Then adjust R-30 until the recorder reads 0.70 ma. When making any calibration adjustments, tap the recorder tape in front of the pen until the pen comes to its final reading. 2.3.2 “Top Hat" Detector Calibration There are three mechanisms for obtaining the desired calibration curve, these are: A. Adjustment of the gain of the photomultiplier tube to obtain the proper slope. B. Adjustment of the bias control to set the low end of the calibration curve. C. Adjustment of the amplifier plate voltage supply to bring the amplifier into the logarithmic portion of its charucteristic. The gain of the photomultiplier tubes is adjusted by selection of the voltage regulator tubes, and setting of th potentiometers R-9 and R-19. 1000 volt corona regulator tubes (V-3 and V-l,) should be used as a first approximation to the proper value. With the range switch at position A, and the detector in a 10 mr/hr radiation field, adjust the low level photomultiplier gain control, R-9, until the recorder reads 0.82 ma. Should it be impossible to reach this current, the 1000 voltage regulator tube, VR-4, should be replaced with one regulating at a higher voltage. 0.01 ma and set the amplifier Then reduce the radiation field to grid bias control (float control) R=-27 to produce a recorder indication of 0.275 ma. (If the recorder current cannot be brought down to this value the photomultiplier tube has too high a noise level and should be replaced). Check the current at 10 mr/hr and if it has changed read just R-9, Then check the 0.01 mr/hr point end make any necessary adjustment with R-27.