Neutron Sources for Radiography

1. Nuclear Reactors:
o Produce large numbers of neutrons at low or medium power levels.
2. Particle Accelerators:
o Generate neutrons using nuclear reactions by accelerating charged particles. LINAC is
commonly used.
3. Radioisotopes:
o Generate neutrons when positive ions bombard specific materials.

Moderators and Collimators

• Moderators: High-energy neutrons are slowed to thermal levels using materials like water,
paraffin, or carbon.
• Collimators: Focus the neutron beam, allowing only neutrons traveling in the same direction to
pass through. The collimator’s L/D ratio (length to inlet diameter) determines image resolution.

Neutron Detection

• Neutrons cannot directly ionize radiographic film; hence, a converter screen is used.
• The converter screen absorbs neutrons and converts them into secondary radiation that the film
or detector can pick up.
• Common detection methods include:
o Regular radiographic films with converter screens (e.g., gadolinium or scintillator
screens).
o Advanced methods like imaging plates and etchable plastic films.

Qn ; Techniques in Neutron Radiography ??

1. Direct Neutron Radiography Technique

o Thermal neutrons hit a cassette containing X-ray
film sandwiched between metal foils (e.g.,
gadolinium).
o Neutron absorption emits particles (e.g.,
electrons) that expose the film.
o Scintillator screens (e.g., lithium-6 and zinc
sulfide) produce light that quickly exposes the
film.
2. Indirect Neutron Radiography Technique (Transfer
Technique)
o Neutrons strike special screens (e.g., indium or
dysprosium), making them temporarily
radioactive.
o The radioactive screen is then placed on X-ray
film to transfer the image.
o This technique is slower but useful in nuclear
applications due to the separation of activation
and image capture steps.

Advantages of Neutron Radiography

• Effective for low atomic number materials in high atomic number assemblies.
• Can detect defects invisible to X-rays or gamma rays.

Qn: Notes on Flash Radiography ??

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2. Double Wall Single Image (DWSI) Technique

• Used when the inside surface of the

pipe is not accessible.
• How it works:
o Both the radiation source and
film are placed outside.
o Radiation passes through
both walls, but only the film-
side weld is visible.
• Suitable for pipes with diameters
larger than 90 mm.
• Requires multiple exposures to cover the entire weld circumference.
• The IQI is placed on the film side.

3. Double Wall Double Image (DWDI) Technique

• Used for smaller diameter pipes (up

to 90 mm OD).
• How it works:
o Radiation passes through
both walls and the source-
side weld and film-side weld
are captured on the film.
o The source is positioned at
an offset to avoid overlapping weld images.
o The IQI is placed on the source side.
o Needs two exposures (perpendicular to each other) to cover the entire weld.

Qn : Real-Time Radiography (RTR) Or Fluoroscopy

What is RTR?

• Real-Time Radiography (RTR) is an

advanced method of radiographic
inspection.
• It eliminates the use of films, instead
capturing digital images electronically.
• RTR allows almost instant viewing of
images, reducing the time between
exposure and image capture.

How RTR Works:

1. Radiation Source: X-rays or gamma rays are used to pass through the object.
2. Fluorescent Screen: Replaces the traditional film and converts radiation into visible light.
3. Conversion to Video Signal:
o The visible light is converted into a video signal using a closed-circuit TV camera
(CCTV).

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3. Annular Array Probes

• Description: Composed of concentric ring-shaped elements.

• Application: Provides improved focusing for high-resolution inspection of small areas.
• Key Feature: Offers depth focusing without mechanical adjustments.

4. Sectorial (Angle Beam) Probes

• Description: Designed to scan using angled sound beams.

• Application: Ideal for weld inspections and angled surfaces.
• Key Feature: Capable of covering a wide area by sweeping the beam at different angles.

5. Custom Array Probes

• Description: Tailored designs for specific applications, such as turbines or pipes.

• Application: Optimized for irregular shapes or unique inspection needs.
• Key Feature: Ensures maximum coverage and accuracy for specialized components.

Qn. Explain the calibration procedure in TOFD?

1. Setup of Equipment:

• Transducer Placement: Position the TOFD transducers (a transmitter and receiver) at the
required inspection points on the material surface.
• Initial Calibration Block: Use a calibration block with known defects or features to verify the
system's accuracy.

2. Adjusting for Material Properties:

• Material Velocity: Ensure that the velocity of sound in the material is set correctly in the system.
This is necessary for calculating accurate travel times of the waves.

3. Establishing Reference Signals:

• Lateral Wave: The lateral wave is generated just below the surface and travels the shortest path
between the two transducers. It's used to calibrate the system for accurate wave travel times.
• Back Wall Signal: This strong reflection from the back wall of the material is used as a reference
point in the calibration.
• Defect Signal: For defect detection, calibrate the system to detect diffracted waves from defects
like cracks or flaws, typically weaker than the back wall signal but important for defect
characterization.

