Free Radiography Exposure Time Calculator for Industrial NDT: Ir-192, Co-60, and Se-75

The answer is their very different half-lives. Iridium-192 has a half-life of 73.83 days, meaning it loses half its activity every 74 days. A source that starts at 100 Ci will be 50 Ci at 74 days, 25 Ci at 148 days, and only 12.5 Ci at 222 days. Below roughly 10 Ci, the shot times required for practical work become economically unacceptable: a shot that took 3 minutes at 100 Ci takes over 24 minutes at 12.5 Ci, which is impractical for production pipeline radiography. Cobalt-60 has a half-life of 1,925.1 days (approximately 5.27 years). A Co-60 source of 200 Ci will still be 186 Ci one year later. Co-60 sources are practical for 3 to 5 years, which is why they are the preferred isotope for permanent or semi-permanent exposure facilities such as industrial radiography bays at fabrication shops. The tradeoff is that Co-60’s much higher energy (1.25 MeV average versus 340 keV for Ir-192) requires significantly larger controlled areas and more substantial shielding for storage and transport.

The RHM value (Roentgen per Hour at 1 Meter per Curie) is the specific gamma ray constant for a radioactive isotope. It quantifies how much radiation exposure in Roentgens a one-Curie source produces at one meter distance in one hour. The RHM is specific to each isotope because it depends on the energy and intensity of the gamma photons that isotope emits. Cobalt-60 emits two very energetic gamma rays (1.173 and 1.333 MeV) almost simultaneously with every nuclear decay, giving it an RHM of 1.32. Iridium-192 emits multiple lower-energy gammas across a broader spectrum, giving it an RHM of 0.55. Selenium-75 emits lower-energy gammas still and has an RHM of only 0.20. This is why a 100 Ci Co-60 source produces about 2.4 times more exposure per hour at the same distance as a 100 Ci Ir-192 source: the RHM ratio is 1.32 to 0.55. The inverse square law then scales this with distance squared, so doubling the SFD quarters the dose rate at the detector regardless of which isotope you are using.

ASME Section V Article 2, paragraph T-274 specifies the maximum allowable geometric unsharpness (Ug) for radiographic examination of pressure equipment. Ug is calculated using the formula Ug = f x b / a where f is the source focal spot size (or source diameter for sealed radioactive sources), b is the distance from the source side of the object to the film, and a is the distance from the source to the source side of the object. The limits in T-274.2 are: 1/32 inch (0.794 mm) for material thickness up to 2 inches, 1/16 inch (1.588 mm) for thickness 2 to 3 inches, and 1/8 inch (3.175 mm) for thickness over 3 inches. These limits apply to all radiographic examinations performed per ASME Section V, which includes pressure vessels under ASME Sec VIII, piping under ASME B31.3 and B31.1, and nuclear components under ASME Section III. When Ug exceeds the limit, the exposure is not accepted and must be repeated with modified geometry, typically by increasing the SFD or using a smaller source.

The film factor is the radiation exposure in Roentgens required to develop the film to the target optical density at the processing conditions you are using. Different film grades have different speeds, or sensitivities. D7 film (fast or Type 6 per ASTM E1815) requires less exposure to reach a given density than D5 (medium) or D4 (slow). The tradeoff is image quality: slower films have finer grain and better contrast, producing radiographs that can detect smaller discontinuities, but they require longer exposure times. For most field pipeline work where production speed matters, D7 is the standard choice. For nuclear or aerospace components where maximum defect sensitivity is required, D4 or slower films are specified. The exact film factor varies by manufacturer (Agfa, Carestream, Fuji, Kodak) and processing conditions (developer temperature, development time, replenishment rate). The default values in this calculator are typical reference values. For production work, verify your specific film factor using your film manufacturer’s characteristic curve and your laboratory processing conditions, then enter that value in the film factor override field.

ASME Section V Article 2, paragraph T-282 specifies that the optical density of the processed radiograph shall be a minimum of 2.0 for single-film viewing and a minimum of 1.8 for composite film viewing when using Class 1 film. The maximum density is 4.0 for single-film viewing. In practice, most radiographic testing procedures target a density of 2.5 to 3.5 because this range provides optimal contrast for defect detection in most industrial metals. Densities below 2.0 produce low-contrast films where subtle changes in thickness (indicating small discontinuities) are difficult to detect on the densitometer. Densities above 4.0 require very bright viewing illuminators to achieve adequate transmitted light and make the film difficult to interpret in typical field conditions. Your written procedure must state the required density range, and the densitometer used to verify film density must be calibrated per your procedure requirements. ASME B31.3 process piping adopts ASME V requirements by reference, as does B31.1 power piping and most vessel fabrication codes under ASME Section VIII.

