Gamma exposure time

A gamma exposure chart gives, for a material thickness and a stated film class and film density, the quantity of radiation the film needs. That quantity is expressed as an exposure factor in curie-minutes at one metre. It is a property of the part and the film, not of the particular source in the pot. To turn it into a shot time you divide by the activity you actually have and correct for the distance you are actually working at: t = EF x d^2 / A.

The d^2 term is the inverse square law. The film needs a fixed number of photons, so backing off to 1.4 m from 1 m doubles the time. The 1/A term is why decay correction matters: an iridium source loses about 1 percent of its activity per day, so a chart time written six weeks ago is already 40 percent short.

Film class changes the exposure by a large factor. Relative to a class C5 medium-speed film such as Agfa D7, a fine-grain D4 (class C3) needs roughly 3.2 times the exposure and a very fine D2 around 12 times, because the finer grain intercepts fewer photons per unit area. That is the price of the higher gradient and lower graininess needed on critical work. Faster films such as D8 cut the time roughly in half at the cost of image quality.

Use your own exposure chart, produced on your own film, processing and screens. A chart read from a textbook is a starting point for the first shot on an unfamiliar part, not a substitute for a step wedge calibration.

t = EF x (film factor / chart film factor) x d^2 / A
EF in Ci.min at 1 m, d in m, A in Ci, t in min
Exposure delivered = A x t  (Ci.min)