4. Calibration Adjustments:

• Fine-tune the time delays and system settings based on the signals from the lateral wave, back
walland defect signals to ensure proper resolution and defect detection capabilities.

5. Verification:

• Test on Known Defects: Verify the calibration by checking the system's response to known
defects, ensuring the signals from cracks or flaws are properly detected and identified.

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Limitations of TOFD

• Weaker Diffracted Signals: Diffracted waves are weaker than reflected waves, requiring
sensitive equipment.
• Requires Skilled Operators: Interpretation of TOFD results needs expertise.
• Surface Preparation: Clean and smooth surfaces are necessary for accurate testing.

Qn. Explain the synthetic aperture focusing technique

The Synthetic Aperture Focusing Technique (SAFT) is an advanced imaging method used in ultrasound
testing to improve resolution and detectability of flaws. It combines data from multiple ultrasonic
signals to create a high-resolution image of the tested material.

Working Principle:

1. Signal Collection:
o Ultrasonic waves are sent into the material from multiple transducer positions.
o The reflected signals (echoes) from defects or boundaries are collected.
2. Data Processing:
o The echoes received at different transducer positions are analyzed.
o A computational process reconstructs the signals to focus on a specific point in the
material.
3. High-Resolution Image:
o The signals are combined to produce a detailed image of the material’s interior.
o This synthetic "aperture" mimics the effect of using a large physical aperture.

Advantages:

• Enhanced Resolution: Provides clearer images compared to traditional ultrasonic methods.

• Improved Flaw Detection: Identifies smaller and more subtle defects.
• Better Depth Information: Allows precise imaging of defects at various depths.

Applications:

• Inspection of welds, composites and metals.

• Suitable for industries like aerospace, automotive and construction.

Qn. What is the significance of probe angle during phased array inspection?

The probe angle in phased array inspection is crucial as it determines the direction and depth of the
ultrasound beam within the material being inspected. Its significance can be explained as follows:

1. Defect Detection:
Different probe angles help detect defects of varying orientations. For example, a specific
angle can make the ultrasound beam perpendicular to a crack, enhancing reflection and
making the defect easier to identify.
2. Coverage:
By varying the angle, phased array probes can inspect a larger area without physically moving
the probe. This improves efficiency and ensures complete coverage of the test object.
3. Material Geometry:
Certain probe angles are more suitable for inspecting complex geometries, such as curved or
angled surfaces, where a fixed-angle probe may not reach effectively.
4. Depth and Resolution:
Shallow angles are used for near-surface inspections, while steeper angles are better for
detecting deeper flaws.

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 • Non-hazardous: Unlike radiography, it doesn’t use ionizing radiation, so no safety zone is
required.
• Efficient: Covers more area with greater precision.
• Versatility: Suitable for complex geometries.

Applications:

• Weld inspections
• Crack and flaw detection
• Thickness measurements
• Corrosion inspection

Qn. Explain the significance of wedges used during phased array inspection.

Wedges play a critical role in phased array ultrasonic inspection by helping to direct and angle the
ultrasound beam into the test material. They serve the following purposes:

1. Beam Steering: Wedges allow the ultrasound beam to enter the material at the desired angle for
better defect detection, especially in angled or curved surfaces.
2. Mode Conversion Control: They help in minimizing unwanted mode conversions (like shear
waves) by ensuring proper coupling between the probe and the material.
3. Surface Adaptation: Wedges provide smooth contact with uneven or curved surfaces, ensuring
accurate and consistent inspections.
4. Coupling Medium: They act as an intermediary between the probe and the test surface, ensuring
the ultrasonic energy is efficiently transferred.

Qn. Write short notes about beam steering of ultrasound waves.

Beam steering in ultrasound refers to the ability to control the direction of ultrasound waves without
physically moving the transducer. This is achieved by electronically adjusting the timing (phase) of
signals sent to the elements of a phased array transducer.

• How it works:
o Each transducer element emits sound waves.
o By delaying the signals to specific elements, the ultrasound waves combine and form
a beam that can be steered at different angles.
• Applications:
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Helium is preferred for leak detection because it’s inert, meaning it won’t react with other materials in
the system.
Since helium is rare in the atmosphere, it minimizes background interference, making it ideal for
accurate leak detection.
Due to its light mass, helium can pass through small leaks more easily than heavier gases, increasing
the sensitivity of the test.
Immersion test
• Small vessels are pressurized with any gas and the vessel is immersed in water or water with an
additive
• Escaping bubbles from leak locations are observed.
• The method locates small leaks.

Pressure change test

• The closed container is pressurized and the pressure of the system is observed for two hours or more.
• After this time, the initial and final pressures in the container are compared
• A change in pressure is indicative of a leak in the system
• It is not possible to locate the leak spot and the test could be time consuming.