No. This calculator is specifically designed for sealed radioactive gamma sources (Ir-192, Co-60, Se-75). X-ray machine exposure calculations are fundamentally different because X-ray tube output is described in milliampere-seconds (mAs) and kilovoltage (kV), not in Curies and RHM. X-ray output is also adjustable in real time by changing kV and mA settings, whereas a sealed source produces a fixed radiation output that decreases only through radioactive decay. The inverse square law still applies for X-ray machines, and geometric unsharpness with ASME T-274 limits applies to X-ray radiography as well (with the focal spot size of the X-ray tube used as f in the Ug formula). A separate X-ray exposure calculator using mAs and characteristic curve data would be needed for X-ray tube calculations. Note that X-ray machines are regulated by OSHA and state health departments, not the NRC, because they generate radiation electrically rather than through nuclear decay. This is a significant regulatory distinction that affects licensing requirements.

A step-wedge exposure (also called a step-wedge calibration shot or calibration strip) is a reference radiograph taken with a calibrated step-wedge test piece of known thicknesses alongside or in the same film holder as the production radiograph. It allows the radiographer to verify that the calculated exposure time is producing the correct film density for the specific combination of isotope, material, SFD, and film processing being used on that job. The step-wedge result confirms whether the film factor (FF) value used in the calculation is correct for the actual film and processing conditions. Without a step-wedge calibration, the exposure calculation is theoretical: it assumes that the film factor corresponds to your actual film and processing conditions. In practice, film age, developer temperature, development time, and processing chemistry all affect the actual film factor. ASME Section V, API 1104, and most quality programs require that exposure techniques be established (demonstrated to produce acceptable film quality) before production examination, and the step-wedge is the standard method for this demonstration.

These terms describe the exposure geometry for pipe weld inspection. Single-wall single-image (SWSI) places the source inside the pipe and the film on the outside. The radiation passes through only one wall thickness, and that single wall is what is being inspected. This produces the best image quality but requires a crawl device or internal source placement, which is not always possible for small-bore pipe or limited-access configurations. Double-wall double-viewer (DWDV) places both the source and the film on the outside of the pipe. The radiation passes through both walls, and welds on both the near and far side of the pipe are imaged simultaneously (though with different effective distances). The exposure calculation for DWDV uses the total thickness of both walls as t in the HVL formula. The Ug calculation for DWDV uses the outside diameter (OD) as b per ASME V T-274, not twice the wall thickness. ASME Section V permits DWDV only under specific conditions and requires that the IQI sensitivity be demonstrated on the thickest wall being evaluated. API 1104 for pipeline welds similarly specifies when each exposure geometry is acceptable.

The controlled area boundary is established by calculating the distance at which the dose rate from the source (after it emerges from the exposure device and passes through the collimator if one is used) equals the boundary limit specified in your written procedure. Under NRC regulations and OSHA 29 CFR 1910.1096, the controlled area boundary is set where the dose rate is 2 millirem per hour or less for unrestricted public access, or 100 millirem per hour for restricted-access controlled areas where only monitored radiation workers are present. The dose rate at a given distance from a source is calculated using: dose rate (mrem/hr) = (A_Ci x RHM x 1000) / distance_meters squared. Rearranging for the required boundary distance: d_meters = sqrt((A_Ci x RHM x 1000) / 2) for the public boundary. For a 30 Ci Ir-192 source (RHM = 0.55) in open air without shielding: public boundary d = sqrt(30 x 0.55 x 1000 / 2) = sqrt(8250) = 90.8 meters. Most field radiography is conducted in partially shielded environments where pipe walls, concrete, and the exposure device itself substantially reduce this distance. Your written procedure should specify the boundary calculation method and your radiation survey meter must verify the actual dose rate at the boundary before locking out the area.

Se-75 has become the preferred isotope for stainless steel pipe walls in the 5 to 40 mm range for two main reasons: better image contrast and smaller physical source size enabling panoramic exposures in confined access geometries. At thin wall thicknesses, Ir-192’s higher average energy (340 keV) produces more radiation that passes through the material without interacting, called scatter, which reduces contrast and makes small cracks and porosity harder to distinguish from the background film density. Se-75’s lower average energy (216 keV) interacts more selectively with the material, producing better differential attenuation and therefore better contrast for thin-wall inspection. The second advantage is physical source size. Se-75 sources are available in very small configurations (1 mm diameter or less) that fit inside small-bore pipe for panoramic exposure, where a single shot exposes the entire circumference simultaneously. Ir-192 sources for similar activities are physically larger, making panoramic shots in small-bore pipe impractical. The tradeoff is that Se-75 is too low in energy for material thicker than about 40 mm in steel, and its 119.78-day half-life means more frequent source replacement than Co-60 (though less frequent than Ir-192). Per the NRC, all three isotopes require the same 10 CFR Part 34 licensing.

NRC 10 CFR Part 34 requires that radioactive sources used in industrial radiography be stored in a shielded container (the exposure device or storage container) that limits the dose rate at the surface to less than 200 mrem per hour, and at 1 meter from the container surface to less than 10 mrem per hour, when the source is in the fully retracted (safe) position. Storage locations must be secured to prevent unauthorized access and must be posted with appropriate radiation warning signs. The source and its storage container must be inventoried at intervals not exceeding 3 months, and the inventory records must be retained for 3 years. Overnight storage of sources in field locations requires either a locked vehicle or a lockable storage location that meets NRC access control requirements. Transportation of radioactive materials is governed by DOT regulations under 49 CFR Part 173, which specify packaging requirements, labeling, shipping papers, and quantity limits that apply whenever the source moves by any vehicle including company trucks. Moving across state lines requires the NRC Form-241 filing discussed elsewhere in this article.

