Model 943-27-31

The crystal is protected from the environment with a 0.0002 inch thick titanium end window. The titanium end window also significantly reduces the noble gas adsorption associated with traditional aluminized mylar end windows. The crystal is coupled to a 1.0 inch diameter, 10-stage photomultiplier tube through a quartz light pipe. To operate over the range required, the photomultiplier tube is operated in the current output mode, instead of the usual pulse output mode. In this mode, the photomultiplier output current is directly proportional to the incident radiation, and may be measured by an electrometer. The detector is provided with an integral high voltage and signal cable. Each cable is six (6) ft long, and terminates on a mating coaxial connector in the preamplifier enclosure. The detector operates at a nominal -500 VDC, supplied by the readout electronics. Secondary standard solid source sets, which slide directly onto the detector body, are available for on-site re-calibrations.

The Model 943-227 Digital Preamplifier is used to measure the detector output current, approximately 1.0 x 10-12 A/µCi/cc, digitize the current value, and transmit the detector reading to a remote readout via a serial data communication port. The preamplifier contains two (2) circuit boards, an electrometer board and a communications interface board. The electrometer board contains an integrating capacitor electrometer, programmable gain amplifier, digital switch, an analog to digital (A/D) converter, an EEPROM and a microprocessor. The microprocessor controls the operation of the auto-ranging, programmable gain amplifier, the charging and discharging of the integrating capacitor and the serial output port. Input to the electrometer is buffered through an ultra low leakage mosfet. The communications interface board contains a regulated DC/DC converter, an asynchronous communication interface adapter (ACIA), a RS-232 serial port driver and a modified RS-485 serial data interface (VICO loop) driver. The DC/DC converter provides on board regulation for the DC voltages required to operate the preamplifier. The ACIA drives the RS-232 serial port and the VICO loop driver. The serial port is used for diagnostic testing, or for interface with a local Model 960 system readout. The VICO loop serial port is used for communication with a remote Model 946A Readout or a supervisory computer system. Optical isolators are provided on the VICO loop transmit and receive signals. The preamplifier enclosure functions as a waterproof housing for the preamplifier circuit boards and provides a method for connecting the detector high voltage (HV) and signal coaxial cables and the digital communication cables to the control room readout.