image credit - @TWI Global

The science of photography far precedes radiography. Mankind is always looking for novel methods of documenting or recording information. Radiography is another methodology that uses ionizing rays to record the inner workings or structure of the subject observed under them. 

A depiction of the electromagnetic spectrum (Image credit: www.nj.gov)

Ionizing rays are waves or particles of energy given out by materials that gain or lose electrons and are often radioactive, belonging to celestial entities. They are given out by nuclear reactions or equipment producing high voltages.

This methodology has proven beneficial in medicine and follows the principle of non-destructive testing.

Most non-destructive techniques, like dye penetrant testing, magnetic testing, etc., cannot be used to detect defects in porous materials; however, due to the nature of ionizing rays, porous materials, and their internal defects, like voids, can undergo non-destructive inspection.

Discontinuities in the crystal structure of a material can go through NDE inspection with correct NDT training by adjusting the orientation of the X-ray beam using radiography.

The radiographic method of non-destructive inspection can also detect inner defects obstructed from plain sight, changes in composition, and thickness variations in the material using electromagnetic radiation of short wavelengths to impinge the materials.

Radiography can be used in real-time or recorded onto specialized films or materials sensitive to radiation. Older techniques used glass plates as a medium to record the image obtained; however, post the first world war, films made of nitrocellulose were introduced into this methodology. 

WHAT ARE RADIOGRAPHIC FILMS

Films used in the radiographic method of NDT inspection consist of a base layer of a material of polyester or cellulose layered on with an emulsion on both sides. This emulsion usually contains suspended particles like silver bromide or silver chloride suspended in gelatin. 

This emulsion adheres to the base film with a subbing layer, and a super coating protects its outer layer. The silver halide particles suspended in gelatin change their physical nature on exposure to ionizing rays like X-rays or gamma rays.

The layers of a radiographic film (Image credit: Pocket dentistry)

Human eyes cannot perceive the physical change in the crystal nature of the silver halides suspended in gelatin. The film needs further processing to be analyzed and to conduct NDT inspection on the image captured using the radiographic method of non-destructive testing. This change in nature is also termed a “latent image.” 

Further processing involves using a developer, a chemical solution to create a chemical reaction that forms black metallic silver.

SELECTION OF FILMS

Different components have different requirements regarding their reaction to different ionizing radiation, different intensities of the resulting image, and varied reactions to the power applied. The factors that determine the quality and intensity of the resulting radiographic image are as follows:

• The physical characteristics and dimensions of the workpiece or test subject undergoing non-destructive inspection under radiographic techniques are of utmost concern when determining the best results.
• The ionizing rays impinged on the material (X-rays or gamma rays) also play a key role in ensuring optimal radiographic test results.
• Priority of the operator in terms of the quality of results versus the time constraint in obtaining them also plays an important role in the final result and selection of film.

Also Read, Destructive Testing Vs Non-Destructive Testing 

PROCESSING OF FILMS

Further processing in the radiographic process is conducted after the change in the state of the materials suspended within the emulsion. To study this latent image, which is invisible to the human eye, the film is developed using the following three techniques:

• Manual Method
• Semi-Automatic Method
• Automatic Method

The processing of films aids in making the particles suspended within the emulsion produce an image of higher intensity. The process consists of the following steps:

• Developer:

A developing agent is used under controlled temperature for a dwell period to convert the silver halide crystals to darker metallic silver for increased visibility. This image formed by metallic silver is called the manifest image. It should be ensured that the developer does not sit for longer than 12 minutes (or max time as specified for the developer in that environment), as it can convert the crystals unexposed to radiation to metallic silver as well.

• Bath:

This step processes the film in a water bath that slows off the excess developer. This, in turn, stops the previous process at a precise time sufficient for highlighting only the radiation-exposed particles to obtain accurate NDE inspection results. This process only lasts around 15 seconds.

• Fixing process:

This process involves another bath for approximately 5 minutes. This bath dissolves the unexposed silver halide crystals and leaves only the metallic silver. This step ensures the storage ability of the image obtained.

• Wash:

In this step, the film is washed with water to remove any chemicals used during its processing.

• Preparation for viewing: 

To view the results, the film is either dried for ease of observation or is wet to swell the emulsion, depending on the material and other vital factors involved.

Care should be taken to ensure uniformity of operation and precise control of environmental and testing factors involved in the process. The operator’s skill and thorough NDT training ensure high-quality results.

TYPES OF RADIOGRAPHIC FILMS

Radiography has versatile uses, which often come up with unique challenges and processing differences, including the type of workpiece on which the test is conducted, the radiation type utilized, radiation energy, intensity, and the extent of monitoring required.

Radiographic films that are used for non-destructive inspection can be categorized into four varieties which include:

• Films that can be differentiated based on the Emulsion layer.

1. A single coating of the emulsion layer

2. Double coating of the emulsion layer

• Films that can be differentiated based on the Screen.

1. Films with an Intensifying screen

2. Films without an Intensifying screen

• Films that can be differentiated based on the Emulsion coating.

1. Blue light-sensitive emulsion coating.

2. Green light-sensitive emulsion coating (Orthochromatic)

3. Red light-sensitive emulsion coating (Panchromatic)

• Films that can be differentiated based on the Speed of the film.

