An x-ray beam with a monitor chamber is calibrated at a point of reference by me

Question

An x-ray beam with a monitor chamber is calibrated at a point of reference by means of an ion chamber for which N_x A_ion is 7.32 times 10^9 R/C for the beam quality used. The air temperature is 19.3 degree C at the (vented) calibrating chamber, and 19.8 degree C at the (vented) monitor chamber. The pressure is 709 torr. For a shutter-timer setting of 100 s the calibrated chamber yields a charge of 1.14 times 10^-8 C at saturation, while the monitor chamber gives 4.26 times 10^-6 C. What is the monitor calibration factor N_M, in roentgens at the reference point per coulomb collected from the monitor at 22 degree C, 760 torr? (Take shutter timing error = 0.) At a later time the x-ray beam in problem 5 is to be used to deliver an exposure of 200 R at the same location. The monitor temperature is 21.4 degree C; the pressure is 720 torr. Assuming N_M to be constant, and that the beam is automatically turned off at a preset monitor charge, what should that setting be? Approximately what would be the irradiation time if the exposure rate is roughly the same as it was when the beam was calibrated?

Explanation / Answer

5)   Monitor calibration factor , Nm = [(7.32 * 109) * 709 * 19.8 * 4.26 * 10-6]/[ 760 * 22]

                                                         =   26181.58 Roentgens

6)   a)    setting =   20 degree celsius , 700 torr and    2.8 * 10-6 C .

     b)     irradiation time   = 100 * 760/720

                                        = 105.55 sec

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