Pulsed radiation fields are encountered in the vicinity of electron Linacs (McCall and Ipe 1988). Most therapy Linacs are operated at repetition rates varying from 100 to 400 pulses per second with pulse widths of about 1 to 10 microseconds (AAPM 1986). The fraction of operating time (that is, pulse width x repetition rate) during which the beam is on is called the duty factor. For example, the duty factor for an electron Linac operating at 100 pulses per second with a pulse width of 10 µs is 0.001. These small duty factors impose severe limitations on the radiation detection instruments. The peak intensity is equal to the average intensity divided by the duty factor, which in this case will be 1,000 times higher than the average intensity. An instrument that normally responds well to the average dose rate spread out evenly in time will not be able to cope with such a high dose rate. The intense photon pulse usually overwhelms any active detector (that detects particle events electronically). Instruments which have long dead times, such as the GM and proportional counters, tend to become saturated in such fields and only count the repetition rate. Thus GM counters should not be used to perform radiation surveys outside exterior walls of the Linac room. Scintillation survey meters may become nonlinear at higher dose rates because photomultipliers cannot handle the high instantaneous currents.Ionization chambers are less influenced; however they must be operated with adequate voltage to overcome recombination losses. There are several commercially available ionization chambers that can be used for Linac radiation surveys
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THE ROOM HAS THICK CONCRETE WALLS PLUS LEAD SHEETS. LEVELS OF EXPOSURE I AM GETTING WITH LUDLUM MODEL 3 SURVEY METER (SM) A 44-9 PROBE ARE BETWEEN 0.1-3 MR/H IN WORST CASE (HIGHER ENERGY AND POINTING BEAM TO WALL). SM WAS CALIBRATED 137CS MY NOMINAL 18 MV (MEAN ~5 MV). WHAT KIND INSTRUMENT MORE ADEQUATE USE FOR THIS TYPE BETTER RESPONSE?