Radiographic Equipment, X-ray Units and Gamma Sources
X-ray units
The tube has a filament cathode, a target anode, a focal spot, a window and both inherent and added filtration. Generators range from self-rectified and half-wave to constant potential; constant potential gives higher output and better definition at a given kV. kV controls penetration, mA controls intensity, and duty cycle plus focal-spot thermal rating limit long exposures. Small focal spots improve geometric sharpness but restrict tube current. Tube heads need warm-up, adequate filtration and verified kV/mA calibration.
Gamma sources
Radioactive sources give discrete energies and an output that decays with half-life: Ir-192 about 74 days, Se-75 about 120 days, Yb-169 about 32 days, Co-60 about 5.3 years. Activity is quoted in becquerel or curie (1 Ci = 37 GBq) and is stated at a reference date. Exposure devices must project and retract the source safely; collimators, guide tubes and cranks must be matched and checked. Panoramic exposure uses a 360° source, while directional exposure uses a collimator to cut dose.
Traps
Confusing source activity with dose rate; ignoring decay correction when setting exposure time; mixing up half-lives in days versus years; forgetting the projected source must be locked and verified before anyone approaches; and assuming the nominal focal spot equals the effective source size used in unsharpness calculations.
Worked example
An Ir-192 source of 3.7 TBq (100 Ci) on 1 January. After 148 days (two half-lives) activity is 0.925 TBq (25 Ci), so exposure time for the same film density must be quadrupled if geometry is unchanged.