaxation time), analysis of the spectra based on the ¹H nuclei associated with water.

Physical quantity to be measured (e.g. actions, displacements, deformations, dynamic structural properties, material properties including mechanical, electrical and chemical properties, relative displacements of the two sides of a crack, etc.).

• Computed X-ray/Gamma Tomography – attenuation measurements, the density of each point of the specimen under study; composition, the structural integrity of concrete samples.
• Neutron Radiography – depth of water absorption.
• Nuclear Magnetic Resonance Spectroscopy – electromagnetic signal measured as a free induction decay, atomic/structural details within the material on spectra; dampness of the structure.

Induced damage to the structure during the measurement

Radioactive and nuclear methods are non-destructive without moving the structure.

General characteristics

Measurement type (static or dynamic, local or global, short-term or continuous, etc.)

• Computed X-ray/Gamma Tomography: local measurement, both short-term or continuous.
• Neutron Radiography: local measurement, both short-term or continuous.
• Nuclear Magnetic Resonance Spectroscopy: local both short-term or continuous

Measurement range

• Computed X-ray/Gamma Tomography:

• structural changes detection at resolution down to hundred nanometres

• Neutron Radiography:

• detection of the liquid channels in range of 100 – 200 μm
• greyscale resolution of the camera – depends on the manufacturer e.g. 4096 gray level (12 bits)

• Nuclear Magnetic Resonance Spectroscopy:

• frequency region 4 - 600 MHz
• wavelength region: 75 - 0.5 m. field.

Measurement accuracy

• Computed X-ray/Gamma Tomography:

• depends on the quality of an image – computerized tomography is capable of producing images of millimetre or submillimetre resolution

• Neutron Radiography

• beam purity indicator (L/D ratio) is a determining factor in the sharpness of an image in a neutron radiograph,

• Nuclear Magnetic Resonance Spectroscopy

• affected by magnetic field fluctuation, quality of calibration process, and preparation of the sample.

Background (evolution through the years)

At first, computer tomography has been widely used for medical purposes to detect pathologies on the internal organs. In 1980 computer tomography technique has been developed for use in geological surveys such as analysis of the internal microstructures of rocks. Since its first use in the medical field, the technique of computer tomography becomes popular in other areas such as paleontology and engineering. Also, In 1980 high-resolution tomographic equipment has been introduced and named a micro CT scan. New sources of the radiation haves been then developed – gamma rays and synchrotron. For medical purposes, the equipment functioning mode is different than for industrial purposes. In the first case, the emitter and receptor revolve, while in the second case the specimen is moved and turned in system emitter-detector. CT method has been then applied in the asphalt mixtures technology for obtaining the geometry of the internal structure of the asphaltic mixtures to improve their properties. Recent advances in Tomography Imaging are the HRXCT – High-resolution X-ray Computed Tomography and micro-CT analysis in geosciences.

Neutron radiography has been used in industrial applications for at least fifty years until now as a non-destructive technique. Recent advances concern automatic systems for neutron inspection.

Performance

General points of attention and requirements

Design criteria and requirements for the design of the survey

Necessary to comply with health and safety regulations taking into account the radiation during measurement. The assessment of the suitability of the method should be preceded with examination of material under studies – its thickness, type, geometry and types of defects to be expected.

Procedures for defining layout of the survey

For radiological surveys:

• selection of energy, type and source of the radiation,
• selection of the direction and method of exposure of the examined object,
• selection of photographic film and exposure time,
• time of irradiation exposure,
• method of developing of the photographic film,
• interpretations of radiograms,
• report from the survey.

Design constraints (e.g. related to the measurement principles of the monitoring technologies)

In the case of the NMR method – NMR equipment used has a specific restriction on usage – laboratory organization for the stability of magnetic field and many others. This requires specific knowledge in the area of Nuclear Magnetic Spectroscopy and permission to work with NMR.

Sensibility of measurements to environmental conditions

The measurements are not influenced by environmental conditions.

Preparation

Procedures for calibration, initialisation, and post-installation verification

For NMR Spectroscopy the experimental set-up, initialization, and procedures of gradient shimming require ground knowledge and training in the use of NMR Spectroscopy. An example of the technical user guide can be found in - . Anyhow it will vary on the type of spectrometer and manufacturer.

Requirements for data acquisition depending on measured physical quantity (e.g. based on the variation rate)

Not applicable.

Performance

Requirements and recommendations for maintenance during operation (in case of continuous maintenance)

For NMR – magnetic field stability has to be verified in specified intervals of time or by observation of any abnormal signals on the spectrum, however, this requires great experience in analysis of NMR spectra.

Criteria for the successive surveying campaigns for updating the sensors. The campaigns include: (i) Georeferenced frame, i.e. the global location on the bridge; (ii) Alignment of sensor data, relative alignment of the data collected in a surveying; (iii) Multi-temporal registration to previous campaigns; and (iv) Diagnostics.

Not applicable.

Reporting

Reports include:

• Computed X-ray/Gamma Tomography – images of the samples with description.
• Neutron Radiography - images of the samples with description.
• Nuclear Magnetic Resonance Spectroscopy – analysis of the spectra of the sample.

Lifespan of the technology and required maintenance (if applied for continuous monitoring)

• Computed X-ray/Gamma Tomography – geometrically continuous monitoring in hours.
• Neutron Radiography – suitable for monitoring of long term changes of water content; starting from at least 240 minutes, neutron images can be taken automatically through time.
• Nuclear Magnetic Resonance Spectroscopy – continuous monitoring in days.

