Table Of Content
- The Principles of Radiography
- Equipment and Testing Mechanism
- Film Capture
- Image Quality Indicators (IQIs)
- Radioactive Material and Handling
- Key Takeaways
- FAQs
The penetrative power of ionising radiations has proved their applicability in industries through Radiographic Testing. Using different kinds of ionising radiation, this technique has provided in-depth full-body assessments of test subjects in the Non-destructive Testing industry.
In 1895, Wilhelm Conrad Roentgen developed the X-ray technique by halting high-energy electrons in their path using a metal target in a vacuum tube, termed the X-ray tube.
X-rays are a spectrum, and are conventionally described with the unit “kV”, representing the peak value of the spectrum, as using the unit “keV” would otherwise suggest that they have a single wavelength.
Gamma rays, unlike X-rays, do not have controllable intensities and are emitted in an isolated line spectrum. Their high penetration, which is undeflectable and unperceivable, can be manipulated using crystalline grids, which have made them applicable within the NDT sphere as well. Numerous principles of physics come into play while implementing these electromagnetic waves for assessing materials, structures and machinery, which play a major role in the quality of results during NDT inspections.
The Principles of Radiography
Radiographic Testing Equipment
The radiation hardness of X-rays and Gamma rays is dependent on the wavelengths of these electromagnetic waves. Harder radiation is characteristic of small wavelengths, while radiation energy is soft when the wavelengths are long. The beam quality of X-ray tubes is affected by the tube voltage range, and the tubes in the market are characterised by the radiation energy.
1. Absorption and Scattering
While impinging on a test subject, these electromagnetic waves reduce in intensity, affected by phenomena like the Photoelectric effect, Compton effect and Pair production. The energy of the incident radiation and the material irradiated causes one of these reactions to predominate, with the attenuation of the X-rays being a combination of the three. In these, the primary X-ray energy transforms into a lower form of energy. These processes often cause secondary X-ray energy with different wavelengths and directions, which do not contribute to the Radiographic inspection processes but may cause a loss in image quality.
2. Penetrating Power
The energy, or radiation hardness, of the radiation is directly proportional to its penetrating power. The mechanisms that cause radiation absorption affect this relationship between energy and penetrating power. Homogeneous, single-wavelength radiation undergoes an intensity reduction when it passes through matter, with its magnitude proportional to the thickness of the material. Softer radiation gets filtered out more than harder radiation when the material thickness is increased, and this phenomenon is called ‘hardening’.
3. Filtering
Materials placed between the source of radiation and the radiographic film can cause absorption and filtering. The position of this median material is vital, as a metal layer may filter out the soft radiation. This helps in inspecting test subjects with varying thicknesses and can reduce contrast. Filtering materials may include Lead, Copper and Steel.
4. Half-value Thickness
The penetrating power of radiation for specific materials has been quantified by the half-value thickness, referring to the thickness required to reduce the intensity of a radiation beam (monochromatic) by half. In applications wherein hard radiation is used, an assumed average Half-value thickness (HVT) is used for accuracy, to compensate for the inconsistencies in HVT for a heterogeneous beam.
Radioactivity and the effects of radiation have been quantified, with its rapid global adoption in various fields, including Radiographic Testing and NDT. The radioactivity of a radiation source (isotope) is calculated as per the number of disintegrations per second. Industrial RT NDT methods also use the SI unit Becquerel (Bq), which is equivalent to 1 disintegration per second.