Six isotopes cover almost all industrial gamma radiography, and the choice between them is made on photon energy against material thickness. Too low an energy and the exposure will not penetrate; too high and radiographic contrast collapses because the attenuation coefficient stops changing much with thickness.

Ytterbium-169 (mean about 93 keV) and thulium-170 (about 84 keV) are thin-section sources for small-bore tube, aluminium and composites. Selenium-75 (about 217 keV) covers roughly 10 to 40 mm of steel and has largely replaced iridium on thin pipe because its lower energy gives better contrast and a much smaller controlled area. Iridium-192 (about 380 keV mean over a complex spectrum) is the workhorse for 20 to 100 mm of steel. Cobalt-60 (1.17 and 1.33 MeV) reaches 40 to 200 mm but needs heavy shielding and produces poor contrast on thin work. Caesium-137 (662 keV) sits between them and is now uncommon in radiography.

The specific gamma-ray constant (also called the RHM value) is the exposure rate in roentgen per hour at one metre from one curie of unshielded source. It is a property of the isotope’s photon spectrum alone, so the dose rate at any distance is D = Gamma x A / d^2. Iridium at 0.48 and cobalt at 1.35 differ by nearly a factor of three for the same activity, which is why a small cobalt source demands a far larger barrier.

Half-life determines the working life of the source and how often the exposure chart must be redrawn. Ytterbium at 32 days is replaced several times a year; cobalt at 5.27 years is effectively a fixed installation.

Worked example

Isotope	ir192
Present activity	40 Ci
Distance	1 m
Half-life	73.83 d
Specific gamma-ray constant	0.48
Mean photon energy	380 keV
Dose rate at distance	19200 mR/h
Dose rate at distance	168192 uSv/h

Ir-192: half-life 73.83 d, gamma constant 0.48 R.m^2/(h.Ci). At 1 m from 40 Ci the exposure rate is 0.48 x 40 / 1^2 = 19.2 R/h = 19 200 mR/h. Converting at 8760 uSv per R gives 19.2 x 8760 = 168 192 uSv/h.