2.2 Mechanical Description The instrument consists of two units, excluding recorders, the “Top Hat" detector and the Control Box. cables. These are interconnected by The "Top Hat" detector is housed in a 12" high by 8" diameter aluminum cylinder; closed at one end ty a welded plated and her- metically sealed to an aluminum base plate at the other, The "Sep Hat" housing provides a hermetic seal *o prevent aiidity from affecting the high impedance sections of the detector and amplifier circuits. The phetomultiplier gain oentrols are Brought through the base plate by sealed shafts. The veltage ragulators for the photomultipliers, the dynode voltage divider networks, and the logarithmic amplifiers are within the housing with the unregulated high voltage fed through the base plate through hermetic seal connectors. The phosphor-photomultiplier assemblies are mounted perpendicularly to the base plate over thinned eut sections of the plate (1/16" thickness to reduce attenuation Of rays being measured). ‘The phosphors are provided with 1" thick peripheral lead shields fy minimize radiation effects should the plane become contaminated. The shielding also provides the characteristic of Figure 9. The control circuit is mounted in a water tight box, on" wide by 7* high by 34” deep. The control box houses a high voltage vibrator supply, the batteries and all controls with the exception of the photomultiplier tube gain controls. The controls that are used during a survey are mounted on the cover of the control box. These are the RANGE switch, B PLUS set adjustment, altitude compensation, SET and the MANUA] ALTITUDE contral. - 18 -