Interpretation and validation of results

Expected output (Format, e.g. numbers in a .txt file)

• Computed X-ray/Gamma Tomography – tri-dimensional image in a greyscale, where darker tones represent lower densities in the object.
• Neutron Radiography - time-dependent moisture distribution in a graphical form as a function of time; three-dimensional projection of the object in two dimensions, averaged over thickness along the path – radiograms.
• Nuclear Magnetic Resonance Spectroscopy – spectra with ¹H NMR relaxometry distribution; peaks on NMR spectra in a Lorentzian curve shape with parameters: amplitude [A], width at half height [Hz], and position in [Hz].

Interpretation (e.g. each number of the file symbolizes the acceleration of a degree of freedom in the bridge)

• Computed X-ray/Gamma Tomography - two-dimensional images of three-dimensional discontinuities are obtained; the images show the shape and size of the discontinuity in a plane perpendicular to the direction of radiation propagation; the difference in the degree of blackness of the radiographs in the place of the discontinuity image and outside this area (contrast) contains information about the dimensions of the discontinuity.
• Neutron Radiography – water penetration depth is obtained after post-processing of the data; the dampness of the structure; radiation attenuation can be related to the water content in a given material.
• Nuclear Magnetic Resonance Spectroscopy – correlation of the sample of interest with the signal intensity representing concentration; water mobility and pore size distribution can be assessed; relaxation time corresponds to molecular mobility and interatomic distance.

Validation

Specific methods used for validation of results depending on the technique

Comparison of the numerical results with the experimental data.

Quantification of the error

For neutron radiography, additional tests should be carried out such as capillary suction tests in order to quantify the error.

Quantitative or qualitative evaluation

Quantitative analysis with the use of finite element methods (FEM and image-based finite elements methods. In NMR Spectroscopy after spectra processing – baseline correction, chemical shift calibration, removal of solvents and other contaminants – statistical methods are used for spectra analysis. Peaks are associated with for example Alkali-Silica Reaction and quantification.

Detection accuracy

The detection accuracy of radiological methods is affected by local temperature variations, presence, and concentration of dissolved salts in the pore solution as well as by the presence of metallic components close to the measurement point.

Advantages

• Computed X-ray/Gamma Tomography:
  • dimensionally accurate analysis,
  • vertical and horizontal overview of the specimen,
  • no interference in the structure of the investigated object,
  • analysis of the shape of a defect,
  • easily accessible source of radiation.

• Neutron Radiography:
  • possibility of imaging light elements.

• Nuclear Magnetic Resonance Spectroscopy:
  • very accurate and detailed technique,
  • advanced NMR techniques use portable magnets that are applied to the object of interest.

Disadvantages

• Computed X-ray/Gamma Tomography:
  • slow in case of use of X-ray radiation,
  • expensive for high power sources,
  • limited resolution.

• Neutron Radiography:
  • expensive for high power sources.

• Nuclear Magnetic Resonance Spectroscopy:
  • limited availability of the equipment,
  • high cost of tests,
  • difficult to operate without training,
  • equipment sensitive to environmental condition,
  • difficult calibration process.

Possibility of automatising the measurements

NMR, Computed Tomography and Neutron Radiography work in an automated manner, however it is necessary to prepare the samples and control the process by an experience operator. Technical improvements focus on the ongoing optimization of standard imaging setups and development of the methods that go beyond the established 3D mapping. The main aim is to increase the potential of neutron imaging and widen the range of applications. Advances in neutron imaging consists of introducing new ways of achieving the image contrast and new neutron imaging concepts .

Barriers

The main barrier in usefulness of the methods on a daily basis is the cost of the equipment. The presence of ionizing radiation forces strict rules during operation which limits radiological methods to laboratory inspections in many cases. In computed radiography that can be overcome with use of imaging plates, which are suitable for laboratory and mobile inspections under various environmental conditions.

Existing standards

• ASTM D2950-91 (1997) STM for Density of Bituminous Concrete in place by Nuclear Methods.
• BS 4408: pt. 3, 1970 Non-destructive methods of test for concrete-gamma radiography of concrete, British Standards Institution, London.
• TGL 21 100/01 Non-destructive testing of concrete buildings and structures – Guideline for the determination of the density with gamma rays.
• NDIS 1401-1992 Methods of radiographic examination for concrete constructions.

Applicability

Relevant knowledge fields

• Civil Engineering:
  • diagnostics of civil engineering structures,
  • high-performance concrete investigation,
  • fiber reinforced concrete investigation,
  • asphalt mixtures testing.

• Palaeontology:
  • ancient relics and heritage.

• Geotechnical engineering:
  • aspects of rocks and minerals.

• 3D Printing:
  • replicas.

• Automotive:
  • inspection of defects after manufacturing.

• Art conservation

Performance Indicators

• cracks,
• deteriorated mortar joints,
• delamination,
• displacement,
• loss of section,
• rupture,
• deformation,
• debonding.

Type of structure

• bridges,
• tunnels.

Spatial scales addressed (whole structure vs specific asset elements)

Radiological and nuclear methods are used especially in bridge components such as abutments, decks, beams, girders, cables, rivets, trusses, pins and hangars, paint, fenders, footings, foundations, and culverts.

• piers,
• rivets, bolts,
• welding joints,
• retaining walls,
• culverts,
• foundations.

Materials

• concrete,
• reinforced concrete,
• steel,
• metals,
• composites,
• wood,
• polymers,
• ceramics.

Available knowledge

Reference projects

No reference projects.

Other

Phoenix Transforming Nuclear Technology – Neutron Radiography Training Description

Phoenix Transforming Nuclear Technology – Neutron Generators

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