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Increased filtration due to tungsten deposits inside the envelopes of x-ray tubes.

Reduced output from X-ray units is often attributed to increased filtration due to tungsten deposits inside the tube envelope (Dunlap, 1977). During 12 years of full-time radiation survey work I have measured the output and quality of radiation under standard conditions and excitation voltage of all the X-ray installations in my area and have not detected any significant change in HVT, even on the most heavily used units, from the value when the tube was new to that after as many as 200000 exposures. The largest change was from 2·6 to 2·65 mm Al, i.e. 0.05 mm Al (within experimental error for a poorly regulated unit). My measuring technique has been constant throughout using the same aluminium filters, standard jig, chamber and dosimeter at 80 kV estimated from Ardran penetrameter results and direct measurement.