Estimate gamma radiography exposure time for an Ir-192 or Co-60 source from its activity, source-to-film distance, steel thickness and film speed.
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Calculator Functions
The calculator estimates how long to expose a film radiograph with an iridium-192 or cobalt-60 gamma source. It also shows the dose rate at the film with and without the object, the source activity after decay and the exposure in curie-minutes.
| Function | Use it when | You enter |
|---|---|---|
| Exposure time | You are planning a gamma radiography shot on film | Source, activity, source-to-film distance, material thickness and film |
| Source decay and material | The activity was measured on an earlier date, or the part is not steel | Days since the activity was measured and the material, in the optional section |
Activity can be entered in curies or gigabecquerels, distance in inches, centimetres, millimetres or metres, and thickness in inches, millimetres or centimetres. The film choice sets the exposure the film needs: pick fast or slow film, or enter your own figure in roentgens.
Radiography Exposure Time Formulas
| T | Exposure time (minutes) |
| E | Exposure the film needs for a density of 2.0 (R) |
| M | Material multiplier, 1.0 for steel |
| B | Attenuation by the object, the number of times it cuts the dose rate |
| X | Dose rate at the film position with no object in the beam (R/h) |
The film needs a fixed dose, so the time is that dose divided by the dose rate that reaches the film through the object. A 50 Ci Ir-192 source 24 in from fast film behind 1 in of steel gives 64.7 R/h in open air and 16.7 R/h through the steel, so 1 R takes about 3 min 35 s.
Dose rate at the film
A is the source activity in curies and d is the source-to-film distance in metres. Γ is the output of the source: 0.481 R/h per curie at 1 m for Ir-192 and 1.30 for Co-60. These come from the gamma factors 0.13 and 0.351 mSv/h at 1 m per GBq [1], with 37 GBq to the curie [1] and 1 R taken as 10 mSv. Doubling the distance makes the exposure four times as long.
Attenuation by the object
x is the material thickness and H is the half-value thickness, the thickness that halves the radiation. In iron and steel H is 1.3 cm for Ir-192 and 2.0 cm for Co-60 [1], so each extra 13 mm of steel doubles an Ir-192 exposure.
Film exposure and other materials
Fast, coarse-grained films such as D7, AA and IX100 need about 1 R to reach a density of 2.0, and the very fine-grained D2 needs about 13 R; other films fall in between [2]. For a film or a density not listed, choose Custom film exposure. For a metal other than steel, the steel time is multiplied by an approximate factor [2]:
| Material | Multiplier on the steel time |
|---|---|
| Aluminum | 0.35 |
| Titanium | 0.9 |
| Iron and all steels | 1.0 |
| Copper | 1.1 |
| Inconel X | 1.3 |
Source decay
A₀ is the activity on the date it was measured, t is the time since then and t½ is the half-life: 74 days for Ir-192 and 5.25 years for Co-60 [1]. An Ir-192 source loses half its output in 74 days, so the same shot then takes twice as long. The Exposure Rate Calculator gives the dose rate at any distance from a source for other nuclides.
Limits of This Estimate
- The result is a starting point for a first exposure, not a procedure. The half-value thicknesses are general shielding values [1], and scatter, screens, film batch and processing all shift the real time. Confirm it with the film maker’s exposure chart or a test shot. These estimates are based on the best available data and estimations.
- The film exposures are for a density of 2.0. A darker radiograph, which many procedures ask for, needs a longer exposure [2].
- Ir-192 suits steel up to about 3 in (76 mm) thick; thicker sections are usually shot with Co-60 [2]. The calculator warns you above that thickness.
- The source is treated as a point with nothing but the object between it and the film. Gamma radiography sources are used only by trained, licensed radiographers working to their radiation safety procedures.
Sources
- International Atomic Energy Agency, Manual on Gamma Radiography, IAEA-PRSM-1 (Rev. 1), 1996: gamma factors, the dose rate and distance equation, half-value thicknesses in iron, half-lives and the curie
- U.S. Patent 6,135,348, Apparatus and methods for calculating gamma radiography variables, 2000: film exposures for a density of 2.0, material multipliers and the working range of Ir-192
