The HVPSs were fabricated at Sandia National Laboratories (SNL), in Albuquerque, New Mexico. They were mounted to the IPCs at Columbia University in New York. The HVPSs convert low voltage power to high voltages using a high frequency step-up transformer. The transformer is driven at a frequency of 60 kHz. The high voltage (HV) output is controlled by a simple step/reset digital interface. The HV control digital to analog converter has 10 bits and the range runs from 0 V to full scale (near 700 V or near 4000 V). Each HVPS has analog monitor outputs which allow the output voltage and current to be measured.
3.3 E-Boxes The E-box contains a high voltage filtering network and preamplifiers. The E-boxes were fabricated at Sandia National Laboratories (SNL), in Albuquerque, New Mexico. They were mounted to the IPCs at Columbia University in New York. After preliminary (but complete) testing, the high voltage components in the E-box were potted. The potting must cover the high voltage feedthroughs on the IPC as well as the high voltage components in the E-box. After potting, it is very difficult to remove the E-box from the IPC without damage to the E-box components. Therefore, the E-box and IPC are treated as a mated pair after potting.
3.3.1 Mechanical Layout The E-box is attached to the IPC via six mounting studs on the IPC. The electrical interface is via 12 feedthroughs and ground contact through the IPC body. The pattern of the feedthroughs is shown in Figure 3.3-1. The two large feedthroughs are used for the Anode and Anti and are rated for 6 kV. The remaining feedthroughs are rated for 1 kV.
The mechanical layout of the E-box is shown in Figure 3.3-2. The E-box contains two printed circuit boards. One is for the high voltage components (the HV board) and the other is for the preamplifiers and other low voltage components (the LV board). High voltage is brought into the E-box via three cables. These are soldered to attachment points within the E-box and have connectors for mating with the HVPSs.
3.3.2 High Voltage Network The E-box schematics are shown in Figures 3.3-3a, 3.3-3b, and 3.3-3c. The circuits for the Anode and Anti planes are identical. Each is powered by a separate HVPS. The input HV is filtered through two stages of RC filtering. The wire plane is isolated by a 500 megaohm resistor. A 1.5 nF decoupling capacitor is used to isolate the preamplifier from high voltage. A high voltage monitor divides the voltage by 10^4.
The single cathode HVPS powers all of the cathode wire planes, the W&S electrodes (including the side anticoincidence), and the field forming rings. The input HV is filtered through two stages of RC filtering. The cathode wire planes and the W&S electrodes are all held at the same voltage. Each wire plane or electrode is isolated from the common voltage point by a 500 megaohm resistor. We note that the two cathodes surrounding the Anti wire plane are both connected to the same feedthrough. The field forming rings are attached to a voltage divider chain so that the applied voltages are 1/5, 2/5, 3/5, and 4/5 of the cathode voltage. Each instrumented electrode is connected to a 3.3 nF decoupling capacitor which passes the signal through to its preamplifier. A high voltage monitor divides the voltage by 10^3.
3.3.3 Low Voltage Circuits Six amptek A250 charge sensitive preamplifiers are installed on the low voltage board. The following names are given to the six electronics channels: Anode, Backanti, Sideanti, Wedge, Strip, and Zee. The Anode and Backanti preamplifiers are connected to the Anode and Anti wire planes respectively. The Wedge, Strip, and Zee preamplifiers are connected to the Wedge, Strip, and Zee electrodes respectively and the Sideanti preamplifier is connected to the copper electrode surrounding the Wedge, Strip, and Zee electrodes.
As shown in the schematics in Figures 3.3-3a, 3.3-3b, and 3.3-3c each preamplifier has a signal input connected to a decoupling capacitor and a test input. The preamplifier FET is protected by a diode which diverts charge away from the FET in case of a spark. The test pulse signals are differentiated in a 1 pF capacitor to produce a charge input of (1 pC/V). The preamplifier operates in a charge sensitive mode. The feedback capacitor is 1 pF, which sets the charge to voltage conversion of 1 V/pC. A feedback resistance of 300 megaohms gives a time constant of 300 microseconds. This limits the maximum useful count rate to roughly 1 kHz. Different FETs are used for the Anode and Anti versus the W&S electrodes due to the difference in capacitance of the electrodes.
In addition to the preamplifiers, the low voltage section also contains an AD590 temperature transducer used to monitor the temperature of the E-box.
3.4 Operation Interaction of an x-ray or charged particle in the IPC will produce signals at the preamplifier outputs. The Anode signal is used to estimate the energy deposited in the IPC for a given event; in addition, the timing properties of the Anode signal are used as a means of background rejection. The Wedge, Strip, and Zee signal amplitudes are used for two-dimensional imaging. Signals are produced by the Backanti and Sideanti only if charge is deposited in the rear anticoincidence region or in the side anticoincidence region above the Sideanti electrode. These two signals are used for background rejection. Next Page