An Image Quality Indicator (IQI), also called a penetrameter, is a reference device placed on the subject being radiographed to verify that the radiographic technique is sensitive enough to detect a discontinuity of specified size. ASME Section V uses two types: wire IQI (per ASTM E747) consisting of wires of graduated diameters, and plaque IQI (per ASTM E1025) consisting of a metal plate with drilled holes of specified sizes. The IQI is placed on the source side of the weld at the time of exposure. After developing the film, the radiographer must be able to see the specified essential wire (for wire IQI) or the 2T hole (for plaque IQI) in the image of the IQI to confirm the technique is sensitive enough. If the required wire or hole is not visible, the technique is insufficient and the radiograph cannot be accepted regardless of what it shows about the weld. IQI selection is based on the material thickness, using ASME V Article 2 Table T-276 for wire IQIs or Table T-277.1 for plaque IQIs. Getting the IQI selection wrong is one of the most common quality nonconformances in radiographic examination programs audited by ASME Authorized Inspection Agencies.

Partially. The source activity decay calculation, the HVL attenuation factor, and the geometric unsharpness calculation all apply equally to computed radiography (CR using phosphor storage plates) and direct digital radiography (DR using flat-panel detectors) as to conventional film. The physics of radiation transport through the material is the same regardless of the detector type. What does not directly apply is the film factor (FF): CR and DR systems use different sensitivity metrics than film. CR systems are characterized by their exposure index, and DR systems use their detector efficiency curves. ASME Section V Article 2 now includes provisions for digital imaging systems with acceptance criteria defined in terms of signal-to-noise ratio and equivalent penetrameter sensitivity rather than optical film density. If you are using CR or DR, use the exposure time output from this calculator as an approximate starting point scaled to your system’s validated technique, and verify with the digital technique-specific calibration exposures required by ASME V Article 2 Appendix VII or your procedure.

Each half-life reduces the source activity by exactly 50 percent. An Ir-192 source starting at 100 Ci follows this pattern: 74 days = 50 Ci, 148 days = 25 Ci, 222 days = 12.5 Ci, 296 days = 6.25 Ci. The practical lower limit for field pipeline radiography is approximately 10 Ci, where shot times at typical field distances become economically unacceptable. Reaching 10 Ci from a new 100 Ci source takes approximately 245 days (about 3.3 half-lives), which is why Ir-192 sources in high-volume field shops are typically replaced every 3 to 5 months depending on initial activity and required shot times. When activity drops below the useful minimum, the source must be returned to the supplier for disposal per NRC regulations. The supplier handles the licensed disposal; the licensee does not dispose of sealed sources independently. Source disposal documentation must be retained as part of the license records. The 180-day activity forecast chart in this calculator helps you plan when to order a replacement source so you are not caught with an unusable source mid-project.

Per NRC 10 CFR Part 34, a radiographer is an individual who has been certified by the licensee to perform radiographic operations. This requires demonstrating knowledge of the licensee’s written procedures, NRC and DOT regulations, radiation safety principles, and the physical operation of the exposure devices and survey instruments the licensee uses. Certification typically requires documented training, experience, and passing a written examination. The radiographer is personally responsible for ensuring that each radiographic exposure is conducted safely and in compliance with the applicable regulations and procedures. A radiographer’s assistant is an individual who works with a radiographer during field operations. The assistant may handle the exposure device under the direct supervision of the radiographer, maintain the controlled area perimeter, position the guide tube, and perform similar support tasks. The assistant may not operate the exposure device independently (that is, move or change the source) without the radiographer present. Both must have dosimetry and both count toward the minimum two-person crew requirement. The distinction matters because only the licensed radiographer bears personal legal responsibility under the NRC license for the safety of each operation.

The exposure time formula in this calculator accounts for primary radiation attenuation through the material using the HVL exponential factor, but it does not explicitly account for scatter. Scatter is radiation that has been deflected from its original path by interaction with the material, the exposure device structure, or surrounding objects, and still reaches the film. In thick sections and at higher gamma energies (particularly with Co-60), scatter can constitute 20 to 50 percent of the total radiation at the film. Scatter does not carry useful image information and acts as fog, reducing contrast and making real defects harder to see. To reduce scatter, radiographers use lead intensifying screens, collimators that restrict the beam to the inspection area, back-lead sheets under the film to block backscatter from the ground or structure, and increased SFD when geometry permits. The effect of scatter on exposure time is that it contributes to film density without improving image quality, meaning that in heavy-scatter situations you may achieve the required density with less primary radiation than the formula predicts, but the resulting radiograph will have lower contrast than expected. Always use a B marker (lead letter B placed on the back of the cassette) to check for significant backscatter, which shows up as a lighter B image on the radiograph if backscatter is excessive.