Qn. Explain types of leak testing?

Types of Leak Testing

Leak testing is a non-destructive testing (NDT) method used to detect and locate leaks in pressure
vessels, pipingand other systems. Below are four types of leak testing methods explained in detail:

1. Hydrostatic Test

• Purpose: Used to check for leaks in welded pressure vessels, piping and fittings.
• Process:
o The pressure vessel is filled with water at high pressure.
o If water seeps out, it indicates a leak or defect in the material.
• Advantages:
o Reliable for identifying weak points or discontinuities.
o Commonly used due to its effectiveness.
• Limitation:
o It can be time-consuming to perform.

2. Liquid Film Test

• Purpose: Detects leaks in pressurized vessels.

• Process:
1. Pressurize the vessel with gas.
2. Apply a thin layer of soap solution (or similar liquid) to the vessel's outer surface.
3. Observe for bubbles, which form at the leak site as gas escapes.
• Advantages:
o Simple and cost-effective.
o Shows the exact location and size of the leak.
• Limitation:
o Only effective for detecting surface leaks in pressurized vessels.

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 o The sensors detect AE signals when damage begins in the material.
2. Strength Estimation:
o The strength (amplitude) of the AE burst is measured to evaluate the severity of the
damage.
3. Locating the Source:
o Multiple sensors are placed on the material in a specific pattern.
o The time taken for the AE waves to reach each sensor is recorded.
o Using wave velocity and arrival times, the damage source is located.

AE Inspection Setup

1. Sensors and Coupling:

o A piezoelectric sensor is attached to the object using a coupling material (e.g., grease)
and secured with tape.
2. Signal Processing:
o The sensor output is sent to a pre-amplifier to eliminate unwanted noise.
o The signal is filtered and further amplified before being analyzed by a computer
system.
3. Signal Analysis:
o When the signal crosses a set threshold voltage, the analysis system records the AE
event.

Wave Behavior in AE

• AE waves spread spherically from the source and bounce off the object’s boundaries.
• The signal detected is a combination of the original wave’s components.
• Resonant waves reaching the sensor produce the strongest detected signal.

Applications of AE Testing

• Monitoring structural integrity in aerospace, automotive and construction industries.

• Detecting leaks in pressurized systems.
• Identifying fatigue cracks in metals.

Advantages of AE Testing

• Real-time monitoring of damage.

• Detects micro-level defects before they grow larger.

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 2. Detection of Defects: Heat-sensitive paints are useful in locating defects such as cracks,
delamination, or uneven heating in materials, which may not be visible to the naked eye but are
detectable through temperature variations.

Qn. Write short notes about the significance of thermo mechanical behaviour of materials during
thermo graphic evaluation

Thermo-mechanical behavior refers to how materials respond to temperature changes and mechanical
forces. Understanding this behavior is crucial during thermographic evaluation, which involves using
infrared thermography to inspect materials for internal defects and stress distribution. Here's why it's
significant:

1. Heat Distribution and Material Response:

When a material is subjected to temperature changes or mechanical stresses, it undergoes

thermal expansion or contraction. The way a material absorbs, conducts and dissipates heat
affects the thermal pattern on its surface, which can be captured using infrared cameras. These
patterns help identify internal defects like cracks or voids that alter heat flow.

2. Influence on Thermographic Image Quality:

The thermo-mechanical behavior determines how well a material’s surface will show up in a
thermographic inspection. Materials with high thermal conductivity, for example, will
distribute heat more evenly, leading to clear thermographic images. On the other hand,
materials with low conductivity might show uneven heat distribution, making defect detection
more challenging.

3. Stress and Strain Effects:

Mechanical stress and strain can cause localized heating due to friction or deformation.
Thermographic evaluation can detect these temperature variations, which are crucial for
assessing material performance under operational conditions. For instance, when materials are
stressed, micro-cracks may form and affect the heat transfer, which thermography can identify.

4. Predicting Material Behavior Under Different Conditions:

The thermo-mechanical properties of a material help predict its behavior under varying
operational conditions (e.g., temperature fluctuations, pressure). By analyzing thermographic
images, engineers can assess the material’s performance, durability and potential failure points,
ensuring better design and maintenance practices.

Qn. Explain the concept of leak testing and its importance in various industries.?

Leak Testing is the process of identifying and measuring leaks in systems or components to ensure
safety, reliabilityand environmental compliance.

Concept of Leakage:

• Leakage refers to the escape of liquid or gas from a system due to a pressure difference between
the inside and outside.
• It can also occur due to permeation, where fluids pass through a material barrier.

Importance of Leak Testing in Industries:

1. Safety: Prevents hazardous accidents caused by leaks.

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