1. Standard speed film

2. Fast-speed film

3. Ultra-speed film

Radiographic films can further be categorized into the following categories based on the modus operandi of the radiographic testing, which can include:

• Salt screen films:

This methodology utilizes a screen made of micro-crystalline metallic salt layered on a flexible base material. This microcrystal metallic salt layer can be made of calcium tungstate and has fluorescent properties under the effect of radiation. This method of NDE inspection is primarily used in the medical field; however, the contrast is lowered.

• Metal screens or no screens:

This technique is called a direct technique and can be used for multiple speeds and industrial and medical uses. The contrast of the resulting image for industrial uses improves with increased density. It has an additional coating that is heavier and almost twice that of the coating used on the films used for medical purposes. 

• Special-purpose films:

include less frequently utilized screens used for specific usage conditions.

1. Fluorographic films:

These are X-ray films with a single-side coating instead of a dual-side coating and are used to photograph a screen on which an X-ray image has been formed. This screen is fluorescent in nature and is sensitive to blue or blue-green colored light emission.

2. X-ray paper:

Modern technology allows for rapid radiographic NDE inspections utilizing an X-ray paper. An instant cycle processor unit is used to carry out this process, and it uses developing agents within the emulsion to obtain prompt results. It can also be cost-effective as the price of the X-ray paper is low.

TYPES OF INTENSIFYING SCREENS

To utilize the full effect of radiation energy that passes through the film, it is layered between intensifying screens to improve the quality of the resulting image. Many options are available in the intensifying screens that can be used to enhance the radiographic image formed. They are as follows:

• Lead screens:

Here, the screen thickness ranges between 0.02mm to 0.15mm and is used to filter backscatter. 

The base film is sandwiched between sheets of lead and increases intensity by a factor of 4.  The back screen of lead is used solely for filtering. 

This method can be used for both X-ray radiation and Gamma radiation. However, care should be maintained in polishing the lead to ensure thorough adherence and maximized contact with the base film. It also ensures that the resulting image is clearer, as any flaws or errors in finishing may disrupt the clarity of the image formed.

• Fluorometallic screens:

This methodology combines the benefits of both lead and fluorescent screens. It contains a layer of lead foil sandwiched between the fluorescent layer and the base. RCF films are utilized in this technique.

This method provides results in a short amount of cycle time. Here, the front screen acts as a filter and intensifier.

• Fluorescent screens:

These are used for materials that emit such a high intensity of light on being subject to the radiographic method of non-destructive testing that create a better-quality result on films that are light sensitive than ionizing rays directly incident on them.

This methodology is persistent for as long as the radiation effect exists. The result's intensity is twice as high as the lead screen's.

Also Read, Acceptance Criteria for Liquid Penetrant Testing

• Copper and steel screens:

In the case of high-energy radiations, steel, and copper intensifying screens are used to conduct NDE inspection. X-rays with a voltage range between 5 to 8 MeV function better with copper and steel screens than lead screens.

• Salt screens:

As mentioned, these are metallic crystal salts layered around a base film. The salt is a rare earth salt and fluoresces under radiation effect. This method drastically reduces exposure time, increasing the image intensity by 50 times.

THE PACKAGING AND HANDLING OF RADIOGRAPHIC FILMS

Radiographic films are commercially available in multiple forms. Selection of the right product is of utmost importance as it can affect how the resultant images are produced. The purchase of films in packaging most appropriate for the operator's intended method of use and handling style can make or break this non-destructive evaluation process.

In-depth knowledge of the intricacies of the process and thorough NDT training can help an operator or organization choose between individual sheets available in a box or singularly packed sheets of radiographic films.

The individually packed sheets are packed in envelopes tightly protected from light. A mechanism like a ‘rip-strip’ is provided for easy removal from its packaging during usage to ensure no contamination and efficient use. This is incredibly easy to use and ensures easy access to the film in a darkroom.

More commonly available film sheets are packed together in a box. Each individual sheet requires careful handling and protection from light; hence it is housed in a cassette which is a film holder that protects the radiographic film from unwanted light exposure.

For complex radiographic testing, wherein materials of cylindrical geometry etc., are to be analyzed, rolls of radiographic films are also available. This gives the operator the freedom to adjust the film length used. This can be economically viable in an industry wherein varied test subjects are observed under radiographic testing.

Radiographic films can be delicate and require careful and trained handling. Any error on the operator’s part in the handling of the film can cause damage to the film and affect the results of the testing process. Uniform pressure should be applied, and strain, friction, and scratches to the film should be avoided at all costs. The borders should handle the film to avoid surface contamination and scratches, which can affect the results irrespective of the technique or voltage used.

Individually packed films can help avoid these issues. Radiography is a boon to technology and can help detect defects deep within a subject. This penetrating nature can, however, be hazardous to human tissues and physical well-being. Operators and personnel should be educated about the hazards of being around these types of machinery and equipped with proper protective gear.

Organizations that peruse such non-destructive testing processes should be aware of the hazards that often come with such advanced technology and take an incentive to ensure the protection of human life and the environment while utilizing this technology to its best.